Unsteady Aerodynamics of Upwind-Sailing and Tacking

Abstract

Predicting unsteady sailing yacht performance is of significant current interest to racing syndicates and yacht designers alike because it is anticipated that optimising boats for dynamic conditions will lead to large performance gains. Such optimisations are usually carried out with the aid of Dynamic Velocity Prediction Programs (DVPPs). These programs require knowledge about the behaviour of the sails in unsteady flow. To this end Part A of this work looks at the simplified unsteady aerodynamics of a yacht that sails upwind in waves. Unsteady thin aerofoil theory is extended to the case of two interacting aerofoils representing the headsail and mainsail. The developed novel method is analytical in nature and is based on representing the sail bound vorticity distributions and the unsteady vortex wakes by planar vortex sheets. The theory is successfully validated against data from the literature and pressure distributions from wind tunnel tests. An application of the theory to the case of an International America's Cup Class yacht that pitches in waves reveals that the time-varying components of the aerodynamic forces are small and that only very little energy can be extracted from the unsteady flow about the sails and converted into thrust. Part B looks at the aerodynamics of a yacht that tacks. Since today's DVPPs usually suffer from a lack of available data on the behaviour of the sail forces at very low apparent wind angles a series of quasi-steady experiments was carried out. Test results for different tacking scenarios (headsail flogging or backed) are presented in a form that facilitates incorporation into a DVPP. The quasi-steady approach used in the wind tunnel tests does not account for unsteady effects like the aerodynamic inertia due to the 'added mass' of the sails. The added inertia of a sail can, however, be estimated by 'strip theory'. Using expressions from the literature and experimental results it is found that such a strip theory over-estimates the added inertia of a mainsail by about 20% because it neglects the three-dimensional flow around the head and the foot of the sails. Using the DVPP FS Equilibrium the influence of added sail inertia on the tacking behaviour of a yacht is studied and it is found that added sail inertia has a small but noticeable influence on tacking performance.