Depends how accurate you want to be, if its not for testing hull and sail designs and is for a game I would simplify. First thought would be just use layers of dynamic steering fields for water current and wind with a turbulence influence and just account for the momentum of the boat and the rudder position and the properties of the sail (Boom angle, % full sail that sort of thing) rather than trying to have the physics determine the force the wind imparts due to its shape (But I am sure that is a solved problem). Then you can have the wind fields affect in a purely visual way with the sail with it modelled as simply a mass point grid.
(I did a simple steering field at the end of this video http://www.youtube.com/watch?v=A3dR6SPMrrc).