Conceptual design of an autonomous and modular naval vessel DEPARTMENT OF MECHANICS AND MARITIME SCIENCES CHALMERS UNIVERSITY OF TECHNOLOGY Gothenburg, Sweden 2025 www.chalmers.se www.chalmers.se Concept design of an autonomous and modular naval vessel Jakob Haga, Christopher Ljungholm Examiner: Per Mottram Hogström Supervisor: Joakim Hill (SAAB Kockums AB) Department of Mechanics and Maritime Sciences, Marine technology Chalmers University of Technology Gothenburg, Sweden 2025 Concept design of an autonomous and modular naval vessel Jakob Haga, Christopher Ljungholm Examiner: Per Mottram Hogström Supervisor: Joakim Hill (SAAB Kockums AB) © Jakob Haga, Christopher Ljungholm 2025. Department of Mechanics and Maritime Sciences Chalmers University of Technology SE-412 96 Gothenburg Sweden Telephone: + 46 (0)31-772 1000 Cover: Rendered image of the concept design. Abstract The private sector and authorities are both showing increasing interest in autonomous ships, making it necessary to examine and determine the best approach for transition- ing toward uncrewed vessels. The maritime legislative landscape regarding autonomous ships remains in a stale mate, as legislators wait for guidance from the International Mar- itime Organization before formulating national laws to govern these vessels, and private companies wait for the legislators and classing societies before investing large sums into autonomous vessels. This study investigates the transition from crewed to uncrewed vessels from both a naval architecture and legislative perspective. Based on these findings, an intermediary vessel is proposed through an iterative design process. The transition pathway is examined in depth, considering the technical, operational, and regulatory challenges involved. The resulting vessel concept is designed to operate flexibly in both crewed and uncrewed modes. Emphasis is placed on modularity, with a fully detachable crew compartment that houses crew essential systems such as toiletry and galley. This approach ensures that the vessel remains streamlined and functionally optimized when operating autonomously. Keywords: autonomous, modular, flexible, concept design, naval, defence, USV. Preface and Acknowledgments We would like to extend our gratitude to the following people: • Our examiner Per Mottram Hogström, Senior Lecturer at Chalmers University of technology, for his help and guidance during this thesis as well as during our pre- ceding studies. • Our supervisor Joakim Hill, Head of Naval Architecture and Signatures at Saab Kockums AB, for his guidance and invaluable insight in the concept development process in the defence industry. • Rebecka Bondesson, Lecturer at Chalmers university of Technology, for her help with navigating maritime rules and regulations. • Independent Defence analyst H.I. Sutton for invaluable insights into the current landscape of naval warfare and future aspects of USV technologies. • Mats Hammander from the Swedish Transport Agency for taking us up to date on the current legislative work within IMO regarding unmanned vessels. Glossary Autonomous A ship that can navigate and make decisions in order to complete tasks in a safe manner without the need for human input. Flexibility A vessel that have the capability to change equipment depending on mission profile at a very short notice. Gross Tonnage Gross Tonnage is a value calculated from total internal volume multi- plied with a logarithmic coefficient also calculated from the internal volume. Large Unmanned Surface Vessel Unmanned surface vessel with an overall length longer than 50 m.. Medium Unmanned Surface Vessel Unmanned surface vessel with an overall length longer than 12 m but less than 50 m.. Modularity A vessel or series of vessels designed to share large parts of its interior, structure and systems in order to facilitate ease of upgrading or class-wide similar- ities. Transportstyrelsen Swedish Transport Administration, governmental authority that formulates legislative text regarding all forms of transportation in Sweden. Acronyms CONOPS Concept Of Operations. DARPA Defence Advanced Research Projects Agency. EW Electronic Warfare. FMV Swedish Defence Materiel Administration. GT Gross Tonnage. IMO International Maritime Organization. LOA Length Overall. LUSV Large Unmanned Surface Vessel. MASS Maritime Autonomous Surface Ships. MUSV Medium Unmanned Surface Vessel. NOMARS No Manning Required Ship. STCW Seafarers’ Training, Certification and Watchkeeping. USV Unmanned Surface Vessel. Contents 1 Introduction 1 1.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 1.2 Aim . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.3 Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.4 Scope and limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 1.5 Stakeholders . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 2 Literature study 8 2.1 Applicable rules and regulations . . . . . . . . . . . . . . . . . . . . . . . . 8 2.2 Crew Logistics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 2.3 Current USVs in development and use . . . . . . . . . . . . . . . . . . . . 15 2.4 Use cases for naval USVs . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 2.5 Modularity and flexibility . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 2.6 Conclusions of literature study . . . . . . . . . . . . . . . . . . . . . . . . . 21 3 Requirements 24 3.1 Mission profile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 3.2 Requirements from stakeholders . . . . . . . . . . . . . . . . . . . . . . . . 26 4 Main particulars 28 4.1 Ship particulars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 4.2 Payload . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 4.3 Energy estimates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 5 Concept Design 35 5.1 Hull Configurations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 5.2 Propulsion system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 5.3 Machinery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 5.4 Chosen machinery system . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 5.5 Modularity and flexibility . . . . . . . . . . . . . . . . . . . . . . . . . . . 48 5.6 Crew compartments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 5.7 Water treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 6 Final concept 52 6.1 Chosen concept . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 6.2 Crew Modules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 Contents 7 Discussion 62 7.1 IMO MASS regulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 7.2 Level of flexibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 7.3 Point defence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 7.4 Scope of operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 7.5 Technological readiness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66 7.6 Autonomous USV, a Vessel or Equipment? . . . . . . . . . . . . . . . . . . 67 8 Conclusion 68 9 Recommendations for future work 69 9.1 Detailed design of the vessel . . . . . . . . . . . . . . . . . . . . . . . . . . 69 9.2 Other concepts to consider . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 A Appendix Thought Experiment I A.1 Alternative C – Medium Sized Unmanned Surface Ship . . . . . . . . . . . I A.2 Alternative D – Large Unmanned Surface Ship . . . . . . . . . . . . . . . . V B Appendix Studied Vessels X 1 Introduction In the automotive and aerospace industries the technology facilitating autonomous vehi- cles have in recent years seen great advances, where aerial drones have successfully been deployed in many missions since the turn of the millennia. However within the maritime sector there has been relatively little progress in the field of automation, especially in comparison to the aerospace sector. One reason for this is that while the International Maritime Organization (IMO) is currently developing regulations for autonomous ships, IMO (2025), clear and distinctive legislation regarding unmanned surface vessels (USV) are yet to be implemented. This has lead to some flag states approaching autonomous ships individually instead of relying on the IMO. For example the Norwegian Maritime Authority have developed their own guidelines for domestic testing of autonomous ships (Sjøfartsdirektoratet 2020). Another nation having a similar program is Japan (with the MEGURI2040 (2020) programme) that have developed a multi-stage plan for the intro- duction of autonomous vessels into commercial traffic. These regulations are based on the existing regulations for conventional ships, the Norwegian government has also cre- ated designated testing areas for autonomous ships to be tested in a real environment (Voldsund 2022). The Russian invasion of Ukraine has shown what strategical potential USVs can provide in a military context. Ukraine has quickly developed a fleet of small USVs that have made a significant impact by attacking several larger Russian warships. H. I. Sutton (2024a) argues that many of these attacks are not possible with crewed vessels, as crewed vessels need extra volume to account for crew compartments and systems related to human needs, thus making them easier to detect and target. With the military implications shown in the war, many other navies have started showing increased interest in the subject of unmanned surface vessels. While much of the recent development is focused on smaller vessels, there is a clear increase in the development of larger vessels such as USA’s NOMARS (Parken 2024), China’s USV-JARI-A, or Thales and Stellers’ TX-Ship programmes, compiled information about those can be found in Appendix B. Saab Kockums AB is likewise interested in exploring the potential of a larger USV and the unique opportunities and challenges that these vessels can provide. The Swedish Shipowners’ Association (SSA) have published their considerations for de- veloping SMART ships (Swedish Shipowners’ Association 2021). SMART being the op- erative word for a collection of advanced assisted or remote controlled vessels. SSA state the overall reason for SMART vessels is: 1 1. Introduction The reasons for developing SMART ships should be to attain increased safety, efficiency, and sustainability. Furthermore SSA provide perspectives from different stakeholders, and state that the IMO must lead the development of the regulations. It is not considered that existing rules and regulations are sufficient for highly automated and digitalized vessels with only a minor, or no crew. This means that rules and regulations for SMART Ships needs to be established and this must take place on an international level; IMO needs to lead this. This creates a regulatory void for shipbuilders that even though some ship builders are willing and capable of manufacturing and implementing autonomous systems, the regula- tions essentially prevent them from developing and testing new systems. This forces the still willing shipbuilders into a potentially costly IMO1455 process, as RISE (2024) have described it, and this is currently is the only viable way to introduce new technologies, such as unmanned surface vessels, into commercial traffic. Naval ships have have additional incentives to increase the level of autonomy as this allows allows for fewer crew aboard a ship, thus minimizing the risk for causalities in the case of an attack. Less crew aboard the vessel also means that less space needs to be designated for crew amenities, leading to smaller and lighter ships. This in turn increases the manoeuvrability and reduces the detectability of the vessel. 