Fiberglass advice

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ar1g3

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Hi I'm looking for a place to get advice on some structural fiberglass work I need to do on my boat.

The transom core is rotten, and needs to be replaced. I don't have a lot of experience with composites, but still I'd like to do this job myself. I have a plan on how I want to tackle this job, but I'm not a 100% sure about it.

My plan is to cut the outside skin, leaving a 10cm offset around the edge. The skin is 0.8cm thick, so that gives room for a 12:1 scarf on the cutting edges. I'd get the old core out somehow, and then put a new one in. This new core will be layered plywood and every layer will be cut in two or three pieces to fit through the hole in the outer skin.
After this I'd glue the old outer skin back on, scarf the cutting edges and fill this 'valley' with new cloth.

I had this plan in my mind, and after jumping online to see what other people suggested I found this: http://epoxyworks.com/index.php/repairing-fiberglass-powerboat-transoms/
It's exactly what I had in mind, but still I'd like the opinion of other people on this since it's pretty crucial this job is done right.

So are there any people on here who know a thing or two about this kind of fiberglass work? If not, does anyone know a place online where I could find advice about this?


Thanks a lot!
 
Watch the Sail Life Athena videos on YouTube. Mads replaces the whole core of his deck. Should give you some useful pointers
 
Your plan seems basically sound to me. I would assemble the core and bond it together with epoxy, but i would repair the GRP with polyester. That'll be more than strong enough and much easier to work with, especially now the weather is colder. When you do the epoxy for the core you'll need some heat.
 
I would consider opening the transom in sections.
The area which carries the outboard needs to be strong. I would want to be sure the core in this area was particularly solid, not crushed by the outboard clamping. It's possible this area is where 99% of the damage is? So maybe the rest of the core does not need to come out? This area can be reinforced externally. If you can leave most of the transom in place, there is less possibility of distortion.
But if the whole core is detached, I'd have it out.
Bonding the core to the inner skin may take a lot of gloop.
It is feasible to atack the inside instead I would. That allows you to carry reinforcement down to the floor.
Particularly if the original GRP is much thicker outside than inside.
 
I would work from the outside. I would recommend you start by fitting a temporary brace across the inside of the transom to maintain the shape.

I would cut the whole outer skin off in one go rather than trying to do it piecemeal.

I would use epoxy rather than polyester as it has superior adhesion to cured polyester resin.

As other posters have said there is a minimum temperature for epoxy work. Consider making a double wall tent over the transom area to maintain the temperature. [ don't let elf and hatey know

You will need plenty of clamps for the top and I would use battens and screws to get clamping pressure low down. It is easy enough to fill screw holes.
 
Wow thanks for all the replies!!

My epoxy supplier has special epoxy for use in cold environments so I will discuss this with them, but if I don't get a guarantee that this stuff is just as strong as other epoxy I will heat up my workshop. I won't be using polyester, after a lot of research I found that the general consensus is that epoxy resin bonds better to old GRP.

I don't have good acces to the inside, so I will have to go in from the outside. I would have prefered to cut the inside open, but there's just no room. It's the same kind of design like an old Piranha speedboat. The core definitely needs to be replaced completely. I stuk a moisture meter in there and it has a humidity of 98.4%. The outboard is only a 28hp, but I just don't trust it anymore.

Thanks again for all the advice! What is your opinion of the scarf angle I should use on the old skin? is a 12:1 angle good or can I go smaller, to a 10:1 for example? Because the less offset I leave around the edge of the transom, the more space I have to get the new core in.
 
in the December edition of pbo I carry out a repair on a hull/deck which got clobbered.
This has a plywood core and was replaced during the repair and I used a mix of epoxy and polyester.
The polyester was used as I needed to match existing gel-coat colour, deck grip etc.
However the techniques used could easily apply to your challenge.
 
Is there any crazing in the gel coat around the base of the transom?
If there is, the core failure has weakened the structure, allowing the outer skin to flex and fatigue.
So you may be scarffing to very degraded laminate.

I would use polyester for this kind of repair, because you are going to use a lot of it, and also mat is easier to work with than cloth for tapering.
If you can't get polyester to stick to the old polyester, you're wasting your time anyway.
If in doubt do a test on a small area.
The old structure may want some serious drying out.

I doubt that getting the old core out without damaging the rim of grp you're trying to leave will be easy.
In fact, despite being 100% humid, it could be remarkably strong and swollen a bit, not keen to leave.
Is the inside a liner moulding bonded to the hull, or laid up directly in the hull?

Also working on a vertical transom, I'd consider either vacuum bagging or some other scheme for pressing the repair into place.
These escapades can go off-message very quickly when you get down to it.
 
So..................we have some marine ply bonded to the inner surface of the inside transom, all the play is bonded to the next bit of ply and the the sides of the hull and anywhere else it touches. The outer skin of the transom is bonded to the ply too. So the back of the boat is now a solid chunk of GRP, ply and epoxy.

The 28hp engine has been happily hanging off of the rotten cored transom for Lord knows how long and even when it was built the transom wasn't as strong as it is now, with all the epoxy bonding. But somehow, using polyester, which is cheaper and far easier to work on for the final finish won't be strong enough ?

