Best Fiberglass For Repairing Boat Transoms: Expert Guide

A boat's transom is the structural section at the rear of the vessel to which the engine is mounted. Transoms can become damaged over time due to repeated pounding into waves, towing strains, improper trailering support, direct impacts, freeze-thaw cycles, age, and general weather exposure. Transom repairs are essential to prevent more serious structural issues and total replacement down the line. The best type of fibreglass to repair a boat transom will depend on the specific type of damage and the boat's construction. Repairs typically involve grinding away damaged areas, sealing and rebuilding the transom, laminating new fibreglass layers, and applying a gelcoat.

Epoxy and fibreglass

Epoxy is a highly effective material for repairing fibreglass boats, offering a range of benefits over traditional polyester resin. Its superior bonding strength and resistance to harsh marine conditions help maintain the structural integrity of the boat, increasing its lifespan.

Epoxy is also a cost-effective solution in the long run, as repairs made with epoxy tend to last longer, reducing the frequency and cost of future repairs. It can be used for both structural and cosmetic repairs, filling cracks, chips, and scratches, and can be easily sanded and painted over.

When repairing a boat transom with epoxy, the first step is to gain access to the transom so you can remove the rot. This may involve cutting the deck, top cap, side walls, or other parts of the boat. Once you have access, you need to completely remove the old plywood core and inside fibreglass transom skin. Make sure to take measurements of all the parts around the transom before making any cuts, as you will need these measurements as a reference later.

After removing the old core, create a template of the transom using cardboard, doorskin, or cheap plywood. This template will be used to cut your new transom core and to draw/cut the wide fibreglass pieces that will go between the new plywood and outer skin. The new plywood core should be the same total thickness as the original.

Before installing the new core, fill the gaps between the core and the sides/bottom of the hull with putty. Also, create a fillet around the joint between the core and the hull, as this will allow the fibreglass to make a smooth turn around the angle. For tabbing the core to the hull, use 1708 Biaxial fibreglass tape, which is stronger and easier to work with than traditional woven tape.

When installing the new core, you can either laminate it in place or install it as a full-size piece. If laminating, coat the faces of the plywood that will be glued together with unthickened epoxy resin, then apply epoxy glue and clamp the layers together. If installing a full-size piece, apply unthickened epoxy to all plywood segments and the abraded fibreglass surface, then use thickened epoxy to fill gaps and bridge layers.

After the new core is installed, you may need to repair the fibreglass skin. If the skin is in good condition, it can usually be glued back in place over the new core. If not, a new skin can be laminated over the core using multiple layers of fibreglass and epoxy. Remember to aggressively sand epoxy-coated surfaces before gluing.

Finally, seal all holes drilled through the transom with multiple coats of epoxy to prevent water ingress and future rot.

Transom support bracket

A transom support bracket is a crucial component for any boat owner, as it provides additional support to the transom and helps distribute the load. The transom is the flat surface at the back of the boat that bears the weight and vibration of the outboard motor. Over time, this can lead to weakening or cracking of the fiberglass.

The purpose of a transom support bracket is to reinforce the transom and prevent damage. It is designed to bolt onto the transom, providing extra support and ensuring safe and efficient boating. These brackets are usually made of heavy-duty aluminum and are tailored to fit specific boat models, so it is important to select the right one for your vessel.

When choosing a transom support bracket, it is essential to consider the size and adjustability. Some brackets are adjustable, allowing them to fit a range of boat and trailer configurations. For example, the Bass Pro Shops Adjustable Transom Support Bracket can be adjusted from 25" to 35" and is compatible with almost all boat and trailer setups. Other brackets may have fixed sizes, so measuring your boat's transom is crucial to ensure a proper fit.

In addition to adjustability, the material of the transom support bracket is also an important consideration. Heavy-duty aluminum is a popular choice due to its strength and corrosion resistance. This ensures that the bracket can withstand the elements and provide long-lasting support.

When installing a transom support bracket, proper surface preparation is key. The transom should be clean, dry, and sanded before mounting the bracket. This ensures a strong and secure bond between the bracket and the transom. It is also important to follow the manufacturer's instructions and seek professional help if needed to ensure a safe and effective installation.

