replacing transom. This is my method. Am I right?

[Crazydawgs29@gmail.com]

First off I have a 1985 Thompson fisherman 240 5.0 mercruiser. The transom was 100% shot so I had to replace it.

I pulled motor pulled out drive. Removed the floor from the front of engine to rear of boat. The stringers were all solid no rot. Unbelievable yes indeed but they were 100%solid. I cut the stringers about a foot and a half back to allow access to transom. I removed transom from the inside. Very easy project took me a few hours. I prepped the entire surface per videos I've watched.

Here's where I'm wondering if the method I was told from the marine shop is correct..

Materials:
* marine system 105 epoxy resin
*marine system 205 slow harder
*marine system 407 filler
* 3 sheets 1/2 inch plywood
* 24 Oz woven roven (enough to do entire transom)
*3/4 inch plywood
*2x4s
*all threads

I plan on using 3 1/2 inch piece of plywood to make the transom. I plan on using the epoxy resin and filler to form a paste to attach the first layer of plywood to fiberglass transom. I will be making clamps out of the 2x4s and all threads to squeeze the plywood to fiberglass. I will then do this same method for the remaining 2 sheets of plywood. Using the 3/4 plywood and roven I plan on rebuilding the strings to the transom. Next I will be using the roven over the entire plywood transom and securing it to the sides and to the strings. And then putting the floor back down.

My question is only on using the 24 Oz woven roven. Will 1 layer be enough? The guys at the shop said this will do just fine.

 

[ondarvr]

Roving wouldn't be my first choice, but would work. You'd be better off with a couple of layers of 1700 or 2400 Biax, it'll use less resin and have more strength.

​Roving leaves an open weave that uses a good deal of resin to be water proof, it can be hard to fill the gaps in the open weave with enough with resin when only one layer is used. If you end up using roving you'll need to apply more resin a few times as it hardens to fill the gaps.

​Epoxy isn't really needed for this repair, but will work well.

 

[Crazydawgs29@gmail.com]

No Title

I plan on previous cutting the outdrive pattern out on all 3 pieces before putting them in

 

[Crazydawgs29@gmail.com]

It's a family boat so the extra cost of epoxy resin to make it stronger is better then a sunken boat with my kid on it.

 

[Crazydawgs29@gmail.com]

No one has 1708 biax around

 

[ondarvr]

It's a family boat so the extra cost of epoxy resin to make it stronger is better then a sunken boat with my kid on it.

​Epoxy doesn't make it stronger, you build with either type of resin to the same strength, you just use less epoxy and possibly a thinner laminate to get to the same point.

​I'm pretty sure it didn't sink in the last 30+ years, even with a rotting transom, and it was all polyester.

​I'm not saying don't use epoxy, either one works, just don't be mistaken and think it will be safer with epoxy. The outcome will be based more on your attention to detail than the resin type, and typically a DIY rebuild is done much better than the original build.

 

[Crazydawgs29@gmail.com]

From what I understand is poly resin is not 100% water proof but epoxy resin is

 

[Crazydawgs29@gmail.com]

And for bonding purpose epoxy is a stronger bond then poly

 

[ondarvr]

Nothing is waterproof, there are just different levels of water resistance, and in an application like the it makes little difference. Water migrating through a laminate and causing wood to rot is not a problem. Unsealed screw holes, areas left unsealed, and poor workmanship are the main causes for the wood to get wet and rot. If the wood gets wet in this way the resin is not going to make a difference, it will rot either way.

​Yes, epoxy does bond better, but the polyester bonds rarely fail on a boat, so is the increased bond strength needed. Bond failures are mainly from poor workmanship, and both resins require the same prep for a good bond, epoxy does help if the prep work is poor though.

​If the bond is going to see a maximum stress of 100psi, the polyester fails at 150psi, and epoxy at 200psi, it means that although epoxy is stronger, neither will fail in that application. (these are just numbers, not the real numbers, they can vary)

​Again, I'm not saying don't use epoxy, my comments are only to let you know what the real difference is between the two. When building wooden boats epoxy is the way to go, when repairing a fiberglass boat it's not required, but can be used.

 

[Crazydawgs29@gmail.com]

Ah OK yes I have done research on both poly and expoy. I just felt that the epoxy is a little bit better of a bonding agent there fore I went with that. Now I'm the type that I don't cut corners on thing. If I can make it better with a few more bucks I do it. I'm sure the poly would definitely work but I felt the epoxy would work a little better is all. I definitely appreciate your insight and the knowledge your sharing with me I'll definitely remember it and use it I'm sure

 

[ondarvr]

The biggest problem you'll have is with the roving and the wide gaps, these tend to let water through, so more coats of epoxy are needed to prevent water from reaching the wood.

 

[Crazydawgs29@gmail.com]

Here's a question will west marine 407 filler be good for making a paste to bond the plywood together or should I have gotten 404? And so what your saying is if I use poly I will be perfectly fine. Wonder if spending the extra for epoxy was really worth it then

 

[ondarvr]

The cost difference between the two resins isn't as much as seems up front, you'll use less resin and less glass with epoxy if done correctly. This is the part where many DIY guys make a mistake, they end up using the same amount of glass and resin with both products, so they end up with an over built and more expensive repair when using epoxy. Polyester is a little easier to laminate with though, with epoxy you need to watch it until gels or it can drain out of the glass.

​404 is better.

 

[Crazydawgs29@gmail.com]

OK I'll probably stick with epoxy. Now the guy at west marine said to me that 1 gallon epoxy would be enough. Is this a true statement or am I going to need to get another gallon. The way he said it is 1 gallon should cover 4 4x8 sheets of plywood front and back.

 

[ondarvr]

You'll need more than a gallon to finish it.

 

[Crazydawgs29@gmail.com]

Not sure how. I'll be using epoxy on first layer plywood to fiberglass transom then in between 2 more plywood then on the last woven. I have a gallon poly I'll use for stringers and the floor as I really don't need epoxy for that

 

[ondarvr]

You will need to pre coat the plywood with epoxy, then mix putty for bedding the ply to the transom, more putty will be needed to fill the gap around the plywood so it can be glassed over, and the roving will suck up a good amount, plus you'll need to recoat the roving to fill in all the gaps.

​The other issue is the polyester won't bond well to the epoxy, so how do you plan to tie the stringers and floor into the transom. And a gallon of resin won't do this either.