I have the same trouble with my sailfish, I've try cranking it as tight as I could, go super slow up the ramp. Even had the weld break on the trailer post, trying to crank it on after it on land, so whatever I do it moves. I have a bunk trailer so once it on the trailer it will not move at all. I just strap it down and move on. Once I get it home I use jack stands lift the stern, and a floor jack and once i get most of the weight up off the trailer i can tighten it up to the post. I could jack and move it on the trailer better at the marina, but I never had yet. So , if you ready want your boat tight on the trailer post , you could pick up 2 flat boat jack stands for the stern and a car floor jack for the center and a few blocks of wood and jack it tight right after your up the ramp and on flat ground.