Replace transom core

My son bought a 16 foot fiberglass boat a few years ago. Don't know the brand or model.
The transom core was soft so he is currently replacing the core. He has asked me to help.

His friend who has done boat repairs told my son to remove the transom core from the inside, while leaving the fiberglass on the outside intact. Then install two 3/4" marine plywood from the inside and finish off with fiberglass cloth.

QUESTION:
My question is what type of polyester resin to use to glue the plywood core to the transom's fiberglass hull? If laminating resin is used, I'm thinking that it will never harden. When I built a Glen-L boat many years ago, I used laminating resin to install the fiberglass cloth on the outside of the hull. That laminating resin never hardened until I applied finishing resin as the top coat.

So if laminating resin is used to glue the plywood to the transom's fiberglass hull, finishing resin will never be applied. So will the laminating resin ever harden, or will it just remain tacky?

Or should we use epoxy for the entire job? But I'm still curious on how laminating resin can cure completely, without finishing resin applied directly on top of it.

 

The resin will cure fine on its own, it's not even a slight concern, forget about it.

The type of resin used isn't going to make much of a difference in this type of repair except in cost, so don't spend more than needed.

And for wood, there actually is no such thing as marine plywood, it's a marketing term. There are many types of wood used to build transoms, your attention to detail is far more important.

And doing it from the inside is typically better, mainly because as soon as you open things up you'll find the floor and stringers are rotting too. So expect the job to get much bigger after day one of the project.

 

Most exterior grade plywoods are fine. I would generally recommend using epoxy for amateur repairs. Precoat the wood, bond with epoxy thickened with cabosil about 1:2.2 by volume, adjust as needed so the mix doesn’t collapse when troweled. I use 1/16” vee trowel.

As Ondarvr has stated; there is a good chance the problem extends to other structural elements, so you also need to evaluate cost/benefit.

If it costs $2000 to fix and the boat is only worth $1500, it may not be wise money and better spent finding a boat not rotted.

 

I agree with the advice to approach the repair from the inside,it is an order of magnitude less work than hacking off the outer laminate and having to finish an external repair to a good standard.Without knowing more about the type of boat I would hazard a guess that a double layer of 3/4" plywood is rather substantial.Perhaps in the Pacific there is no such thing as marine plywood,but I have the good fortune to live in a country where there is an official specification for marine plywood.It requires that all veneers are of a durable species and that they are bonded with a glue that will resist immersion in boiling water for 72 hours.There may be rogues selling low grade stuff with a falsified official stamp in some places but that doesn't mean that the grade hasn't been specified.If in doubt buy Bruynzeel.

 

What I'll say about resin choice.

In the 60 years of hands on lamination building just about everything you can imagine, and flying around the world fixing stuff, 99% of the time using a similar resin to what it was built with originally works out fine. There are a few situations where upgrading the resin system pays off.

It helps when repairs need to be done without full access to what needs to be repaired and the physical properties are important. The next is when you are going to add significant stresses to something that far exceeds the original design, but even then, if weight isn't a concern you can increase the strength with fabric choice and amount and typically get to the same place.

These old boats frequently went 30 years before someone noticed the transom and stringers were pulp and they didn't actually fail. So upgrading the resin system doesn't add much benefit.

These old boats failed (rotted away) from poor quality in the build process, the resin choice is irrelevant if water reaches the wood, it turns to pulp no matter what resin is used.

 

Stay away from the epoxy, because you will not be able to follow-on with polyester or vinyl ester resin.

I prefer to use vinyl ester resin, because it has better adhesion and strength characteristics and stays in a workable state longer than poly. As for the issue with curing . . . resins with wax are intended for finish coats, as the wax creates a layer on the surface, keeping the air out, allowing the resin to cure. Not what you really want with laminating resin though, as the idea is you will add more layers as you go. If a final cure is needed, you can always coat the exposed resin with PVA and then wash it off. But, as the others have mentioned, curing is not really going to be a problem.

 

Thanks for all of your responses.

I just watched a YouTube video by "Tiny Boat Nation" on replacing the transom core. His method resolves the concern I had about using laminating resin to bond the plywood core to the transom's fiberglass hull.

What he did was to glass both sides of the plywood core using laminating resin. Then he applied the finishing resin on only the back side of the core to fully cure that side. Then after removing the wax on the backside and roughing up the surface, he used epoxy to bond the core to the transom's fiberglass hull. That way, the epoxy used is sandwiched between the core and the hull and can fully cure. Also, epoxy is said to have better bonding than polyester resin per Google. In addition, the epoxy is not exposed to further use of polyester resin on the rest of the job. I can see how the rest of the job can be covered with finishing resin to fully cure the exposed laminating resin.

