Derivation of Forces on a Sail using Pressure and Shape Measurements at Full-Scale

Abstract: Aerodynamic forces are usually computed numerically or measured in a wind tunnel. These forcesmay be used to predict the performance of a yacht at full-scale. The aim of this project was todevelop a new method to measure the forces and moments directly at full-scale. The theory wasthat by simultaneously measuring the shape of the sails and the pressures around them the forcenormal to the surface could be derived by summation of the product of the pressure, unit normalvector and the relevant area of the discretised sail shape.A code was written for the interpolations of the discrete shape and pressure measurements to thefull sail shapes and pressures over them. The shape was then discretised and the pressure at thecentre of each cell was assumed constant over it. The results were validated through testing atmodel-scale. The calculated forces and moments were compared directly to the results obtainedfrom a force balance on which the model yacht was mounted. The results were also used in a VPP toassess the accuracy when predicting heel angle and boat speed.The first test was conducted on a set of semi-rigid sails with surface mounted pressure taps. A studyof the pressure distributions found using a VLM was used to optimise the pressure tap locations. Theresults were encouraging; however it appeared that the interference on the flow caused by the tapsresulted in large under-predictions of the forces and moments. This lead to a second round ofmodel testing using sails with internal pressure taps leaving the sail surfaces smooth. The resultsshowed good agreement, although there was still a significant under-prediction of the drive force.Full-scale testing was carried out using a purpose built pressure measuring system. Although fullscaletesting is extremely difficult due to the unsteady environment, good results were obtainedwhere clear trends in the boat speed and heel angle were also seen in the calculated drive force andheeling moment.