Hull thickness for 16 foot skiff

Discussion in 'Fiberglass and Composite Boat Building' started by willfishforbeer, Mar 4, 2007.

  1. willfishforbeer
    Joined: Jun 2006
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    willfishforbeer Junior Member

    I'm building a 16 foot by 6 foot shallow water poling skiff and i need to know how thick the hull should be. I found a weight per square foot chart of finished layers of different weights of fiberglass and made some very rough area calculations on weight based on a 1/4 inch hull thickness and come up with about 250 lbs hull weight. I really want to have around 200 or even less than that. What is the thinnest hull i could possibly go with without risking anything?
     
  2. Raggi_Thor
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    Location: Trondheim, NORWAY

    Raggi_Thor Nav.arch/Designer/Builder

    It depends on your planned speed.
    For a slow boat 1/6 inch of traditional fiberglass and polyester shpuld be OK.
    If it flexes to much you need some extra stringers or frames.
     
  3. willfishforbeer
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    willfishforbeer Junior Member

    The boat will have a 25-60 horse outboard motor on it and top speed will be 30-50 mph. I'm going to core the transom with 1/4-1/2 inch marine ply and brace it with fiberglassed knees of 1/4 to 1/2 inch ply.
     
  4. Raggi_Thor
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    Raggi_Thor Nav.arch/Designer/Builder

    Then you have to look more carefully into it. If you reduce thickness you must have more frames. How many frames and stringers?
    I hope someone here with more experience with this kind of boat can answer.
    I suppose shallow water means mostly flat water?
    Why not switch to plywood? It's less than half the weight, so you get a lot of stiffness, for example with 1/4 inch with a thin layer of glass on both sides.
     
  5. Raggi_Thor
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    Location: Trondheim, NORWAY

    Raggi_Thor Nav.arch/Designer/Builder

    I looked up some values in "Nordic standard".
    For 16 feet, LOA ~5 meter, typical dimensions would be
    Bottom : 9mm (3/8 inch)
    Topsides : 6mm (1/4 inch)
    with frames every 400 to 450mm (one and a half foot).
    NOW, this is for heavy duty workboats :)
     
  6. willfishforbeer
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    willfishforbeer Junior Member

    Yes shallow water means mostly flat water, but it has to be able to withstand some light chop. I dont think I'll be able to afford a solid hull anyways.
     
  7. trawler builder
    Joined: Mar 2007
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    trawler builder Junior Member

    hull thickness

    i would reccomend 8 layers with 3 being woven roven and 5 being 1.5 oz mat ,you can go with six layers wIth 2 roven woven but if you need strength go with 8 and you will find you can use less bracing
    TB
     
  8. Raggi_Thor
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    Raggi_Thor Nav.arch/Designer/Builder

    TB, how does that translate to kg/m2?
    1.5oz per square feet?
     
  9. Roly
    Joined: Jul 2005
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    Roly Senior Member

    1.5* 3.28*3.28*28.4 = 458.3gm/m2
    Just researched to convert a csm layup to a tri.,uni. & woven kevlar layup.
    Surprising just how much resin csm sucks up.A triaxial of the same weight is a little over half the thickness.
     
  10. Raggi_Thor
    Joined: Jan 2004
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    Raggi_Thor Nav.arch/Designer/Builder

    Thanks :)
    I think csm is used as a cheap way to build thckness.
    A more expensive but much lighter way is of course some kind of core, or something like spheretex.
     

  11. Kobus Potgieter
    Joined: May 2004
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    Location: South Africa

    Kobus Potgieter Naval Architect

    A small boat like this scantling will be approx.
    m ft
    Lh length of hull 5 16.41
    Lwl Lwl @full load 3.85 12.63
    Bh hull Beam 1.6 5.25
    Bwl waterline beam 2.5 8.20
    Bc chine beam 1.27 4.17
    Tc immerse depth of canoe body 0.3 0.98
    Htc scantling depth above baseline 0.30 0.98
    Hsc scantling height above DWL 0.30 0.97
    Tk draft of keel below canoe body 0 0.00
    M displacement(light) (kg) 500 1102.30
    ▼ displacement volume (m³) 0.49 17.23
    V speed in knots 34.95
    bottom deadrise in deg. 10-30 15
    running trim angle.min 4º 4

    Above is the assumption;
    Scantling will be (min standard):
    Hull bottom (keel area) -5.84mm
    Hull bottom - 5mm
    Topside , deck-4mm
    Frames/stiffners
    Needs to have at least 3 - 4 transverse stiffners
    Needs to have at least 2 longitudinal stiffners
    Hope this help
     
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