By using a transom support bracket, boat owners can effectively distribute the load on the transom, reduce the risk of damage, and improve the overall safety of their vessel. With proper maintenance and care, a transom support bracket can extend the lifespan of your boat and allow for many enjoyable trips on the water.

Repairing a rotted core

Step 1: Assess the Damage

Before beginning any repair work, it is important to assess the extent of the damage. This includes measuring the thickness of the fibreglass, plywood, and other materials used in the boat's construction. It is also crucial to ensure that the hull is properly supported during the repair process to avoid deformation.

Step 2: Gain Access to the Transom

The next step is to gain access to the transom, which will involve cutting through the deck, top cap, sidewalls, or other components. The specific approach will depend on the design of the boat. It is important to carefully remove the old plywood core and interior fibreglass transom skin, ensuring that you have access to the entire transom, as well as at least 12 inches up the sides and bottom of the boat.

Step 3: Prepare the Template

Once the old core and skin have been removed, create a template of the transom using cardboard, doorskin, or cheap plywood. This template will be used to cut the new core and to draw/cut the wide fibreglass pieces that will go between the new core and outer skin.

Step 4: Install the New Core

The new plywood core should have the same total thickness as the original. Cut and prepare the new core, treating it with CPES (Clear Penetrating Epoxy Sealer) to prevent future rot. If necessary, make adjustments to the thickness of the core to accommodate any design changes, such as higher engine mounts or an enclosed transom.

Step 5: Fill Gaps and Create Fillets

Before final glassing of the core, fill any gaps between the new core and the sides/bottom of the hull with putty. Additionally, create a fillet, which is a radius of putty that allows fibreglass to turn around angles smoothly. A fillet should be applied around the joint between the core and the hull.

Step 6: Tabbing the Transom

Use 1708 Biaxial fibreglass tape to tab the transom core to the sides and bottom of the hull. This tape is stronger than traditional woven tape due to its fibre orientation. Apply multiple layers of tape, depending on the specific requirements of your boat. It is recommended to work "wet on wet" by performing the filleting and glass tabbing in one session for better lamination quality.

Step 7: Final Steps

After the epoxy has cured, you may need to perform additional finishing work, such as sanding or painting. Consult with a professional or refer to detailed boat repair resources for further guidance on finalising the repair process.

Removing the old transom

Step 1: Measurements and Support

Before starting the removal process, take detailed measurements of all the components around the transom, including the fiberglass thickness of the outside skin, plywood thickness, inside transom skin thickness, stringer location, and stringer thickness. These measurements will be useful later during the repair process. Ensure that the hull is properly supported to avoid deformation. The best support is a bunk trailer with additional blocks under the keel and transom.

Step 2: Gain Access to the Transom

The next step is to gain access to the transom so you can remove the rot. This may involve cutting through the deck, the top cap, sidewalls, or a combination of these elements. In some cases, removal of the cap may be necessary to reach the entire inside of the transom. The goal is to gain access to the entire transom, plus at least 12 inches up the sides and bottom of the boat. Taking pictures and seeking advice from experts or forums can help you determine the best places to make cuts and avoid time-consuming fairing work.

Step 3: Remove the Old Plywood Core

Completely remove the old plywood core using appropriate tools such as a reciprocating saw, grinder, hammer and chisel, or an air hammer if you have an air compressor. Be careful not to damage the outside fiberglass skin during this process. Remove any bonding putty in the corners by grinding it out. Make a template of the transom using cardboard, doorskin, or cheap plywood, which will be used for cutting the new core and drawing/cutting the wide fiberglass pieces.

Step 4: Make a Template

Create a template of the transom using cardboard, doorskin, or cheap plywood. This template will be crucial for cutting the new transom core and drawing/cutting the wide fiberglass pieces that will be used for the new inside transom skin.

Step 5: Remove Old Putty and Make Repairs

Remove any old putty or filler material from the corners and joints of the transom. This step is important to ensure a clean surface for bonding the new core. If there is extensive damage to the outside fiberglass skin, you may need to reinforce it with a layer of fiberglass before installing the new core.

