Using Ethylene Glycol on rot damaged GRP covered stringers and bulkheads to arrest further decay

Sorry I meant do know the internal structure of these boats fallguy?

 

No, but it appears to be some wood cores with light chopper gun work. Hopefully the hulls are frp and not cored. You really need to verify the hull is solid frp before spending a minute or a dollar. A balsa cored hull with all the issues you show is almost certainly going to have core rot.

 

Hull is solid glass

 

Sorry for saying that but generally polyester guys have not the knowledge background about the chemical interactions between wood/plywood and polyester resin. A 60 years experience tells these two materials are promised to divorce and after some time there is a very poor adhesion between the GRP skin and the wood.
For prolonging the boat life for a short time there is a very amateurish fix, which needs only patience and few tools. Disclaimer no pro would use that as the results are at best mediocre but for a boat used only in calm weather on a nice sea near the shore that can work.
The first thing is to a fix of the transom an essential structural piece and absolutely needed for the waterproofing of the boat. It's rather easy and that will give you a taste of the job to do.

First step is just to cut the external GRP skin of the transom, leaving around 8 cm at the top and the sides so the glass joints with the hull are not touched. Use an 4" angle grinder with a diamond disk or simply the thin disks for stainless steel. Peel the skin with the help of a wood chisel. You'll be surprised how easily it will peel.
Now you can assess the extension of the rot and the general humidity of the wood. You may be surprised by the degree of humidity of the wood. Take out all the rotten plywood gently with a good margin into the apparently good plywood. Clean up very well.
Make holes diameter 1" 25 mm, depth 2/3 of the plywood thickness. Take care not going too far and piercing the internal GRP skin. Pattern: a grid of holes spaced about 3" 7.5 cm starting close to the GRP strip. These holes have two functions: to help to dry, to create a mechanical key between the outside skin and the plywood.
Let it dry slowly, protected from sun, you do not want any warping or checking, as only one face is exposed to dry. That can take some time at least one week, maybe more. Resist to the disastrous hot gun itching, be patient. An ordinary fan helps a lot.

When dry, repair the missing rotten plywood with pieces of external or marine plywood. Do not use any epoxy glue which inhibits polyester, use simply polyester resin with a bit of cabosil. There is no need of a very strong glue. Glue well specially to the hull and the inside skin.

You are going to make a new external skin. You have several possibilities but I'll describe the simplest one.
With the angle grinder and a sanding flap disk grit 60 or 80 sand and bevel the 8 cm strip of GRP that remained attached to the hull. That will permit to make the joint between the new skin and the hull.
Impregnate all the holes with resin diluted with 10 to 20% styrene with a good dose of peroxide, about 1%. No other solvent only styrene which cures with the resin. Impregnate until the wood refuses to suck the diluted resin. Immediately fill the holes with a sauerkraut of chopped glass and bit of aerosil to control the thixotropy. Fill well with no air bubbles and let it protrude of the surface of the plywood. It's ugly. It's a messy job, do not worry.
Let it cure well, at least 1 day.

Now sand the plywood and glass plugs with a 40 to 60 grit sanding paper. The surface must be rough to give a mechanical key on the plywood, but perfectly flat.
Now glass the transom. It's a classic lay up of mat and cloth until you get the good thickness.

Finish the job and paint.

After you can see how to fix inside, you have learnt the basics.

 

yes

 

Ah Brendan (sorry the convo thread is a bit hard to follow) so you do know the southwinds?
If so could u please explain the internal structure of the stringer/floatation tanks at all?
That would be fantastic.

 

With tubes to block the wet timber it would work, I wasn't aware that epoxy that can set underwater wouldn't go off in that environment, but I wasn't thinking of that.
I think your polyester alternative is much better anyway, it is easier to sand . Your explanation adds necessary details. Stopping the original leaks is another. Drying out the timber by venting at high points and/or cutting out patches, and heating to extract moisture/humidity. and emphasis on mechanical bond to the outside skin. I knew I should have deleted that post ,..drats ..ha

 

Point taken I rushed that post , and yes I know flock wont inject, drying out the timber adding re enforcement with more glass, getting a good mechanical bond , sealing original water ingress points if there are any, yes polyester isn't waterproof ,. The photos weren't up when I posted, I wouldn't be cutting out much glass.
Contact points between the outside skin and the inside skin are important.
Strengthening corners with triangles or box shapes, glassing in an extra C channel or box shape across the transom/bulkhead , I think I'd stick with polyester and rely on good matt thickness and structural bond and shapes,.. as a DIY. Epoxy is so hard everything around it is like butter to sand. Anyway I shouldn't have responded in the first place , I'll put my indignant grumpy old man back in the box.

 

no sorry, i was replying to a comment from fallguy. i had a friend who used to sell southwind boats in his dealership, thats about as close as i have been to them. but i did remember a book i have with a southwind sf20 rebuild article. it is f&b trailer boat renovations and makeovers. it has a few photos of the stringers and layout with the floor removed. google sea media website, they should have a copy. if not pm me and i will post you my copy.

 

Some photos would be great. Is this the article you mean Brendan?
https://www.australianboatmags.com.au/pdf/previews/RFBDIY62_prev.pdf
It’s just a preview but I can buy the article which i think is fair enough.

 

yes thats

yes thats the one martin. its not an in depth article but its an interesting read with a few photos which may give you a bit more of an idea. there problems were exactly the same as yours but a lot worse. the bottom was flexing on waves.

 

i can't imagine yours flexing with the thickness of the glass on your stringers.

 

Hey Brenden, just finished reading the three articles ... super informative . Thanks so much for the lead on it!
The photos of the stringers was very useful and gives me a great understanding of the construction of the boat. The rot damage in my boat is currently nowhere near where they were at.
Thanks so much. Cheers

 

With regard to the use of ethylene glycol. Whilst it may act as a rot inhibitor there are several downsides I have discovered.
1. Pure ethylene glycol seems to be hard to get (radiator fluid is normally watered down with other anti corrosive additives),
2. Its toxic (to consume)
3. It doesn’t fix existing rot although I do think (from what I’ve read) it can help prevent rot by preventing fungal /microbe growth.
4. And as fallout guy said “Do not use glycol. You will never be able to get rid of that slimy ****.”

 

Hey everyone , I just wanted to say a BIG thankyou to you all for taking the time with your advice and opinions. Your feedback has been fantastic!
Ilan I have never seen a transom construct described so well without even one photo .

I have a bunch of questions that are more around dealing with the timber degradation etc that are away from the initial thread topic I posted. Is it worth asking them here or starting a new thread?