BTW, regarding using marine plywood, marine plywood is available in our area. Coosa or similar sheets is not available locally so would have to be special ordered with expensive shipping. Regarding marine plywood vs exterior grade plywood, marine plywood has more layers than exterior grade plywood which is better. But most of all, it does not have any internal voids like non marine exterior plywood can have. I don't doubt that exterior grade plywood could work, but it's just a personal choice to use marine plywood for the transom core.

Thanks again for your responses.

 

Is this the reference video?



The polyester glassing and waxed resin coating of the surface of the plywood going towards the boat's existing fiberglass -only to grind it back before epoxy-seems extraneous. I would have just used the same filled epoxy he used to glue the plywood pieces together, and make sure all edges and holes in the new plywood get resin treated before they are buried under the final glassing or hardware.

 

Oh well, I guess the guy never heard that a chain is only as strong as its weakest link. Anyway, fortunately for him he will probably get away with it just fine. Just don't go repeating his mistakes.
If you want to use epoxy to bond the core and laminate over the inner transom face with polyester all you have to do is work really clean. Mask the edges with wide tape and clean the squeezed out thickened epoxy really well while it's still soft then remove the tape.

As to your concerns how the laminating resin cures, well it's quite simple, there's no air between the core and the existing laminate, so the resin cures. Prewet the core and transom with laminating resin and use a structural filler to bond the core.

 

You are vastly overthinking this.

Yes epoxy is stronger, but the polyester used in 99.9999% of all transoms doesn't fail, so what are you gaining.

Transoms fail due to water reaching the wood, it makes no difference which resin is used.

You can do it that way, but it requires extra work and cost with no benefit.

Again, marine ply is a marketing term, yes there are certain characteristics that tend to be found in something called marine plywood. But it doesn't need to meet any spec or pass any testing. I'm not saying what you are buying is a bad product, just don't expect it to do anything special in this application. For at least the last 25 years the industry has used kiln dried pressure treated ply in new construction.

 

Often called "boat board" . . . See if you can get 'Greenwood XL' or 'Perma Panel' , which are the KDAT plywood sheets that boat builders use.

The main advantage of Coosa (or similar products) is that it won't readily absorb water and won't rot. Coosa is not as strong as plywood, but is more dimensionally stable. There are also some products similar to Coosa . . . Airex T92 comes to mind. I am wondering if you expand your search area if you will find a specialties plywood dealer within reasonable driving distance, and can get what you need. I often drive a couple of hours to get the right materials.

 

I always chuckle when I see a video like that talking about building it back "stronger" when the failure mode was rotten wood.

The boat didn't fail in decades of use from being under built, it failed from poor attention to the details of keeping the wood dry, its as simple as that. Building it back "stronger" is of no value.

 

The only reason I recommend epoxy for amateurs is ease of use, and sometimes for waterproofing, say as ply deck coating.

It would not be preferred where gelcoating is desired.

I think the OP has chosen a decent path forward mixing systems. The epoxy eliminates any concerns over resin not curing. He should coat all sides of the plywood prior to bonding putties to avoid resins being sucked from the putty and leaving a dry bond.

I would add, the most common failure in transoms is penetrations not sealed well.

For all transom penetrations, typically the absolute best method is to overbore the core 3x the drill hole planned. Fill with thickened epoxy all the way. Allow it to cure, rebore the desired hole and apply sealant. This offers an extra sealant to the wood.

 

The issue is there is no concern of resin not curing, it is a made up imaginary problem.

Epoxy is not easier to use, that's one reason why it's not used nearly as much in production, it slows the process and adds additional steps, plus costs more.

The only way to really take advantage of what epoxy has to offer is have the project engineered and use the exact right fibers, precise laminate schedule, and attention to detail.

Nothing in this project comes even close to needing anything epoxy has to offer. Can you use epoxy...yes...is there any benefit....no.

 

Here's a link to the referenced video.
He was actually using Vinylester laminating resin.

Per a comment I made, his response was that the epoxy procedure was: "actually suggested to me by US composite... and they are the ones who manufacturer the glass products for a lot of big name boat companies." So it appears that even US Composite has concerns about laminating resin not fully curing between the core and the hull. So it seems to be a very valid procedure. At any rate, it's what we'll use on my son's boat.
Thanks.