quoted from Knuttle...."From the picture, I do not think you can get enough downward force on the hull to cause the indentations. Look at the height of the winch and compare to the height of the bow eye. It looks like mine and that is about two inches difference. Most of the force from the winch is against the rubber bow support. I have not measured the forces, but I will bet that between the angle, and the rubber bow support, little downward force is being transferred to the hull at the front of the bunk."
on the line of level, as determined by the bunks which the boat is riding on, I can tell there is at least 5 inches of downward pull from your winch. measured from the inside of the bow eye, to where the strap rests on the winch. if you were to slacken the winch there would probably be more.... to properly measure, you need to crank the tongue jack so the front of the bunk is level with the rear of the bunk, and then measure from the bow eye to the ground,... and then from the where the winch strap leads onto the drum, to the ground... and see what the difference is....
it may seem like not enough difference to cause your problem, but its the ANGLE of the pull that is the problem.
its obvious the PO has already determined this was a problem as he has added a cross member and put a wood block in place, although probably not high enough.
and just because the boat is tight against the bow chock doesnt mean there is no downward pull on the bunks.... whenever you have to crank the boat up to the chock that last 4-6 inches after pulling it out of the water, and the boat doesnt slide on the trailer but an inch, yet the gap closes, that is due to the trailer flexing (springing) to come up and meet the boat, rather than the boat being pulled into the proper position.
AND when that happens the winch does in fact rise closer to the height of the bow eye, but all that means is its trying to pull the boat downward on the bunks to meet the "at rest" level of the winch..... it will forever have an overloaded downward pressure on the bunks, AND thats where the cross member and block of wood come in to prevent the bunk damage on the boat.... if its high enough
and that mast is not original....it may work perfectly and hopefully so. all im saying is that the profile is wrong and the 21's didnt have twin spreaders, and the profile of the spreaders are wrong also, from what macgregor used... you should stand the mast and put it all together in your driveway and hoist the sails to see if everything fits and tensions up properly.... best of luck