I'm in the process of completing a 31' Lake Michigan cruiser made of 5086 marine grade aluminum. It has been an excellent journey so far requiring numerous extra tools that I had to make for the fabrication. One necessity (if you don't have a large crane) is some method of lifting the boat so the trailer can be moved into place for fitting. Since I wanted to build a custom trailer I decided I needed to build the jacks to lift the boat. Commercial jacks are quite expensive, and rightly so, there is a lot of work and expense to make them. I figured I could fabricate a set which was a compilation of the best commercial designs. The pictures below are the jacks I built. I've substantially increased the thickness and size of some of the members to make them sturdier. The main post is 3 1/2" X 3 1/2" steel tube, 1/4" wall. The bases are 3/4" steel plate. The hydraulic jack boxes are 1/4 steel tig welded for strength. The rollers which ride along the support posts are 1 1/2" steel with bronze oilite bushings inside for bearings. Each jack is equipped with an 8 ton long stroke hydraulic jack from Northern Tool. The jacks have worked flawlessly, easily picking up a 7,000 boat like it wasn't even there. I had the jack assemblies hot dip galvanized to make them weather resistant in case they had to be left outside. All in, I built them for about a third of the cost of commercial models.