When boats are hand built the hulls and decks are moulded weeks, maybe months, before final assembly and fitting of the internal bulkheads etc. Do they fall apart because polyester is used ? According the the West system website they must do, according to the real World hey are still floating around decades later.
 
ar1g3,
If the work you are doing is structural then I would advise getting your work examined by a surveyor and copying his (hopefully good) report to your insurers.

If your amateur work is not good it may compromise the vessel and crew safety and render your insurance invalid.

Even if you employ a professional to do the GRP work my advise is the same.

I don't recall his aspect of structural repair being addressed on "Sail LIfe"
 
Last edited:
PaulRainbow said:
Hmm. 20 years old, referring to the "new 2 part car paint", which was isocyanate, not really a good DIY suggestion. Gelcoat would be the way to go.
Click to expand...

I'd certainly use gelcoat on a polyester repair, if the colour could be matched adequately.
On an epoxy repair, it's not so easy.
Painting with 2 pack paint like perfection or awlgrip is a good option. Other paint might be plenty good enough?
 
lw395 said:
I'd certainly use gelcoat on a polyester repair, if the colour could be matched adequately.
On an epoxy repair, it's not so easy.
Painting with 2 pack paint like perfection or awlgrip is a good option. Other paint might be plenty good enough?
Click to expand...

According to West, poly gelcoat adheres to epoxy layup as well as it does to poly layup.I'd get the colour as close as possible and re-gelcoat the whole transom.But then i'd still do the final layup with polyester resin and CSM.
 
Here's a picture of the transom:
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I will look into getting the work checked out, and see if the cost of this is worth it.

There's no crazing in the gelcoat on the transom.
The inside layer is laid up directly into the hull. The inside layer is not as thick as the outside layer, I think it's primary function is to cover the core. I think most of the strength comes from the outer skin.

The 28hp engine is new, I bought the boat as a restoration project. I don't know what kind of engine was on there before, but you can see that the clamps on the outboard bracket damaged the fiberglass. (Yes, that will be repaired aswell!)

So you guys are saying to use polyester for the GRP lay up? The fact that it's a lot cheaper makes me consider it. As I said, most of the strength comes from the outer skin so it's vital that it's as strong as possible. That's why I'd use epoxy, I really don't want to take a chance with this.

Also, I'm not going to gelcoat it. The original gelcoat was painted over and damaged, so I'm repainting the whole boat with a 2K paint. I don't want to spend money on gelcoat if I'm going to paint over it anyway. Matching the gelcoat with the paintjob is no option either, the paint is high gloss so you will always see a difference.

Here's a picture of the bottom which is already finished:
UCgALzoTPd4pkxQcSV0leKuCqZmuHzhup7JN4-vggdI%2CBSQUhboABG1iI9HkpFtkoKsGw-2puYJledhsEfgAnEQ


Really, thanks for all the comments!
 
Right, scrub the gelcoat then, it makes no sense now you've shown those pics.

I would still say use polyester for the layup, but to use epoxy to bond the new core in/together. The strength will not be in the outer layer of GRP alone, but the transom as a whole. You could do a lot worse than get a copy of the Dec PBO and read the "T-Boned deck repair" article by Javelin, of the forum here. Much of the technique that Mike uses to fix the damaged hull will directly transfer to your project. At the final stages, instead of gelcoat, you will be painting.
 
Looks like the deck moulding is separating from the hull stbd side?
Looking at that, the strength, in terms of resisting the thrust of the motor, largely comes from the sides of the well bracing the transom?
That is the big thing, you have the thrust of the motor trying to bend the transom, plus its weight every time you hit a wave etc.
The reinforcement in the transom outboard of the well is not doing much.
I would chop out the inner within the well, reinforce with new plywood and then laminate over it.
The inside of the well does not have to look as pretty as the outside, I'd settle for fair and strong.
Not sure exactly how the deck moulding comes into this, but looking at that joint, I'd take the opportunity to re-engineer things a bit, so the two faces of the transom and the deck moulding (which will also be the sides of the well) have some integrity. Looks like they are just put together with 'bog' in the time-honoured fashion and it's past its sell-by date?
This is a repair where I would use epoxy, I have a dinghy that went like that on the gunnel after I hit a steel pile at full chat. Fixed with WEST.

I suspect all this slotting in of cunningly cut ply panels from the outside will get messy.
It's one thing to find the recipe on the interweb, another to find someone who's done it.

Just my opinion, I'm happy to be proved wrong, but I'd rather give my opinion up front than be saying something unhelpful later on.
 
I will try to get my hands on a copy of that edition of the PBO magazine, thanks!

I will definitely be replacing the whole transom core, I just don't trust it anymore and if I'm going to do this work I want to do it right the first time. My plan, at first, was indeed to just replace the plywood core at the outboard well, but the core is so soaking wet that I don't want to take a chance on this.

The deck moulding and the hull are rivetted together, so no risk there. Admittedly that joint dus look ugly, but it's fine. There's a rivet every inch, so I'm not scared of that coming apart ever!
 
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