What wood to use to replace transom?

[LDUBS]

Why not use an exterior grade plywood or marine plywood and treat it with pentrating epoxy to make it waterproof? A quart of this stuff goes a long way.

https://www.totalboat.com/products/penetrating-epoxy-sealer?variant=43757090963680

Conventional wisdom on these boards is it is perfectly OK to use exterior grade ply. The consensus is that the added benefit of a marine grade, which is basically elimination of voids, adds quite a bit of cost (translate, not worth the extra $$ to many of us).

In my mind sealing the new transom wood has got to be a #1 best practice. I"ve used the Total Boat penetrating epoxy sealer. Like you say, coverage is pretty good. Another product highly recommended by long term member @DaleH is Raka epoxy.

 

[LDUBS]

I suspect the wood is for cushioning, to prevent the alum from cracking. Surely the alum is stronger than plywood. If or when I replace the transom wood, I'll likely go with exterior ply and a coat of either fiberglass resin or spar varnish. Spar varnish is basically made for this application isn't it?

The transom wood is providing the structure/rigidity. The aluminum skin is thin & flexible. It keeps water on the outside but would not stand up to the stresses of an outboard motor very long without the added structure. So in effect it does prevent the aluminum skin from bending/cracking.

 

[Colorado1135]

The grade of plywood doesn't really matter as long as it's an exterior. On the last one I did I added fiberglass layers all around the outside. Not sure if do that again, added cost, was messy and took way longer. I'm not sure how much it added to it really strength wise. Just smother it in epoxy resin and let it soak in, flip it over and do the other side. If you want it super smooth then sand it after it dries and apply another layer. It's all going under the aluminum anyway and won't be seen. Probably not a bad idea to paint just in case it could be exposed in places to UV light. Most originals weren't treated with any sealing compound at all and can last decades.

 

[user 23839]

Just to be clear, we are talking 1 part spar, 1 part boiled linseed oil and 2 parts mineral spirits as the ingredients of Old Timers' Formula, right? How does this stack up against a sealing-grade epoxy?

 

[Robert Smith]

Most aluminum boats use plywood to provide rigidity to the transom. The ply runs completely across the width of the transom. How far down from the top of the transom it extends depends on the boat design.

Like @Tinhead1986 says, most of the time when folks on this site refer to replacing the transom, it means that plywood. I"ve seen at least one thread that replaced the aluminum transom skin, but that is really an exception to the normal process discussed here.

Here is a link to another transom replacement thread - the first post includes a pic that shows the transom wood:

Lowe 16 transom replacement or renewal

3 layers 3/4 plywood, glue together with gorilla glue, screw together and let dry, cut to fit. Use fiberglass resin, extra hardener and thin with acetone, really thin, roll it on with paint roller, lots of it, it will completely soak it, let dry. IT WILL BE A FOREVER TRANSOM, I've used this method for 40yrs, NEVER had one go south on me. Always predrilled your holes and seal when mounting anything.

 

[DaleH]

FWIW my good friend owns a now 4th generation wooden boat business and they use nothing but TiteBond-III for gluing woods. They even tried them all, glueing togther scrap pieces and placing them in their steaming water tanks (used for bending woods), but the water wasn’t heated.

The TiteBond-III glued joint was the ONLY one that couldn't be pulled apart after water immersion for several days.

 

[hennessyktm]

What kind of wood do I need to use to replace transom on my 14’ Jon boat? Also do I need to treat it with something to keep it from rotting? And is there a type of wood that will last that requires no treatment?

I recently bought a 1560 Jon boat with a completely rotted transom. What was left was literally dirt and completely rotted. The idiot hung a 9.9 mercury outboard off it, anyways. Incredible the thin aluminum managed, but not without some damage. Another story. Anyways, after much research, discussion with lumber salesman (not big box), and not trying to spend a fortune on the awesome $300 Jon boat I purchased, I used the following:
- Roseburg 113 RP AC plus 23/32 RPC Radiata Pine. Used for underlayment, cabinets, etc. I bought a 4x8 sheet and cut two pieces for my transom.
- Titebond III wood glue to glue the 2 transom pieces together. Used clamps and a couple of trolling motor batteries to press them together while they dried.
- Once test fitted, having to make a few mods, I coated the transom with multiple coats of Raka Inc. 127 Low Viscosity Epoxy resin (Part A) and their 606 Slow Epoxy hardener (part B). I made this decision after tons of research, and, after I called the guys at Raka and told them what I was doing. Worked GREAT. Damn thing is near bulletproof now. Hard part was doing one side of the transom at a time and trying keep it from sticking to the blocks I used to suspend the transom off the table I was working on.

This worked so well. It was also beautiful and I was tempted not to sand and paint the transom before I installed it. It looked like a nice piece of furniture or custom countertop. It was sanded, painted brown, installed, and now supports a 2003 Johnson 25hp short shaft outboard on my fishing/duck boat like a champ.

Happy to discuss further or in more detail if you have any questions.