1.1 Background Vessels not intended for use in private leisure and exceed 24 meters in length currently needs a trading certificate and subsequently a decision on minimum safe manning and thus the assignment of a crew to ensure safe operation (Regeringen 2003). In Swedish national waters these rules are governed by the Swedish Transport Agency, and by the IMO in international waters. Many nations and thus the aforementioned agencies follow the IMO guidelines for the domestic regulations as well, prompting the need for crew in domestic waters. Current Swedish maritime regulations are derived from the guidelines set out by the IMO. While the IMO have announced the development of a regulatory framework for Maritime Autonomous Surface Ships (MASS), this regulatory framework is not expected to be finalized before 2032 (IMO 2025). The absence of clear international regulation contribute to an uncertainty for shipbuilders and shipowners, where investments in de- velopment and research risk being redundant in the near future because of regulatory changes. Some adaptations and research have been proposed, such as Bolbot et al. (2025) where the authors identified commonly referenced acceptance principles in the maritime regulatory framework related to the new MASS framework. This can be used to aid and support the decision-makers regarding the safety of implementing autonomous ships. Addition- ally crew wages contribute only a small fraction to the total cost of operating a vessel, Kretschmann et al. (2017), thus reducing the financial incentive to achieve full autonomy. The risk for the crew aboard a surface vessel is also significantly lower than in other sec- tors such as underwater or aerospace, where unmanned technology has progressed further 2 1. Introduction (Nakashima et al. 2023). In addition an autonomous vessel will have increased costs in certain areas as costs for remote control centres, large shore based maintenance crews, boarding crews for port calls, etc, (Kretschmann et al. 2017). These factors has lead many shipbuilders to hold off on developing autonomous systems until the IMO releases new guidelines. The Russian invasion of Ukraine has significantly accelerated the development and opera- tional deployment of small USVs, marking an acceleration from the previously incremental pace of advancement in the field. Previous uses of unmanned systems have been limited to patrolling and surveillance, but now USVs are actively engaging in combat operations, one example being the Ukranian use of USVs to eliminate Russian fighter jets, as re- ported in the Kyiv Post by Zakharchenko (2025), an image from the instance is included in Fig. 1.1. Ukrainian naval forces have demonstrated notable success in their use of USVs against larger, conventional Russian warships (Rishko 2024). These operations demon- strate that USVs can pose a substantial threat while enabling the operator to remain at a safe distance, thereby reducing the risk to personnel and lowering the operational cost of engagement. The tactical success of the Ukrainians puts into question if the current typical layout and capabilities of modern naval vessels in fact are best suited for modern naval warfare. Figure 1.1: Still image from a recording of the moment an Ukranian USV eliminates an Russian Su-30 Fighter jet. Image adopted from English (2025) How to implement large USVs effectively into a navy is a question that still needs to be explored more with many navies exploring different concepts. The US Navy has developed USVs with the intention for them to provide surveillance and logistical support. The surveillance ship Sea Hunter is intended to locate and track submarines for extended periods of time, Turner (2018), while the vessel Nomad is a former patrol boat that currently serves as a unmanned support ship for the US Navy (Swiftships 2022). The American defence agency Defense Advanced Research Projects Agency (DARPA) has developed a No Manning Required Ship (NOMARS) called Defiant that is intended to be able to extend the range of manned ships by acting as a launch platform, Serco (2022), that can return to port autonomously after expending its payload. This allows for an increased presence in waters far from a ships home port, such as in the Pacific ocean. Another use case for larger USVs is mine countermeasure missions such as mine hunting and mine sweeping, Turner (2018), as these are missions that are time consuming and 3 1. Introduction could pose a potential threat to human life. As naval ships are often designed for a long service life, often more than 30 years, there is often a need for a mid-life modernization where different systems and components are upgraded and replaced with more modern systems better suited for the ever evolving requirements a navy faces (Schank et al. 2016). This has made modular and flexible ship designs popular among navies around the world as it simplifies and lowers the costs of modernizations of systems and allows the ship to easier adapt to evolving requirements Largiadèr (2001). Schank et al. (2016) states that modularity and flexibility are similar but different concepts and describe the main differences between them as: Modularity entails partitioning a system into modules that consist of self- contained elements. It hinges on a systems engineering process that stresses functional analysis and identification of key interfaces. Typically, the concept calls for using common industry standards for key interfaces. Flexibility is a broader, less-precisely defined concept, but generally means constructing ships in such a way that they can more readily adapt to chang- ing missions and technologies. Modularity can be a subset of flexibility and together they contribute to adaptable ships. Schank et al. (2016) further define different types of both modularity and flexibility. This thesis focuses on two of these subtypes of modularity: • Self-contained modules that provide a plug-and-play capability for the equipment inside the module. • Modular installations that provide a basic ship structure and services that allow various mission packages to be installed and interchanged as needed. Self contained modules can be made very versatile where a standard interface secures the module to the ship and provides communication and power. Standard shipping containers are an example of this type of modularity where the containers have a standard interface and the contents of the containers is largely irrelevant for the ship. There are also more specialized examples of this type of modularity in a naval context. Schank et al. (2016) brings up the Vertical Launch System of the Arleigh Burke class destroyers as it can be fitted with several types of missiles using the same boundaries and interface. Modular installations differ from self contained modules in that they are larger more focused payload packages consisting of several assemblies that are closely associated with each other (Schank et al. 2016). Modular installations also contribute to the structural integrity of the ship and the ship can typically only carry one type of modular installation at once. 1.2 Aim The aim of this thesis is to generate a concept design for Saab Kockums AB, of an autonomous and flexible ship that is able to take crewed modules and allow for a crew to command the ship in accordance with current regulations. The intention is that once 4 1. Introduction regulations allow for the ship to operate fully autonomously the crew modules can then be removed from the ship to allow for additional cargo and expand the ships capabilities. This thesis also investigates the challenges and opportunities that face large unmanned surface vessels in a naval context. It investigates the current rules and regulations regard- ing autonomous vessels and how these can be addressed while still maintaining a high operational efficiency. Furthermore the thesis studies what other actors are currently exploring in this area. The thesis focuses mainly naval ships of similar size, however civilian ships and ships of other sizes are also considered as well, The level of modularity is also investigated. This is an important aspect to consider for the ship as a high level of modularity can provide a greater scope of operations, however it can also be associated with decreased effectiveness and increased costs. The requirements set out by Saab Kockums AB are to generate a concept design of a ship with an approximate length of 50 m that is able to reach speeds of 30 kt and has 14 days endurance. 1.3 Methodology This thesis conducts a literature study to investigate the current rules and regulations that apply to large unmanned surface vessels. The literature study sources information from scientific databases such as Web of Sience (2025) and Scopus (2025). Laws and judicial texts are collected from sources such as JUNO (2025), Transportstyrelsen (2025) and Regeringen (2025). As the field of autonomous vessels is progressing fast additional information is gathered from journals and news reports from international sources. In- formation about different conceptual options is also gathered. This includes the hull configuration, the propulsion system and different types of flexibility and modularity. The literature study also studies current USVs and other naval ships of similar design in use and development today. The literature study establishes the current requirements and constraints facing USVs, utilizing these identified parameters as the foundation for concept development. Oper- ational considerations including manning requirements and command structure are ex- amined, as these factors significantly influence design alternatives. Crew accommodation and habitability standards are also analyzed since the vessel must maintain capability for manned operations. Additionally, various modularity approaches are investigated, given the diverse range of existing market solutions, each presenting distinct advantages and implementation challenges. Some potential use cases for larger USVs are explored in the literature study as the intended mission profile will dictate the design of the vessel. As only some dimensions are specified from Saab Kockums AB, naval architectural prin- ciples according to Lewis (1989) are used to estimate the remaining main particulars to be able to perform estimates for power requirements of the ship as well as range calcula- tions. These calculations are to be used to support the selection and dimensioning of the machinery system as well as the number of crew that are required aboard the ship. 5 1. Introduction The information gained from the literature study and the initial estimates act as a foun- dation for the concept generation. The concept generation individually evaluates different aspects of the ship such as hull configuration, machinery and propulsion system, modular- ity and crew compartments. The concept evaluation follows naval architectural practices and evaluates based on several criteria such as performance, cost, size, weight, service- ability and durability. The selected concept is further explored and preliminary layouts of payload and machinery systems is suggested. 1.4 Scope and limitations This thesis focuses on the regulatory challenges that face an autonomous ship today, as well as differing design criteria to handle the unique challenges and opportunities that a large modular and autonomous ship faces. This thesis does not consider on the autonomous control system as there are already several solutions on the market, such as Saab Autonomous Ocean Core, Saab (2024), that are in use today. The thesis only focuses on the conceptual design of the vessel and thus no detailed solutions are considered. The payload that will be carried by the vessel is only be considered at a high level, such as approximate size, weight, and if the payload needs access to the environment around the ship. As many systems require differing prerequisites, this needs to be studied closer in the detailed design of the ship, which this thesis does not focus on. No greater in-depth analysis of stealth or low radar/IR signature capabilities are made. For any parts designed all surfaces are instead be designed with flat surfaces and incor- porate an angle of approximately 13◦. This is due to the limited ability to find openly available information within the field. Furthermore details about radar signatures are outside of the scope of this thesis. The design process is conducted at a high level where the concept itself is proposed, thus this thesis does not include any in depth studies on the hydrodynamics or the load distribution, structural integrity or any of the structures proposed of the vessel. 