Development and Optimization of Underwater Exhaust Outlets for Sailing Yachts

Abstract

Although sailing yachts primarily rely on wind for propulsion, engines are used for propulsion in unfavorable sailing conditions and maneuvering in the marina. On sailing yachts, exhaust gases are typically expelled through outlets positioned above the waterline. This can contribute to noise, vibration, odor and increased piping requirements, which may negatively affect onboard comfort, weight and space utilization. One solution to reduce these issues is to relocate the exhaust outlet underwater and closer to the engine. While underwater exhaust outlets are commonly used on motor yachts, their application on sailing yachts remains largely unexplored. This thesis investigates the development and optimization of an underwater exhaust outlet for sailing yachts using a structured product development approach supported by computational fluid dynamics. A representative 33 m sailing yacht hull was modeled and in total 30 exhaust outlet concepts were generated and evaluated with over 60 CFD simulations in OpenFOAM. The final concept, called the Quarter Bell-Mouth, consists of a tapered underwater outlet combined with an interceptor positioned near the outlet. The interceptor creates a low-pressure region that helps reduce back pressure and promotes a steadier exhaust gas flow. Simulation results indicate that the concept can expel exhaust gases underwater without negatively affecting the propeller or rudders while maintaining acceptable back pressure between approximately 8 and 13 knots at heel angles of ±5◦. However, oscillations at lower speeds indicate that a bypass exhaust is required below 8 knots. The results of this thesis show that underwater exhaust outlets are a promising design alternative for sailing yachts, with potential benefits in comfort, weight and space utilization. Future work should include validation on additional hulls, material and manufacturing studies and sea trials with a physical prototype.