I wouldn’t call myself an expert on this subject, at all.
But, correct me if I’m wrong, should the expected amount of heel not be a part of the equation? I have made an illustration to show what I mean. I have looked at the forces developed by the sail plan and the submerged hull and projected the forces in two directions forward, and sideways.
So from the sail there is a heeling force and a driving force which pushes the boat forward acting at the CE. The submerged hull incl appendages will also develop a heeling force and a drag force from water resistance, acting at the CLR.
When sailing without accelerating or slowing down, the driving force and the drag force will be equal.
I think the whole reason why we need the lead is to cancel out the torsion caused by the force couple F.Drive and F.drag. As you can see if the heel angle is increased, the distance between the force couples increases, thus the torsion increases. To cancel out this torsion, we need the heeling forces to generate a torsion in the opposite direction, which is done with the lead.
So in some way form-stability of the hull could also play a role. So if we consider a near cylindrical hull shape, it would need more lead compared to a more “boxy” one. Simply because it would have less resistance to the heel force, an thus move the two force couples further apart.
But don’t ask me how to calculate the exact % of lead. I guess it will also depend on the expected weather conditions.
But in Alan’s case with 3 different boats which is properly going to be compared in the same conditions, I would look into the “form-stability” to decide in which area the of Lead scale it should be…
I think you can calculate something called the Metacentric height which can be used to make a stability curve. But I would have to read up on that.
Hope it makes sense, and that I am not all wrong.
Again I’m not an expert; I just read a book at a pool in Egypt back I november.
P.S. How do you make pictures big when inserting them inline?