Repairing a cracked hull

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smert

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We noticed a small puddle would form in the cabin floor of our Shetland 535 if we were out and got banged about a bit in rough water. I figured it was a small crack that was getting stressed as the boat flexed in the waves as she could sit for weeks at the mooring and be dry as a bone!

So last weekend I got her out onto dry land to have a look, and I discovered a small crack (the anti foul had all dried apart from a small patch where there was water seeping out!). There has been a repair done here already by a previous owner evidenced by the epoxy lump on the inside of the hull.

My plan is to sand out an area and glass in a patch on the outside and similar on the inside. My question is, what should I use in terms of resin and glass cloth/mat?

I have some polyester resin, some CSM and 400g cloth, but I'm sure I read somewhere that polyester will shrink slightly as it cures so is probably not the best for the outside repair. As far as I am aware the hull is polyester GRP so bonding new to old shouldn't be an issue for my resin.

Also, is there anything I should be on the lookout for?
 
Better to use epoxy and glass cloth rather than polyester. Difficult to get a good chemical bond on old GRP with polyester. Difference in cost is minimal on a smallish repair. If you can get at the inside, then do the reinforcing from the inside having first ground out the crack on the outside. Then fill the outside again with epoxy - you might need some cloth if the crack is deep - then prime and paint. Good information on repairing GRP using epoxy on the Wessex Resins website. You don't have to use their West resin, but it is good.
 
Download the relevant manual from West System, it's the 3rd one down on the list. Although the book is based around West Products, it is really the repair methods that you are interested. The techniques are applicable to other epoxy glues. A proper repair requires a certain amount of grinding out of the crack and rebuilding the hull using various layers of glass cloth and finishing the outside with gel coat. Some folks on here have good tips for getting a great finish on gel coat that involves applying gel coat to the inside of a plastic barrier placed over the outside of the crack. I have done a lot of repairs now (minor ones) using West's methods, I recommend you at least read the section of the book on fixing cracks, which is free.

http://www.westsysteminternational....rt-guidance-for-specialist-epoxy-applications

Fibreglass Boat Repair & Maintenance
A complete guide to repair fibreglass boats with WEST SYSTEM epoxy. Includes illustrated procedures for structural reinforcement, deck and hull repair, hardware installation, keel repair and teak deck installation.
Click to expand...
 
Achtung! Personal opinions follow.

Chances are that the cracks will extend further on the inside than what might be apparent from the outside.
I would start on the inside and grind along the crack for around 4 inches all round or until you are well clear of any damp fibres by the same amount.
The resin that I have used for similar repairs has usually been polyester and has always been effective. As you have yourself seen as "evidenced by the epoxy lump on the inside of the hull" the use of epoxy is no guarantee unless there is proper preparation of the substrate. Use epoxy if you must but I don't consider it necessary on such a small repair/hull.
Wipe the ground area clean with acetone (copious quantities) and apply two layers of CSM and resin. Add more layers alternating between CSM and woven, widening the strips as you proceed until the area that you have ground is well filled. Use a metal roller to eliminate excess resin and any bubbles.
The following day repeat process on the outside, only this time you would not need to go so deep. The colour of the new (inside) lay-up will show you when you have gone deep enough. Build up with CSM until you are just shy of the surrounding area using tissue for the last two layers.
Fair with a good epoxy filler and sand fair. I use Veneziani Epomast Pro ( http://www.ellcee.com/en/products/webshop/32/veneziani-epomast-pro-compounds-aandampb.htm ) but there are others.
At this stage you will need to decide on finishing. Even if the hull is in a very good state I would not go for gelcoat since the damage is below the waterline. I would use a two-part painting epoxy (eg. Veneziani Aquastop) to seal the filler and make it even more impervious to water.
Prime and apply antifouling.

If this has sounded like teaching a grandmother how to suck eggs, please accept my apologies.
 
PuffTheMagicDragon said:
Achtung! Personal opinions follow.

