I do what Sabre
recommends. You can get away with a 5 ton floor jack for most boats. Just tighten the stands a lot, give a little pressure on the keel
with the jack (make sure to put a board between the keel
and the jack), re-tighten the stands, repeat until you can move the boards. I feel a little better about it if I have to force the boards to move along the keel, not have the boat
lifted so high they move completely free.