1.5 Stakeholders As this thesis is made in collaboration with Saab Kockums AB to design a vessel for the Swedish Defence Materiel Administration (FMV), there are several different stakeholders interested in this project. First and foremost Saab Kockums AB as the client for this thesis and main stakeholder when setting performance and operational criteria for the vessel. Saab Kockums AB are also interested in further investigation in developing a similar vessel and could be a potential constructor should the vessel be built. Chalmers University of Technology is another major stakeholder in the project. Chalmers University of Technology is interested in the regulatory challenges facing autonomous 6 1. Introduction ships as well as how different aspects of the ship design interact with each other. FMV, is a stakeholder as they are responsible for the materiel and equipment for the Swedish Defence Forces, including the Swedish Navy. Their interest is related to the performance of the vessel as well as the maintenance and cost aspects across the lifetime of the vessel. The Swedish Transport Administration also have an interest in the project from a regu- latory perspective. While not an active stakeholder in the project, the ship is designed to be compliant with the rules set out by the Swedish Transport Administration. Further, as regulations for autonomous ships does not yet exist, there is extra interest from the Swedish regulatory bodies. 7 2 Literature study Many different factors need to be considered for the design and operation of a vessel. While naval vessels have the capacity to bypass particular regulations, their design in many cases must remain fundamentally compliant with current regulatory requirements in order to ensure crew safety, asset protection, and environmental preservation, Klesaris et al. (2017). While regulations for USVs are still under development, there are still many regulations about the crewing and watchkeeping of a ship that are of interest. The logistics of the ship are also be studied, especially pertaining to what the crew needs, such as accommodation and provisions. In addition, a study of other nations similar vessels in development or already implemented is conducted and displayed in Appendix B. This part of the study mainly focuses on naval vessels but civilian counterparts are also of some interest. Similarly modular and flexible solutions in use are also studied. 2.1 Applicable rules and regulations Unmanned vessels are still considered a novel idea, but regulations regarding operation of unmanned vessels are still not enacted, IMO (2025). This means that in the mean time, before accurate regulation is ready to be implemented, USV’s must comply with rules relating to crew and safety that need to be followed by any vessel. As the vessel will mainly be operated in Swedish waters it needs to follow Swedish laws such as Sjösäker- hetslagen 1994:1009. Furthermore, as it will be used by the Swedish defence forces it also needs to be compliant with "Regler för militär sjöfart", the ruleset developed for naval vessels in Sweden (Försvarsmakten 2002). IMO and Seafarers’ Training Certification and Watchkeeping (STCW) rules are also considered as Sweden is a member state of the IMO. The Swedish regulatory system builds upon a constitution, which trickles down into openly formulated laws. These laws are then interpreted into governmental regulations by the Swedish Government. Here after the regulatory administrations, such as the Swedish Transport Administration, take action and formulate the regulatory provisions, or "Föreskrifter", that governs as the actual guiding documents and actions of enforcers. These regulatory provisions are the most practically applicable legislative documents. In the Swedish legal system, legislative texts are written in an open-ended provision. This means that the practical interpretation is guided primarily by the intent of the law, which 8 2. Literature study may also be documented in preparatory works (förarbeten), which in many cases can be found and used to better interpret the law. One example of an open-ended provision of writing the law is the Fartygssäkerhetslag (Sea Safety Act), which states: Fartygssäkerhetslagen (2003:364)(SFS) - 2 kap. 4§ ”Ett fartyg ska vara be- mannat på ett betryggande sätt.” This law translates into: "Any vessel must be crewed in a safe and ensuring manner." which does not explicitly state that there must be any crew onboard the vessel. In many cases certain administrative offices, such as the Swedish transport Administration (Transportstyrelsen)(STA), break down and formulate guidelines of how to interpret the laws regulating their administrative responsibilities. These guidelines are then formulated as a the governing rules, "föreskrift". In the joint industry and agency symposium "Policylabb smarta fartyg" by Burden et al. (2022), a cooperation between Swedish Transportation Authority (STA), Saab Kock- ums AB, the Swedish Transportation Authority Ferry Company, ABB and other partners concludes: Om vägfärjan däremot inte har någon befälhavare ombord eller enbart har en reducerad bemanning som inte kontinuerlig navigerar och övervakar färden finns krav i både lag, förordning och föreskrifter kopplat till vakthållning och bemanning som i dagslägen är svåra att uppfylla, åtminstone om man utgår från en strikt tolkning av hur kraven är formulerade Which translates into meaning: A road ferry lacking a captain or has minimal crew not continuously overseeing navigation, current laws and regulations on watchkeeping and manning are difficult to meet under a strict interpretation of current regulations. In Burden et al. (2022) the authors have conducted some thought experiments in the papers appendix, where they explore the needs for different adherence for two smaller USV’s. These thought experiments are continued for medium sized and larger USV’s in this thesis Appendix A, the nomenclature in the appendix is a continuation from Burden et al. (2022), where thought experiment A and B can be found, as such the Appendix A in this thesis are named C and D, as these appendices approximate continued evaluations in the same manner but for increasingly larger unmanned surface vessels. There are ways to construct and test alternative designs within the current regulatory framework. Questions like these are handled individually within the framework of the IMO1455 "Alternative Design", mentioned in Chapter 1, where the ship builder can test the case of new equipment or concepts via scientifically proving the new concept inherit at least equal safety to already proven designs. Currently there exists several aspects that can be challenged this way. As a few examples, the STCW Manilla Regulation VIII/2.1 state that the bridge must be manned at all times, but they do not specify the location of the bridge. The regulations further require specific outlook criteria, however it does not mention the possibility of using cameras or remote viewing, leaving the possibility of a remote bridge open for interpretation. By implementing an IMO1455 process to vindicate a remote or an autonomous surveillance system could be a good first step. 9 2. Literature study Swedish maritime law does not explicitly state the manning or crew required, however the IMO rules that the Swedish laws are derived from does mention the capabilities the crew of the ship needs to possess. The IMO Resolution A.1047(27) Principles of minimum safe manning state: 3 Principles of minimum safe manning 3.1 The following principles should be observed in determining the min- imum safe manning of a ship: .1 the capability to: .1 maintain safe navigational, port, engineering and radio watches in accordance with regulation VIII/2 of the 1978 STCW Con- vention, as amended, and also maintain general surveillance of the ship; .2 moor and unmoor the ship safely; ... .6 provide for medical care on board ship; ... .9 and operate in accordance with the approved Ship’s Security Plan; The regulations does not include a strict number but instead ensures that functional capabilities are reached, which creates the possibility to argue for equivalent safety on many points. While some regulations might prove more difficult to achieve without a crew, such as the regulations regarding mooring, IMO A1047(27) 3.1.1.2. If the rule IMO A1047(27) 3.1.1.6 in the same IMO regulations could be intended to extend to shipwrecked individuals happened upon while the vessel is out at sea, the situation could prove troublesome since no crew member can aid or attend to those in need. Lastly, the issues of legal responsibility in the event of accidents, and the authority over navigational decisions, remain central challenges that both regulators and stakeholders advocating for unmanned vessel technologies must address before new regulations can be implemented and unmanned vessels fully integrated into operational service. 2.1.1 Specific regulatory limitations Swedish maritime legislation does not prescribe explicit minimum levels of manning for vessels. Instead the governing laws and regulations are again written in a vague form, stating that there must be a plan of safe manning for each vessel. This plan is submitted to the Swedish Transport Agency and then an individual evaluation is done for, at least, each class of vessels. Mention of explicit physical presence on board the vessels is appear in the STCW Manilla regulations, where concrete regulations requiring crew members to be physically present on the bridge as well as stand by to physically enter the machine room. Specifically the VIII/2.1 and VIII/2.3 regulations. STCW Manilla Reg. VIII/2.1 - officers in charge of the navigational watch 10 2. Literature study are responsible for navigating the ship safely during their periods of duty, when they shall be physically present on the navigating bridge or in a directly associated location such as the chartroom or bridge control room at all times; STCW Manilla Reg. VIII/2.3 - officers in charge of an engineering watch, as defined in the STCW Code, under the direction of the chief engineer officer, shall be immediately available and on call to attend the machinery spaces and, when required, shall be physically present in the machinery space during their periods of responsibility; As stated in the STCW Manilla regulations, an appropriately skilled operator must phys- ically be located on the bridge or prepared to enter the machine room. With an USV, the position of the bridge could be argued that if the USV is in fact remotely controlled the actual bridge is not located on board the vessel itself, but rather on another vessel or even on land. This would possibly fulfil the direct need of manning on the bridge. However the possibility to physically enter the machine room is impossible to achieve remotely. This also holds true for unmanned machinery spaces or partially unmanned machinery spaces, however in these cases a high level of automation in the onboard machinery systems is a prerogative, allowing alarms to be sounded in the cabins of the crew responsible for the machinery instead of requiring constant supervision. This means that the crew is still needed to be physically onboard the vessel. 2.1.2 Specific regulatory exemptions The regulations set out by the IMO and other agencies could inherently reduce specific systems effectiveness or in some cases add complexity to systems in need of simplicity. This is specially true for naval vessels, where regulations about crew and environment could effectively render the design of small and nimble warships or submarines mute. In Sweden this is solved by the possibility for state owned and operated vessels exempt from certain regulations, one example being "Fartygssäkerhetslag" (SFS) (2003:364) Ch 3, 12 §, where it states that the agency that operates the ship shall decide the minimum safe manning needs of the vessel, in collaboration with the Swedish Transport Agency. It also leaves the possibility for direct governmental intervention into the matter in special cases. Fartygssäkerhetslag Kapitel 3, 12 §. För fartyg som ägs eller brukas av svenska staten och som används uteslutande för statsändamål och inte för affärsdrift skall säkerhetsbesättning fastställas av den myndighet som förvaltar fartyget, om inte regeringen föreskriver eller för särskilda fall beslutar annat. Myndigheten skall samråda med Transport- styrelsen före beslutet. Lag (2008:1378). Similar exemptions are found in several other legislative texts, where the government have secured a regulatory freedom in how the state owned vessels can be operated. Yet another example can be found in "Transportstyrelsens föreskrifter och allmänna råd om fartyg i nationell sjöfart" (TSFS) (2017:26 Ch 1, 3.7 §) where it states that the regulations stipulated in does not pertain to naval vessels. 