Theory of planing

Discussion in 'Hydrodynamics and Aerodynamics' started by sandhammaren05, Jan 13, 2019.

  1. sandhammaren05
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    sandhammaren05 Senior Member

    New ideas often come from outside an established field. Kolmogorov, v. Weizsäcker, Heisenberg and Onsager's four parallel papers on turbulence provide an excellent example. This particular paper ends with a discussion of the mistakes in the standard ideas of planing that float around unchallenged within the naval architecture literature.
     

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  2. CT249
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    CT249 Senior Member

    The paper says "A boat with no beam at the stern (fig. 1) cannot plane and is thereby limited to very low speeds".

    I have shown pics of several craft with no beam at the stern that DO plane and can plane at over 30 knots. This pic is of a windsurfer of a type that did over 40 knots when sailed by Eric Beale. It has no beam at the stern but it does plane.

    [​IMG]
     
  3. CT249
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    CT249 Senior Member

    The paper says "Sailboats with the appearance of a transom generally have a strongly rockered bottom so that the transom is always above the waterline."

    This appears to be incorrect, or at the very least unproven. The transom of J/24 (world's most popular racing yacht) is not 'always above the waterline'. It's the same with the Catalina22, the most popular yacht. Nor is the transom of the Laser or Optimist, the two most popular dinghies. Even the original Windsurfer, still perhaps the most popular sailboard, is not always above the waterline at the transom.

    Where is your evidence for this claim?
     
  4. Earl Boebert
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    Earl Boebert Senior Member

    Plus "Baby Bootlegger:"


    [​IMG]

    Cheers,

    Earl
     
  5. DCockey
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    DCockey Senior Member

    Previous thread which was also started by sandhammaren05 with the same paper attached in the original post: Lift onset, planing, and lift coefficient https://www.boatdesign.net/threads/lift-onset-planing-and-lift-coefficient.60847/ Related is this thread started by sandhammaren05: Froude and planing https://www.boatdesign.net/threads/froude-and-planing.57340/ A review of these threads may lead to questions and possibly conclusions about the source of "mistakes".
     
  6. Joakim
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    Joakim Senior Member

    Here we go again! You are still writing about "onset of lift", Kutta condition, no lift with backflow although in previous threads your theory about those have been clearly demonstrated to be false.

    Now you have studied some work of Savitsky, but claim he has it all wrong. Although his model is known to work well and used by many to design planing boats, including racing boats. You claim that "Savitsky's formulae for drag ignores lateral wave making and spray, which are the main causes of drag". Wave making is included in drag since it is caused by pressure and pressure drag is included in the model. Spray and aerodynamic drag models were developed later and presented in this paper.

    Savitsky was mainly interested in rather heavy boats at relatively low speed. For those whisker spray drag is not important. It is more important for high speed racing boats, which seems to be your interest. For those the basic Savitsky model will fail, since it assumes that chines touch water at the transom. This can be quite easily dealt with.

    Note also that Savitsky model is mostly a fit to experimental data. It has a lot of experimental proof. And naturally he does not mention "onset of lift", which doesn't exist in experiments nor in theory like you decsribe it. Nor does he mention Kutta condition, which is not related to the lift of a prismatic hull.

    You also mention a boat that porpoises at some speed, but then stabilizes at higher speed. Savitsky model can predict that. At least for the two boats that I have tested know to behave like that. One of those was mine, 3.8 m long and 48 knots top speed with 40 hv engine. It porpoised rather wildly from 15 to 30 knots, but was stable above that. This behavior and the top speed was quite accurately predicted by my Savitsky model based program (includes aerodynamic drag, whisker spray drag, drive leg drag and chines off the water).

    Yes Savitsky method is quite complicated to use, but in 1964 computers were not really available. Applying the method as a program makes it efficient and simple to use. In order to understand what is going on, you must make several runs. E.g. for optimizing LCG, deadrise etc.
     
  7. sandhammaren05
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    sandhammaren05 Senior Member

    Cheers yourself. Look at a picture of the bottom. Fooled by a photo.
     
  8. sandhammaren05
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    sandhammaren05 Senior Member

    How's it run with a motor?
     
  9. Earl Boebert
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    Earl Boebert Senior Member

    Well, if you want to claim a step is a transom, I stand corrected.

    Cheers,

    Earl
     

  10. sandhammaren05
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    sandhammaren05 Senior Member

    It's clear that the step where the transom should be located serves as the transom: it provides the sharp separation of the flow from the bottom necessary for lift. Kutta condition, etc. This boat is a very dumb design, which is why it was not produced later. The idea of having a lizard tail sticking out behind the transom to add weight and drag water is silly. Crouch's formula for speed-power-weight is also wrong. I have derived the correct scaling law from hydrodynamics.
     
    Last edited: Jan 20, 2019 at 4:09 AM
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