Foil shaped hulls

Discussion in 'Boat Design' started by ChrisPie, May 9, 2020.

  1. ChrisPie
    Joined: Aug 2017
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    ChrisPie New Member

    This guy has built a cat with foil shaped hulls. If your only goal was minimum resistance in the speed range of 0.7 to 0.8 of hull speed. What is wrong with this design? Obviously something is otherwise all boats would be this shape?
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    Unfortunately, I have the attention span of a small gnat, can you precis the presentation for us ? The fish-shaped hulls (in plan view) aren't anything radical, but they look to have a fair bit of wetted area for small displacement.
  3. ziper1221
    Joined: May 2018
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    ziper1221 Junior Member

    If you look at the water plane section of a round bottom beach cat, it looks suspiciously like a foil section in reverse, with the pointy end in front. In addition to the wetted surface concern Mr Efficiency has, the lack of rocker may make maneuvering difficult.
  4. Dejay
    Joined: Mar 2018
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    Dejay Senior Newbie

    I really like that as a tutorial. I'm a novice, but I think there is nothing wrong with hulls like this. You could add a rocker too if you'd cut the plywood properly. You can look up "U shaped vs round hull" to find some threads here and info.
    The larger wetted surface of a U hull / flat bottom hull has (slightly) more friction resistance at slower speeds.
    At faster speeds the wave making resistance gets larger exponentially, so how suitable they are depends on how fast you'll go. For slow canoes, kayaks or row boats a rounded hull is better. At faster speeds almost only length and displacement / weight matters. I think rounding over the chines helps.
  5. upchurchmr
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    upchurchmr Senior Member

    Nothing new about the hull shape from above.
    The only thing strange is calling it a foil shape, I've never seen a foil with a sharp nose.

    A flat bottomed hull is really poor design.
    Poor for turning and poor for resistance.
    Why bother? Just to make something easily?
    Why not just use plastic coke bottles glued together - that would be easier.
  6. tlouth7
    Joined: Jun 2013
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    tlouth7 Senior Member

    At a simple level, this is applying a 2D concept to a 3D situation. Additionally:

    - foil shapes are optimised to produce lift. This hull has no need to produce lift (sideways).
    - foils are designed to be fully immersed in a single fluid. Hulls operate at the boundary between two fluids.

    So while it may not be terrible, there is not reason to believe that a foil shape will be optimal in designing a hull.
    Ilan Voyager likes this.
  7. Dejay
    Joined: Mar 2018
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    Dejay Senior Newbie

    Would the lack of rocker actually be bad for resistance with this very slender displacement catamaran?
    I imagine a rocker could help reduce buoyancy in the front and above normal hull speed help slice through the bow wave.
  8. Ilan Voyager
    Joined: May 2004
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    Ilan Voyager Senior Member

    There is a lot to say about this "design".
    First what do you call hull speed. Because this hull speed is extremely variable by the factors of displacement/length ratio, hull's width, placement of the center of displacement, angle of water entry at the bow, longitudinal repartition of volumes and a few others. The infamous coefficient 1.34 means nothing on modern boats and even less than nothing on multihulls. The Gerr method for predicting hull speed is a bit better but if I remember well it does take in account the total displacement/length ratio but not the width at LWL/LWL ratio.
    In short, a light displacement long slim hull with fine bow entry with a CD somewhere 52 to 60 % of the LWL will have a far better hull speed than a short fat hull of same displacement. To give you an idea a 40 feet 2.5 metric tons catamaran goes easily to 23-24 knots with 200 HP without any symptom of planing. A 21 meters 6.5 tons trimaran with a very thin main hull (17/1 ratio) makes a max 28 knots with 240 HP and it does not plane, it's just at the beginning of its max hull speed.
    Planing is another song and asks for a different analysis and shapes.
    Boats are at the interface water/air with waves. That means that the hull will have vertical sollicitations to damper. Some volume is needed forehead, but not to much and some volume is needed aft but not too much so the hull pitch is not too violent and dampens by itself . In "displacement" speeds that puts the center of displacement around the middle zone of the hull. I am not sure that the hulls in the video have the CD at the right place.
    As the hull has vertical movements in the water a flat bottom at the bow and middle hull is not the optimal shape. And a straight keel line does not help for the yaw ie turning. The repartition of the volumes is not optimal with that inverse profile with a bad behavior on agitated water.
    So I do not see any interest in that "innovation" specially on a small motor or sail catamaran whose good foolproof shapes are known since more than 30 years.
    BlueBell and Dejay like this.
  9. mudsailor
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    mudsailor Junior Member

    I’d also add that a lot of transom hung rudders use a different foil at the surface than the ‘regular’ NACA’ foil used to produce lift, IIRC a foil with max thickness at 50% is good to minimize wave making, then transition to the NACA ahape pretty quickly for the rest of the foil.
    Dejay likes this.

  10. kleenbreeze
    Joined: Jun 2020
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    kleenbreeze Junior Member

    interesting video. He is correct in many ways but boating doesn't occur in a lab, the real world has lots of exceptional elements thrown in.
    But what can anybody say about moving the COB forward in a sailing cat? That does seem like a good idea.
    BTW there is a little geometric trick that would transform his foil shaped concept into something with rocker and no hard chines......while still retaining the hydrodynamic efficiency he is claiming.
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