Modern hull shape vs ideal hydrofoil shape

Discussion in 'Multihulls' started by JohnGB, Feb 19, 2021 at 2:16 PM.

  1. JohnGB
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    JohnGB Junior Member

    It does, but it still leaves the question of why the general airfoil shape is backwards. Essentially the pointy side is at the bow and the more rounded side is at the stern. I would have thought it would be the other way around, but I'm sure it's just me not understanding why rather than the actual boat design being incorrect.
     
  2. bajansailor
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    bajansailor Marine Surveyor

    Look more closely at the waterlines, and the sections shown on the side profile of the lines plan.
    They show the more rounded side at the bow, and pointy side at the stern.
     
  3. Ad Hoc
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    Ad Hoc Naval Architect

    The objective is to avoid/prevent flow separation for these type of hulls...as that leads to added drag!
     
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  4. Mr Efficiency
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    Mr Efficiency Senior Member

    I may be misinterpreting, but I imagine the OP is wondering why the underwater part of of displacement hull isn't shaped more like a fish like a mullet, blunter forward, tapering very gradually to a pointy end. One explanation would be that boats are operating at the interface of air and water, and fish are evolved for full immersion, if that isn't what he means, then carry on, and I apologize for the interruption !
     
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  5. bajansailor
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    bajansailor Marine Surveyor

    That is a good point - and that was the theory behind the 'cod's head & mackerel tail' approach to hull design in the 19th and early 20th centuries.
    But tank testing and practical application since has shown that this is not the case.
    A basic explanation here -
    Design terms https://classicsailor.com/2019/07/design-terms/

    And here -
    Why did ship designers from the early 17th century onward start building ships with a bluff bow and the widest beam quite forward? - Quora https://www.quora.com/Why-did-ship-designers-from-the-early-17th-century-onward-start-building-ships-with-a-bluff-bow-and-the-widest-beam-quite-forward
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    There's nothing like testing and experience in the field to inform opinions, it does seem a bit strange that the "cod's head and mackerel's tail" people did not realise that fish don't have to contend with the pesky consequences of waves interacting with things floating on the water surface, and those floating things creating waves when they move. And let's not even bother with the minor detail that fish are not rigid objects when they do propel themselves through the water. I guess submarines when submerged, are a reasonable analogy to fish. Surfaces vessels much less so.
     
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  7. JohnGB
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    JohnGB Junior Member

    That's exactly what I mean. Thanks for expressing it in a clearer way.
     
  8. JohnGB
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    JohnGB Junior Member

    Thanks, that was both illustrative and informative.
     
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  9. redreuben
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    redreuben redreuben

    A boat doesn’t just move through the water it also performs other functions like load carrying and living space. Not all of these loads and functions can be squeezed into the max chord part of the shape, like engines and bunks so the shape is expanded to best fit the functions.
    And unlike a fishes body it is fixed in shape not flexible, if a fish needs to reverse direction it bends its body around and swims the other way, if a boat is pushed backwards down a wave or simply has to motor in reverse then it needs buoyancy aft as well.
    And so the “ideal” hydrodynamic shape is not so ideal.
     
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  10. Mr Efficiency
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    Mr Efficiency Senior Member

    Not to mention that without ballast, a fish-shaped hull would be unstable, dead fish float on their sides.
     
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  11. redreuben
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    redreuben redreuben

    Except it’s a catamaran.
     

  12. Mr Efficiency
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    Mr Efficiency Senior Member

    If it is a catamaran, but catamaran fish are a rarity.
     
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