Development and Optimization of Underwater Exhaust Outlets for Sailing Yachts

Sammanfattning

Although sailing yachts primarily rely on wind for propulsion, engines are used forpropulsion in unfavorable sailing conditions and maneuvering in the marina. Onsailing yachts, exhaust gases are typically expelled through outlets positioned abovethe waterline. This can contribute to noise, vibration, odor and increased pipingrequirements, which may negatively affect onboard comfort, weight and spaceutilization. One solution to reduce these issues is to relocate the exhaust outlet underwaterand closer to the engine. While underwater exhaust outlets are commonlyused on motor yachts, their application on sailing yachts remains largely unexplored.This thesis investigates the development and optimization of an underwater exhaustoutlet for sailing yachts using a structured product development approach supportedby computational fluid dynamics. A representative 33 m sailing yacht hull was modeledand in total 30 exhaust outlet concepts were generated and evaluated with over60 CFD simulations in OpenFOAM.The final concept, called the Quarter Bell-Mouth, consists of a tapered underwateroutlet combined with an interceptor positioned near the outlet. The interceptor createsa low-pressure region that helps reduce back pressure and promotes a steadierexhaust gas flow. Simulation results indicate that the concept can expel exhaustgases underwater without negatively affecting the propeller or rudders while maintainingacceptable back pressure between approximately 8 and 13 knots at heelangles of ±5◦. However, oscillations at lower speeds indicate that a bypass exhaustis required below 8 knots.The results of this thesis show that underwater exhaust outlets are a promisingdesign alternative for sailing yachts, with potential benefits in comfort, weight andspace utilization. Future work should include validation on additional hulls, materialand manufacturing studies and sea trials with a physical prototype.