Boat yards use jack stands to stabilise the hull of a boat in dry dock. Keel blocks are used to support the weight of the boat. Jack stands are used to prevent the boat from tipping during repairs or maintenance. Typical boat jack stands are similar to auto jack stands. The primary difference between these jacks is the plate on a boat jack stand. The plate conforms to the angle of the hull when the jack stem is raised in place. DIY boat stands are made by retrofitting this type of plate to auto jack stands.
Remove the stem from the base of an auto jack stand. Use a mitre saw with a steel cutting blade to cut the stem just below the support plate and remove the plate from the stem. Secure the stem in a bench vice with the cut end extending to one side of the vice.
Drill a 3/8-inch diameter hole through the jack stem, 1 inch from the cut end. The hole is used for a shoulder bolt that carries the load of the floating plate. The plate and bolt will be installed on the stem in the final step.
Have a steel fabricator provide a 12-inch square piece of 1/4-inch thick steel plate for each stand you plan to make. Have the fabricator provide a 6-inch long piece of 2-inch wide 1/4-inch steel plate for each stand.
Cut the 2-inch wide piece to make a pair of 3-inch long flanges. Drill a 3/8-inch hole, one-half inch from the end of each flange. Measure the width of the jack stem and position the flanges in the centre of the 12-inch square steel plate. The flanges should be on the end with the drilled holes at the top. Make sure the stem will insert between the flanges and align the drilled holes.
Weld the flanges to the steel plate. Allow the welds to cool. Turn the plate over and apply contact cement to the surface of the plate. Attach a piece of carpet remnant to the plate and trim any excess off the edges with a utility knife. The carpet prevents the steel plate from scratching the hull of the boat.
Reinsert the jack stem into the steel jack stand base. Position the plate flanges over the end of the stem and insert a 3/8-inch diameter shoulder bolt through the flanges and the drilled hole at the end of the stem. Thread a nylon locknut on the end of the shoulder bolt and tighten it with a box wrench. Repeat the steps to make additional boat jack stands.