Originally Posted by peterchech
PS As many of you may know my boat's a Cherubini Hunter 25 masthead sloop, shoal draft fin keel (no cb) and spade rudder. It will not move under main alone, no matter the wind strength this boat simply won't move without slot effect of the jib. The boat also has far too much weather helm, you have to nearly flog the main upwind to keep the boat balanced and once the wind is over 8-10 knots you still have to keep pressure on the tiller to keep the boat from rounding up. Not sure if this may have something to do with the heaving-to issue...
Sounds as if somthing is not quite right. Mast rake can effect weather helm. The mast should be raked towards the stern maybe a 1/2 of a mast diameter off vertical. Maybe you should also look at rig tension.
It sounds like you have tried just about everything as far as sail trim. With my mast head sloop, if I have too much main up or jib, I will have severe weather helm. With just the right amount of sail, the boat nearly steers itself with no helm pressure. If I over sheet the main sail I will get heavy weather helm, but ease off just a few inches of sheet and she steers great.
Maybe get a sailboat "racer" to go out with you and see if they can help you look at the rig tune and sail trim.