Help with hull weights

Discussion in 'Boat Design' started by Saqa, Dec 14, 2014.

  1. Saqa
    Joined: Oct 2013
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    Saqa Senior Member

    Guys, need your help please

    pontoon length 6m, width 50cm, height 70cm

    Have I got this calculation right (approx) ?

    2000L hull volume

    Skin weight approx 45gsm double bias/epoxy = 15kg

    8lb density poured foam core = 256 kg pontoon core weight

    6 coosa26 1" bulkheads with 50% mass removed with holesaw = 11kg

    Total hull weight = approx -300kg

    Secondly, how does this weight compare to a glass over ply hull built to similar size and strength?
     
  2. TANSL
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    TANSL Senior Member

    A pontoon has to endure a lot of punches and generally harsh working conditions. Build it in GRP does not seem very good idea.
    I do not know what material is the skin that you have studied, but a weight of 45 grams per square meter seems very little. (Perhaps I have misunderstood "gsm")
     
  3. Saqa
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    Saqa Senior Member

    Sorry, that was a typo.... 450gsm double bias glass

    The pontoon will be a pair used for a pontoon cat boat
     
  4. TANSL
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    TANSL Senior Member

    Keep in mind also the weight of the resin may be of the same magnitude as the fiber.
     
  5. Saqa
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    Saqa Senior Member

    Yeah I used 50/50 best case to calculate skin weight
     
  6. Saqa
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    Saqa Senior Member

    2000L or 2 cubic meter volume for each hull

    Work duty will include weathering storms while stored at mooring and rot free and fatigue resistant

    I am comparing vessel weights with the Supercat Silver 29 @ 1350kg

    Project hulls each -300kg......pair = -600kg

    Power 2x Yamaha 40XWTL each 80kg... pair = 160kg

    Pontoon totals with motor = 760kg

    Leaves me with 590kg to play around with to build the bridge deck including centre console, railings, controls and such. I haven’t started working out if that is feasible with the 590kg

    I am working towards 20-30' vessel with each pontoon having a starting volume of 2 cubic meters, may increase that if required. Starting length of 6m, and waterline beam on an individual pontoon starting at 50cm and decreasing that while increasing waterline length when calculating resistance and sea-keeping as required

    For weathering abilities I am leaning towards 8lb density solid core with a skeleton structure formed with 26lb density coosa. Six bulkheads crated with 3 longitudinals. The 6 frames reduced 50% in volume with a holesaw so the foam can flow through and attach better to the coosa and be better connected through the baffles. The ends of the frames to stick out of the pontoon as a flange to connect the open, flat and rigid bridge deck. I am leaning either alloy framework with top deck alloy mesh fore and sheet aft as option one. Option two being all coosa box beams with coosa top deck. Coosa transoms
     
  7. Saqa
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    Saqa Senior Member

    Hi guys
    Any educated guesses as to some ply hull weights?
     
  8. waikikin
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    waikikin Senior Member

    Saqa,

    Seems your foam "core" weight is a very big proportion of the total, there must be a better way. Also the skin is light especially for the bottom & beach groundings.
    As a comparison a foam sandwich tri I worked on had with hulls of 8.5 meters the "floats" were only 85-100kg two people could lift them(just-awkwardly).

    Jeff.
     
  9. Saqa
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    Saqa Senior Member

    Jeff, thats true. The hull weights with 'solid' foam are a lot. Using minimum open deck and superstructure might offset that at total boat weight. That is why I am seeking opinions on weights of ply boats built to a similar strength

    Lol if it be possible to have motorcycle helmet strengths and rot free without sinking the boat or moving the earth instead of the boat!
     
  10. rwatson
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    rwatson Senior Member

    You're making a lot of expense and trouble trying to do it out of solid foam - it just isn't needed.

    Check out http://www.bruce-roberts.com.au/grp_news.htm for example. A mould is better way to go - have you ever tried to get a lump of foam fair and even ?

    Also, if you look at the that page, you will see that the foam isn't carried through around the chines for reasons of strength.
     

  11. Saqa
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    Saqa Senior Member

    Yeah looks like it will be expensive and making a concrete mold to pour 8lb PU to make a solid core will require a lot of effort. Wasted effort if glassed solid core is weaker then foam sandwich. Fairing shouldn't be too bad though as mold should be fair to begin with
     
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