8ft free running model building

Discussion in 'Fiberglass and Composite Boat Building' started by xinguo, Oct 27, 2021.

  1. xinguo
    Joined: Oct 2021
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    Location: Texas

    xinguo Junior Member

    Hello,

    I'm new to this forum. I'm going to build an 8ft planing hull for free running model test. It will be tested at waves with a max significant wave height around 7''. I'm wondering what material I should choose for my model? The CNC milled foam of course is the easiest way. As my rough strength estimation, one inch-thickness foam with epoxy coating should be fine, but the estimation could be wrong, I do not have 100% faith on it. Does anyone have experience on it? Should I use foam and get it laminated by glass fiber for safety reason? And one concern about the glass fiber lamination, I think it can be done awfully if the operator is not experienced. Is a female mold necessary if I want to get a quite precise hull shape. Thanks in advance!
     
  2. bajansailor
    Joined: Oct 2007
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    Location: Barbados

    bajansailor Marine Surveyor

    Welcome to the Forum Xinguo.
    Your project sounds interesting - would you like to tell us some more about it?
    What scale are you working to (ie how big is the vessel that you are modelling at 8 ft long)?
    And what is the desired free running speed?
    I presume you know about Froude Number for scaling?

    By this do you mean that the hull thickness of the 8' model will be 1" of foam, with just a layer of epoxy resin (ie no cloth) on the outside?
    If this is what you are proposing, I don't see how this can be strong enough - you will need to include some glass cloth / matt with the epoxy.

    It sounds like you are not planning on doing the glass fibre laminating yourself?
    A female mould would be a LOT of work to build if you are only going to build one hull.
    If you CNC cut your hull shape from foam, it should be possible to glass it on the outside - it will probably require some fairing, but it should still be pretty accurate / precise re the designed hull shape (so long as you don't go wild with the filler!) .
    @fallguy is the gentleman to talk to about this.
     
  3. xinguo
    Joined: Oct 2021
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    xinguo Junior Member

    Bajan, thank you for your advice. The model will be used to investigate the hydrodynamic characteristics of planing hull at open sea instead of towing tank. The full scale boat is 42ft, the maximum desired running speed for model is 23 knots. I'm not familiar with composite manufacturing. Could you tell me what method should I use for lamination without employing a female mold, so I can find some related videos to learn about it. Another question is how can I decide how many layers of glass should go for the model?
     
  4. bajansailor
    Joined: Oct 2007
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    Location: Barbados

    bajansailor Marine Surveyor

    Do you really want to run the model at 23 knots?
    That is a Froude Number of 2.4 - the corresponding speed for the full scale boat would then be 52 knots.

    If the full size boat is run at 23 knots, then the Froude Number is 1.05, and the model speed is approx 5 knots.

    Will you be operating the boat with a radio controlled unit? I am guessing that it will have twin electric engines for propulsion?
    If so, then you will need to build some beds in the model for the engines.
    It will probably be worthwhile glassing the interior of the hull as well (?) - maybe one layer of thin cloth (not a fancy stitch mat).

    You will effectively be building something like a male mould - here is a video that only lasts a minute, which explains it all (re glass and fairing the outside) quite succintly (I hope).
     
  5. Rumars
    Joined: Mar 2013
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    Rumars Senior Member

    It depends on what your cnc can actually do, and the precise geometry of the boat.
    A 5 axis cnc that has sufficient portal height can mill the whole boat in one piece. Make a plywood box smaller then the boat, glue on some foam sheets (high density PU or XPS) and mill it. Depending on surface quality you may need some hand sanding and filling.
    If the boat is developable a 3 axis cnc can cut the panels from plywood and you assemble and glue it by hand with fillets. 3-4mm ply for the skin, 6-9mm for the frames, construction ply, painted.
    If the boat is non developable and you don't have a high enough cnc you can cut plywood frames and strip plank with cheap wood and screws, then sand it by hand.
    None of the above methods require fiberglassing.
     
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  6. xinguo
    Joined: Oct 2021
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    Location: Texas

    xinguo Junior Member

    It is pretty high Froude Number. The 23knots speed is not firm, , I'll increase speed gradually to 23knots to see if it's possible. The model will be RC controlled. Is there any other way instead of a male mould? Like hand layup.
     
  7. xinguo
    Joined: Oct 2021
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    Location: Texas

    xinguo Junior Member

    A 5-axis CNC is available for me. So you mean the plywood will be strong enough and don't need any glassing, right?
     
  8. Rumars
    Joined: Mar 2013
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    Rumars Senior Member

    Yes, no need to glass anything, just dimension it correctly for the expected loads. If the boat is developable then it is the simplest option, the surface is already fair and smooth, you just need to clean the joints and paint it.

    If the cnc is big enough for the whole boat you can use a double milling technique, mill the foam (or wood if you want to use that) undersize, glass by hand then apply a thick coat of structural putty and mill again to final size. It will still require some hand filling and sanding, but the results are very accurate.
     

  9. xinguo
    Joined: Oct 2021
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    Location: Texas

    xinguo Junior Member

    Thank you, Rumars. I think double milling is great for improving the accuracy.
     
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