I'll draw a diagram when I get a chance.
In the meantime, follow these links to quench your thirst for knowledge...
Ackermann Steering 1 Wikipedia - Ackermann Steering 2 Use this information carefully, the principle is correct but the geometry is slightly different.
My initial estimations based on basic ship manoeuvring indicates that the point to aim your tillers at (not your rudder blades, these should still be straight) should be just forward of the centreboards at the centreline due to the effect of leeway angle (which cars don't have).
regards,
Nick.
