A larger head sail can cause weather helm? and about that clew height...

[a64pilot]

My main was replaced by the PO with a furling boom, boom is a little longer than original was and the sail is full battened and has a lot of roach. I don’t know how much larger the main is, but it’s larger.
In short with the main fully up and properly trimmed, the boat is not trimmed well at all, to get a neutral rudder position it took either easing off the main sheet or reefing the sail a few feet. That was with a 110 Genoa.
So I had a 135 Genoa made reasoning that the bigger head sail would move the center of pressure further aft and I could properly trim the main.
Well guess what, it didn’t change anything at all. I can only assume Mack Sails knew what they were doing and cut the bigger Genoa so that it wouldn’t change trim.
Now I have a cruising code zero, from what I can tell essentially a lightweight fuller cut 170 Genoa. Now surely this 170 Sail would move the center of pressure aft and cause the Boat to trim much different?
Well guess what, it doesn’t.
I can only assume that a sailmaker can cut a sail so that even a bigger sail does not move the center of pressure much?
The clew height does appear to be higher on the 135 than the 110, and the code 0 even higher, but that is just an observation, no measuring.

[Jim Cate]

A64, with many designs the center of effort has little to do with helm pressure... more to do with heel angle. You may recall that DH has reported that his helm remains balanced with no main at all... until the boat heels steeply at which time weather helm develops.

We've found much the same with our boat... a very different hull shape to DH's.

In your case, are you attempting to get a truly neutral helm... that is, going straight with a zero rudder angle? Most authorities suggest that (when going to weather at least) best VMG will be achieved with a few degrees of weather helm, say 3-5 degrees. Perhaps that is what Mack was aiming at...

Jim