Rot Doctor


Subject: Transom repair on a fiberglass boat
Date: Thu, 22 Apr 1999


I have an 18′ Larson fiberglass boat (circa 1969) with an 85 hp outboard motor. The transom is rotted and I do not think that the boat is worth the time or effort to remove the transom and re-install it after a repair. This is the only solution that I have been offered thus far. When I asked why the wood that is rotted couldn’t be removed and the entire void merely filled with epoxy resin the only answer I got was that this just wasn’t done. Given the small size of the boat and the small motor, isn’t this a possibility and if not, why not? Thank you for your willingness to share your expertise. I very much enjoy your web site.

David and Linda,

Your advisor was misinformed. What you wish to do is done commonly and best done using our products and methods, and with boats larger than yours and bigger engines. It’s this simple:

1) Remove the motor and cut the top edge off the transom. If you can, save it (or the pieces) and you can reinstall them later. Buy, borrow or rent a Sawzall and put in a metal cutting blade — that’s the easiest method to cut fiberglass.

2) Hook out all the bad wood you can. Use whatever you can to hook — a piece of bent rebar, whatever.

3) Allow whatever is left inside to dry, or force-dry with a hairdryer or other heated blower.

4) Apply our CPES (Clear Penetrating Epoxy Sealer) to whatever wood is left inside. You can just throw it in with a brush, or squirt it in with a syringe. This will consolidate the rotted wood into a hard, epoxy-saturated matrix that is highly resistant to future rot.

5) Figure out the thickness of your transom and get some exterior plywood to match or come close to matching the thickness. You can use two pieces bonded together. Cut them to a size that will fit, more or less, and coat them generously with CPES, especially the edges. Give them all the CPES they will take. After the CPES has cured (a couple of days) them bond them together with our Layup & Laminating Resin. Wait 24 hours for that to cure out.

6) Now, mix a batch of our Layup & Laminating Resin (you can extend it somewhat by adding sawdust — just keep it so it will pour) and pour it into the transom and at the same time drop in your wood. Let the wood settle where it will and then just keep adding the Layup & Laminating 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 that you can spread with a putty knife and finish off the inside top of the transom.

It’s important to use our L&L Resin in this process. It is very slow-setting so it has time to fill vacancies, and when cured is more flexible than standard epoxy resins. All epoxy resin is expensive, which is why you’re putting in the wood, to take up the space. But the wood will now be protected from future rot problems by the CPES treatment.

7) Put the top back on the transom, or a new top made from aluminum channel, or an epoxy/glass cloth laminate.

You’re done. Transom fixed. Never rot again. It will easily support your motor. Estimated materials: 2-quart unit of the CPES, two or three of the 2-quart units of the L&L Resin. The sawdust you can get locally or from us in 5 lb units. Cost of material estimate: $200 to $300, depending on how much of the L&L Resin you buy. The exterior ply should not be very expensive.

Come back if you have additional questions. We’ll be happy to answer them.


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