Ayrton Oneill
Replacing the transom of a fiberglass boat is a complicated but rewarding process. The transom is the first part of a boat's structure to rot, and it is also one of the most important structural parts of the boat. Before beginning the process, it is important to take measurements of all the parts around the transom, including the fiberglass thickness of the outside skin, the plywood thickness, and the inside transom skin thickness. The first step is to gain access to the transom, which may involve removing the deck cap or cutting through the deck and sole. Once access is gained, the old plywood core and inside fiberglass transom skin can be removed. The new transom core is then installed, and the gaps between the new core and the sides/bottom of the hull are filled with epoxy putty. The transom is then tabbed to the sides and bottom of the hull using fiberglass tape. Finally, the new inside transom fiberglass skin is laminated, and the transom is sealed with epoxy to prevent water from reaching the plywood core.
Characteristics and Values Table for Replacing a Fiberglass Boat Transom
| Characteristics | Values |
|---|---|
| First Step | Gain access to the transom. |
| Tools | Circular saws, pry bars, chisels, large drill bits, rotary rasps, grub hoe, adz, angle grinder with a cut-off blade, hairdryer or other heated blower, sawsall with a metal cutting blade. |
| Next Steps | Remove the old plywood core and inside fiberglass transom skin. |
| Materials | Epoxy resin, polyester ortho resin, marine ply, exterior ply, fiberglass cloth, epoxy putty, silica, wood flour glue, laminating resin, putty knife, screws, pop rivets, fiberglass tape, gelcoat, fiberglass skin, plywood. |
| Repair Process | Cut out the rotten wood, dry the remaining wood, apply epoxy sealer, cut plywood to size, bond plywood with epoxy resin, fill gaps with epoxy putty, seal with fiberglass tape, laminate new inside transom fiberglass skin, sand, fill holes with epoxy, coat with epoxy, paint. |
| Tips | Limit repairs to the transom to reduce costs, repair from the inside, stagger joints for maximum strength, seal all holes with epoxy, coat screw holes with epoxy before installing screws. |
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What You'll Learn
Removing the old transom
Firstly, you need to gain access to the transom. This will vary depending on the boat's design and construction. You may need to remove the deck cap or make cuts through the deck and sole to reach the entire transom. It is important to take measurements of all the parts around the transom, including the fiberglass thickness, plywood thickness, and stringer location.
Next, remove all internal structures, such as seats and tanks, to create a clear workspace around the transom. If your boat has an inboard/outboard (I/O) engine, it will need to be taken out. This step is crucial to access the transom and perform the necessary repairs.
Once you have cleared the area, it is time to cut away the inside layer of fiberglass. This step requires extreme care to avoid damaging the exterior fiberglass layers and gel coat. An angle grinder with a cut-off blade is a suitable tool for this task. If the plywood is severely deteriorated, the fiberglass may peel off easily. Otherwise, you may need to use pry bars to carefully detach it.
After removing the fiberglass, address any remaining wood inside the transom. Remove as much of the deteriorated wood as possible, using tools such as a bent piece of rebar or a similar implement. Once you have extracted the wood, allow the exposed areas to dry thoroughly. You can use a hairdryer or a heated blower to speed up the drying process.
Finally, treat the remaining wood with a consolidant to prevent future rot. A clear penetrating epoxy sealer (CPES) is an effective option for this step. Apply the consolidant generously to any exposed wood, ensuring complete coverage. This step will help strengthen the remaining structure and prepare it for the next steps in the replacement process.
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Measuring for the new transom
Measuring for a new transom is a critical step in the process of replacing a fibreglass boat transom. The transom is the flat, vertical section at the rear of the boat where the outboard motor is mounted. Getting accurate measurements is essential to ensure the proper functioning of the boat. Here is a comprehensive guide to help you through the process:
Before beginning the measurement process, it is important to gain access to the transom. This can be done by removing any components that may obstruct your access, such as the motor well bulkhead or the deck cap. The specific steps may vary depending on the design of your boat. Once you have clear access to the transom, you can start taking measurements.
Firstly, determine the height of the transom. This is calculated by measuring from the bottom of the hull to the top of the transom, using the centre line as a reference point. The height of the transom is crucial as it dictates the size of the outboard motor that can be used. Industry standards specify that short shaft engines require a transom height of 15 inches, long shaft engines require 20 inches, and extra-long shaft engines need 25 inches.
Next, measure the transom angle, which is the vertical incline of the transom. Transom angles can vary, with flat transoms having zero degrees and angled transoms ranging up to 30 degrees. The average transom angle is around 14 degrees, and it impacts the trimming abilities of the boat. Understanding how the transom angle affects the boat's performance will help you determine the appropriate angle for your new transom.
Additionally, take measurements of various components around the transom. This includes the fibreglass thickness of the outside skin, the plywood thickness, the inside transom skin thickness, stringer location, and string thickness. These measurements will be important when installing the new transom to ensure a precise fit.
