Double diagonal planking strength/repair

Discussion in 'Boat Design' started by motorbike, Mar 6, 2011.

  1. motorbike
    Joined: Mar 2011
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    motorbike Senior Member

    Hi, Im thinking of purchasing a boat that was constructed of glued diagonal planking (3 layers) not glassed. Some repairs are needed in soft areas around the chainplates etc. Would the strength of the hull be compromised by substituting the rotten timber with appropriately scarfed and faired in fibreglass sections?

    thanks for your input
     
  2. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Your description of the repair isn't specific enough to offer much of an answer. Can you be more specific about the laminate schedule you intend to employ, fabric choices, resin system, how deep, how many layers of planking are affected, etc.
     
  3. motorbike
    Joined: Mar 2011
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    motorbike Senior Member

    Difficult to ascertain without grinding it all back, but the hull is 3 layers of 1/4" epoxy glued. The rot is about a foot square
    The boat is a 34 foot sloop.

    Plan A is to grind all the affected area away, the rot varies from 2 planks to an 1/8" but its irregular, create a scarf area of 10:1 and fill with layers of heavy boatcloth and epoxy.
     
  4. motorbike
    Joined: Mar 2011
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    Location: Beam Reach

    motorbike Senior Member

    The selection of cloth and/or any reinforcing system whether that is more planking scarfed and glued in or carbon or even additional reinforcing in the form of doublers etc is what I would like to know.

    I realise that without inspection no real advice can be given but i may be able to proceed with an idea of what an accepted method may look like....
     
  5. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    You can use fabrics (biax) to build up the missing material. Diagonal build methods are difficult to repair properly, but the generally accepted method is to cut back to the first seam that is unaffected by rot or damage, which exposes the layer beneath it. With the first layer of planking ground out and cleaned back to the diagonal seams, you can then do the same for the second layer (if necessary).

    Ideally, you'd replace the missing material with more of the same, epoxied in place as the original was. Naturally, you'll have to stagger the seams to prevent any stress concentrations, but this is all common sense stuff.

    You're asking if you can skip the new wood on old wood part of the process and the answer is yes, but it costs more and is more difficult to do correctly.

    If this is the route you want to take, then the proper method is to feather out each damaged layer of planking at 10:1 (at least) then bulk this up with biax (45/45) or triax to the same thickness as the wooden layer. Each layer of planking has to be feathered back and again the seams not aligned between the layers. This is of course finished with a fairing compound, which considering it's location should be a fairly hard mixture, for compressive strength. Epoxy is the only resin choice recommended.
     

  6. Boston

    Boston Previous Member

    any particular reason you don't want to fix it in wood ?

    seems that when you use a similar material you get similar expansion and contraction and avoid unexpected problems

    cheers
    B
     
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