11 2. Literature study "Transportstyrelsens föreskrifter och allmänna råd om fartyg i nationell sjöfart" (TSFS) (2017:26 Ch 1, 3.7 §) 2 Föreskrifterna gäller inte [] ... [.7] Örlogsfartyg. This opens up for a cooperation with the different Swedish agencies, such as the FMV, the Coast Guard or the Swedish states road ferry company under the transportation agency, to successfully implement USVs and develop the technology. The Swedish defence force has an internal process to setup rules of manning and staffing marine vessels called "Försvarsmaktens Interna Bestämmelser", which translates to Swedish Armed Forces (SAF) Internal Directives. In FIB (2018) the manning requirements state: 1 §En besättning på ett örlogsfartyg ska vara sammansatt så att den kan: .1 hålla säker vakt på brygga, maskin och radio, .2 klara förtöjnings- och losskastningsarbeten, ... .6 hantera livräddningstjänsten och sjukvården ombord, ... .9 bistå eventuella passagerare ombord, 1. efterleva viloregler för att inte uppnå kritiska nivåer av sömnbrist, samt 2. med hänsyn till örlogsfartygets avsedda användningsområde och i en- lighet med CONOPS, kunna genomföra resa i aktuellt fartområde. 2 §Ett örlogsfartyg ska alltid bemannas enligt relevant bemanningsplan så att kraven enligt .1 krav på behörigheter enligt 4-5 kap. .2 utbildningskrav enligt 7 kap. .3 det lägsta antalet tjänstgörande i besättningen som krävs för att örlogs- fartyget ska få framföras. För örlogsfartyg med tekniska system som, utöver skeppstekniska system, kräver särskild kompetens ska kompetenskrav framgå av aktuell bemannings- plan. Comparing the SAF’s internal rules with the IMO A.1047(27) it is clear that SAF have interpreted the IMO in order to safely man their vessels. However the SAF directives deviate in certain areas where it deems it necessary to maintain effectiveness without compromising safety. Further more the SAF have the mandate to accept their own esti- mation of safe manning of their vessels, creating the possibility to implement an unmanned naval surface vessel. 12 2. Literature study 2.2 Crew Logistics All vessels requiring a crew have logistical challenges related to human necessities. This includes living quarters and storage, food and water facilities, and waste. As the ship is relatively small compared to many civilian cargo ships, there are several requirements that are exempt such as the need for several toilets or showers, since the crew is small and distances within the living quarters relatively short. 2.2.1 Living quarters and personal space Since many of the regulations are built on each other, and all humans require the same basic needs, many of the rules regarding the living spaces onboard a ship is quite similar in many different regulations. According to the ILO MLC 2006 Reg, all crew must have access to a personal locker, there must be a toilet per every 12 passenger, the indoor height must be at least 2m, and many other specific requirements are listed in Chapter 3. In addition to the ILO MLC 2006 Regulations more logistical needs have been gathered from Naval Sea Systems Command (2016). 2.2.2 Crew estimates Given the vessel is intended to operate at sea for at least 14 days, there needs to be a watch rotation on board. This means that there needs to be a commanding officer, a chief and at least four other crew members, depending on power production of main engines. This means that each watch needs to contain • Commanding officer • Coxswain • Engineer Providing there is two separate watches, this yields a minimum crew of 6. Given the number of crew members on board and awake at all times is three, two crew are always be available for docking or mooring operations which is one of the big issues for the safe handling of the vessel, until autonomous docking is implemented. During these operations it will be all hands on deck and seeing as the crew will either muster on or off at the same time to possibly switch the crew or perform checks and maintenance of the vessel, this will be a negligible problem. In order to reduce the watch on board, there is an possibility to incorporate the onboard sensors instead of having sailors standing around. Sensors such as cameras can be fitted with night vision technology or even IR and heat sensors, giving them much better visual observation in many conditions. Only in clear daytime conditions the human eye could rival the modern sensors given the resolution of said systems might be lower than the humans eyes perception, this is however something that might have to go through a IMO1455 Alternative Design process. Regler för militär sjöfart (RMS) allows for multiple berths in the same cabin, and lists dif- ferent requirements for free floor area depending on the number of berths, Försvarsmakten (2002). The floor area requirements are listed as follows 13 2. Literature study • Cabins with one or two berths: 2 m2 • Cabins with three to five berths: 2 m2 + 0.7 m2 for each berth above two • Cabins with more than five berths: 4 m2 + 0.5 m2 for each berth above five They further list requirements for the berth itself as • The dimensions of the bed shall be at least 80 x 200 cm • All crew shall have their own bed • There shall be at least 65 cm of free space above the top of the berth • Berths placed longitudinally shall be placed with the foot end forward • Berths separated by less than 60 cm shall be separated by a divider • The cabins should be located so that they can be used even during MCR-operations in unfavourable weather conditions MCR stands for Maximum Continuos Rating and refers to the maximum sustained power the ship can operate at for extended periods of time, as stated in MAN Energy Solutions (2023). One way to save space is the use of bunk beds, meaning that the berths are stacked vertically on each other. This way the footprint of the berths is reduced as well as using otherwise dead space for something useful. Assuming that the headroom is kept at 203cm as required by ILO MLC 2006 Reg 3.1.6a, and assuming that each berth itself is 10 cm tall, to comply with the requirement of 65 cm of free space above each berth, it is possible to fit two berths atop each other. This creates several different possible cabin configurations to consider Configuration Floor area per cabin Total floor area for all cabins Total room area including beds Three cabins with two berths each 2 m2 6 m2 10.8 m2 One cabin with two berths and one cabin with four berths 2 m2 + 3.4 m2 5.4 m2 10.2 m2 One cabin with six berths 4.5 m2 4.5 m2 9.3 m2 Table 2.1: Free floor area required for different cabin configurations As the floor area of a standard 20 foot container is 13.86 m2, it would be possible to have all configurations listed in Table 2.1 above, however for simplicity the solution with one cabin with six berths is chosen as this allows for more open floor space for the crew, as well as eliminating the need for interior walls that would otherwise take up space. This is then coupled with another 20 foot container housing the galley, food storage, as well as bathrooms and showers. 14 2. Literature study 2.3 Current USVs in development and use Currently there are very few autonomous ships in civilian operation. This is in large part due to regulations not accommodating the unique challenges that autonomous vessels face. Some countries, such as the UK and Norway, have however granted exceptions from certain rules to aid with the development and testing of new technologies, though most of this has been most prominent in military applications. Many military projects are however kept secret, meaning that reliable information about said developments can be difficult to find. An overview of autonomous ship concepts, operational and conceptual are presented be- low. More information about these as well as other ships of interest in the current study can be found in Appendix B. 2.3.1 Yara Birkeland One of the leading countries when it comes to civilian USV technology is Norway, where several ships are currently being tested and developed. The worlds first electric and autonomous container ship, the Yara Birkeland, as can be seen in Fig. 2.1, has been in commercial operation for over 2 years along the Norwegian coast (Yara 2024). While the ship was initially planned to have a crew supervising operations for the first two years before transitioning to fully autonomous operations, this transition has been delayed by technical as well as regulatory issues. Knut Midtsian, Interface Manager for Yara Birkeland says that the regulatory aspects of autonomous ships are challenging as no regulations yet exist. Figure 2.1: Yara Birkeland in 2020. Adopted from Yara (2020) Mr. Midtsian further comments that for innovative projects such as autonomous vessels, the technical solutions must come first, and the robustness and reliability need to be presented to the authorities for approval of the regulatory requirements. While Yara Birkeland has a completely different mission profile to a naval vessel it still showcases many of the challenges any USV faces. Extensive trial periods with crew aboard 15 2. Literature study the vessel will most likely be required as there are many situations that could potentially lead to disaster without a crew on board to remedy it. 2.3.2 Sea Hunter The US Navy is also far along in the development of autonomous vessels. The 40 m long prototype Sea Hunter, as seen in Fig. 2.2 has a range of 10.000 km and can reach speeds of 27 kt. It is currently being tested with a crewed module onboard for a crew member to monitor the ship in case of any issues. The ship is intended to carry out submarine surveillance missions and currently is no plan for the ship to carry any weapons. Figure 2.2: Sea hunter during RIMPAC 2022. Adopted from Freutel (2022) 2.3.3 NOMARS Defiant The Defence Advanced Research Projects Agency (DARPA) have also developed the the No Manning Required Ships (NOMARS), a modular autonomous ship platform that has been designed from the ground up with the purpose of not considering the human element to further push the development in the field. A 55m prototype ship, Defiant, was launched in February 2025 (Parken 2024). A computer rendering of the Defiant can be seen in Fig. 2.3. The ship is intended to be highly flexible with the ability to quickly swap between different cargo types or seemingly even weapon system modules. 16 2. Literature study Figure 2.3: Computer rendering of NOMARS Defiant. Adopted from DARPA (2022) 2.3.4 JARI USV-A China has constructed a 58m USV with a small bridge for crewed operations, called the JARI-USV-A, see Fig. 2.4. Interestingly both the American Sea Hunter and the Chinese JARI-USV-A are both trimarans, allegedly to improve stability in rough seas. The mission profile of the ship is not made public, however defence analyst H. Sutton (2024), speculates that it might carry both advanced sensors as well as sophisticated weapons. Figure 2.4: Image of JARI-USV-A docked in harbour. Adopted from H. Sutton (2024) 2.4 Use cases for naval USVs There are certain areas of operation and tasks that larger USVs are well suited for that cannot be completed effectively by smaller vessels. High risk or mundane operations can benefit from being handled by autonomous vessels so that personnel can be utilized 17 2. Literature study effectively in other areas instead. In this section some proposed use cases for large naval USVs are discussed. This section is an inventory of identified areas where an autonomous or unmanned vessel can operate. 2.4.1 Logistical The US Navy have an ongoing project that aim to solve autonomous logistical solutions, foremost intended to be used within the Pacific Ocean, where the logistical chain is longer and replenishment can take weeks. This can be solved with autonomous supply ships. Similarly the same missions could be adopted within the Baltic region, where either logistical support can be offered to allied nations or units out at sea without the need to induce a break in their own operations. 