Henry, 'cause nobody else has pitched in, after doing a fair bit of research a while back before modifying our rudder, I'll add my $0.002 - at current exchange rates for what it's worth..
Originally Posted by heinzir
To simplify my original question: Ignoring the benefit of increased rudder draft, would my boat perform better if the rudder was vertical instead of raked forward? Why or why not?
One additional piece of information: the boat has a swing keel which rakes aft. Combined with the forward rake of the rudder, the profiles of the two together form an open ended "V"?
Assuming the keel position doesn't change, the thing that matters most is the Center of Effort (CoE) and it's proximity to the turning axis of the rudder - basically a straight line through the pintles. To keep the boat going in a straight line, the lift from the rudder (rudder force) must equal the lift from the keel, less any sideways forces acting on the sails - one reason yachts need bigger rudders than stinkpots!
Anyhoo, assuming a rectangular blade shape and a typical NACA cross-section, the CoE will be approximately 1/3 of the way back from the leading edge ie. just forward of the center of the blade.
If the CoE was precisely on
the turning axis of the rudder, you would have the least
amount of turning force on the tiller (ie. it would feel sloppy). If the CoE is too far from the turning axis of the rudder, the turning force on the tiller would be too great one way or the other (too far aft = weather helm; too far forward = lee helm). As it turns out, you want a bit of weather helm (to be safe), so most boats have the turning axis right at or very near the front of the rudder blade - certainly within the first 1/3rd.
It then follows that if you move the rudder toward the keel, for a given amount of rudder force the turning rate would increase (ie. the boat turns faster) if you move it back, it turns slower.
Thus, if your rudder is currently raked forward, I suspect that moving it to a vertical position (CoE slightly further aft) would make little or no difference, but would probably not be better.
Apologies for the long reply..