By following these steps, you will have successfully removed the old transom and prepared the boat for the next stages of the repair process. Remember to take your time, work carefully, and seek guidance from experts or online resources if needed.

Installing the new transom

The new transom 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.

The new core can be either made in one piece, then bonded into the hull, or you may laminate one layer at a time into the hull. It is generally easier to laminate the core outside the boat and bond it into the hull once, but on very large transoms, this may not be possible due to weight and handling difficulties.

To laminate two pieces of plywood, first coat the faces of the plywood that will be glued together with unthickened epoxy resin. This is a thin coat of epoxy, just enough to let the face of the wood soak up a little. While this epoxy is still tacky, mix a batch of epoxy glue. Using a notched spreader, apply even amounts of epoxy glue to each plywood face. Now clamp the two layers together using weights, clamps, or drywall screws (temporary, removed after glue cures). Do not apply too much pressure! 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 and wipe off excess epoxy glue.

Before final glassing of the core, the gaps between your new core and the sides/bottom of the hull will need to be filled with putty. 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. All inside corners of the transom should have a round fillet.

The next step is the tabbing of the transom core to the sides and bottom of the hull. For tabbing, use 1708 Biaxial fiberglass tape. This tape is stronger than traditional woven tape due to its fiber orientation (45/45 degrees). It is also very easy to work with, wets out easily, and is epoxy compatible. Use multiple layers of the 17 oz Biaxal tape; the amount will depend on the particular boat. We highly recommend doing the fillet and glass tabbing all in one work session, which is called working "wet on wet" and results in a much better lamination quality with less cleanup and no grinding/sanding between layers! Just be careful of heat buildup if your putty is thick in some places.

The original fiberglass skin can be laminated over the new core or, if necessary, a new skin can be laminated over the core. Remember to aggressively sand the epoxy-coated surfaces and the backside of the fiberglass skin in preparation for gluing.

Glue the skin in place using the same laminating techniques previously described. I normally use temporary drywall screws driven through oversized washers or a small plywood block to spread the holding power of the screw over a larger area. This prevents dimpling in the fiberglass skin, which would require considerable filling and fairing later. Apply auto paste wax or cooking spray to the washers or plywood ahead of time to allow for removal later, or apply a layer of plastic sheet over the fiberglass skin before applying the plywood cauls.

After the epoxy has cured, remove the screws and fill the holes with slightly thickened epoxy, injected with a plastic syringe.

If you were able to reuse the fiberglass skin and glue it in place over the new plywood core, there is one last step to complete the structural part of the repair. The fiberglass skin damage created when we cut the fiberglass away to gain access to the damaged core needs to be repaired.

Prepare the cuts in the skin by grinding a minimum 12:1 bevel. If the fiberglass skin is 1/8″ thick, the bevel on each side of the cut line needs to be at least 1½” long. Because the repair involves two bevels, one on either side of the original cut, the dished-out repair will be at least 3″ wide.

Laminate the repair patch with multiple layers of fiberglass and epoxy until the dished-out cavity is full. Remember to apply the largest fiberglass layers first and apply progressively smaller layers until the repair is flush with the surrounding fiberglass skins.

Once cured, wash the repair with water to remove amine blush and grind or sand the fiberglass smooth. Fill low spots with thickened epoxy and recoat as necessary to seal exposed fiberglass and sanded low-density fillers.

Be sure to coat and seal all holes drilled through the transom with multiple coats of epoxy. This is not a waste of time. If the holes are not properly sealed, plan on replacing the core again in the future. If all sources of water are eliminated by sealing the wood in epoxy, the repair should be better than new and last indefinitely.

Now the transom is ready for paint or gelcoat. I strongly recommend paint, not gelcoat, for amateur repairers. It is much easier to achieve satisfactory results. To prepare for paint, tape off the transom along the edge of the corners to keep paint off the sides of the boat. Find a paint that matches the original finish reasonably well and apply it using techniques that you are comfortable with. I personally like using a paint roller and tipping brush approach to apply a quality two-part linear polyurethane paint.

Frequently asked questions