Custom 19' all weather, minimalist, strip plank composite 'go fast'

Discussion in 'Boat Design' started by socalspearit, Sep 2, 2021.

  1. fallguy
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    fallguy Senior Member

    No. I just think modeling is needed for the OP. The boat is going to heel even from helmsman as drawn.
     
  2. TANSL
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    TANSL Senior Member

    Yes, of course, but with such a simple hull it would not even be necessary to create a 3D model. With 4 or 5 transverse sections, some calculations could already be carried out and, of course, moving the console and the pilot to the center, unless there is a great weight to port that compensates it. But that is just another of the several serious problems that this hull could have. Some of them could be solved without using any software.
     
  3. fallguy
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    fallguy Senior Member

    But the OP doesn't seem to realize any issue, so if he modeled one person trying to board and had an understanding of the heel angle; it may help.
     
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  4. sharpii2
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    sharpii2 Senior Member

    You are correct thar the boat is more stable than it looks.

    This is because initial stability comes from average Beam, not maximum beam.

    That being said, I think the height you put the cockpit sole at is a major design decision.

    The higher you put it, the less stable the boat is going to be.

    If it were my boat, I would have it maybe two inches above the WL at maximum displacement (say 1800 lbs).

    You can narrow the air boxes somewhat, bu I wouldn't make them any less than 8 inches. And if I did that, I would definitely extend them from the cockpit sole to the sheer. This would give you somewhere near 30 inches of sole width to work with.

    When I was once watching WAYFAIRER dinghies race, I noticed that they had circular holes cut into the transom. These were about 2 inches in diameter, and the lower edges of them were at the cockpit sole. Stuffed into these, from the outside, were two funnels. These funnels were held in place by shock cords inside the boat, that attached to the funnel's apex. This made a handy, self-tending, one-way valve. If the cockpit flooded, the weight of the water would push the valves open, so it could flow out the back.

    If the air boxes extend to the sheer, the will provide a great deal of bending strength. Twisting strength can be provided by by installing bulkheads inside them. These can have large holes in them to save some weight. But if it were my boat, I would want to divide each airbox into three compartments of near equal length. The forward and aft ones could be stuffed with foam. The center ones could have hatches to store some gear in.

    Due to the narrower air boxes, I would increase the beam by making it 46 inches at the chines instead of at the sheer.

    I would also extend the airbox portion of the hull about 12 to fifteen inches past the transom.
    This would give the engine very little support, while the boat is sitting still. But once the boat starts moving fast, they will do wonders to keep the boat from squatting.
     
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  5. fallguy
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    fallguy Senior Member

    Okay, where are the 'air boxes' in this picture? It is a vee bottom jet ski like a Yamaha Waverunner. And OP is talking about putting 4-6 people in it and loading off the side for the beam and outboard.

    D5921929-4802-4F0B-A245-DE7DD0688ED0.jpeg
     
  6. fallguy
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    fallguy Senior Member

    Further, an inflatable is essentially a twin hulled beast with zero bdeck clearance. How anyone can compare a vee hull to a rib eludes me. I love your twin hulled air box vee hull?

    there is a disconnect...is it mine?
     
    Last edited: Sep 12, 2021
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    It has the dimensions of a freighter canoe, but a freighter canoe would be more seaworthy. The bottom line is that with small boats on open water, having to be seated is a punishment, unless perhaps your are well aft and steering a tiller outboard. The bent knees are a good shock absorber that can't be utilised when seated. This is particularly true of lightweight, easily driven boats. It is strictly for the daredevil set.

     
  8. fallguy
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    fallguy Senior Member

    You think anyone would board a freighter canoe from the side? I say no; not even with swim fins.
     
  9. bajansailor
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    bajansailor Marine Surveyor

    Socal, take ANY inflatable dinghy, and compare it against a plywood (or fibreglass) dinghy of the same length and breadth - are you claiming that the stability of the 'hard' dinghy will be as good as that of the inflatable?
    If you are, I think you might be disappointed if you do manage to do a fair comparison trial.
     
  10. fallguy
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    fallguy Senior Member

    Worse. He is claiming a 3 degree deadrise vee hull will be as good as an inflatable. No?
     
  11. bajansailor
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    bajansailor Marine Surveyor

    I think so. And I think that yes, it will be worse.
    Especially as Socal's boat has 13 degrees of deadrise at the stern.
     
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  12. fallguy
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    fallguy Senior Member

    I thought 3 degrees only. Oi vey.
     
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  13. messabout
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    messabout Senior Member

    No fair Bajan Sailor. If the geometry and the weights of the sections of either types are the same, there will be no difference in the instantaneous righting moment

    In fact if the inflatable has rounded sections, that would be the chine, and the wooden boat had a more squarish chine area, the woodie would win.
     
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  14. fallguy
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    fallguy Senior Member

    Maybe, but the OP is talking about a vee hull vs an inflatable. And the board members keep priming him with just enough vapor to keep chuggin instead of talking reality; his design is only longer than a 12' alumacraft from the 60s..otherwise very similar and many people drowned in those. Yeah, maybe not divers with fins, but on lakes; not ocean.

    Also, the idea is the soft middle of the inflatable creates a bit of a tunnel/catamaran, so this is how they differ intuitively.
     
    Last edited: Sep 13, 2021
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  15. bajansailor
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    bajansailor Marine Surveyor

    Messabout, I know that 'in theory' there should be no difference - but in practice I have found that an 8' inflatable is more stable than an 8' plywood pram dinghy of the same beam.
    And as Fallguy notes, the 13 degrees of deadrise will have an effect on the hard dinghy, not to mention the 'double bottom' which effectively raises the CG of the crew and gear (whereas on an inflatable everything is literally on the bottom of the boat).
     
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