Re-using old catamaran hulls

Discussion in 'Boat Design' started by Phlames, Jul 23, 2018.

  1. fallguy
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    fallguy Senior Member

    A couple thoughts.

    The step idea - not a fan. Landing/boarding door as rail opening that doubles as water exit (or rolling thermos) - yes

    I don't at all like the distance of the vee to WL. I have a feeling you will have hull drag in any chop as the vee waves will crash into the hulls. Rethink that and get some clearance by flattening the vee or raising the deck. Those tornados aren't that high up on the water.

    Another poster questioned the cutdown. I'd be inclined to agree and only cut them to do the beam work (easier said than done!) You may need to cut them down and build them back for best hull/beam interface or strength.
     
  2. fallguy
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    fallguy Senior Member

    This does beg the question, 'why is there any opening between the hulls and sole at all?'
     
  3. Phlames
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    Phlames Junior Member

    I agree there will be extra drag created by having the vee in the water which will reduce the top speed. With the opening 'between the hulls and the sole'
    and the vee interacting with the hull waveforms, I will be fascinated to see what the resultant water flow actually does. Will the opening provide a venting
    effect and actually reduce the overall drag?
    Not sure what you mean by the 'cutdown'.
     
  4. fallguy
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    fallguy Senior Member

    5258FDEE-EBEC-42C0-9867-94BA9C5AADE4.jpeg Isn't the hull cutdown from original where I squiggled?
     
  5. upchurchmr
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    upchurchmr Senior Member

    Tornado hulls cause very little bow wave.
    It does depend upon the speed you are driving and how much the hulls are submerged.
    But remember that in sailing, at high speed, all the boat/ sailors weight is supported on one hull.
    Motoring with the boat flat, means each hull takes 1/2 the weight, reducing the potential bow wave.

    Personally, the V seems like a good idea, proven by large motor catamarans where it was added after damage caused by waves + bow wake hitting a flat underside.
    I'd suggest raising the V as much as possible (even by reducing under sole storage) and raising the whole pod as much as possible.
    Seems to me that the top of your pod would not be more than 6 to 7' off the ground on the trailer as you show it.

    I use to tow a Tornado tilted up at 45 degrees and never had a problem. That is 10' off the ground.
    Lots of anxious moments going under bridges and trees - but never a problem.

    I think that Brown must have a good reason for the height of his boat.

    Nice model
     
  6. fallguy
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    fallguy Senior Member

    Is not two bow wakes interfering with each other greater interference than a single bow wake causing no interference?

    Of course my original point is interference of three hulls!

    And then we agree up with the middle.
     
  7. upchurchmr
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    upchurchmr Senior Member

    Absolutely true about the two hull interference.
    But its only going to be 2x a single hull, the original single hull wave is small, and will be smaller due to each hull being sunk less.
    Just my guestimate.

    The OP already has the hulls, perhaps he can measure the height of the two waves interfering.

    IMHO, he should be setting the V to not have any contact with water in normal conditions - only when in unusually heavy seas, or the odd wave.
     
  8. BlueBell
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    BlueBell Ahhhhh...

    I agree.
     
  9. fallguy
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    fallguy Senior Member

    Don't forget he is running narrower. So the two hulls will interfere; my point is a third would result in multiple drag forces.

    The wave interference point is theoretically at 5.4 meters from the bow, using his 2.1M width and 19 degree bow wake. Right about at the motor..
     
  10. upchurchmr
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    upchurchmr Senior Member

    Then he doesn't need as long of a shaft on the motor.
    Great Design.

    From that, the lowest point of the V will not contribute to 3 hull interference.
     
  11. Phlames
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    Phlames Junior Member

    The original tornado hulls remain. The upper forward portion has been added.
     
  12. Phlames
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    Phlames Junior Member

    upchurchmr, you have almost convinced me that I knew what I was doing! When completed, it will be interesting to observe what goes on between the hulls and the pod
    but I will still use a long shaft 4-stroke outboard even if I have created a tornado trimaran.
     
  13. Rurudyne
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    Rurudyne Senior Member

    A suggestion: leave provision for changing your mind in how you mount your bridgedeck structure, if not actually build it to be adjustable. That way if you find you're getting too much drag from the hulls being closer together than ideal you can space them out. A GoPro like camera mounted under the bridge deck to record what the hull waves are doing would give you the evidence to know how far and maybe also how far is excessive.
     
  14. fallguy
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    fallguy Senior Member

    Not sure if it matters
    How would he do that?

    Essentially some type of beam would carry the decks.

    Curious how is all.
     

  15. Rurudyne
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    Rurudyne Senior Member

    For adjustable, there are a number of swing or scissors designs for beams to normally allow catamarans or trimarans to be trailerable that could be adapted. These seem more common with trimarans.

    But it might be as simple as having the main beams, as you suggest, be replaceable with longer or shorter ones and the hulls attach to these from below, where the deck etc from above. The bonus advantage disassembly for transport is possible if the assembled cat is too wide to tow. Not as convenient as a structure actually designed to do that without disassembly, but serviceable.
     
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