Latest News: Forums Technical Repairing outer skin of hull (W World)

  • This topic is empty.
Viewing 7 posts - 1 through 7 (of 7 total)
  • AUTHOR
    POSTS
  • #4091

    Hiya
    still getting my new hull ready for the water. The main issue with the boat (and the reason it was going cheap) is that there has been a substantial impact at the waterline near one of the self-bailers. It looks like the usual sort of crazing at first glance, but looking closer it appears that the skin has actually broken and that this will be a source of leaks.
    The World outer skin seems to be a very thin material. The usual advice on repairing a GRP hull seems to assume a much thicker layup and that you have to make a cut-out with chamfered edges, and lay down new glass in a series of layers. Somehow I don’t think this will apply to such a thin hull skin as the World?

    #9630
    admin
    Member

    No one seems to have come picked this one up, so what follows is partly from a book I have and partly from experience, although the damage I repaired like this (in a buoyancy tank on an Enterprise) was relatively small – sort of saucer sized.

    With a large star craze, the cracks can extend quite a distance from the point of impact and as you say, can result in the gelcoat flaking away to reveal the lay-up underneath. If the damage has penetrated the lay-up, damaging the chopped strand and polyester resin, you may need to consider reinforcing the hull with an additional layer of glass and resin before rubbing down and refinishing the outside surface, alternatively you could cut out the damaged area and replacing a section of the skin of the hull, which is a bit serious but can be done.

    I understand that the World has a moulded in buoyancy tank where you describe and the damaged area is inaccessible from inside/above. Unless you decide to cut a hatch in the deck/side tank to gain access (possible but I imagine there must be stiffeners and beams under there), then going in from outside may be the best option.

    So if you decide that is what you are going to do…

    Take your hack-saw blade and your courage in your hand and cut out the damaged part of the hull until you get back to sound material, then chamfer the edges 12:1 (so if the hull is 2 mm thick the chamfer should be about 24 to 30 mm wide). Go and have a stiff whisky, ‘cos that was scarey.

    Next laminate a flat piece GRP made of two layers of woven mat impregnated with epoxy on a piece of waxed glass, leave it for a day then part it from the glass, wash it with fresh water and scrub it with a Scothbrite to remove the amine blush and abrade it to make it rough enough to form a key for the adhesive that follows. The flat piece you now have will form an inner patch. Cut the laminate so that it overlaps the gaping hole in your boat by about 50 mm all around. I think the hull there should be a two-dimensional curve, meaning it is bent but not bowl-shaped. If you prepare the patch fairly soon after laying it up on the piece of glass it is quite soft and can be cut with strong scissors or tin-snips. Drill a couple of holes in your patch and tie some string to it.

    Mix up some epoxy with enough fibres to make it ketchup thick. Buy a tub of fibres from your epoxy supplier, it looks like powder but is in fact ground glass strands – keep it away fro your cup of tea). Clean the inside of the hull around the hole and a wipe with acetone would not go amiss, give the inside surface a good rub with coarse abrasives and by jiggling the patch around (holding the string) get it inside the hull. Apply the thickened epoxy liberallyto the patch and the hull and stick the patch to the inside of the hull, use the string tied to a local immovable object to hold it in place while the epoxy goes off. Leave it a couple of days (especially in this cold weather which slows down the epoxy).

    You have now cut out the damage and patched the hole from behind, all that remains is to restore the outer skin and finish. Abrade the patch and lay in progressively larger pieces of cloth and epoxy, eventually overlapping the chamfer you cut on the hull to bring the surface up to as close as you can to the fair hull shape. Make sure that the chamfer is overlapped by the new glass and epoxy, this will restore the strength of the hull to close to the original; the nice thing about GRP is that a repair can be as good as the original in terms of strength and durability.

    The repair is daunting but not too difficult once you have summoned the vourage to cut the hole but restoring the hull surface is time consuming and may not result in a perfect colour match. You will probably have to paint the patch as polyester gelcoat will not adhere to epoxy. There are white epoxy marine fillers that might not be too bad a match for the original hull colour. Alternatively now might be a good time to paint on a waterline.

    I hope you can solve it by simpler means than this, simply rubbing the surface down and doing a cosmetic job could be storing up problems for later, this way you have reinstated the strength and the water-proofing.

    #9631

    Thanks- great reply!

