Boat transom replacement tutorial


People who are restoring old boats often face the problem of rotten wood inside their boat’s core. One of the most frequent repairs on older boats is replacing or at least repairing a plywood cored transom. I had a similar problem when rebuilding my boat and since I didn’t find any good How-To’s I decided to write one and try to illustrate how to replace the transom plywood core on a boat.

Preparing for the boat transom replacement

The first thing to do, before you start to remove parts from your boat, you should take some pictures of how everything looked in the start, so you will have some guidelines for later on. Also you should take some measures of parts around the transom, such as fiberglass thickness of the outside skin, plywood thicknes of the transom itself, inside fiberglass skin thickness, stringer location and thickness and so on…

Be sure to support your boat’s hull well, becouse when you remove structural parts from the hull it becomes flexible and it can twist. You will end up with a twisted boat, and that can’t be undone. Keep in mind that the keel should be heavily supported.

Gain access to the transom

Boat transom replacement tutorial

I started with cutting away the outboard motor well and removed all the fiberglass that covered the transom. In my boat, the whole top back end of the boat had to be removed, but this may be different in other boats.

Next I cut out the inner fiberglass skin that covered the transom in order to get to the plywood core. It was very rotten. In fact it was already in little pieces. I cut the the inner skin in one piece and used it later on as a template for the new plywood transom core. If your transom is not completly rotten, you should remove it carefully, becouse it will save you a lot of time if you can use it as a template for the new one.

Old plywood core must be completly removed. I used all sorts of tools to do that, but an old crowbar,a hammer and a chisel did the best job for me. If you have an air compressor, a small air hammer is a good investment.

After you remove every bit of the old transom, take a good sander and sand the outer fiberglass skin to perfection. If you make it thinner with sanding, you should add a layer of glass and resin over it and sand it again. You must end up with a completly flat surface to bond your new peace of plywood to.

Once the old plywood is removed, make a template of the transom using cardboard or cheap plywood or construction foam. You will use this template to cut your transom core and also to draw/cut the wide fiberglass pieces that will make up the new inside transom skin.

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Old transom removed and outer fiberglass skin sanded.

Installing the new plywood core

Boat transom replacement tutorialYou should make your new plywood core at least as thick as your old one was. I made mine thicker. Originally it was 38mm thick  and I made two new identical pieces that were 22mm each and were made out of waterproof marine graded plywood. I also took some measurements of my engine to see if it will fit thicker transom. Everything seemd to be OK so I started laminating the first piece of plywood in.

I positioned the first piece into the beat and started laminating. I used 330gr/sqm mate and 600gr/sqm roving (cloth) to do the job. When you place your first piece in the boat be sure to laminate it with one layer of glass mate and use a lot of epoxy on it. Install the piece while still resin is still wet so it will bond to the old skin. Clamp everything together and leave to cure.  Do not apply too much pressure when clamping! Epoxy is gap filling and extremely strong, you want the epoxy to remain between the plywood layers. Once the epoxy begins to squeeze out the sides evenly it is time to stop the clamping pressure. Do the same with the other piece of plywood.

Boat transom replacement tutorial Boat transom replacement tutorial Boat transom replacement tutorial
First piece in place and two layers of fiberglass laminated over it.

Laminate new inside transom fiberglass skin

After you laminated your new transom you will laminate your  new inside transom fiberglass skin. The lamination schedule for each boat will vary, but in general, you will be looking to build up to the original thickness. I did a little extra on my boat, but this is optional.  The new inside skin will be much stronger, and better bonded to the core because of the superior materials (epoxy and biaxial fiberglass). Work slowly and precisely and be sure to remove any trapped air from your lamination layers, becouse they can start holding water in next couple of years and you will then have to do all over again.

Boat transom replacement tutorial Boat transom replacement tutorial Boat transom replacement tutorial
Both pieces in place and the new inner fiberglass skin laminated.

Your new transom is now complete and the only thing you need to do now is to tie it to your boat’s stringers and at the end coat everything with gelcoat to make it really waterproof. Use heavy biaxial cloth to tie the stringers to the transom and repeat the lamination several times. This could be a big weak point of your boat if not done right.

Put it all back together

Now it is time to look at those pictures I told you to take in the beginning. They sould give you some directions where things that you removed belong. Laminate the motor well back in place, mount all parts back where they were, mount your engine and you are ready to go. Below are some pictures of how my boat turned out, although this is not the only repair I had done.

Boat transom replacement tutorial Boat transom replacement tutorial Boat transom replacement tutorial
Boat transom replacement tutorial Boat transom replacement tutorial Boat transom replacement tutorial

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