Transom Repair: Replacing Wood In A Boat

Replacing the wood in a boat transom is a common task for boat owners, especially those with older boats. The transom is often the first area to rot in trailerable boats due to water intrusion, and this can lead to safety concerns. This guide will cover the steps and materials needed to replace the wood in a boat transom, ensuring a strong and long-lasting repair.

Removing the old rivets

First, identify all the rivets that need to be removed. In the case of a boat transom, there are typically around 16 rivets. Mark the rivet heads with a black dot using a Sharpie to ensure you only drill out the necessary rivets.

Next, use a spring-loaded punch to create a divot on the rivet head. This will help your drill bit stay in place and prevent it from slipping. Place the punch in the centre of the rivet head and strike it with a hammer to create a small dent. You may need to do this several times to create a deep enough divot.

Now it's time to drill out the rivets. Choose a drill bit that fits neatly into the divot and is long enough to penetrate the rivet. Apply lubricant to the end of the drill bit to prevent overheating and damage. Put on safety glasses to protect your eyes from sparks and metal fragments. Hold the drill against the rivet and apply firm, steady pressure while drilling at a low speed. Continue until you have drilled through all the marked rivets.

After drilling, the transom should be loose and ready for removal. You may need to apply a little force to remove it completely.

It is important to note that removing rivets can be done in other ways, such as grinding the rivet off or using a hammer and chisel to break off the head. However, drilling is a precise method that minimises the risk of damaging the surface around the rivet.

Remember to take your time and work carefully to ensure a safe and successful rivet removal process.

Removing the old plywood core

Firstly, ensure you have the right tools for the job. A reciprocating demo saw, grinder, hammer and chisel are all useful. If you have access to an air compressor, an air hammer is a good investment. You will also need safety equipment, such as gloves and eye protection.

Next, gain access to the transom. This may involve removing the deck cap or cutting through the deck and sole. The specific approach will depend on the construction of your boat. It is important to only cut where necessary to avoid time-consuming fairing work later.

Once you have access to the transom, carefully remove the old plywood core. Be cautious not to inflict too much damage to the outside fiberglass skin. While damaging it is not ideal, it can be repaired with a layer of fiberglass later. Use your tools to cut away the old plywood and any bonding putty in the corners. The new core needs to be bonded directly to the old outside skin, so ensure all the old material is removed.

Make a template of the transom using cardboard, plywood, or any other suitable material. This template will be used to cut your new transom core and to create the wide fiberglass pieces for the new inside transom skin.

Before removing the old plywood, take measurements of the transom, including the fiberglass thickness of the outside skin, plywood thickness, inside transom skin thickness, and stringer location. These measurements will be useful later when installing the new core.

Finally, completely remove the old plywood core. Be careful not to damage the outside fiberglass skin any more than is necessary. Once the old plywood and putty are removed, inspect the area and ensure all the old material has been taken out.

Remember to work safely and carefully throughout the process, and don't be afraid to seek advice or help if needed.

Installing the new plywood core

The new plywood core should be the same total thickness as the original plywood. If you are changing anything about the design of the transom (higher engine mounts, enclosed transom, bracket, etc.), you may need to make adjustments to the thickness of the core.

Most powerboat transoms are made by laminating multiple layers of 1/2" or 3/4" plywood. It is not a problem to make a very thick transom by using epoxy to laminate many layers together. In fact, it is recommended.

Plywood selection is another topic in itself, but in general, you will be looking for plywood that is void-free, with tight knots, and an high ply count for its thickness. Marine-grade plywood is a good option, but if you can't find it, then exterior-grade plywood with an A- or B-grade ply on one side is a good alternative.

To install the new plywood core, you can either laminate the core outside the boat and bond it into the hull once, or you can laminate each layer one by one into the hull.

If you choose to laminate the core outside the boat, first coat the faces of the plywood that will be glued together with un-thickened (neat) epoxy resin. While this epoxy is wet, mix a batch of epoxy glue. Epoxy glue is made from the same epoxy with wood flour mixed in to reach a "ketchup" consistency. Using a notched spreader, apply even amounts of epoxy glue to each plywood face. Now clamp the two layers together using weights, clamps, or dry wall screws (temporary, removed after glue cures). Do not apply too much pressure! Epoxy is gap-filling and extremely strong, so 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.

