Rot Doctor


Subject: Outboard Transom Height Extension
Date: Sat, 24 Jan 2004


I read your article on transom rot repair and found it to be very interesting and doable. I may have a similar problem, but don’t know that yet. My problem is that I had a 125 HP outboard motor (Force) without a boat to mount it on. It was a spare motor I acquired when I had a Bayliner Trophy with twin Force 125 outboards. Well, I sold my Bayliner last summer and had the spare motor left over. I decided to buy an un-powered outboard motor boat with trailer to mount the motor to. I failed to do all my homework and realized (after the purchase) and some research that the anti-cavitation plate of the outboard when mounted to the transom should be even with the bottom of the keel of the boat. I went to measure the transom, and sure enough, the transom measures 20 inches from top to bottom. The outboard motor is a 25 inch shaft motor, putting the anti-cavitation plate on the outboard 5 inches below the keel. Not good! I’ve looked at outboard motor Jack Plates which allow the motor to be raised/lowered on the plate. I am really not crazy about going that route. I am wondering if there is a way to raise the transom height by performing a similar repair as outlined in your article on transom rot by adding wood and glassing over? What do you think?

Thank you for your time and thoughts!

K. U.

That is certainly a doable project. The single most important consideration would be the structural design of the extension. If you do not design the extension properly, the motor could break the extension off the transom. The area where the motor mounts to the boat is one of the most if not the most high stress areas on the boat.

For the extension itself, you want to make sure that you overlap the joint. By this I mean stagger the new plywood where it meets the old plywood. You want a 7:1 overlap. This means that if the old plywood is one inch thick, you would want to cut half of the old plywood back seven inches. If the old plywood is 1.5″ thick, multiply 1.5 x 7 to get the required stagger. You would then use two pieces of 3/4″ plywood to build the extension, with one piece being taller by the amount of the stagger. This will make the joint as strong as a continuous piece of plywood. You do basically the same thing with the fiberglass, except you grind a bevel into the old glass and use larger and larger layers of fiberglass until the last layer goes to the outer part of the bevel in the old glass. Use epoxies for all your plywood laminating and fiberglassing. You can spray gelcoat over the finished fiberglass, or a high quality two part polyurethane can be sprayed or brushed on.

Let me know if you have further questions.