1)Try a 6:1 pulley.
2)Drill 2 wholes in a 1/2 to 3/4 inch metal bar. Feed the rope up through the side facing the sail and back down through the next whole then back through the loop you made at the top between the 2 wholes. The harder you pull the tighter it gets, as the pulling motion of the loop tightens the rope feed through on the bar. There is no chance of it locking up either. Very hard to explain in writng.

Very simple when you see it.

3) Straighten your leg rather than using your back and give it heaps.
Deano