A great deal of research, both computational and experiment based, has been carried out on the optimisation of hull geometry for minimal hydrodynamic resistance; however, rather less effort has been expended on the equivalent problem of optimising the sailplan geometry. The goals of the optimisation procedures which have been adopted for sail optimisation are somewhat diverse; some researchers have concentrated on the maximisation of aerodynamic drive force, with or without constraints on heeling moment, whilst other approaches have included simple representations of hydrodynamic sideforce and induced drag. Much of the work has concentrated on optimisation of the distribution of aerodynamic lift over the sailplan without specifying how the lift distribution may be achieved in practice if indeed the lift distribution can be obtained within the practical constraints of conventional rig technology.
It is argued here that, from the point of view of yacht design, an appropriate goal for the optimisation is the selection of the sailplan geometry which yields the best performance for a given hull form over a suitably weighted range of windspeeds. In this work, the aspect of performance considered as being of primary importance is upwind speed. A method is presented by which the principal dimensions of the rig - luff length, foot length etc. - may be chosen so as to maximise upwind or close reaching speed for a particular yacht in a given wind strength.
The aerodynamic performance of the rig is predicted using a non-linear vortex lattice model; viscous effects, including the effect of the mast, are included using an ad hoc approach based on two dimensional experiment data. The hydrodynamics and hydrostatics of the hull are obtained using a performance prediction approach based on the well known Delft yacht hull series. Optimisation of the rig geometry is carried out using a stochastic technique known as a structured genetic algorithm.
Results are presented for two variations on the sloop rig; the first employs a mainsail with an essentially triangular planform, whilst the second allows the inclusion of a substantial roach, leading to sails of the form normally associated with full length battens. The hull forms considered include typical examples of both modem and slightly older racing yachts. The sensitivity of the geometries obtained to constraints unrelated to upwind speed - such as structural integrity or offwind sail carrying capability - is also illustrated.