Predicting Yacht Performance in Waves Using a CFD Velocity Prediction Program

Doktorsavhandling, 2025

This thesis describes the development of a CFD-VPP procedure for predicting the performance and time-history of motions and forces on a yacht sailing upwind in regular waves. It is the first 6DOF CFD-VPP procedure to handle incoming, quartering waves, and the first CFD-VPP procedure to include a comprehensive unsteady aerodynamics model. In order to accomplish this, modern CFD capabilities and techniques have been utilised to handle the large motions experienced by a yacht while maintaining grid quality and reasonable computational effort, resulting in a state of the art simulation setup. A number of sub-studies have been undertaken, concerning unsteady aerodynamic effects on sails, the prediction of the unsteady sail forces using CFD, as well as the development of a low-order model for this. The low-order unsteady sail force model is capable of handling arbitrary motions, and has been shown to result in improved prediction of the sail forces for a three-dimensional, pitching sail plan. The unsteady sail force model has been coupled to the CFD-solver, and subsequently used to predict the dynamics of a yacht sailing upwind in quartering waves. A single upwind sailing case has been simulated, using the unsteady sail force model as well as a conventional quasi-static sail force model. The differences in the dynamics predicted by the two models have been analysed, shedding light on the complex interactions that occur as a yacht encounters waves, and proving the relevance of the procedure developed in this thesis to the study of sailing in waves. The results reveal significant differences in average boat speed, in turn driven by differences in the dynamics in roll, affecting the yaw balance and ultimately, the rudder angle and resistance components associated with the rudder.