2.4.2 Anti-Surface Warfare (ASuW) The Armed Forces of Ukraine (AFU) have proven the case for using USVs in modern warfare. What started as One Way Attack (OWA) USV’s have quickly evolved into carriers of different UXV’s, anti-air platform, torpedoes, mine layers and other capabilities that have allready been mounted upon them. While much of this success is attributed to the size of the vessels, making them hard to detect by both radar or visuals, the development of these Ukranian USVs have steadily increased in ambition and the latest iterations now boast of using torpedoes, anti-air missiles and launching drones from them. All these added systems might make an increase of vessel size imminent as all systems increase the mass and require energy, and increased energy demand require increased power production and more fuel. 2.4.3 ISTAR Intelligence, surveillance, target acquisition, and reconnaissance could potentially be dis- tributed to unmanned systems, especially in combination with crewed stealth warships. Warships or aircraft built for stealth often utilize passive radar receivers to reduce their chances of detection as an active radar emits electromagnetic waves. This prompts the stealth vessel to instead rely on other radar detection systems to gain awareness of their surrounding. Airborne early warning and control (AEW&C), such as the SAAB Global- Eye (2025) can provide naval vessels with over the horizon detection of adversary units at longer ranges, but many navies still mount large radar units on their warships to enable stand-alone operations and increase detail resolution. An unmanned system could potentially relieve the stealth warship of some of the risk of detection by instead acting as a distributed forward ISTAR unit that sends the data back to the warship. 2.4.4 Anti-Submarine Warfare (ASW) There already exist USVs intended for ASW, such as the American Sea Hunter and Chinese JARI-USV-A. These vessels are designed to have extensive endurance, allowing them to operate far from home for extended periods of time. This type of mission is well suited for USVs as the main operational mode consist of loitering in an area and collecting 18 2. Literature study data. Larger vessels are well suited for this type of missions as they are able to carry more sophisticated sensors, as well as carry more fuel for increased endurance. 2.4.5 Minelaying For mine laying operations the size comes into play since larger vessels can carry more mines and also since mines can be placed defensively as well as offensively. Where larger USVs could facilitate an form of loitering mine field, for defensive purposes, the operation of laying mines is in many cases quite easy for an adversary to monitor and thus avoid. USVs could aswell instead offer an solution for an offensive mine strategy, where mines are used to deny an adversary from leaving their ports or naval bases. Partaking an offensive mining mission is an very risk filled and dangerous operation, therefore it is better suited for an unmanned vessel. 2.4.6 Mine Counter Measure (MCM) There are two main strategies for MCM, the first being mine hunting, where mines in singular form are carefully disposed of. The second is mine sweeping missions where vessels clear paths of mines en masse. Mine sweeping have previously been troublesome to carry out in a modern context due to the incredibly high risk these types of missions pose for the crew. The emergence of USVs, and especially larger ones that can either sweep with nets or sustain little to no damage when triggering a mine open up new avenues for these tactics. 2.4.7 Carrier or communications relay Within areas with heavy electronic warfare (EW), often called EW bubbles, the use of direct radio communication is impaired, thus by posting an relay vessel outside the bubble and utilize optical fibres to an advanced units within the EW bubble, the effect of the EW bubble could be negated. In the Russo-Ukrainian war advanced electronic warfare have increasingly been implemented due to the increased presence of unmanned remote controlled drones (Joseph Trevithick 2024). USV’s that carry UAV drones have already been implemented in the AFU and used in the Russo-Ukrainian war (H. I. Sutton 2024b), with examples of successful operations within the black sea, where USVs have acted as an remote launch platform for UAVs that can either inspect areas out of sight of the drones, or perform precision strikes on enemy forces and installations. 2.5 Modularity and flexibility Large naval vessels are typically very costly and time-consuming to build, and they are usually designed for a prolonged service life, often designed with the intention to be upgraded in the future (Klesaris et al. 2017). Due to the significant initial investment, it is common practice to perform a mid-life upgrade, where systems and capabilities are modernized to ensure the vessel remains operationally effective over an extended period. These mid-life upgrades are themselves expensive and often time-consuming, and because much of the work is performed while the ship is in dry dock, the vessel is rendered inoperable for an extended period. For example, the mid-life upgrade of the USS John 19 2. Literature study Paul Jones, an Arleigh Burke-class destroyer in the U.S. Navy took approximately 10 months to complete according to Schank et al. (2016). Another approach to the ship design process is to instead design the ship with the intention of being easy to modify during its lifetime from the initial design phase. This is the pretence of designing naval vessels with modularity in mind (Largiadèr 2001). Since the implementation of modular design reached the naval sector the approach have spread and today each navy, or naval ship builder, have different definitions of exactly what it means, but the overall intention is still to mitigate both the time spent and cost of modernizing a vessel. Some examples of this can be seen both in Europe, such as the Danish Absalon class and the German MEKO class vessels, and the US with the Littoral Combat Ships and several classes of aircraft carriers. In Schank et al. (2016) the differences between modularity and flexibility is specified that modularity is the possibility to construct ships in the same series with different purposes, and that a ship can be repurposed into the different variants in short notice. This operation would include reconstruction and moving of parts such as bulkheads, walls or decks that might interfere with the new components and layouts. Flexibility on the other hand is that the ship can be set up on a mission basis, with the best load out for that specific mission, with minor to no actual changes to the ship structure itself, but maybe adjusting of an movable internal wall or floor in order to house different equipment within each void. These movable walls and floors are often called flexible decks and walls, and are implemented in vessels such as aircraft carriers, logistical vessels or amphibious assault vessels in order to provide the best internal spaces for the planes, armoured personnel carriers, boats or other equipment based in the internal hangars and storage spaces. Schank et al. (2016) defines modularity as: Modularity involves creating fixed boundaries, defined interfaces and defined ship services (such as power and cooling) to standard portions of a ship, which are termed modules. Modularity is then divided into three different types of modularity. • Common modules used across multiple ships • Self-contained modules that provide plug-and-play capabilities • Installations that provide basic ship structure and services that allow various mission packages to be installed and interchanged Schank et al. (2016) also define flexibility as Flexibility involves the ability to change boundaries, whether they are physical or related to ship services. Which also is divided into three distinct types of flexibility • Flexible infrastructures that allow changes to the boundaries of ship spaces to be made quickly 20 2. Literature study • Additional space within a ship • Additional ship services within a space Designing a modular and mission-flexible vessel has the potential to introduce additional logistical complexities, particularly concerning the storage, maintenance, and deployment of interchangeable mission-specific equipment. To fully take advantage of a modular/flex- ible system, these modules must be readily available ashore between deployments, which necessitates dedicated infrastructure and inventory management. At a certain point, the operational and logistical costs associated with maintaining flexibility may outweigh the benefits. An alternative approach could involve deploying multiple vessels, each configured for a specific mission profile, thereby reducing the need for duplicate equipment stock- piles. This shift in strategy reflects a broader cost trade-off—where savings are achieved by minimizing hull production, yet significant resources are still allocated to maintain multiple high-value equipment sets that cannot be simultaneously utilized. 2.5.1 Modular and flexible ships in use today There are several examples of modular and flexible ships in use today. The STANFLEX system was developed by the Danish navy in the 1980s as a way to reduce the costs of replacing an ageing fleet according to Rear Admiral Søren Torp Petersen in Harboe- Hansen (1992). The system consists of non-permanent equipment such as guns, torpedoes or sonars that are mounted to standardized containers with a standard interface that is able to be swapped out quickly with the help of a crane. According to Rear admiral Knud Brock this flexibility allows the Danish navy to perform a much broader scope of tasks with fewer ships than a traditional navy as the ships can change their capabilities depending on the current need (Harboe-Hansen 1992). Another example of a modular ship design is the SIGMA-class of corvettes and frigates designed by the Dutch company DAMEN that is designed to allow for different sections to be added or subtracted to the ship, as well as the locations of the different sections (Naval Technology 2014). This allows the ship to easily be customized to a specific need and also makes potential future upgrades simpler as the ship is already sectionalized. 2.6 Conclusions of literature study The initial literature study indicates that, in Sweden, a larger civilian unmanned surface vessel is not likely to be adopted or allowed into service other than under governmental agencies. The legislation within the field is simply not mature enough to allow for un- manned vessels. For unmanned air vessels the situation looks different, since the governing rules have had longer time to mature. As the IMOs process to formulate new regulations is inherently slow to ensure safety and clarity, it is clear that this is causing shipowners to be cautious about developing large USVs ahead of regulatory decisions. This is even though there is an obvious interest from the merchant fleets, as cutting crew costs and reducing the risk of personnel on board ships. Different companies do however try to position themselves into this new technology. As an example there are already companies that are starting to offer solutions to remotely controlling larger merchant vessels such as SeaQ Remote from Vard (2025). This could be the first step into creating large uncrewed 21 2. Literature study vessels that can navigate themselves from shore to shore, but as one can see from the Norwegian autonomous vessel Yara Birkeland, the de-crewing takes time. In the PolicyLabb initiative Burden et al. (2022), conducted by the Swedish Transport Administration in collaboration with its partners, two analyses were carried out regarding test operations for autonomous surface vessels (ASVs). These analyses, referred to as Al- ternative A and Alternative B, see Appendix A, explore the current legislative frameworks applicable to the implementation of small USVs. Through a series of thought experiments, the study illustrates both the regulatory opportunities and the legal challenges associated with different USV configurations, providing a concise overview of potential operational constraints and enablers. The results of these adapted analyses are presented in Appendix B. It is important to note, however, that the PolicyLabb report considers only the most restrictive interpretations of current legislation and jurisdictional authority, not taking into account the possibility for governmental agencies to own and operate the vessels. In this report, these thought experiments, as can be seen in Appendix A, have been extended to assess larger autonomous vessels. The thought experiments from the Pol- icyLabb does not extend beyond vessels larger than 12 m, thus the replicated though experiments in this thesis follows the nomenclature from the PolicyLabb but instead fo- cus on a Medium Unmanned Surface Vessel (MUSV) example with a length from < 12 m to ≤ 50 m, and a Large Unmanned Surface Vessel (LUSV) with a length < 50 m. Since this also is a question about the strict legislation, there are different answers for each question, where the question are answered in the manner of strict adherence to the law, a questioning stance to the law, and a completely naval perspective, as written into the Swedish SjöLag, vessels owned and operated by governmental agencies, such as FMV or Swedish Armed Forces, does not have to comply strictly with all regulations for civilian vessels. Some certificates gets valid depending on gross tonnage, as gross tonnage gets cal- culated from a logarithmic coefficient based on the internal volume of the ship, multiplied with the internal volume of the ship. However for the cause of these thought experiments the MUSV are considered below all gross tonnage requirements, meaning below 400 GT, while the LUSV might fit within those ranges. 