Is this Image about boat resistance calculations correct?

Discussion in 'Boat Design' started by Driiftwood, Aug 5, 2022.

  1. Driiftwood
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    Driiftwood New Member

  2. gonzo
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    gonzo Senior Member

    Any system will self destruct when stresses exceed its strength. It is not a particular characteristic of displacement hulls. For example, look at hydrofoils crashing and getting shredded. Also, narrow beam displacement hulls have a less pronounced curve.
     
  3. BlueBell
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    BlueBell . . . . .

    Welcome to the Forum Driiftwood.

    Yes, the images are correct but with the caveat:
    "Boat types exist that don't fit into this model."
    Leaves a lot of wiggle room doesn't it.

    What book is this from?
     
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  4. Driiftwood
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    Driiftwood New Member

  5. Kayakmarathon
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    Kayakmarathon Senior Member

    This image is from "Principles of Yacht Design" byLarsson and Eliasson.
    The curve for the displacement hull actually has several smaller humps at lower speeds. This occurs as the number of transverse waves in a boat length decrease.
     
  6. jehardiman
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    jehardiman Senior Member

    I think they are OK if you are working from available model test data and have a argumentative conclusion you want to reach (like any good teaching point). They allow those who haven't waded through all the hydrodynamics to get a feel of the complexities of resistance and differences between the two major hull regimes. But from a point of absolute correctness they are a bit dated; that said, they are consistent with most early modern (pre 1980's) texts and thoughts and perfectly adequate for most design work...with appropriate withheld reserve. ;)
    IIRC, "Principles of Yacht Design" dates from the late 1980's early 1990's and has been revised many times (Amazon offers a 2014 4th edition and there is a 2022 5th edition) .... this could be from an older edition.
    Also FWIW, the powering line for a displacement boat is rarely shaped like the first figure. As Gonzo and Kayakmarathon say there are more humps and shape to the line and it never becomes truly asymptotic as shown (it can't, the hull would cavitate). That's just a poor curve fit.
     
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  7. Driiftwood
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    Driiftwood New Member

    Thanks all!

    I understood from the image already the bottom part is from a book teaching boat design. I was mostly thinking perhaps the top part with the self destruct and we don't know lines was totally out of whack.
     
  8. jehardiman
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    jehardiman Senior Member

    Not really out of whack, but definitely provocative.
    There is no such thing as maximum hull speed...a vessel will go as fast as propulsion power will allow it. Additionally all hulls are supported by total pressure (static and dynamic) over its wetted surface in the water
    That said, all propelled vessels reach a point in speed (often referred to as hull speed) where there is a dynamic pressure crest at the bow and a dynamic pressure crest at the stern with a trough midships. These dynamic pressures (which form the visible wake waves on the surface but also exist subsurface) retard the hull and are caused by the four main inflection points in a typical hull, the bow, the forward quarter, the stern quarter, and the stern (see the Wigley Hull).
    What we call "planning" vessels have a short-wide-shallow hull form and a power to weight ratio that allows them to easily "climb up" out of this trough and balance (most of the time, hence the hydroplane comment) on this bow wave using their hull form and the dynamic pressures to support and "lift" them.
    What we call "displacement" hulls are longer-narrower-deeper which have a harder time "climbing" their bow wave. But many do, such as the old liners and many, even very large, combatants. The issue becomes one of power density to the dynamic pressure on the hull retarding the ship as the ship remains generally supported by the static water pressure. Given enough propulsive power, even a "displacement" hull will "lift" onto plane provided its hull form and the total pressures are stable.
    Finally, there are the special cases like hydrofoils, surface effect ships, and supercavitating torpedos.
     
  9. Driiftwood
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    Driiftwood New Member

  10. jehardiman
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    jehardiman Senior Member

    Not even a Flicka, but even a rowing dinghy towed behind a semi-displacement power cruiser. Recall my statement
    Most displacement hulls are stable "in" the water, not "on" plane.
     
  11. Ike
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    Ike Senior Member

    From the point of view of someone towing a Flicka (I'm retired Coast Guard) the real problem is not that it would plane or not plane. The problem is that they become dynamically unstable and begin yawing left and right and doing other crazy stuff, like heeling over to one side and then back to the other side. If this continues they usually end up destroying themselves. Typically when towing, no one is allowed to be in the vessel being towed, so there is no one to steer and keep it going in a straight line. This is because tow lines are notorious for snapping, and like a giant rubber band, on snap back, cleaning the decks.
     
  12. BlueBell
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    BlueBell . . . . .

    Ike,

    No polypropylene tow ropes in those days?
    No stretch, no snap-back, no deck clearing.

    Cheers
     
  13. jehardiman
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    jehardiman Senior Member

    No strength... and nylon has its own problems.
    I remember my Father (the on scene corpsman) talking about the USS Gilmore nylon rope accident in La Maddalena when I first started in the yards, two cripples and the XO was killed. Dacron only for mooring in the US Navy was mandated soon after that. There are reasons that ploypro and other things are not used for heavy load towlines.

    VMH: PAUL R. KLINEDINST, JR., CAPT, USN https://usnamemorialhall.org/index.php/PAUL_R._KLINEDINST,_JR.,_CAPT,_USN
     
  14. mc_rash
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    mc_rash Junior Member

    @BlueBell you never ever want to be in the area of a tow snapping back regardless the material.
     

  15. BlueBell
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    BlueBell . . . . .

    What causes snapback, stretch?
    No stretch, no snapback.
    (Thread drift!)
     
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