Chances are that the cracks will extend further on the inside than what might be apparent from the outside.
I would start on the inside and grind along the crack for around 4 inches all round or until you are well clear of any damp fibres by the same amount.
The resin that I have used for similar repairs has usually been polyester and has always been effective. As you have yourself seen as "evidenced by the epoxy lump on the inside of the hull" the use of epoxy is no guarantee unless there is proper preparation of the substrate. Use epoxy if you must but I don't consider it necessary on such a small repair/hull.
Wipe the ground area clean with acetone (copious quantities) and apply two layers of CSM and resin. Add more layers alternating between CSM and woven, widening the strips as you proceed until the area that you have ground is well filled. Use a metal roller to eliminate excess resin and any bubbles.
The following day repeat process on the outside, only this time you would not need to go so deep. The colour of the new (inside) lay-up will show you when you have gone deep enough. Build up with CSM until you are just shy of the surrounding area using tissue for the last two layers.
Fair with a good epoxy filler and sand fair. I use Veneziani Epomast Pro ( http://www.ellcee.com/en/products/webshop/32/veneziani-epomast-pro-compounds-aandampb.htm ) but there are others.
At this stage you will need to decide on finishing. Even if the hull is in a very good state I would not go for gelcoat since the damage is below the waterline. I would use a two-part painting epoxy (eg. Veneziani Aquastop) to seal the filler and make it even more impervious to water.
Prime and apply antifouling.

If this has sounded like teaching a grandmother how to suck eggs, please accept my apologies.
Click to expand...

Don't use epoxy with CSM, or at least the common powder bound stuff.
The key is grinding out all the rubbish and the contaminated/oxidised surface.
Polyester resin will be fine if you abrade and clean up properly.
Make sure it is dry, maybe rinse with clean water before abrading and dry with a hot air gun?
Do a trial bit and check that it sticks well.
As the hull has cracked, you may want to ask why. It could be the trailer stressing the hull, or old beaching damage.
It might be a good idea to reinforce over a fairly wide area, the materials are not that expensive.

Top tip, get some polythene pipettes and actually measure the catalyst, instead of guessing the number of drops.
I bought 10 on ebay for about £1.50.
Then gel time will be consistent.
I would add one layer and let it gel before adding the rest, this should prevent the split from moving which will possibly create bubbles and weakness.
Mask the outside so resin does not drain through.

If you let it set for more than a few hours /overnight, I think it's best to abrade before adding more layup.

Get a load of polythene gloves, some cheap scissors to cut mat and throw-away paint brushes.

HTH.
 
lw395 said:
Don't use epoxy with CSM, or at least the common powder bound stuff.

HTH.
Click to expand...

I do hope that I did not give that impression! :eek:

For the benefit of the OP: The strands in CSM are held together by a powder binder that dissolves in polyester resin, thus allowing them to be more easily applied around and over contours; without it the strands would be impossible to control before they are wetted. Epoxy resin does not dissolve this binder and should only be used with woven fabric or with rovings that are held together with stitching.
 
Thanks for all the info folks. There looks to be some good stuff in the West Systems booklet - I'll read and digest this evening!

I have done a fair bit of foam sandwich laminate fibreglassing before (which is why I have some resin, CSM and cloth), but never had to do a repair or used epoxy before!
 
Well this is turning out to be a bigger job than I was expecting!

I started by grinding out the existing repair on the inside (looked like epoxy putty, about 7cm across). As I got through to the balsa wood core, the water started to almost pour out! Bearing in mind it's been dry and fairly warm for the past few days, I hadn't expected quite so much water! So take it back a bit further to find some dry core.... and a bit further..... a bit more... further yet.... I now have an area about 40cm x 40cm where I have had to remove the balsa core as it was completely shot, as well as opening up the keel a bit.

There is still quite a lot of moisture in the core that's left, although it is still serviceable, so we dumped a couple of kilos of rice into the hole to help draw out the water!

So what I had hoped was going to be a few hours, is actually going to be a few days. At least the repair is getting done properly this time though!
 
I would reiterate use epoxy resi and woven cloth. You may need to fill the space left by wet balsa with a mix up 2 pack polyurethane foam. This is horrible stuff as it expands hugely so try painting it in place after mix and expect to have to do several layers. Finally level it out to the skin contour.
The actual skin inside and out should have a tapered edge (shamfered) so that new GRP has a long edge to attach to. Outer new skin should be as thick or thicker than the original. On the inside an even thicker skin would be advisable considering that the original failed. good luck olewill
 
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