Repairing delaminated sandwich, advice needed

Hi all,

I am considering purchasing a 28ft cat that has some delamination issues.
The hulls are made from flat panels of 600/300 glass and 12mm 60-80kg foam and, where the beams meet the port hull sides, there are 2 areas of delamination, each about a metre in diameter.

The gougeon brothers boatbuilding manual recommends drilling through the outer skin of glass in several places, filling the void with epoxy and then clamping it all together with screws through the drilled holes. Seems to be a very straightforward method of repair but I wonder about the reliability.

However, it has also been recommended to me that I should cut and grind away the delaminated outer glass and reglass. This gives the advantage that I could get a good look at the condition of the foam and I could prep the foam to ensure a good bond. The downside of this method is that the hull sides are flared, meaning I'm glassing on an overhanging surface and I obviously don't want any voids!

Does anyone have experience of repairing similar problems, what method would you recommend? Would be interesting to hear of successful repairs and opinions on methods.

Cheers
Ben

 

I would probably redo the affected area. Drilling holes and filling might work for small and less important areas, or as a means of last resort, but I would definately like to take a look and see what is going on in that laminate.

 

The injected epoxy method works only if there is no water or other contaminants. I think it is virtually impossible to determine unless you cut some sections out.

 

I've done both and the holes (on 2" - 3" centers) does work, though it's a tedious affair. I usually bag or weight the affected area, rather than use screws. The problem with this method is you can't really be sure how effective the repair is. On a small area, I'll use this route, but on a repair as large and as highly loaded as yours, I'd rip out the delaminated portions of the laminate and reskin it. This way you can be sure.

 

fill with epoxy ?? you must be joking !!

First the load carrying structuall beams should never be glassed just to the innerskin specially if the core is foam !! balsa maybe but never foam !! never !!
i have said many many time the peelablity of glass off foam is completely and totally scary!! and once its starts there nothing to stop it !!
So you really need to cut the delaminated glass away and feather the edge then where the beam is dig out the foam to the outter skin for at least 150 mm wide all round the shape of the beam and taper the core so you can reglass .
You need to get Glass to glass bond to get some guts in there !! glassing to an inner skin of foam is absolute stupidity !and who ever did that job should have there bump feeling !!
once the glass pulls away from the foam core there nothing to stop it spreading and getting worse very rapidly .
Have you checked the other side ?? .
Who ever made that sure as hell didnt understand what they were doing thats for sure !!
Drilling holes and filling with epoxy from the outside is deffinitly never going to last much longer than it will take to do that job !!

 

Before we jump to conclusions Tunnels, it's probably best to get a picture of the areas in question, though your points are valid.

 

A beam !! a main structuall part of the boat and its just laminated to a inside skin on top of a foam core is absolutly asking for trouble !!
Hasnt anyone done any testing of the almost non resistant peel ablility of foam core !! its just peels off like pages of a book in the wind !!
I tried to make this point clear here on one on the threads last year some time and mr watson just about had kittens and pooh poohed the idea !!
its never the adhesion of the glass skin to the core its the core its self that lets go just a mil below the surface of the foam .
In 2000 i was involved in a company with making big panels all with Foam core and as we lifted a panel off the bench an edge caught and one of the guys grabbed it and simply ripped off 90% of the top skin almost effortlessly . we all just looked at each other in complete amazement !! so after that incident i began doing some testing on scrap paneling !, and saw first hand how easy it just came apart !
The density hard or soft core made very little differance to the peel resistance ,so after that i had a complete change of mind about foam cores and where and what thye should be used !!

Even the glass skin to skin laminating should be a minimum of 100mm or more and dont use woven roving its the wrong glass but something like double bias to laminate with !!

 

The beams are indeed bonded to the inner glass of the sandwich only, I figured that was the cause of the delam. My intention, to stop it happening again, was to attach the beam to the inside of the deck and maybe add another bulkhead, to share the load over a much greater area.
Not bonding the beams to the inner skin only seemed like a no brainer to even me with my very limited experience but, the funny thing is, the boat was designed and built by a well known and well respected designer.

The other mistake I have noticed on the boat is that the plywood bulkheads are right up against the hull instead of floating on foam spacers. This has caused a bit of a hungry horse look from the outside, you can see hard spots where the hull has hinged on the bhs. I've been told by a surveyor friend that this shouldnt be too much of a problem though?

I'm afraid I don't have good photos of the areas and am currently several hundred miles away.

So, the consensus seems to be strongly against injecting epoxy. My worry with reglassing is I'm not 100% sure I can do it with absolutely no voids, due to the fact that the outer hull sides are overhanging. My experience with glassing on upside down surfaces is that the glass attempts to peel away due to that, usually helpful, gravity problem! How do you guys usually combat this problem, it seems to me trying to vacuum bag upside down would also be troublesome?

Well, I haven't bought the boat yet, so there is one easy way for me to deal with the problem... Shame though, she is a nice design and going for an absolute song, would be nice to fix her up if I can

 

He maybe a respected designer but when it comes to actual construction he's a fruit cake !!
Bulkheads against the hull just need to space away 5 mm all round then bog cove with a 15 mm radius (30mm dia pipe size ) do this both sides and force the bog under the bulkhead !!! the amount of glass used and the catalyst ratio of the resin has a lot to do with making a imprint on the outside of the hull .Any glass going round a corner as the resin gells and hardens want to pull the quicke the resin the more heat the more heat the more the shrinkage !! get the picture !!
So even if you took the old bulkheads out now the mark would still be visable !! its cured that shape and could remain like that for ever more .

