A reader recently asked about a project he was contemplating. He had been given an old outboard hull and was thinking about restoring it. He mentioned that water was leaking from several holes in the transom. That usually means part of the transom will need replaced.

The Basics

Certain areas fiberglass hulls need extra reinforcement and the transom is one of the main ones. This most often consists of one or more layers of plywood, cut to shape and bonded in place with resin. After the plywood is in place additional layers of fiberglass and resin are applied.

As long as there are no places for water to reach the plywood, things are fine. But, it the case of an Inboard/Outboard installation, a big hole is cut through the transom. In other cases, holes are drilled to mount swim ladders, transducers or other accessories. When the sealant fails around these holes, water begins to leak into the plywood core.

After a while, the plywood begins to rot and delaminate.  Long term, the plywood in the transom looses strength and becomes very flexible, allowing the gelcoat and fiberglass to crack. While it is a pretty complicated job, replacing the transom core isn’t impossible.


Transoms are usually replaced from inside the boat. This means removing all the internal structure, seats, and tanks, etc. so you can work on the transom. If the boat is an I/O, that means removing the engine.

Once you can get to the transom from the inside, it is time to cut away that inside layer of fiberglass. This needs to be done in a way that doesn’t stress or crack the exterior fiberglass layers and gel coat. An angle grinder with a cut-off blade works well.

If the plywood is in bad shape, the fiberglass will probably be easy to peel off. Other wise, judicious use of pry bars may be necessary.

Once the inside layer of fiberglass has been removed the plywood core can then be taken out. While much of the plywood will be wet and some of it may be rotten, there will be sections that will still be bonded to the hull. These will need to be carefully chiseled off, again without damaging the exterior hull.

Remove all traces of the old plywood then sand and clean with 80-grit sandpaper. Be sure and sand the edges where the inner layers of fiberglass attach to the hull, it will make bonding the new transom in place much easier. Vacuum up the dust and wipe down the fiberglass with acetone.


The next job is to make a pattern of the inside of the transom so the new plywood can be cut to shape. A good choice for pattern material is artist’s foam board. You can tape smaller pieces together with masking tape to get the sizes you need. Brown wrapping paper also works as long as you tape it in place so it doesn’t slip around. The pattern (and the plywood cut from the pattern) needn’t fit precisely around the edges. In fact, the radius of the corners may well prevent a tight fit. Any gaps will be filled in when the plywood is bonded in place

Most transoms are on the order of one and a half inches thick. If there is any curve to the transom, this thickness is best made up of two three-quarter inch or three half inch thick layers of plywood.

You can use top-of-the line marine plywood or lumberyard exterior ply. The marine ply will be expensive and somewhat hard to get. The exterior lumberyard plywood will be cheaper, easier to get but will be rougher and poorer construction and will probably have voids. Both will be made using exterior glue.

A better choice would be MDO plywood. This plywood is designed for use in exterior signs, has few or no voids and has a smooth exterior surface of phenolic resin paper. This surface accepts epoxy and paint very well. The plywood is made using exterior glues. It is cheaper than marine plywood and much better than lumberyard exterior ply. It can be ordered at most good lumberyards.

You may have to make the new transom in several pieces if fitting it back in the boat is a problem. If you need to make the transom in several pieces or you are using multiple layers of plywood for your new transom, stagger the joints for maximum strength.

Once the plywood is cut to size, you can start the assembly process.  The transom should be bonded in place using epoxy resin and appropriate fillers. Epoxy will give a solid, strong bond to the old fiberglass while polyester resin, though cheaper, will not. Dry fit the first layer of plywood in place. You will need to apply some pressure to the plywood, to hold it in place, seat it in the epoxy and to conform to any curve in the transom.

This can be done a number of ways, but one of the best ways is to simply drill several holes though the plywood and the fiberglass transom and bolt it in place. Be sure to spray the bolts with a cooking spray so the epoxy won’t stick. These holes can be filled in with epoxy/filler after the epoxy has set and the bolts popped out.

Mix some epoxy and pre-coat the plywood surface that will be going against the fiberglass transom. Add filler to the epoxy and trowel onto the fiberglass transom. A notched plastic squeegee will help get a uniform layer of epoxy. It is better to use a little too much epoxy and clean up the squeeze out than to risk any voids or poorly bonded areas.

Press the first plywood layer into the epoxy and apply pressure.  Use any squeezed out epoxy to fill in the voids around the edges. Make sure you don’t leave any drops or lumps of epoxy on the surface of the plywood, as this will cause problems when installing the next layer of plywood.

The second and third (if used) layers can be epoxied in place. Use self-tapping screws to hold the layer in place while the epoxy cures. Use the squeezed out, or extra epoxy/filler if necessary, to fill in the voids around the edges.

With the plywood bonded in place, it can then be covered with layers of fiberglass reinforcement and epoxy. The reinforcement can be cloth, biaxial cloth with mat, roving or a combination of all these. The intent is to build up a thickness equal to the one you cut out. Mix a little colloidal or foamed silica to the epoxy to keep it from sagging. A grooved roller is an excellent tool for getting any air bubbles out of the fiberglass.

The fiberglass should lap over the inside of the hull all around the edges of the transom. This seals the plywood core and forms a strong structural bond to the hull. Once the epoxy has cured, it can be scrubbed with warm soap and water to remove any amine blush and then painted if desired.

Final Steps

If the boat is an I/O model, you will need to cut out the opening for the stern drive as well as the necessary mounting holes. Even an outboard installation may require drilling holes in the new transom. Unless you want to repeat this project in the future, you will need to take some care with these holes.

All edges of any openings should be completely sealed with epoxy. This will prevent any water from reaching the plywood core. In the case of fasteners, it is a good idea to drill the holes oversize, fill them with epoxy/filler and then re-drill the holes to the proper size. This way any fastener hole is drilled through solid epoxy and the plywood is protected from any water leaks.

Replacing the plywood core of a transom isn’t the simplest project in the world but is certainly within the reach of most boat owners. Take your time and do it right, this is one job you don’t want to do more than once.


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