If you choose to laminate each layer into the hull, the method is the same as above, but you will be clamping each layer into the hull one by one.

In the example boat, the customer decided to laminate each layer one by one into the hull. After three layers total, we have the total core thickness epoxied into place.

The gaps between your new core and the sides/bottom of the hull will need to be filled with epoxy putty. The epoxy putty is made from the very same epoxy and wood flour glue, but to make the filling and fillet putty, you will want to add a little silica to make the putty easier to spread. After filling the gap, use the same putty to make a fillet all around the joint between the core and the hull. A fillet is a radius of putty that allows fiberglass to make a turn around an angle.

Tabbing the transom core to the hull

Before beginning the tabbing process, it is important to have the right materials and tools. You will need 12 oz biaxial fiberglass tape, epoxy resin, a notched spreader, weights or clamps, and a drill. It is also recommended to wear protective gear, such as a full-face respirator, gloves, and a tyvex.

The first step is to prepare the transom core and the hull for tabbing. Ensure that the core is securely clamped or weighted in place and that the hull is well-supported to prevent deformation. Clean and degrease all surfaces to ensure proper adhesion.

Next, apply a thin coat of unthickened epoxy resin to the surfaces of the transom core and the hull where the tape will be applied. This will help the tape adhere better and create a stronger bond.

Now, you are ready to begin tabbing. Starting at the bottom of the transom, apply a layer of epoxy glue to the surface of the hull. Take a length of the 12 oz biaxial fiberglass tape and carefully lay it onto the epoxied area, smoothing out any air bubbles or creases. Use a notched spreader to ensure the tape adheres properly.

Repeat this process, working your way up the sides of the hull and overlapping each subsequent layer by about 2 inches. It is recommended to work "wet on wet," meaning you apply each new layer of tape before the previous layer has cured. This creates a stronger lamination with less clean-up.

Once you have applied several layers of tape and reached the desired thickness, you can finish up by applying a final coat of epoxy resin over the taped area. This will help seal and protect the tabbing. Allow everything to cure completely before moving on to the next steps of the transom replacement process.

By following these steps and allowing adequate curing time, you will have a securely tabbed transom core that is strongly bonded to the hull of your boat. This will provide a solid foundation for the rest of the transom replacement process and ensure the longevity of your boat's transom.

Finishing up

Your transom is now structurally complete. All that remains is to tie your transom into the stringers using stringer splices. If you are not replacing the stringers of your boat, you will need to epoxy glue wood stringer extensions that match the thickness of the core of the stringers. Grind back the fiberglass covering of the existing stringer stub at a beveled edge. Repeat the epoxy fillet and tabbing procedure used to tab in the transom core, ensuring you overlap the old stringer by at least 6". The stringer is also tabbed to the new inside transom skin.

Use the same 12 oz biaxial tape for the stringer tabbing. After tabbing, wrap the entire stringer splice (overlapping up the old stringer) with enough heavy biaxial cloth to build up to at least the same thickness as the skin of the original stringer.

The boat is now ready to be put back together. While the structural aspects of the rebuild are the same from boat to boat, each boat design will be a little different in how it should be reassembled. You do not need to follow the same procedure as the example in this tutorial.

In the case of the example boat, the owner simply epoxied into place the cut-out portion of his motor well. Butt blocks were epoxied to the underside of the parts to create a lip for gluing the two edges of the fiberglass. Using epoxy fairing compound, the joint was then faired out smoothly.

Many customers use the rebuild as an opportunity to completely change the layout of their transom. This could include changing from a single to twin, closed transom for bracket, or raised transom for a longer shaft engine.

If you have made any significant changes to the design of your transom, such as higher engine mounts, an enclosed transom, or a bracket, you may need to make adjustments to the thickness of the core. Post the details on a forum and ask for advice.

It is important to only use biaxial/matt that is epoxy compatible. Not all cloths are epoxy compatible, so be sure to check this before use.

Once you have finished restoring your boat's transom, you can bolt on your motor and get back out on the water.

Frequently asked questions