The beam is a main and very nessasary part of the whole structure so some carefull consideration needs to be taken as to how to get it cleaned up and reglassed better than it was so it should last for a longtime if not forever . The choice of glass is one part of the key and the other is bonding to the outer skin not on top of the core !! ,
Working upside down is an art and needs to be done in strips and start at the lower point and work to the top first with a wide paint roller then a hard roller and peel ply it out and over the edge of where the glass finishs !! Peel ply has a lot going for it !,
One it makes the glass more transparent and able to see air voids easyer !! Two it gets rid of surplus resin and brings it through the sirface so when its peeled the resin goes with it as well
Four its light and when you roll take the peel ply outside the edge of the glass and resin it !!
Once its all hard then peel it off and is ready for another layer of glass .
Step the joins of the glass half the width of the strips you are using no need to overlap just but the strips when you get the idea how to do it you could lay 2 layers of glass at a time plus the peel ply !!
Its the weight of resin in the glass thats the problem and making very sure you have the edges wet out and rolled down 100% so it wont curl and fall off!! You should be able to use strips 300mm wide but the full length of you repair wet out first the whole strip and roll it up leaving enough to start with then roll onto the boat and gradually unroll as you go till its all on and becarefull the paint roller dosent pick up strands and make the glass want to come off .
Double bias with a 225 csm on one side is a better glass to use than terrible woven roving that falls apart with strand dropping every where !! the double bias is normally availible as a tape any width you want so is easy to use !
also Peel ply can be in roll widths as well ,but can be cut with good sharp scissors .
Taking your time and getting into a system makes working upside down a breeze . just dont try to do to bigger bits all at the same time !!

 

Thanks for your advice, most helpful, I have a couple more questions though.
Part of my worry with the upside down glassing was if I do all the layers of glass in one go the weight will pull it off. You seem to be suggesting I get round this problem by letting the glass cure before doing the next layer? If I do this it will only be a mechanical bond as opposed to a chemical one. Will that be strong enough? It wont risk the glass delaminating?

 

Yes you have to make it by bits, and to leave to cure until it hardens enough to be sure that the next layer won't soften or move the previous layer. Lets say about a few hours, thats depends on your resin (epoxy? polyester?) and the catalist or hardener used. On isophtalic polyester resins it's generally about 8 to ten times the gel time with 2% peroxide catalyst at 20-25° Celsius. For example if your resin gels in 20 mn, you'll have to let it cure between 160 and 200 mn, about 3 hours. It's pretty conservative, an experimented guy will go faster but as you seem beginner take no risk. It needs only care and some common sense, nothing very difficult.

Tunnels gave good advice and an excellent method. Re-read it it's clear. You'll have a partial chemical bond even 24 hours after because of the peel ply.
The use of peel ply helps a lot, and leaves a "weave" that makes a fantastic mechanical bond. and in case of using a paraffined polyester resin it insures that no paraffin will stay on the bond zone. Do not worry, that will be strong.

Even on polyester I use always epoxy resin for reparations as the epoxy glues far better, that asks for some precautions. Attention; the epoxy bonds to polyester, polyester or polyester gelcoat never bonds to epoxy (inhibition of the chemical cure of the polyester by the amines of the epoxy).

 

How do you think a boat is made?? in one foul swoop !! come on !! its done on layers and each lot of layers is left to harden some times for 2 or 3 days of even longer before the next lot goes over the top !!. and if theres a core ot could take 3 of 4 days to do and then the inside skins go on !! that why they use inwaxed resins that stay tacky for long time !! mechanical or chemical bonds it depends on the amount of contaminants the get on the surface of the sticky resin ,such as grinding dust etc !!and then its usually only on the down side of the hull if the mould is moveable !!.

If you try to do more than a couple of layers at a time and it falls off you have no one to blame but yourself!!! I could sit here and give you advise by the book load but if you dont want to take the advise seriously then its your choice if it craps out !!
Do what you want !!

 

A peeling failure requires little effort. Once going it is easy to lift the face off a sandwich surface, but the ability of the beam or sandwich to resist bending is far greater. The laminate to foam bond only has to take shear loads which are many times less than what the laminate faces take. Peeling is a special case. While it is remarkably unsettling to watch a surface lift off foam the beam is much stronger than easy peeling would suggest.

If you are still reticent maybe you can cut holes through the sandwich up to the opposite inside face. Prepare a slurry of syntactic foam (filler and epoxy). Trowel into hole and glass new face over it. The buttons will restore your confidence, allow you to inspect more of the sandwich and increase the beams resistance to shear loads. I suspect though that total refacing might require less effort.

Dino

 

Cool off Tunnels. He is beginner, has never seen a shipyard. And the questions are sensate. He is also nervous in front of a thing he has never managed.
A remark: he has not bought the boat...and that's a luck.
One meter diameter delaminations are not a little problem...The other side must be showing the same disease.
The description of the bulkhead and delaminations shows a very poor building and/or an agonizing boat ready to fall apart.
Structural reparations have to be carefully weighted...Are there other problems, after a very close examination? Is the boat truly worth?

 

The description of the problem looks like a structural failure. And more can be discovered. I do suspect that the general building is insufficient...
Buttons are good on very dry flat surface like a deck for just a local fixing, not for that kind of problem..