Fibreglassing a marine ply hull - costs

Discussion in 'Boat Design' started by kitkats, Aug 21, 2009.

  1. kitkats
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    kitkats Junior Member

    I'm very new to boats, so please forgive any lack of acumen.

    I'm thinking of buying a boat to convert into a weekend-houseboat. It will be permanently moored in shallow (0 - 16ft) sheltered coastal waters (an estuary).

    I have come across a hull which I think may be suitable, a 24" x 9" motor yacht (I think). The present owner has suggested that the whole hull be fibreglassed, which seems sensible.

    I'm looking for advice on what type of glass would be best, and roughly how much I'd need, and how much that would cost. I'm based in the UK so if anyone knows of the best suppliers here that would also be useful.

    Thanks in advance.

    [​IMG]
     
  2. apex1

    apex1 Guest

  3. kitkats
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    kitkats Junior Member

    I only just noticed the sub-forums... Guess this should be moved to the Wooden Boat Building and Restoration forum. Apologies. Perhaps an admin can move the post?
     
  4. FAST FRED
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    FAST FRED Senior Member

    The present owner has suggested that the whole hull be fibreglassed, which seems sensible.

    WHY?

    Slathering on a layer of GRP does almost nothing for a leaky teaky in terms of adding structural strength , it mostly adds weight.

    The GRP also give problems as the wood wants to move , expand and contract which breaks the bond to the GRP, even is EPOXY (the only choice) was used.

    I would contemplate weather the boat is suitable AS IS , a wooden vessel, and if so simply maintain what you get.

    FF
     
  5. petethai
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    petethai Junior Member

    Doesn't wood contract and expend only when level of moisture is changing.
    If we apply epoxy over the wood we make this level constant.
    The movement from the mechanical stress, can make some damage.
     
  6. FAST FRED
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    FAST FRED Senior Member

    Doesn't wood contract and expend only when level of moisture is changing.
    If we apply epoxy over the wood we make this level constant.

    Doesnt it rain snow and at rare times the sun shines in your place?

    Any and every hull or deck penitration is simply sealed with a goop which may pass moisture , new or old.

    The downfall of the various Tiawan boats with teak overlay decks was the hundreds of fastenings, which eventually watered the incapsulated ply below.

    YRMV

    FF
     
  7. kitkats
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    kitkats Junior Member

    I'm not planning on this boat being powered, strange as that may seem. So the internal stresses would be less than normal perhaps.

    I'm not planning on the GRP adding any strength, and adding weight could even be an advantage if it increases her stability. The reason for coating it in glass and epoxy would be to provide greater scratch resistance and seal the hull so that I don't have to take her out and repaint her every few years. It would be a thin coating not an inch thick. She will be moored up near a marsh, beached roughly half the day. I'm planning on towing her out there and then ideally leaving her in situ for as long as I can. So the idea is to reduce the frequency of maintenance.

    I've not heard people suggest that marine ply would come away from a GRP coat before... but perhaps that is a danger.
     
  8. rwatson
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    rwatson Senior Member

    Marine ply and GRP are a bad mix. They separate eventually because Polyster resin permists water to pass thru it. There are dozens of discussion about this in the forums.

    Epoxy over a sound surface is the most reliable way, and adding even very lightweight fibreglass will vastly improve the epoxy lifespan.

    But thats not enough - you also need a good paint to keep the UV off the Epoxy (or Polyester) - so that needs to be factored in also.
     
    Last edited: Aug 23, 2009
  9. PAR
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    PAR Yacht Designer/Builder

    GRP structures include epoxy folks.

    Plywood should always be sheathed in fabric, if good durability, increased abrasion resistance and water proofing is desired. Of course as has been mentioned, epoxy is the choice, though if done properly polyester can offer an option. This said, polyester has it's issues and personally I would never use the stuff on wood, but if you happen to have barrels of this goo laying around, then it too can be employed, though you'll pay a weight and application difficulty penalty using polyester.

    The photo you've posted Kitkats, doesn't appear to be one of a plywood boat. Are you sure it's plywood?

    Since you intend to moor the yacht, it should have a fairly thick sheathing, to protect the wood from the likely infrequent haul outs, it'll have to tolerate for the remainder of it's life. I would recommend at least 24 ounces of fabric sheathing, in multiple layers on the outside of the hull. A considerable increase in abrasion protection can be had if you use Dynel instead of regular 'glass fabrics. It will use twice as much resin, but it is a lot tougher then 'glass.
     
  10. Fanie
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    Fanie Fanie

    Polyester is not the material to use for glass to wood, you have to use epoxy.

    I had a discussion recently with someone on this. The polyester does not penetrate the wood sufficiently thus does not really seal it and the glass may delaminate from the wood due to a more 'shallow surface' bond.

    Epoxy does make for better penetration in wood and a better seal and bond process is achieved, even better still if you can vacuum it to the hull.
     
  11. petethai
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    petethai Junior Member

     
  12. Commuter Boats
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    Commuter Boats Commuter Boats

    Gentlemen, there is a completely different track on covering wooden boats that does deserve some consideration. Now I'm not suggesting that somebody builds a beautiful cold molded 44 foot hull and then goes after it with polyester but a vessel that has put in a good service life and is a little on the tired side can have a good bit of life restored with a structural polyester-fiberglass skin applied. There are enough successful examples that the technique should be given consideration when the vessel warrants it.
    A brief rundown on the technique, replace rotten wood, removed through-hulls fittings, apply a minimum laminate of one mat and one roving, mechanically fasten the laminate to the wood, complete the structural laminate in such a way as to prevent freshwater from getting to the wood. Although not wet the hull is not dried ( kept as swollen as it will be) and little effort is done to promote bonding. There is at least one boat ( I think it was a 76 foot herring schooner) were a product similar to 3M 5200 was troweled over the hull and C-flex was laid into it and wet out, fastened, and overlaid.
    There is a good book on the subject and I have scanned the cover.
    Gerald
     

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  13. kapnD
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    kapnD Senior Member

    I like the idea of the 5200 layer between the wood and glass--an expansion joint of sorts that adheres to both sides yet allows some movement without delamination. Still sounds expensive though.
    Getting back to the original question/requirements, it looks more like a floating dock than a boat, so
    Since weight is not an issue, only strength, how about cement plaster?
    It can be mixed with polymer admix to be very strong, somewhat flexible and quite waterproof. It is also sprayable, could be a one day job.
    Also, if it is to be stored in the same location always, you could build a cradle on the seafloor for the boat to rest comfortably on at low water. A couple of pilings to guide and moor it, and there's your floating concrete dock!
    Good Luck, Don
     
  14. cowobyisking
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    cowobyisking New Member

    I am learning about boat building, and I wanted to know if I can only locate marine plywood online or can it be found in most hardware stores?
     

  15. kitkats
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    kitkats Junior Member

    Just wanted to say thanks for all the replies. It sounds like epoxy is the way to go, although I'm hoping now to simply find a fibreglass hull instead. If I don't and I go and see this boat, I'll maybe post back here for more specific advice on the different types and weights of fabric, as there seem to be far more available than I realised.

    If anyone cares to comment on the surface preparation, that might also help. Presumably ALL the paint would need to be sanded off, and back to bare ply - to what grit? C-Flex etc etc?
     
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