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  1. #1
    Join Date
    Jan 2005

    Default Fibreglass deck getting soft

    I have a 8m GRP boat and built in 1980 and the fiberglass deck is getting soft and tends to flex when you walk on it. Ply has been bolted to the deck in some places to reinforce it.

    Is this normal for a boat of this age?

    What are some methods for fixing this?

  2. #2
    William_H is offline Registered User
    Location : West Australia
    Join Date
    Jul 2003

    Default Re: Fibreglass deck getting soft

    I first assumed that there is a core material of some sort between the outer deck layer and the inner layer which permits a compression of the outer to inner layer. Sometimes you can see islands of extra thickness over large flat areas which is core in strategic places or it may be core over all the area. If it is solid fibreglass it is unlikely to give any trouble. The core can be a foam or balsa wood. It probably means that the core has deteriated due to water ingress.
    The answer is to cut away one layer of fibreglass either outside from the top of the deck or from underneath. Gouge out all the rotten core and replace with new core or if you don't mind the weight put solid glass and resin in. Lay up fibreglass over the core to match the existing glass.
    Obviously it is far easier from the outside but you will destroy the non skid pattern and will probably have to repaint the whole deck to make the end job look good and add a grit to get the non skid. So if your deck looks pretty tatty do it from the top but if it is pristine then it is worth trying to preserve the surface.
    If it is the whole deck flexing not just a local compresion then it needs reinforcing. I don't imagine it is deteriation of the fibreglass but rather a design weakness or a bit of both. Get rid of the plywood as I imagine it looks bad. Support is provided by depth of the reinforcing across the weak area. A piece of urethane foam about 30 mm square that is sanded down to hemisperical cross section is glued to the under side of the deck from near one gunwhale to near the other .( or wherever seems appropriate. Fibreglass cloth is then laid over this and onto the existing glass each side by about 30 mm. A thickness of at least 3 mm of glass is needed. Get some waxed resin or Flow Coat resin to use like a paint to smooth it over when the glass is hard. You can buy a cloth weave called "twill" which will curve more easily to follow the shape of the foam. Or alternatively ordinary weave cloth is layed at 45 degrees to the foam length which makes it easier to follow the curve.
    The depth of the foam provides the geometric stiffness so deeper the stronger, but of course may not go well in the cabin. An alternative not nearly so stiff would be to buy Carbon fibre cloth and lay flat over the area needing reinforcing. this means you don't lose headroom but being thinner does not have the geometric advantage. You can use polyester resin or epoxy. Epoxy is more expensive but sticks better to old F/G. Warmth for the curing is vital as is roughing up the area to have resin attached.
    If you are not familiar with F/g work get a book or read some of the references on this forum and it is probably worth doing a dummy run on to a piece of the foam onto ply to understand the problem of getting the glass to conform to the foam without air gaps. A bead of resin with filler along the inside edge of the sides of the foam to deck will change that rightangled bend to a gentle curve to assist glass attachment. It won't follow sharp angles only gentle curves.
    All this assumes you want to fix it. The working in the cabin upside down with resin is awefull and there may not be any harm in a little flexing. Fibreglass doesn't fatigue so as long as it doesn't fail with too much weight it may be ok for years. Regards will

  3. #3
    Join Date
    Jan 2005

    Default Re: Fibreglass deck getting soft

    Thanks for the reply and all the tips.

    I'll check but I do not think there is any core in the constructions just glass about 6mm thick. The softness ( slight give when you walk on it.) it is in the foredeck above the achore locker, some spots on deck which get a lot of traffic, and cockpitsole.

    The cockpit sole which has a trap door to the engine and driveshaft I wouldn't even attempt to glass up side down for sheer lack of accessability.

    I am reasonably handy but would do a fair bit of practice and use the glassing from the top option.

    I was thinking to just add some more layers of cloth and resin, would this help at all?

    As it is a fair way off I might look at doing all the areas and respraying the deck. I want to do the boottop when I go out to anitfoul anyway. [img]/forums/images/graemlins/smile.gif[/img]

  4. #4
    oldsaltoz is offline Registered User
    Location : Australia, East coast.
    Join Date
    Jul 2001

    Default Re: Fibreglass deck getting soft

    G'day Dug,

    If your foredeck is solid glass then the ply may be backing plates for attachments on deck.

    If you do decide to stiffen the area I would suggest you use an epoxy resin, it will cost more per litre but you will use less and finish with a stronger and lighter job.

    A suitable stiffener should be added to the underside, but only after you check the support at the back of the anchor locker has not been damaged or broken, or rotted.

    As for bending glass around corners, don't try to bend it too soon, let the resin break down the bonding agent in the glass first and will lay around a curve without problems; just put a radius on external corners and a fillet on internals, the latter will also make it easy to paint and keep clean. Use only glass designed for use with epoxy, not chopped strand mat or you will use too much resin and end up with a weak and heavy job.

    fairing can be done with a mix of Microballoons or spheres mixed with epoxy to a about toothpaste thickness, apply a layer of resin, let it go tacky and apply the filler, let it cure overnight, sanding is very easy.

    Any glassed or filled areas must be painted to prevent UV damage, this applies to epoxy and standard wax resins.

    Growing old is unavoidable. However, growing up is still optional.

  5. #5
    Join Date
    Jun 2001

    Default Re: Fibreglass deck getting soft

    [ QUOTE ]
    I'll check but I do not think there is any core in the constructions just glass about 6mm thick. The softness ( slight give when you walk on it.) it is in the foredeck above the achore locker, some spots on deck which get a lot of traffic, and cockpitsole.

    [/ QUOTE ]The majority of GRP yachts built around 1980 did have balsa-cored decks, and if mistakes were made with adding fittings so that water could get in, in time the balsa would rot to mush, leaving the deck spongy and flexible. Your symptoms do sound very like this, but drilling a small test hole in the worst affected area will reveal the truth. There's useful information on

    As William_H says, the usual treatment is to renew the coring from inside the yacht, but its a tricky job for DIY if you want a nice finish. Never heard of anyone ignoring it and simply adding reinforcement - sounds like it might be just storing up trouble.

  6. #6
    Ifraser is offline Registered User
    Location : Poole Dorset
    Join Date
    Feb 2004

    Default Re: Fibreglass deck getting soft

    Dug, I am just purchasing a 1976 Conway 26 which has softness and delamination of the deck. I am aware of the problem and negotiated the price to suit. Having done a lot of research into the problem I'd be surprised if your deck isn't cored. On my deck there is a smooth area around the tread pattern area so I intent to cut out the whole of the foredeck area by cutting in this smooth area, remove the core material and replace with a modern alternative and rebond the treaded deck panels back down. I ( or if it proves too troublesome a local fibreglass expert) will then blend in the small cuts with gel coat which I understand is not too difficult on smooth areas. For good measure I will probably glass a few extra ribs under the foredeck with epoxy and foam formers. My surveyor has agreed that this is a good repair technique and also recommends wooden knees on the coachroof to deck corners. As has been stated Mark Pascoes site is very good and out of interest have a look at this one as well
    Good Luck, Iain


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