Rebuild a Rotten Transom

A wet transom core is not good. Here's how to replace it yourself.

I was looking for a center console hull that had great seakeeping ability in order to fish offshore in Florida. I found a used 28 Whitewater hull, which was perfect because I wanted to put some low-hour Yamahas on it. A new fully rigged 28 Whitewater costs about $150,000, but I got the hull for $15,000. With engines, I would have a rugged, reliable offshore boat for about $60,000.

When I drilled a hole in the transom, water leaked out (1), which was, to say the least, really depressing, because a surveyor gave the boat a clean bill of health. Further delving revealed the transom’s wood core was totally rotten (2). Hiring a boatyard to replace the transom’s core would totally blow my budget, so I did it myself.

Replacing the transom’s core from the inside is the preferred method, because it keeps the boat’s original structure intact. The first step is to gain access to the interior of the transom, so I used a Rockwell Sonicrafter Oscillating Multi-Tool to cut the cap and deck off the rear of the boat, which proved to be one of the easier steps. The key is to not cut into the corners; doing so makes it very difficult to repair and can create a weak spot. When cutting back the knees/stringers that were connected to the transom, I had to cut back until I got past the rotted wood. I cut off about six inches of a stringer (3) and could see by its appearance I was past the wet wood. If you are uncertain, use a pin-type moisture meter to measure the wood’s water content.

Removing the old core of wet wood from the transom proved to be the most difficult part of this project. I started using a chisel, hammer and crowbar to remove small pieces at a time. Some parts will be super easy if they’re really rotten. At this point, the fiberglass transom had bits of leftover wood (4), so I used a grinder to grind it all down to bare fiberglass, then used a belt sander to create the flattest possible surface. Make sure to cover up completely. I taped my sleeves to my gloves and used a respirator. Somehow, the glass still got to my skin and itched like crazy. If — when! — that happens, use packing tape to pull the ground glass off your skin.

I began beefing up the transom by adding a couple of layers of fiberglass to the existing fiberglass transom shell. I started with 1708 biaxial cloth, which has a 45-degree weave. Then I laid down 1808 biaxial cloth, which has a 90-degree weave. I used vinylester resin, which is orders of magnitude better than polyester resin. I laid the cut piece of fiberglass down on a piece of plywood and applied the resin until the mat was soaked through before putting it in place on the transom. The key to creating a strong bond is to make sure all the bubbles are rolled out and let it cure for at least 24 hours.

I used Coosa Board Bluewater 26 to replace the wood core, because it doesn’t absorb water and has very high compression strength. I made a cardboard template of the transom inside where the new core was going, which I used to mark the one-and-a-half-inch Coosa Board Bluewater 26 (5). I cut the board with the Rockwell Multi-Tool (6). Holding the board in place, I used a half-inch drill bit to drill through the existing mounting holes whenever possible in the transom, through the cured fiberglass I just laid and then into the Coosa board. I then placed the two-by-fours and drilled through them, so everything lined up.

I created a thickened resin with Cabosil and put it on the fiberglass, laying it on thick with a plastic trowel and being careful to leave space at the edges for it to spread out. I placed the Coosa board core against the transom and bolted on the two-by-fours (7), which acted as a vise to create a very strong bond when fully cured. If there are any spaces between the core on the sides and bottom, apply more thickened epoxy and smooth it down, so it’s even with the Coosa board. After 24 hours, I removed the two-by-fours, filled the mounting holes with thickened resin and sanded down any areas that weren’t smooth.

Again, using the vinylester resin, I glassed down the transom with 1708 and 1808 fiberglass mat — in this order: 1708, 1808, 1708, 1808 — all the way to the top of the transom and out four to eight inches on the sides and bottom (8). It cured overnight before I trimmed excess fiberglass off the top of the transom.

I cut out new pieces of Coosa board for the stringers/knees, to fit the old ones, and glassed them in with a layer of 1708, then 1808. Make sure the glass extends four to six inches onto the transom, old stringers and bottom.

Last, I sanded and painted the area with Interlux BilgeKote primer and paint (9). The final result is an incredibly strong transom that won’t absorb water whenever I drill a hole.

4 thoughts on “Rebuild a Rotten Transom

  1. Did the same with a 19′ Cobia; took out wet transom yet replaced with two bonded and screwed layers of 3/4″ PT ply. Has worked well holding the 115 on it for past 10 years.

  2. Great advice, but I used epoxy paste to sandwich 2 3/4″ marine plywood pressed against outer skin (from inside) using EMT/turnbuckle devices that I fab’ed. I also used air tools like chizzles to remove rotten wood. I just rebuilt 270 SR AJ’s transom. Marine surveyor was incompetent and didn’t identify rot before I purchased. I covered inside plywood with 1808 (3 layers) and allowed for the same 1808 to also tab about 6″ over outside edges. It was a pain in the butt to glass 3 pieces of 1808 onto an area approximately 4′ x 6′. Lots of cursing and lots of epoxy!!! Instead of applying with brush, I used 4″ roller. E-bond epoxy was forgiving as far as curing time, and I was able to apply 2 more layers of 1808. I found it best and messy to coat the mat side of 1808. It stuck better. Sorry, but I didn’t have any 1708. After rolling out all three layers, I tabbed on 3 more layers of 6″ 1808 tape around the 3 pieces mentioned above. I used metal roller to squeeze out bubbles but I found very little due to rolling on epoxy instead of brushing it on. I also epoxy coated inside 7/16 bolt holes and cutout for MC transom assemblies. I purchased marine grade plywood for 80$ a sheet vs 300$ a sheet for synthetic stuff. My project required 2 pieces. I am 70 years old. I’ll be dead before my epoxy coated plywood rots. Good luck!

  3. A local boat repair place shared a secrete in polyester coating plywood. It doesn’t absorb into plywood well, unless thinned. And neither poly nor epoxy is going to soak past the first glue in the plywood well. But, if you thin polyester 400% or MORE with styrene or acetone, plywood sucks it up. I know some out there will disagree, but thinning has worked for me.


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