Dynamic effects on yacht sails and rudders

Licentiate thesis, 2019

In search of increased performance, modern sailing vessels are becoming more and more complex, introducing additional challenges in the design process. The dynamic behaviour of the vessel is of great importance to safe and efficient operation and thus needs to be accounted for when evaluating the performance. To accomplish this, the forces acting on the vessel in dynamic conditions must be predicted and evaluated.

The purpose of this thesis is to develop methods to predict the forces acting on sails and rudders in dynamic conditions.

RANS CFD is used to predict upwind sail aerodynamic forces. A comprehensive study of the numerical setup, physical modelling and uncertainties in the CFD solutions is presented. The spatial discretisation uncertainty is shown to be low for steady conditions, and the forces are well predicted. For dynamic conditions, mean values are predicted with reasonable accuracy. However, the instantaneous quantities show more scatter and a higher comparison error. Following careful analysis of the numerical simulations and experimental procedures, possible error sources are identified. For example, in the post-processing of experimental measurements, added mass forces had been removed incorrectly. For the numerical simulations, poor performance of the turbulence model may explain the error.

The dynamic characteristics of rudders for the Finn dinghy are investigated using full-scale towing tank tests. Considerable differences in both performance and handling are revealed. Also, the study highlights challenges faced when performing experimental testing of dynamic effects, such as the introduction of artificial constraints. The prediction of dynamic effects introduces additional uncertainties, regardless of whether experimental or numerical techniques are utilized, that need to be carefully controlled. Further studies are proposed to investigate these.