2.6.1 Alternative approaches When researching the subject of how to best incorporate an unmanned surface vessel some different feasible approaches have been found. The different approaches are either to strictly follow the current rules and regulations, or challenging the current equivalent safety measures in an IMO1455 process, or disregarding the regulations for civilian use and instead incorporating the vessels into governmental use as research or naval vessels. Strict interpretation of current regulations The vessel can be designed in full compliance with current rules and regulations, this however necessitates significant compromises with respect to its autonomous configura- tion, as the design must accommodate crew presence. All systems required to support human habitability must be included, thereby diminishing the advantages associated with an unmanned or automated solution. This solution could either be implemented into an 22 2. Literature study new vessel or into an midlife upgrade of a vessel in service by upgrading the automated systems to be able to control the vessel while under human supervision. This way the vessel could conduct tests, data collection and system improvements while still in service. IMO 1455 The second approach is to design the vessel and satisfy the regulations by getting approved as an alternative or equivalent design. This allows for novel approaches to the design as long as it can be proven that it is safe. So while following the regulations is not necessary, the approval process can in this case be lengthy. Especially if there are many or large deviations from the current accepted systems, and large deviations can demand intermediate solutions like actually placing a crew on the vessel during testing. The process of proving equal safety follow a scientific model, where tests and documentation is performed and gathered in order to provide for a statistical equivalent safety. This means that a IMO1455 process quickly becomes a costly and time consuming venture where detailed data of every system must be gathered. This approach would mimic a project that is close to what the MS Yara Birkeland is achieving in the Norwegian tests. Naval vessel The regulations set out by the IMO and other agencies can inherently reduce specific systems effectiveness or in some cases add complexity to systems in need of simplicity. This is specially true for naval vessels, where regulations about crew and environment could effectively render the design of small and nimble warships or submarines mute. In Sweden this is solved by the possibility for state owned and operated vessels to neglect the regulations, one example of this is the TSFS 2017:26 §3.7. If the vessel were to be incorporated into a cooperation between a shipbuilder and governmental agency, either within SAF or one of the sub-agencies as FMV or FOI, or even within the Swedish Coast Guard, the vessel could be designed to follow all the current rules and regulations, with the exceptions that being a governmental owned and operated vessel allows. 23 3 Requirements In this chapter the requirements for the ship are discussed. These requirements include the requirements set out by Saab Kockums AB, as well as regulatory requirements with regards to manning and crew necessities. Different example missions are discussed in this chapter. 3.1 Mission profile Here the mission profile of the ship are discussed. The mission profile depends on the requirements set by Saab Kockums AB, as well as what the rules require in terms of equipment and safety. Furthermore, the fact that the ship must accommodate some crew aboard the ship dictates certain aspects as well. Saab Kockums AB have defined the operational profile of the vessel is to be able to stay out at sea for at least 14 days with varying speeds and operational modes. Following are two different missions with descriptions of the modes of operation. 3.1.1 Example mission 1 The first example mission derived for the vessel mimics an intelligence or surveillance mission. The mission starts with a high speed transit to the area of operations (AOE), then operating within the AOE at lower speeds for a prolonged period of time, ending in a burst of speed to transit back to home base. This mission description excludes what kind of operations the vessel carries out in the AOE, but could resemble patrolling in a designated area. For this mission the time spent in the area of operation is key, therefore the distance is denoted as x as it is of less importance. The ship operates at 5 kt within the AOE. 24 3. Requirements Activity Insertion OP RTB Speed [kt] 30 kt 5 30 Distance [km] 277 x 277 Distance [NA] 180 x 180 Time [h] 5 h 326 h 5 h Figure 3.1: Example mission 1, transporting from Karlskrona naval base to area of operation in high speed, conducting operations within AOE in low speeds but prolonged time, then and returning base in Karlskrona in high speed. 3.1.2 Example mission 2 The second example mission derived for the vessel mimics interception and escort of another vessel. The idea is to mimic a counter to the merchant vessels dragging the chains on the bottom of the Baltic Sea, destroying sea floor data or energy cables in the process. With a mission profile like this, the suspected vessel could be intercepted before it reaches the Swedish economical zone east of Copenhagen and escorted to Finnish waters, where upon it leaves the vessel and returns to base. The mission starts from port and makes it’s way towards the target vessel in high speed, then follows along the target vessel in it’s speed. Here assumed to match the typical merchant tanker vessel of 12kt, then upon reaching the another country jurisdiction abort the escort and return to base. The return trip is assumed to also be done in high speed. This mission could also be reversed, as to intercept at the longer distance and return to base (RTB) at a shorter distance. Activity Intercept Escort RTB Speed [kt] 30 12 30 Distance [km] 200 736 530 Distance [NM] 108 397 286 Time [h] 3.4 34 10 Figure 3.2: Example mission 2, starting Karlskrona naval base, intercepting a vessel south of Malmö, escorting the vessel to Finnish waters to then return to Karlskrona. 25 3. Requirements 3.2 Requirements from stakeholders In this section the requirements from different stakeholders are presented. As the different stakeholders are concerned with different perspectives 3.2.1 Saab requirements Saab Kockums AB have stated a set of requirements that the ship needs to fulfil. These requirements only concern very high level aspects of the ship, and more detailed require- ments are up to the authors to decide. • Length ≥ 50 m • Service speed ≥ 30 kt • Endurance of up to 14 days at sea • Vessel should be capable of both autonomous and remote controlled operation 3.2.2 Regulatory requirements There are many rules dictating the design of ships. While certain issues with the current rules with regards to autonomous operations have already been discussed in Chapter 2, there are other rules that have implications for the overall design of the vessel. • ILO MLC more often regulate ships above 3000 gross ton, with less restrictions for smaller vessels. • Keeping the ship below 500 gross ton provides the opportunity for a single person to keep watch from the bridge. (TSFS 2012:67:3.1.2) • Keeping the power below 3 MW avoids the need for 2 machinists aboard. Keeping the power below 1.5MW there is no need for a machinist aboard. (ILO MLC 2006 Reg A3.2.5) • DNV High Speed and Light Craft and Naval Surface Craft, part 2 and 3 are to be applied 3.2.3 Requirements for crew modules As the ship needs to accommodate a crew the requirements of the each crew member in terms of supplies and space is found in texts such as Naval Sea Systems Command (2016) and the regulatory text from the International Labour Organization. Below is listed hard requirements for safely housing the crew mentioned in those sources. • 152 L fresh water per person per day • 10 L cold food storage per person per day • 0.5 L freezer food storage per person per day • 10-60 L black water capacity per person per day. Possibility to pump black water directly overboard outside of 12 nautical miles from shore. • Minimum 203 cm headroom (ILO MLC 2006 Reg 3.1.6a) • Minimum berth size 198x80 cm (ILO MLC 2006 Reg 3.1.9e) 26 3. Requirements • On ships with less than 10 crew a cook is not required, but anyone handling food in the galley must be trained in areas of food and personal hygiene as well as handling and storage of food (ILO MLC 2006 Reg A3.2.5) • Ventilated personal locker of >500 L per crew • 1 Toilet per 12 crew. If crew >8, 2 toilets are recommended. • 1 Shower per 7 crew. • 1 sink per 6 crew, not including toilet sinks. • Washing machine and dryer is a must for longer missions. • Cabinets for cleaning appliances must be well ventilated and close to accommoda- tions. • Indoor temperature must be between 18 and 24◦C. 27 4 Main particulars The main particulars are dependent on what type of mission the ship should be optimized for, as different hull shapes are optimized for different tasks. In the following chapter the main particulars of the ship are discussed, and initial estimates that give the required performance are shown. 4.1 Ship particulars The requirements given from Saab Kockums is that the ship should be around 50 m long, achieve a speed of 30 kt, and have an endurance of around 14 days at sea. This would put a category of ships generally called Fast Patrol Boats, and they are popular with many navies around the world for tasks such as coastal defence and border security. They are generally smaller than corvettes but usually fulfil similar roles by focusing on coastal operations rather than ocean-going journeys. Table 4.1 shows a list of similar ships together with their main dimensions as well as their length-to-beam and beam-to-draught ratios. According do Lamb (2004) the length-to- beam ratio is an important metric when it comes to the resistance of the hull. A higher ratio means a more slender ship which is advantageous from a resistance perspective, however a lower ratio tends to yield a more manoeuvrable ship. In Table 4.1 it can be seen that similar ships usually have a length-to-beam ratio of around 6.5 to 7.2, which would mean a beam of around 7 m for a 50 m long vessel. Another important metric is the beam-to-draught ratio. This has an effect on the trans- verse stability of the vessel, as well as the residuary resistance. According to Lamb (2004) most ships have a B/T between 2.25 and 3.75, however they note that draft limited ves- sels can have a ratio of up to 5. This can also be seen in Table 4.1 that all monohull designs have a B/T between 3 and 5, and as these are ships meant to operate close to shore, a shallower draught is preferable. For a ship with a beam of 7 m, a 1.8 m draught corresponds to B/T = 3.9 which is in line with other similar ships. 