[sailingdog]

Rockter—

Gui is correct. The sail is not rigid, and as such the outer edge of the sail and boom move as the wind pushes on them. If the outer edge of the boom is connected to the high side of the boat, it will have more leverage to pull the boat over with... than if it is connected to the low side of the boat. If the boom and mainsheet are on the leeward side, as the boom tries to rise, it is actually going to be pull up on the leeward side, helping counteract the heeling force caused by the wind a tiny bit.

You're making the mistake of treating the sail, mast and boom as a single rigid unit—which it clearly isn't—and because it isn't it won't act as one.

If you look at the forces exerted on the boom, they'll be roughly in line with the mainsheet...or the mainsheet would be pulled in a different direction.... so the cyan lines represent the force on the boom of the boat...and therefore the boat...



As you can see from my illustration, the forces on the right-hand boat are clearly less likely to cause it to heel because of the mainsheet's location relative to the boom. While the mainsheet on the right-hand boat does contribute a bit to the heeling force, it also contributes a greater segment to the righting force on the boat, as it has a greater vertical component than horizontal component and is to the leeward of the center of buoyancy... so will tend to right the boat more than heel it—that is simple physics...

The force on the left boat has a much greater tendency to heel the boat, as it has greater horizontal component above the center of buoyancy...and a much vertical component pulling upward on the hull to windward of the center of buoyancy... which means that the total of its components are working to heel the boat...

It ain't rockter science...