quadamaran - 4-canoe body

Discussion in 'Multihulls' started by magwas, Sep 6, 2012.

  1. magwas
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    magwas Senior Member

    This is an attempt at designing a trailerable crusing cat with reasonable interior space.
    The main points of the idea:
    - one ama and aka is a trailerable unit
    - an ama keeps upright in nice weather, thus no cranes and such are needed to assemble the vessel

    The attached linesplan is a rough sketch of the concept, I haven't even drawn a deck or cabin top.

    The main point is that each ama floats on two canoe bodies to give them some initial stability. This creates a hull form somewhat resembling to tunel boats. However I am thinking about a sailing vessel, and I guess at these speeds there is not much point in hoping for lift from air, especially with this high hull.

    I would be interested in any comments on the idea, especially about considerations about the height of the tunnel. Higher tunnel means less cabin space, and I guess a low one would mean less performance, and maybe other nasty things due to wave effects. But what kind of effects, and how severe the performace penalty?
    [​IMG]
     

    Attached Files:

  2. cavalier mk2
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    cavalier mk2 Senior Member

    It reminds me of a cross between bathtubs and rocking horses. You could call it the "Wet Rocker Tandem Hot Tub" ! Check out Chinese Dragon boats for a similar bottom channel treatment and see what they've found out.....sorry for the humor:)
     
  3. magwas
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    magwas Senior Member

    Thank you for the aestethical evaluation :cool: and hint. I guess it is different from the dragon's W as the top of the tunnel is above waterline. I have made an estimate of the wetted surface increase. A straight-lined W shape would have sqrt(2) times the surface of a straight-lined V. Based on this I guesstimate the wetted surface to be somewhere around 1.5 times more due the above and shape differences.

    I have no idea however how the wave drag would behave. I guess the difference in shape and the effects of waves made by one half on the other half should be considered, but how? And which is the more significant with this hullform and speeds, surface drag or wave drag?
    Is there an RTFM for these questions?

    Another thing I don't know how to take into account is the effect of sea waves on the W. I guess it would mean increase of wetted surface at least due to the fact that some waves reach the top of the tunnel. With high-frequency waves maybe the effect of air pockets enclosed by two top-reaching waves should also be considered. But again, how?

    And what about the mechanical effects of splashing on the tunnel top? I would guess it could raise higher surface pressures than on a traditional bottom, because of the enclosed space. However there is no hidrostatic pressure there due to depth. Should the bottom thickness be sized up accordingly, and if yes, by what factor?
     
  4. cavalier mk2
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    cavalier mk2 Senior Member

    Robert Harris outlined a inexpensive model comparison technique in his book on trimarans. If 2 models are towed from a beam set up like a scale it will be obvious what has less drag. Another low budget model test uses a fishscale hooked to the tow line and readings taken off at various speeds. While scale affects results spending an afternoon on a boat doing these sorts of things can help you learn how everything interacts when breaking new ground. A basic V hull can be a good test control, models should be close in terms of size and weight should be equal. There are probably computer programs to do this but it would be best to compare the findings with real tests.
     
  5. gonzo
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    gonzo Senior Member

    It a bizarre shape that will have a lot of resistance, behave and handle poorly. Why do you want to depart from shapes that work well?
     
  6. ThomD
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    ThomD Senior Member

    Well he said why, he wants to maximize internal space, and create a shape that is stable in the water prior to linking up with the second hull, so that he can assemble the boat post launch. At least that is how I see it. Horrible hull though, and what you want to achieve has been done by others is somewhat more normal ways. Bernt Kohler's 650 has flat hull, and he also has a clever way of allowing the hulls to spread post trailer. There is a long tradition of dory hulls in cats, and any of those that does not have a badly ballanced wing deck will probably stand up in the water by itself. It should also be relatively easy to design some kind of support system, like a skiff that mates to a wing deck, to provide a very stable pontoon,

    You could call this a quad, though for me that should have four separate hulls, but they used to call the speedboats with three unified hulls cats or tris... The quad I liked had minimal cat beam, and two small amas, for final stability.
     
  7. magwas
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    magwas Senior Member

    exactly

    I am thinking about a design where one hull would be within trailer limits, but two are above. So the hulls would be in disconneced state after trailer.


    Thank you for the hints, maybe these are better ideas than Wet Rocker Tandem Hot Tub.
     

  8. gonzo
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    gonzo Senior Member

    That's a 12m long boat. You will need a crane to assemble it and some help. Also, at that size it makes sense to have accommodations above deck and not just in the hulls. The shape of the hulls has no advantage I can see over any of the usual types. That shape also has more draft, another drawback. A dory hull is not ideal, but easy and cheap to build; particularly if you are not too skilled. If you are skilled, an elliptical or semi-circular bottom section is more efficient.
     
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