 

[LDUBS]

3 layers 3/4 plywood, glue together with gorilla glue, screw together and let dry, cut to fit. Use fiberglass resin, extra hardener and thin with acetone, really thin, roll it on with paint roller, lots of it, it will completely soak it, let dry. IT WILL BE A FOREVER TRANSOM, I've used this method for 40yrs, NEVER had one go south on me. Always predrilled your holes and seal when mounting anything.

I don't think I've every seen a small aluminum boat with 2.25" thick transom wood. That is heavy duty. On my boat that would be overkill.

 

[pathunter]

The grade of plywood doesn't really matter as long as it's an exterior. On the last one I did I added fiberglass layers all around the outside. Not sure if do that again, added cost, was messy and took way longer. I'm not sure how much it added to it really strength wise. Just smother it in epoxy resin and let it soak in, flip it over and do the other side. If you want it super smooth then sand it after it dries and apply another layer. It's all going under the aluminum anyway and won't be seen. Probably not a bad idea to paint just in case it could be exposed in places to UV light. Most originals weren't treated with any sealing compound at all and can last decades.

Marine Grade plywood. I believe "exterior" plywood is only waterproof glued around the original edges. Once cut, the cut edge is not waterproof.

 

[Colorado1135]

In marine grade they get rid of the voids, if you're sealing it anyway that doesn't really matter, in fact I think those open voids helps in taking in the epoxy so it could be a benefit perhaps. My reference was to the grade of exterior. ABCD refers to how finished one surface is, if it's all gonna be enclosed in aluminum or fiberglass who cares if one side came super smooth from the factory?
It's not bad to use marine grade, but I'd rather put money where it counts, and in my opinion this is a really marginal place that isn't worth the extra expenditure. I've done a few and never had an issue, please if you have examples to the contrary let us know, I'm always up for learning more if I can.

 

[nate665]

I just finished mine, about to reinstall.
Not exterior, but the higher grade of plywood on the premise likely fewer voids.
Glued to two 19/32 pieces together with Titebond 3 (clamps all along edge, a dozen or so screws as well.
I was able to salvage the old rotted transom in one piece so I could use as a template.
Cut to size.
Mock it up on the boat to mark all holes and pre-drill.
Remove the screws put there for the bonding process.
I coated the whole thing with fiberglass resin, 2 coats, making sure to apply very well to the cut edges. I did not thin or use extra hardener, but might have been easier if I had. I applied with a brush.

 

[lckstckn2smknbrls]

Marine Grade plywood. I believe "exterior" plywood is only waterproof glued around the original edges. Once cut, the cut edge is not waterproof.

I've not heard this before.
May I ask where you found this?

 

[thill]

Like Robert Smith, I personally like thinned polyester or vinlyester resin for soaking transom wood. In my experience, it penetrates much deeper than any epoxy I've tried. Never used (3) layers of 3/4", but I have used 3 layers of 1/2" with fiberglass in between.

That being said, almost ANY kind of sealer, applied carefully, will get the job done for a long time. Most factory boats don't have any water sealing done in the transoms. But some of the older marine plywoods were treated with formaldehyde, which helps prevent rot.

 

[Al U Minium]

What kind of wood do I need to use to replace transom on my 14’ Jon boat? Also do I need to treat it with something to keep it from rotting? And is there a type of wood that will last that requires no treatment?

I used Alabama Cedar, cut, fit and clear coated. Will surely last a long long time

 

[Allieballieboo]

Great thread. We are also getting ready to replace a transom on a ‘61 Lone Star Big Fisherman my son bought on the side of the road this month. What is the reasoning for not using pressure treated wood? I’m not sure that’s what the kid is doing but I’d better make sure & tell him why.

 

[DaleH]

What is the reasoning for not using pressure treated wood?

The wood preservative used corrodes aluminum ...

 

[thill]

Marine Grade plywood. I believe "exterior" plywood is only waterproof glued around the original edges. Once cut, the cut edge is not waterproof.

That is not correct. The glue they use is the glue they use. They don't glue the middle one way and then glue the edges another way. Exterior grade plywood has waterproof glue throughout.

That being said, SOME of the better commercial exterior sheathing has waxed/sealed edges. They basically roll a green or red wax like substance on the sides of the plywood. Maybe this is what you are thinking of.

35 year Class A contractor, so I've seen a lot of plywood. They do vary greatly in quality, but truthfully, for a tin boat transom, if it's exterior ply, and if you take some time to glue and seal the plywood, any of them will be fine.

 

[LDUBS]

Oops - duplicate response.

 

[Allieballieboo]

The wood preservative used corrodes aluminum ...

Thank you. Pretty sure that is what they bought and are now returning.

 

[airshot]

I used Alabama Cedar, cut, fit and clear coated. Will surely last a long long time

My question is how strong ?? Any cedar I have used was not strong and could easily split with little force. Is this Alabama stuff stronger ?? I agree with life expectancy but strength would be my concern....