Round full blunt bow on a cruising catamaran

Discussion in 'Multihulls' started by valery gaulin, Jan 20, 2017.

  1. valery gaulin
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    valery gaulin Junior Member

    I think people are just affraid in unorthodoxe solution. Of curse there is always some draw back to any design option. But the idea of a blunt bow on catamaran might not be as bad as people think it could be. I need time to be able to try it!!!

    @BJN : Where do you live? I am in the Montréal area we seam to be on the same thinking, of you dont live to far we could probably test a scale model?
     
  2. upchurchmr
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    upchurchmr Senior Member

    Quit talking about water and illustrating with air.

    They are a little different.

    Great idea about a model, so long as you compare to something traditional, with some way to measure the difference, at multiple different speeds.
     
  3. daiquiri
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    daiquiri Engineering and Design

    Boats are not cars - whether it's about aero/hydrodynamics, powering, structure, materials or else.
    They are completely different animals, having completely different operational, constructive, structural and maintenance requirements - and hence their shapes cannot be compared in such simplistic way.
     
  4. bjn
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    bjn Junior Member

    There are similarities though. The turbulence aft of the stern for example. When racing monohulls we try to distribute the crew to make the stern float just on the surface. Too low, we get a turbulent wake, similar to the illustration with the car. Too high the waterline length becomes shorter.

    Maybe there are similar lessons to learn about the front part of the vessel as well?

    valery gaulin
    I'm on the wrong continent!
    When I find the time I will see what a Michlet (free software) simulation will show.
     
  5. daiquiri
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    daiquiri Engineering and Design

    Of course there are similarities, but not to the point where you want to swap bow for stern in order to make the boat resemble an airfoil. It doesn't work that way.

    For example:
    "The waterline length becomes shorter."
    - that one. That's one important point of divergence between boats and cars. Waves, which do not exist in land-vehicle's aerodynamics, are of paramount importance for determining the shape of submerged parts of water-based vehicles.

    IMO, the only really valid reasons for a blunt bow on a catamaran would be a gain in the amount of internal volume of the vessel (if the overall length is limited), and to get a smaller draught.
     
  6. Emerson White
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    Emerson White Junior Member

    A boat exists at the interface between two fluids. Modeling it in a volume of air will not give you the full picture. Also, there is a reason the front end of a fish that swims fast in open ocean tends to come to a point.
     
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  7. DennisRB
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    DennisRB Senior Member

    You have continued to ignore the fact a boat exists between the medium of high density water and low density air, and that wavemaking occurs because of this, and wavemaking is large proportion of hull resistance. You need to think about that for a while so you can understand your graphical representations of shapes moving in a single medium are not applicable.

    Also its a leap of faith to say it will work as well on a cat as the racing mono. The waterplane is very lightly loaded on this carbon racing boat. Its more like a surfboard sitting on top of the water. You will struggle to get your cruising cat to lightly float with little hull below the waters surface, instead you will end up with deeply immersed waterbrake bows which act nothing like a foil immersed in a single medium as explained in my first paragraph.

    So I'm afraid you reached your conclusion using confirmation bias, by ignoring all arguments against it, and finding arguments for it that may not even be applicable as proof its a good idea.

    However, if you made a racing cat with this bow shape that was extremely light it might be fast in flat water by planing. There were some planing cats that were very fast in some conditions. But as a cruiser I'm not so sure.
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    Where's Ad Hoc ? He insists bow shape in a catamaran hull is not that critical, resistance wise, in the scheme of things, but length/displacement and slimness ratio are.
     
  9. DennisRB
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    DennisRB Senior Member

    I have followed those threads with great interest. I don't even think he said BT is a much of a factor. I seem to remember him arguing the opposite, that BT (beam to depth) means nothing compared to DLR. That is what this paper seems to show too. I agree the DLR is the overriding factor. I have the read this paper and understand that DLR is the king, and hull shapes should always be compared to hulls of the same DLR as this factor will override any other.

    http://eprints.soton.ac.uk/46442/1/071.pdf

    Therefore any arguments about bow shape should be made to hulls which have the same DLR. A hull with the same DLR that has waterbrakes attached is going to have a lot more resistance. Therefore its only fair to compare hulls actually designed to be real vessel hulls with low resistance in the SOR. Anyone could design a very unfair hull on purpose to have a poor resistance even at the same DLR.

    Look below, here is a series of actual hull shapes used on real vessels that compares resistance. You will see all the real hull shapes have quite similar resistance. There is one hull shape which sticks out as much worse. The crude hull. I don't know exactly what this shape is but I believe it was a pointed rectangular block.

    I'm afraid this scow cat hull will likely not be much better than the crude hull in this test if its bow is deeply immersed.

    [​IMG]
     
  10. Mr Efficiency
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    Mr Efficiency Senior Member

    We'd need to know exactly what the "crude" shape, was. If it was a rectangular block, the submerged stern drag would be significant.
     
  11. DennisRB
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    DennisRB Senior Member

    The point is the DLR was constant, so shape does play a part if the hull shape is totally neglected for performance. But even then the difference is not gigantic.

    Here is the post:

    http://www.boatdesign.net/forums/hy...-hulls-has-least-drag-45172-2.html#post589959

     
  12. Mr Efficiency
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    Mr Efficiency Senior Member

    It is a matter of degree, and what speeds are unacceptably disadvantaged, if the opening poster is prepared to trade off for what he sees as the advantage of a shorter boat. But the words "shorter" and "advantage" are uneasy companions in a displacement cat, performance wise !
     
  13. DennisRB
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    DennisRB Senior Member

    As seen with the real hull shapes, some had lower resistance and low speeds and high resistance at high speeds, and others were the opposite. Only the crude hull was considerably worse at all speeds! What I get from this is that you need to keep away from crude hullshapes, and any regular hull shape is going to be decent compared to it.

    There was no cucumber hull modelled here so we can only be left guessing :p
     
  14. Mr Efficiency
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    Mr Efficiency Senior Member

    A flat fronted hull isn't the same as his semi-circle, maybe he needs to make some models and test them against one another. What is certain is he will have a nice fountain feature with a plumb round bow.
     

  15. DennisRB
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    DennisRB Senior Member

    If he has a semi circle section it will at least beat the "crude hull" in WSA. But the pointed block crude hull probably created a better entry!
     
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