Modifying a dinghy size

Discussion in 'Fiberglass and Composite Boat Building' started by fallguy, May 21, 2020 at 11:49 PM.

  1. fallguy
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    fallguy Senior Member

    I have a dinghy plan I wish to take from 120" to 96". It is a three seater and I wish to make it a two seater.

    It is a tortured foam panel hull. I think we'd do fine with the reductions.

    My question is what is done with the beam?

    If the beam is say 54" to start; certainly I won't reduce the beam by 20% or I'd end up with a 43" beam which seems a bit narrow.

    or is that right?

    pointers welcome; thanks

    I did ask the designer and so far he has not replied.
     
  2. bajansailor
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    bajansailor Marine Surveyor

    What design is she please?
    Does she have a pointed bow, or a pram type (or garvey type) bow?
    The narrower she is, the easier she will be to row, albeit at the expense of being more 'tippy'.
    Do the plans give the load displacement and draft with 3 persons on board for the 10' boat?
    If so, you could scale down to see if your smaller size still has enough buoyancy available for 2 people (and their bags).
    Volume of Displacement = length x breadth x draft x Block coefficient
    Probably best to use waterline L and B, rather than overall.
     
  3. fallguy
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    fallguy Senior Member

    0FEEA080-D8AD-4BCD-8D3A-217A0FE85E9D.png It is a Merten's design and proprietary.

    The displacement is 400 pounds@ 6" draft as drawn with the 120" length, max beam is 51", waterline beam more like 40-42".

    I took a pic that doesn't give away much, but should help with deciding what to do on the beam question.
     
  4. bajansailor
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    bajansailor Marine Surveyor

    Here is a link to the TD3 dinghy on the Bateau Forum where Fallguy asks Jacques Mertens various questions about it -
    New Design - TD3 dinghy FOAM! - Bateau2 - Builder Forums https://forums.bateau2.com/viewtopic.php?f=8&t=64162

    Re a LOA of 120" (10'), let's assume a LWL of say 115" (9.58').
    You have noted that while the max beam is 51", the WL beam is more like 42" (3.5').
    For a draft of 6" (0.5'), the displacement is 400 lb.
    Using 1 cu. ft = 64 lbs, the volume of displacement = 6.25 cu ft.
    The block coefficient = 6.25 / (9.58 x 3.5 x 0.5) = 0.37

    The ratio of LWL /LOA = 115/120 = 0.96.
    For the smaller boat the LOA is 96" (8').
    Using the same ratio, we get an LWL of 92" (7.67').
    If we keep the same max beam of 51", and WL beam of 42" (3.5'), and keep the depth D of the smaller dinghy the same, and using the same block coefficient of 0.37 then we have for a draft T of 6" (0.5') to keep the ratio of T/D the same :
    Displacement = 7.67 x 3.5 x 0.5 x 0.37 x 64
    = 318 lbs.

    If the dinghy weight is say 40 lbs, then your payload is approx 280 lbs - two x 140 lb persons?
    How heavy are you and your crew?
     
    Last edited: May 22, 2020 at 3:11 PM
  5. fallguy
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    fallguy Senior Member

    Will the boat float at 400 pounds persons and gear then? I assume my cousin and his wife are 350.

    The goal of the boat is to be ultralight and capable of taking down a rough beach to his mooring. But we definitely want it to fit two people and a picnic basket.

    The td3 said 400# displacement at 6" draft and the original boat had freeboard of 8?" iirc and a ppi of 140#.

    So, the shorter boat at 8' long would have say a 100# ppi and sink about an inch deeper loaded to 400.

    No big deal.

    Question still remains, would I be wise to make it narrower or foolish?
     
  6. fallguy
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    fallguy Senior Member

    Seems now like keeping the beam is the best idea.

    corrections welcome

    I am starting to wonder if this boat could become a commercial endeavor in the 8' version. I have to work on the nesting, but two sheets of foam and some 400g glass and I could sell them unpainted.
     
  7. bajansailor
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    bajansailor Marine Surveyor

    I think it would be better to keep the original beam.
    Even with this beam, you might only have 6.5 - 7" of freeboard for a loaded displacement of 400 lbs.
    Which is not a lot really!
     
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  8. fallguy
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    fallguy Senior Member

    Thank you for your contributions.
     
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