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 Combines on-water sail pressure and shape measurements to determine forces  International collaboration involving researchers in France and New Zealand  First published results of such measurements on asymmetric spinnakers  Accuracy of the shape predictions from the highly curved sails is shown to be good  Results give fine details of the pressure distributions along the height and chord  Full-scale results from measurements on both a Stewart 34 and a J80 An innovative method combining simultaneous on-water pressure and sail shape measurements for determining aerodynamic forces produced by sails is described and used on Stewart 34 and J80 Class yachts flying asymmetric spinnakers. Data were recorded in light and medium winds in order to check the reliability, accuracy and repeatability of the system. Results showed similar trends to the published literature. The accuracy of the system was investigated by wind tunnel tests, with determination of the entire sail shape from the stripe images recorded by the camera-based (VSPARS) system, and was found to be relatively good. Generally the pressure distributions show a leading edge suction peak, occurring at 5 to 10% of the chord length, followed by a pressure recovery and then a suction increase due to the sail curvature, with finally a reduction in suction near the trailing edge. The drive force coefficient measured on the Stewart 34 is lower than for the J80 because of a non-optimal sail shape due to light winds. On a reaching course, the standard deviation of the pressure signals was largest near the luff, reducing in the stream-wise direction, while it was high on the entire sail section when sailing on a running course. This project has received funding from the European Union’s Seventh Programme for research, technological development and demonstration under grant agreement No PIRSES-GA-2012-318924, and from the Royal Society of New Zealand for the UK-France-NZ collaboration project SAILING FLUIDS.

This paper describes a method of deducing aerodynamic force components produced by individual sails. This is achieved by measuring the pressure distribution at a number of discrete locations over the sail and extrapolating these measurements into a distribution across the entire sail surface. The sail shape is measured using the camera-based VSPARS system and the force distribution over the sail surface is then determined. Wind tunnel tests have been conducted to validate the accuracy of the model. Full scale testing has been undertaken to investigate how aerodynamic effects of trimming sails affect yacht performance.

The interference between two yachts sailing in several conditions is investigated in the wind tunnel by using two similar yacht models, one of which is mounted on a force balance and the other moved around the test section. The yachts were configured to sail close-hauled upwind at 20° apparent wind angle, downwind under asymmetric spinnaker at 60° and downwind under symmetric spinnaker at 120° apparent wind angle. The regions of positive and negative interference are determined through aerodynamic force measurement and flow disturbance measurement, and the sources of these effects investigated.