Experimental and numerical prediction of foiling monohull dinghy performance

The rise in interest in large foiling yachts, such as those in the America's Cup, has spurred a corresponding interest in foiling applications in monohull sailing dinghies, both for boats designed specifically for foiling, and through retro-fit foil kits. The present study considers the assessment of foil systems for such boats, and specifically the prediction of the necessary foil performance of t-foils with and without flaps. Towing tank and wind tunnel experiments are used alongside numerical simulations. The accuracy of different simulation approaches is examined and the corresponding sensitivity of the predicted boat speed to the numerical method adopted is identified. Results are presented via two case studies, predicting performance for a foiling International Moth, and for a Europe dinghy retro-fitted with hydrofoils. Benchmark data for the numerical simulation is obtained using two approaches. The main lifting foil for a moth dinghy with flap was tested at full-scale in a towing tank at a range of speeds, trim angles and flap angles. Results are compared with published experiment data for a similar moth foil. In a complementary set of tests, the rudder of a Europe dinghy was tested in a wind-tunnel, initially in the original design condition, and subsequently with a T-foil rudder designed for a retro-fit foiling solution. A range of numerical techniques of varying levels of complexity, fidelity, and numerical intensity were then used to predict the performance of the foils. The techniques adopted include 2D predictions using XFOIL allied to a simple correction for 3D effects, a numerical lifting line theory, a fully 3D panel code and RANS CFD. The data from these simulations is used in two ways. The data is first used to examine the accuracy of the different simulation techniques by comparison of predictions with the measured data. The simulated data is then deployed in a 5-DOF Velocity Prediction Program (VPP) in order to predict the performance of a foiling moth and a concept design for a foiling Europe over a windward-leeward course, in order to illustrate the sensitivity of the predicted boat performance to the choice of foil simulation technique. Conclusions are drawn regarding the accuracy of the simulation techniques, and the pros and cons of wind tunnel and towing tank testing of T-foils are discussed. The VPP study demonstrates the trade-offs which may be exploited in the design process when computational resource is limited: between a small number of high fidelity simulations with high numerical intensity and a large number of moderate-fidelity simulations which are computationally less intensive.