28 4. Main particulars Ship Class Nationality Length Beam Draught ∆ L/B B/T Visby Sweden 72.8 10.4 2.4 650 7 4.33 Stockholm Sweden 50 7.5 2.6 320 6.67 2.88 Flyvefisken Denmark 54 9 2.5 320 6 3.6 Buyan Russia 62 9.6 2 420 6.46 4.8 Hamina Finland 51 8.5 1.7 250 6 5 Kılıç Turkey 62.4 8.6 2.82 552 7.26 3.05 Roussen Greece 62 9.5 2.6 580 6.53 3.65 FC50 Qatar 43 9.2 2 - 4.67 4.6 FS56 France 56 8.2 2.6 - 6.83 3.15 Skjold Norway 47.5 13.5 2.2 274 3.52 6.14 Tuo Chiang Taiwan 60.4 14 2.3 567 4.31 6.09 Table 4.1: Ships of similar size and operational roles. Note that the Skjold and Tuo Chiang classes are catamaran designs and therefore significantly wider than their monohull counterparts The displacement is largely a factor of the size of the ship, and the amount of equipment it carries. As the approximate size of the ship is known, but not the amount or types of equipment, given that naval vessels of similar size, such as Stockholm-class corvettes1, the since out of service Swedish missile boats2 or the recently laid down Dearsan 50 Meters Fast Attack Craft3, the estimated of displacement of the vessel is assumed to be close to 300 t. This assumption is also based on a scaled down version of the Visby-class corvette FMV (2025), as this ship is already operated by the Swedish navy with a similar operational profile, the biggest difference is that the Visby-class is considerably larger at 72 m in length and weighing 650 t. The Visby-class is designed to be operated by a crew of 43, meaning that a lot of interior space must be designated to crew accommodation, workstations, and mess. As can be seen in Fig. 4.1 the crew accommodation (yellow), mess (green), and workstations (pink) take up a considerable amount of space. Approximately 20 m of the midship section is used solely for crew facilities, meaning that the remaining 50 m are used for payload, machinery, and auxiliary systems. As a ship with the same proportions as the Visby-class would also be scaled down in beam and height, the weight would also need to be scaled down. The scale factor can be computed as SF = LNew Ship LV isby = 50 72.8 = 0.69 (4.1) Linear scaling of all three dimensions gives displacement as ∆New Ship = ∆V isby ∗ SF 3 = 210.6t (4.2) By applying a linear scaling factor in all three dimensions the displacement for the 50 m vessel is calculated as 210 t, however as the crew facilities that are being removed are 1. https://sv.wikipedia.org/wiki/HMS_Stockholm_(K11) 2. https://sv.wikipedia.org/wiki/HMS_Norrk%C3%B6ping_(T131/R131) 3. https://www.navalnews.com/event-news/dimdex-2024/2024/03/dearsan-signs-contract-with-qatar- for-2-fast-attack-craft/ 29 4. Main particulars comparatively light in comparison to many other systems it is assumed that the new ship weighs around 300t in loaded condition. This assumption is further supported by looking at similar ships that can be seen in Table 4.1, where it can be seen that ships such as the Flyvefisken and Skjold classes have similar main particulars and similar displacements as well. The construction material of the hull also has a large effect on the total system weight. Many modern naval vessels are constructed from composite materials due to the superior strength to weight ratio such materials provide compared to traditional steel structures (FMV 2025). Composite structures are however more expensive to construct, but they also offer other advantages such as a lower magnetic signature. Figure 4.1: Visby-class corvette general arrangement. Adapted from Städje (2024). Note the large internal volumes designated for the crew. 4.2 Payload The point of any vessel is the payload, and by removing the crew from a vessel can be seen as an exercise in optimization, where all the space used for crew facilities are repurposed directly into capability enhancing volumes. Since the ship is designed to be flexible it is reasonable to expect it to be able to carry different types of payloads dependent on what mission profile the vessel needs to operate. Having a flexible configuration where specific capabilities are connected to certain modules allows the possibility to quickly swap out modules containing weapons or sensors in case of changing operational profile or different mission parameters. Initially, until regulations are in place, this could mean crew modules and modules for search and rescue, mine sweeping, surveillance or regular logistical support. When the regulations for unmanned vessels are in place the crew modules can be switched out to instead incorporate modules that further increase the effectiveness of the vessels. To fully utilize such a system the estimated time required to swap modules needs to be kept at a minimum. This also creates an unfavourable situation where several unused modules with different systems could be forced to be stored ashore, rendering the investment in those capabilities unused. It is also important to consider whether to use an existing system for the modules, or if it is better to design a new system. There already exist several systems that promise to be able to quickly swap payloads in port, such as the Cube system from SH Defence, or the STANFLEX system, both of which are currently in use by the Danish navy. 30 4. Main particulars For the purpose of this design study, the time required to swap a module for another is allowed to take several hours, excluding docking and manoeuvring actions. This time constraint is derived from collaborative discussions with experts at Saab Kockums AB. It is however desirable to be able to be able to swap out modules as quickly as possible to allow the ship to spend less time in port and more time conducting mission specific tasks. 4.3 Energy estimates In this sections initial estimates are presented. These estimates serve to provide an idea of what the ship needs to perform its duties to a satisfactory level, both from performance and regulatory standpoints. To ease the calculations the onboard systems that handle the operations of the vessel, the sensor suite and computers, have been given a set value at 4kW. This number is given from SAAB Kockums AB and represent the whole suite of internal systems, meaning if an external larger radar module is mounted on the vessel this is not considered. The total energy requirement for a 4kW system adds little in the context of the ship at large, so this is neglected in future calculations. 4.3.1 Power estimate An initial power estimate is calculated to get an idea of the approximate power needed for the ship. The initial power estimate is calculated using the following parameters: • L = 50 m • B = 7 m • T = 1.8 m • ∆ = 300 t • Vkt = 30 kt The Froude number is calculated as Fn = Vm/s√ L · g = 30 · 0.5144√ 50 · 9.81 = 0.70 (4.3) This puts the vessel in the semi-displacement speed range. The Reynolds number is calculated as Rn = Vm/s · L ν = 30 · 0.5144 · 50 1.19 · 10−6 ≈ 648 · 106 (4.4) The block coefficient is calculated as CB = ∆ L · B · T = 300 50 · 7 · 1.8 = 0.46 (4.5) 31 4. Main particulars A low block coefficient like this is consistent with similar high speed vessels as it reduces resistance at higher speeds. From Papanikolaou (2014) the wetted surface area is estimated as: S ≈ (3.4 · ∇1/3 + 0.5 · L) · ∇1/3 = 316m2 (4.6) With this the two biggest resistance factors can be calculated, the frictional resistance RF and the wave-making resistance RW The frictional resistance coefficient is calculated as follows according to Papanikolaou (2014): CF = 0.075 (log Rn − 2)2 = 0.0016 (4.7) And the frictional resistance as: RF = 0.5 · ρ · V 2 m/s · S · CF ≈ 63kN (4.8) For the wave-making resistance a coefficient of 0.004 is assumed as this is on the higher end of values for semi displacement hulls according to Yeung (2005). The wave making resistance is then calculated as: RW = CW · ρ · g · B2 · L ≈ 99kN (4.9) The total resistance can then be calculated with a form factor value of k = 0.2 which is typical for semi displacement hulls. RT = (1 + k) · RF + RW = 173kN (4.10) From this the required power at the design speed is calculated. Molland et al. (2017) states that a system propulsive efficiency of η = 0.6 can be assumed. Preq = RT · Vm/s η ≈ 5350kW (4.11) A collection of different estimated power requirements can be seen in Table 4.2. With the current legislations mentioned in Chapter 2, this power requirement indicates the vessel indeed needs an engineer in addition to the chief engineer in order to pass the current regulations in the STCW code, also shown in Chapter 2. 32 4. Main particulars Speed Estimated Power requirement 6 kt 520 kW 8 kt 860 kW 12 kt 1380 kW 18 kt 2360 kW 24 kt 3650 kW 30 kt 5350 kW Table 4.2: Power estimates for different speeds in calm water conditions. The values presented in Table 4.2 represent the calm water resistance, however the in- stalled power needs to be slightly higher to account for sea margin as well as to avoid having to run the engines at full power when sailing at the ships service speed. Commonly a sea margin of 10-20% is used according to ITTC (2008), meaning that the installed power needs to be at least 6400 kW. 4.3.2 Fuel estimate One of the requirements from Saab Kockums AB is that the ship should have enough endurance to be able to stay at sea for at least 14 days, meaning that the sizing of the fuel tanks must be considered. To do this the example mission 1, described in Fig. 3.1, is considered where the ship patrols for 14 days at 5 knots, and also spends 10 hours at 30 knots. The choice of running the ship at 30 kt for 10 hours is made to simulate the ship crossing the Baltic Sea, as the Baltic sea is on average just over 100 nautical miles wide. Designing the vessel for 30 kt and a 150 nautical mile to and return ranges would allow for the ship to cross the Baltic Sea in around 5 hours. From Eq. 4.11 the calm water power is calculated for 30 kt, the same approach can be used to calculated to get an idea of the power required for patrolling at 5 kt as well. It can be assumed that for longer missions most, if not all, operations consist low speed patrolling and surveillance. On the other hand missions such as when relocating to another port or crossing the Baltic sea, a significant portion of the operation is conducted at high speeds Some extra power is also needed to account for sea margin and auxiliary systems, so the calculations are performed assuming the power required is increased with 15%. This gives 6,152.5 kW when operating at 30 kt, 1587 kW for 12 kt and 598 kW when operating at 6 kt. Given that a high speed marine diesel engine typically has a Specific Fuel Consumption (SFC) of around 200 g/kWh, (Zamiatina 2016), the fuel consumption can be calculated as: mfuel = ∑ P · t · SFC 1000 (4.12) 33 4. Main particulars Where P is the required power in kilowatts at a given speed and t is the amount of hours spent at that speed, SFC is the specific fuel consumption in g/kWh. For a gas turbine the SFC is slightly higher at around 280 g/kWh according to Vericor (2025). For the 14 day operation in example mission 1 the necessary fuel capacity is calculated to 51,295 kg, calculated using Eq. 4.12. It can also be seen that even though the time spent at 30 kt only accounts for 3% of the total mission time, it consumes over 20% of the total fuel, meaning that the time spent at high speed is a major driving factor for the sizing of the fuel tanks. For the example mission 2 the same fuel estimation for the propulsion system can be calculated to 27,280 kg if the same addition of 15% increased resistance to account for weather, making the fuel estimate for Example Mission 1 the determining factor for the fuel capacity requirement. With the fuel requirement for Example Mission 1 the total range of the vessel would yield a total range of over 3500km, which would be more than enough to patrol the entire length of the Swedish coastline. Alternatively it would be enough fuel to drive at 30 kt for 42 hours uninterrupted. To be noted is that this estimated fuel consumption is without considering neither the internal autonomous systems or sensors. With values given from Saab Kockums AB the complete sensor suite and autonomous systems on board can be assumed to demand around 4 kW of power at anytime the vessel is