It is also important to inspect the transom for any signs of damage or weakness. Check for movement or flexing by applying gentle pressure to the transom while the boat is on a trailer. You can also use a moisture meter to detect any hidden water damage. Inspect the boat's floor near the transom area for soft spots or delamination, as this could indicate water damage that may have affected the transom.
Finally, take pictures of the transom from multiple angles and make additional measurements as needed. This will help you during the installation process, ensuring that your new transom aligns perfectly with the existing structure.
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Cutting and fitting the new plywood
Before cutting and fitting the new plywood, you will need to gain access to the transom. This will vary depending on the boat, but generally, you will need access to the entire transom plus at least 12" up the side and bottom of the boat. You may need to remove the deck cap or make a cut through the deck and sole.
Now, onto cutting and fitting the new plywood. First, determine the thickness of your transom and source exterior plywood of a similar thickness. You can use two pieces bonded together with epoxy resin and appropriate fillers. Cut the plywood to size, ensuring it will fit into the transom. Coat the plywood generously with CPES, especially the edges. Once the CPES has cured, bond the pieces of plywood together with a layup and laminating resin.
Next, mix a batch of layup and laminating resin and pour it into the transom. Drop the plywood into the resin and continue adding resin until it has filled the vacancies around the wood and reached a level near the top of the transom. Allow a couple of days for it to cure hard. You can now make a paste of sawdust and L&L Resin and spread it with a putty knife to finish off the inside top of the transom.
If you are using multiple layers of plywood for your new transom, stagger the joints for maximum strength. Drill several holes through the plywood and the fiberglass transom and bolt it in place. The fiberglass should lap over the inside of the hull all around the edges of the transom, sealing the plywood core and forming a strong structural bond. Once the epoxy has cured, it can be scrubbed with warm soap and water to remove any amine blush and then painted if desired.
Finally, if your 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 with an outboard installation, you may need to drill holes in the new transom. Ensure that all edges of any openings are completely sealed with epoxy to prevent water from reaching the plywood core.
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Applying epoxy and resin
Before applying epoxy and resin, you must gain access to the transom. This can be done by removing the fiberglass skin from the outside or inside of the transom. Once you have access, you can begin the process of repairing the damaged area.
To apply the epoxy and resin, start by mixing a batch of Layup & Laminating Resin. You can add sawdust to this mixture to extend it and make it more spreadable. Pour this mixture into the transom and drop in pieces of wood to fill in any vacancies. Continue adding the resin until it has filled the vacancies and reached a level near the top of the transom. Allow a couple of days for it to cure and harden.
After the resin has cured, you can mix in a paste of sawdust and Layup & Laminating Resin to spread over the inside top of the transom with a putty knife. This will help to protect the wood from future rot problems. Once this is complete, you can put the top back on the transom, or install a new top made from aluminum or an epoxy/glass cloth laminate.
It is important to note that all holes drilled through the transom should be coated with multiple coats of epoxy to prevent water infiltration and ensure a strong bond. Additionally, when working with epoxy, it is better to use too much and clean up any squeeze-out than to risk any voids or poorly bonded areas.
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Sanding and refinishing
Before beginning the sanding process, it is essential to gain access to the transom and remove the old plywood core and the inside fibreglass transom skin. This requires tools such as a reciprocating saw, grinder, hammer, chisel, or a flat pry bar. It is important to be careful during this process to avoid damaging the outside fibreglass skin.
Once the old plywood and putty are removed, the next step is to sand the surfaces of the exposed fibreglass skin. This step helps to expose fresh fibreglass laminate, creating a smooth surface for the next steps. It is important to be thorough during sanding to ensure that every part holds well and is sturdy.
After sanding, the next step is to repair the cuts made in the fibreglass skin. This is done by applying multiple layers of fibreglass cloth and epoxy. It is recommended to use the "wet on wet" technique, where the fillet and glass tabbing are completed in one work session, resulting in better lamination quality and less cleanup. The largest fibreglass layers should be applied first, followed by progressively smaller layers until the repair is flush with the surrounding fibreglass skin.
When the fibreglass layers have cured, it is time to sand again. Fill the gaps with putty, and sand once more before applying the gel coat. This step helps to create an even surface and prepare the transom for painting. After sanding, apply a coat of acrylic paint to the transom.
Finally, remember to sand the excess epoxy on the edges to avoid reviving the fibreglass skin. Sanding is necessary to create an even surface on the edges as well. The number of fibreglass layers required for the transom depends on the size of the opening, typically ranging from three to four layers.
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Frequently asked questions
The first step is to gain access to the transom. This will differ depending on the boat, but it may involve removing the deck cap or cutting through the liner/sole.
You will need a variety of tools such as a saw, pry bars, a chisel, a drill, and a hairdryer or other heated blower.
Materials needed include epoxy resin, fillers, plywood, fiberglass cloth, and a fiberglass repair kit.
