Hi Rob and Everyone,

An explanation of my theory and a caveat that I gave to Rob: I'm a mechanical  engineer, not a sailor.

The torque generated at the CLR by a rudder may be small but I figure it is the same as the opposing CofE torque. Let go the tiller and a boat luffs up naturally, hold the tiller to maintain direction on a reach and the rudder is presumably generating the same torque as the opposing CofE torque.
If lack of torque IS the problem, then consider the equation Torque = Force x Distance (T= F x D) where force is the force generated by the rudder and distance is the perpendicular distance of the rudder force vector from the CLR. Rudolf has moved his rudders forward 80 cm and 80 cm to the windward, both causing a reduction in D. To maintain the same torque opposing the CofE the rudder can either be made larger or the angle of attack can be increased. Increasing the angle of attack might induce stalling. 

Going the other way, moving the rudders more to the stern will increase D and therefore force (and angle of attack) can be reduced. 

On the subject of where the CLR is, I had drawn it for Rob as being somewhere around the centre of the boat and along the fore and aft axis of the lee hull. On further thought it occurs to me that while it is somewhere near the symmetrical axis of the boat, it is may actually be somewhere in between the two hulls as they both resist lateral motion. Assuming the windward hull is 2/3rd the length of the lee hull then the CLR should be approximately 1/3rd (or is it 2/5th?) the distance between the lee hull and the windward hull. This gives a larger value for D, and a so larger torque for the same force F. Up until now I have been unable to get my head around why Harryproas seem to steer OK in spite of the large hull to windward; could this be a reason?

Cheers,

Robin.

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