powered on. This is quite the small power drain, only adding another 1.344 MWh to the 14 day mission, or if the vessel is equipped with additional equipment that increases the passive power consumption significantly. Active radars can require power in the tens to hundreds of kilowatt range, and power consumption numbers on other weapon systems are undisclosed. Estimating the power consumption of these additional systems to 100 kW the total power requirement increases to approximately 35 MWh. Using the same equation for fuel estimation as before, Eq. 4.12, this additional power consumption yields another 6,988 kg, resulting in the final total estimated fuel requirement of 58,283 kg for example mission 1, and 28,266 kg for Example Mission 2. Required Fuel Capacity: 58, 283kg 34 5 Concept Design In this chapter different concepts are explored and advantages and disadvantages are dis- cussed. The chapter starts of with discussions regarding the different evaluating aspect of the concept generation, starting with hull forms and shapes, to then continue on with other aspects of the major design alternatives. In total the concept generation phase potentially can generate just above one thousand unique concepts, due to the study in- cluding three different hull forms. Four different hydrodynamic features, being simple displacing hull, foil assisted hull, foiling hulls, and surface effect. Five different machinery layouts with the main differences being the electric motor propulsion with different gen- sets or direct shaft on combustion engines. Six different propulsions alternatives, ranging from straight shaft with CPP or FPP, waterjets, pods and azimuths. Lastly three levels of modularity or flexibility was recognized as major categories that could be separated and used as a base line for the concepts. The first being a fully ISO container based system, where all future modules and equipment. The vast majority of these concepts are simply not feasible, or fall outside of the strict parameters set upon the design. Two of the main design criteria that yield high significance is the structural simplicity and low maintenance requirement. As the Froude number for the specified length and top speed is approximately 0.7 the vessel does not reach planing, thus the distinction of displacing hulls have been made to separate from planing hulls. 5.1 Hull Configurations Different hull forms provide different characteristics and most of them have strengths and weaknesses that can be used to optimize a vessel designated for a specific task (Molland et al. 2017). Traditionally the monohull is the most common shape for all but a few merchant vessels, where certain passenger or RoPax ships use configurations with several hulls such as catamarans or trimarans. In addition to the hull form, certain hull features can be implemented to further optimize the hydrodynamics of the vessel, such as the hydro planes or stabilizers. In this section the different aspects are discussed. 5.1.1 Monohull With monohull design the idea is to have a traditional hull that operates in displacement and planing conditions. There are many advantages to this design, it is a well proven design that is efficient in a wide range of conditions, as described in Molland et al. (2017). This is a very common design that is found all over the world, from small pleasure craft 35 5. Concept Design displacing in the hundreds of kilograms, to large merchant vessels of several hundreds of thousands of tonnes. The Swedish navy currently operates several types of ships of this design, such as the Visby-class corvette. The Visby-class is designed to be able to perform several different tasks such as submarine hunting, surface combat, minehunting, and assisting civilian ships in crisis (FMV 2025). The monohull design of the Visby-class as seen in Fig. 5.1 is able to reach a speed of 35 kt, while also being able to operate undetected at slow speeds. Figure 5.1: The Swedish Visby-class corvette is an example of a monohull design. Adopted from FMV (2025) One of the main advantages of the monohull is the strength and simplicity of the structure, while at the same time offering large internal volumes, thus reducing the centre of mass. A simple design is also cheaper to construct, potentially making it easier to find a wharf that can construct the vessel. Some disadvantages of a monohull is the wave induced rolling motion, since the initial transverse stability is lower than in multihull configurations. This has effects both for crewed missions, where the comfort of the crew is affected, as well as for autonomous operations where sensors might be negatively affected by excessive rolling of the ship. Further, the increased hydrodynamic pressure the monohull creates, since all mass is dis- tributed within one hull also increases the hydrodynamic wave-making resistance, which reduces the hull efficiency in higher speeds. 5.1.2 Catamaran A catamaran design has the potential to provide several valuable advantages. Catamarans have lower resistance than monohulls and due to their increased beam they are transver- sally stable and allows a large deck area (Molland et al. 2017). However the structure needs to be heavier and more complex to support the span between the hulls, leading to increased construction costs. Catamarans are also generally less comfortable in beam and quartering seas. For an unmanned vessel the comfort on board is of lesser importance, and a valid advantage for catamarans is the lower tendency to roll by having a steep initial GZ-curve, since the platform becomes more stable and thus induce less noise into 36 5. Concept Design the sensor data. Another aspect of the catamaran is the area in between the catamaran hulls that creates a very protected environment. Between the hulls equipment can be housed and lowered into the sea, or located to reduce risk of exposure. As an example, the Swedish Transport Agency’s fairway maintenance vessels MS Fyrbjörn operate within shallow coastal areas, and are designed so that the propulsion system are mounted on the inside of the both hulls, thus protecting propellers from striking underwater obstacles. Figure 5.2: The South Korean Tuo Chiang-class corvettes is an example of a catamaran design. Adopted from Teh (2019) 5.1.3 Trimaran This concept consists of a central hull, with two outriggers off the side to provide roll stability. The advantages of such a system is that the central hull, which carries most of the buoyancy, can be made narrower and thus reducing the resistance according to Yun and Bliault (2012), while the transverse stability is aided by the outriggers that can provide a large righting moment as they are situated far from the centreline. Trimaran designs are currently in use by both the US Navy autonomous test ship Sea Hunter, as well as the Chinese autonomous vessels JARI USV-A, see Fig. 2.4, and Thales TX Ship, see Appendix B. Potential negative features with this design is that it could potentially restrict payloads that need to be launched from the side of the ship, such as torpedoes or rescue craft. Deck space is also restricted as the hull is narrower, and the outriggers might need support structures on deck, further limiting cargo space. The structure of a trimaran is more complex and generally heavier than the same size monohull, as well as the coherent inter-hull volumes are reduced. The JARI-USV-A is an example of a trimaran design as seen in Fig. 2.4. 5.1.4 Foiling monohull Another way to reduce resistance at high speed is to fit the vessel with hydrofoils that instead of hydrostatic lift from buoyancy relies on hydrodynamic lift when the vessel makes speed through the water. Once the ship reaches a certain speed the hydrofoils generate enough dynamic lift to heave the hull up and out of the water. This greatly reduces the hydrodynamic resistance due to the reduction of wetted surface area and exposed 37 5. Concept Design water-displacing elements. Tests with hydrofoils were conducted as early as 1861 and has since been used on many different types of ships through the years (Yun and Bliault 2012). The concept requires higher levels of maintenance and either significantly increases the draught of the vessel if not mounted with retracting foils, which in turn significantly increases the weight and complexity of the vessel. Foils also require an increased level of maintenance to remove fouling in order to maintain the required lift. This might be hard to achieve on an otherwise autonomous or unmanned vessel, but can be solved by shorter missions and increased maintenance intervals when in or close to port. Figure 5.3: The American Pegasus-class was a class of fast patrol craft fitted with hydrofoils. Adopted from Hurst 5.1.5 Foil assisted multihull A foil assisted catamarans or trimarans have the potentials to lower the hydrodynamic resistance forces even further than the regular respective hull shapes due to a foil that can be designed to span between the hulls. This foil only assists the vessel, not lifting the entirety of the hulls out of the water, but aids in increasing the ride height of the vessel in the water. This in turn reduces the wetted surface area, thus reducing drag. The point of not raising the entire catamaran out of the water is to reduce the complexity of the system, by just incorporating a passive foil optimised for a designated cruising speed. 38 5. Concept Design Figure 5.4: The Superfoil40 passenger ferry is a foil assisted catamaran. Adopted from Yun and Bliault (2012) 5.1.6 Surface effect ship Surface effect ships (SES), also called sidewall hovercrafts, are essentially catamaran hulls with the bow and stern gaps sealed by flexible skirts to create an air cushion between the hulls. This air cushion helps lift the ship out of the water, thus reducing drag. Yun and Bliault (2012) notes that development of SES started in China in 1957 and was initially based on pure marine versions of regular hovercraft, the main difference being that the sidewalls of a SES are submerged in the water at all times. Since then several countries such as China, USA, Japan, UK, and Norway have all developed SES vessels for both civilian and military use. The Norwegian navy currently operates several classes of SES, most notably the Skjold-class corvette as seen in Fig. 5.5, which is able to reach speeds of 60kt, making it the fastest combat ship in the world (Umoe Mandal 2025). However the design and construction of a SES is complex and expensive as the structure needs to be both light and strong to be able to benefit from the air cushion. The ship also needs a lot of power installed as the fan for the air cushion needs to be able to pressurize the void between the hulls (Yun and Bliault 2012). Performance in seaway can also be an issue due to air leakage. 39 5. Concept Design Figure 5.5: The Norwegian Skjold-class is a Surface Effect Ship. Adopted from Forsvaret (2023) 5.1.7 Chosen hull concept The hull shape that is determined as the most suitable for the vessel is a monohull as described in Section 5.1.1. A monohull is a simple structure that is cost effective and easy to maintain. A monohull design also provides large amounts of usable volume low down in the ship to mount machinery and other heavy components, aiding with the stability of the vessel. A conceptual sketch of the chosen monohull design can be seen in Fig. 5.6. The hull form is a semi displacement hull that incorporates design aspects from both displacement and planing hull forms. Figure 5.6: Conceptual sketch of the chosen monohull design The downsides of a monohull design include worse roll stability in beam seas compared to multihull designs, and higher resistance at high speeds compared to other designs such as SES or hydrofoil concepts. However these downsides are considered minor in comparison to the advantages the monohull design brings. 40 5. Concept Design 5.2 Propulsion system There are many different solutions for main propulsion systems for any vessel, where each offers different advantages and limitations. This section describes the propulsion systems considered within the design process in broader terms, some specific solutions depending on hull configuration, are not considered here and is a subject left to future work. There exists many different types of propulsio