Transom Construction Best Practices

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Nick Eschenbrenner

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May 19, 2024
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Hello i have an old starcraft islander im replacing the transom on. I was wondering what the best practices would be for gluing up the new transom and reinstalling.

Im going to use marine grade plywood.

What should i use to bond the 2 sheet together?
Should i put a sheet of fiberglass in between?
Can i use screws for clamping?

Stuff like this is what curious about?
Waterproof glue obviously. Titebond III (adequate) to epoxy (best).

As far as adhering the two sheets together, I do not see a need or benefit to putting fiberglass in between.

I've frequently used screws to "clamp" two sheets together. Drill the pilot holes before applying the glue. For a transom, I would remove the screws and fill the holes with epoxy. It is common to successfully "clamp" by using weights (whatever is handy). You can also use clamping cauls but for a one off project they probably are not worth making.

You might consider gluing the two parts together before cutting to final shape. That just makes it easier because things tend to slide around until the glue grabs, making things harder to align.

Even with marine ply, you should seal it (especially the edges).

Initially I wasn't going to comment because you didn't ask, but decided to mention it just in case. The general consensus is it is perfectly OK to use much less expensive exterior grade ply instead of marine grade. Exterior grade uses waterproof glue. The primary difference is, unlike marine grade, exterior grade will have voids, which won't be an issue for a transom.

Don't use pressure treated plywood. Whatever stuff is used in the process has been shown to cause corrosion in aluminum. There is some discussion that new pressure treating processes use chemicals that do not have this problem. Until the consensus changes, I would err on the side of caution.

My two cents/opinion.
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I bought the plywood before I started researching.

Would west systems 105 epoxy and 206 hardener be a good combo for this job? Should I thicken the epoxy when I bond the 2 sheets together?

How much epoxy would u need for a job like this?

If you're going to use the TB III wood glue then I'd recommend fabricating your transom like this. Apply the glue liberally to both pieces of the plywood and then use 2x4 cauls spaced 6" apart to clamp them together. Let cure for 24-36 hours. Then apply "OLD TIMERS MIX".

Old timers formula is a mixture of 1 part oil based spar varnish, 1 part boiled linseed oil and 2 parts mineral spirits. Coverage area -(50" × 15" plywood) = 12 oz. mineral spirits, 6 oz. Spar varnish, 6 oz. mineral spirits. Basically wash this over the wood on all sides until the wood doesn't absorb any more. The idea is you are thinning the varnish and linseed oil, so that it penetrates deep into the wood and waterproofs it to the core. It takes a long time to dry, approximately 3 to 4 days. Then seal it tight with straight varnish. Ideally, 5 coats on all sides is great, especially the edges. Sand lightly between coats with 400 grit.. Sand lightly before you finish up, and paint with a oil based paint (color of your choice).

First Three Coats of Spar Varnish
After all the light sanding of the OLD TIMERS was completed, start applying spar varnish. Use a foam brush, and apply the 1st coat of Spar Varnish, working with the grain if there is one. Otherwise (products like plywood, MSB, etc ) spread your varnish from left to right. After 48 hours, and after sanding (see below*), apply second and third coats of Rustoleum Marine Spar Varnish. You will do this by rolling and tipping. Remember to always tip from the freshly applied wet edge to the last section painted, overlapping the two sections. Also remember that tipping off is to level a surface and is not used to apply paint or varnish. *Lightly sand with 400 grit after each drying time of the 3 coats. (approximately 48 hrs between coats). The objective of lightly sanding, is to get rid of any speckles and bumps and leave a perfectly smooth surface. If you have any drips or bumps in the varnish, this would be a good time to sand those down.

Applying Spar Varnish Finish Coats
After you have sanded the last two coats of varnish with 400 grit sandpaper, you can move on to the "finishing stage". At this stage, you could sand by hand rather than use a power sander. Sanding by hand gives more control over how much to remove over a given area.
Once you have sanded the work piece down, vacuum up the sanding dust. It is best to let everything sit for 24 hours to let dust settle but you can go right into it if you prefer. Wipe everything down with mineral spirits. After the piece dries, give it a once over with a tack cloth before varnishing. Begin applying Spar Varnish by Rolling and Tipping as you did previously. Repeat the above steps when adding additional coats.
The more "finished" coats you apply, the more depth and gloss you will get.
I bought the plywood before I started researching.

In my opinion, you have about the best wood based sheet good you can use for your transom.

Would west systems 105 epoxy and 206 hardener be a good combo for this job? Should I thicken the epoxy when I bond the 2 sheets together?

How much epoxy would u need for a job like this?

I would be looking at the mfg website for info on how to use the epoxy. My guess is it would need to be just thick enough to pour on and spread. How much product depends on the surface area of the pieces being glued together. If it were me, I would think in terms of how much wood glue I would need and use that as a guide.

BTW, I hope my earlier comment didn't sway you away from considering Titebond III waterproof glue. While technically it won't provide the same "structural" strength as epoxy, it will work great for gluing flat pieces of plywood together (they won't come apart). I personally would use it unless I already had epoxy on hand. It is readily available, easy to use, and much less expensive. Pour it on then spread a thin layer on both sides. Make sure the surfaces are completely covered with a thin wet film. Move fast and get the pieces together as quickly as possible.

Whatever kind of glue is used, after the pieces are glued up you can coat them with the Old Timer's Formula explained above by Bay Beagle or any other sealer of your choice.
I redid the transom in my old Grumman 16ft boat, it was a full length/depth transom that was originally just two sheets of 3/4 ply that lasted about 30 years or so with normal use stored in an old barn when not in use.
When I redid it, I used exterior plywood, a sheet of 8 ply exterior ACX grade sheathing. I traced and cut out the two panels using the old panel, and glued the two panels together with tight bond 3. I followed that up with a full sanding, and a single layer of fiberglass on the inside, and then a full coating of epoxy over the entire panel.
I then sanded and painted the panel to give the epoxy better UV resistance although in the end I cut a sheet of aluminum to further block the wood from sun exposure. It was big time overkill but it cost me nothing since I had everything but the plywood. I also redid the two full bench seats with some red oak vs the Masonite looking stuff it came with. I sold the boat a few years after but only because I moved and didn't feel like making 30 trips to bring all my boats here, so about half got sold. (Which was likely the right move because boats don't sell well here at all).
I miss that boat but I miss the first earlier Grumman I had before it more.
I'd cut your pieces, glue and screw them together, and let it cure then remove the screws. Install it in the boat, drill all your through holes and do any final trimming/fitting. Then seal it.
To add to the above, the Titebond III is similar to the glue that holds the plys of the plywood together, so you are essentially turning the two sheets into one. Using screws to pull them tight is acceptable. Use stainless steel screws if you want to leave them in, otherwise you can remove them and fill the holes later.

Or, you can use 2x4s and clamps as described above.

Sealing the wood is the most important thing for longevity. I prefer to put the transom in place, cut and drill holes as needed, pull it back out and THEN seal the wood. But if you seal it and install it, you can use a squirt bottle to seal inside holes you drill. I've done both, and the squirt bottle trick works well.

If you use the methods above, your transom will far outlive the original one in most boats. So, if your boat made it 30 years before needing a transom, you have at least that long before you have to worry about it again.

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