Adding more sail aft of the center of lateral resistance increases weather helm
. It's the same effect a sailboard has when he rakes his sail aft causing the board to head
higher into the wind
. I think in your case your boat was balanced assuming you would be flying the mizzen, and without it you have lee helm.
What you might be noticing is that as the boat heels, some or all of the lee helm goes away in a good breeze. This is caused by the hull
shape changing as the water
sees it, the leeward side has more curvature wanting to round you up, while the lee side has relatively much less curvature. So only sail in a still breeze and the problem will correct itself.....kidding.
I don't know how your boats rig is set up, but if you can rake your mast
further aft, the sails
center of effort would also me moved aft accordingly reducing your lee helm.
It's not practical to move the keel
forward, which would shift the center of lateral resistance forward reducing or eliminating lee helm. So really the only other solutions are to somehow move the center of effort of the sail area aft, either through raking the mast
aft, moving the mast aft or changing the sail configuration so you have more sail area aft.
I think the most practical solution is to put up with using your mizzen so your boat is better balanced. That is probably not what you wanted to hear.