Experimental measurement and simplified prediction of T-foil performance for monohull dinghies

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 flaps. 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, and immersions, to measure vertical and lift and drag. Results are then compared with two simplified models typical of those utilized in Velocity Prediction Programs for preliminary design. The first model uses section lift and drag data obtained using the well-known XFOIL code allied to a simple correction for 3D effects while the second model deploys a numerical lifting line theory in conjunction with section data. A number of practical conclusions are drawn regarding the set-up and sailing of foiling dinghies with flapped T-foils. Results show that whilst the simple and rapid models typically used in basic VPPs may not accurately represent the relationships between angle of attack / flap angle with lift and drag, the predicted relation between lift and drag is reasonably accurate in most cases