    Matching the colours at the end of the job will probably be a job for paint: the waterline on my boat is in yellow, with white above and below- and the area to be repaired, whilst quite small (about 6″ by 4″) would have three bands of colour in it.

    The existing skin is all there, though, and all the right colour- it just has cracks in it. So I was starting to wonder if I could manage a neater job by going in ‘from behind’ (via an inpection hatch), building up glass/epoxy from the inside, and simply ensuring that the outer hull adhered well to this and returned to a being nice flat surface with the cracks closing themselves up.

    I hadn’t even thought of stiffeners getting in the way, though. Would it be daft to try and find these wih a stud detector? Or does anyone have enough knowledge of the design/construction of the World to find out if this is indeed a problem?

    Cutting a hole for an inpection hatch won’t require quite such a large dram! But will it lead to a bodge job? It does have the advantage that any future repairs might be accessible from the same hatch.

    I would attach a photo but alas the camera was drowned earleir this year whilst launching a Wayfarer off a beach (don’t leave them in the thigh pocket of your oilies…).

    #9632
    admin
    Member

    Well it’s certainly an option. No idea about detecting stiffeners and beams, best to get advice from other World owners.

    Check the extent of the cracking, you may find they extend some distance from the site of the impact. Try painting on some thinned dark paint over the area then wipe it off to reveal the cracks. Outside of the damage area the paint will wipe off cleanly but the cracks will show up as hairlines.

    The trouble with doing nothing to reinstate the gelcoat is that water will get into the laminate and sooner or later will weaken it. Cracks that have closed up do not mean a waterproof construction. So if you do decide to do the inside job I still think you should seal the outside, maybe by grinding the gelcoat off and rebuilding it, even though that will leave you with the same problem finishing the outer hull.

    Good luck

    #9633
    Colin Parkstone
    Participant

    How about doing the above repair with Polyester resin and then finish off with white,polyester gel coat.
    Then just paint the waterline and that then will be the only paint on the hull !

    C P

    #9635
    admin
    Member

    Well I have said this before, but please check other sources in case I have it around my neck…

    Polyester resin works by cross-linking long-chain molecules which gives a chemical bond. The great thing about it is that the resin really does become a solid with molecular bonds that join around and through the reinforcing fibres, making a matrix that is extremely strong. That process only works while the underlying resin is fresh. This is why the gelcoat is only allowed to go partly off before the main resin lay-up begins on a new boat and for preference the whole of the lay-up has to be as continuous a process as possible (very large hulls excepted which I suppose are not worked on overnight, so a few hours between laminations must be acceptable).

    The hull of No Disgrace (great name BTW) is “old” GRP, meaning that all of the molecular bonds are solid and “dead” and new polyester resin will not cross link with the old meaning that the patch you apply is never going to be part of the existing construction, but will lay on top of it. Given the exposure to water and UV, it will possibly/probably not be a permanent repair and at some point the gelcoat and resin of the outer repair will part company. Not sure if we are talking months or years, if it’s the latter then who cares?

    Epoxy on the other hand is an adhesive (as in Araldite) and a mechanical bond is formed which, given a good abraded surface sticks extremely well. The downside is that polyester resin does not stick to epoxy, so the received wisdom is that Gelcoat will not adhere to it very well. I don’t have practical experience of this because I have never tried it, maybe others have.

    My sources for all this BTW are two books by an American name of Don Casey, Hull and Deck Repair and Sailboat Refinishing. Good reading, well laid out and very empowering.

    If painting /finishing is the worry, why not make it a feature, paint a big hole there with a shark’s head coming out, I’m sure I have seen that on a film…

    #9680

    A wee update on this.
    I used a dremel to carefully grind away the gelcoat where it was cracked. To my surprise, the area underneath was sound. So all I have done is replace the gelcoat with plastic padding gelcoat filler. I have just used white filler so it is quite obvious where the repair runs into the yellow waterline, but it should be perfectly functional to get me on the water. Maybe at a later date I will try to colour-match a paint to hide this.

    Good forecast for Saturday which is when I hope to finally get the boat launched. Still have to cut the slot in the centreboard and make up a retainer, but otherwise all good to go. I’ve been really pleased with how some of the minor dings and chips have come up with a bit of filling and sanding.

Viewing 7 posts - 1 through 7 (of 7 total)
  • You must be logged in to reply to this topic.