Definition of Planing

Discussion in 'Hydrodynamics and Aerodynamics' started by Leo Lazauskas, Nov 2, 2012.

  1. sottorf
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    sottorf member

    Don, how do apply you trim criteria for a non-prismatic planing hull?
  2. Leo Lazauskas
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    Leo Lazauskas Senior Member

    I agree that planing is not a clearly defined point on a curve, but,
    at least for 2D, the transition to planing seems to occur quite quickly.
    See, for example, Figure 6 of the manual for the simple planing program

    In that figure, FrX is the Froude number based on the distance from the
    trailing edge to the centre of pressure. This is a clearly defined
    distance (mathematically) because the moment of the lift must act at the
    centre of mass of the vessel for it to be in equilibrium.
    alpha/alpha0 is the ratio of angles required to maintain constant lift
    (= vessel weight), and it can be seen that transition is accompanied by a
    large change in the trim angle, which accords with part of your criteria.
    (And so I think you are definitely on sound ground with your method.)

    I'm not sure if the transition occurs quite so quickly in 3D.
  3. Kestrel
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    Kestrel Junior Member

    I substantially agree with response 4.3. Even if different hull shapes could have a slightly different behaviour (depending on a lot of factors: deadrise angle in planing surface (s), beam, aspect ratio, trim angle (s), Lcg positions, displacement, stepped or unstepped, etc.), for sure when volume based Froude nr. crosses a value of >2, a suited hull (hard chines, or stepped, or both, for example), with positive angle of attack, normally developes at least 50% or more of total lifting forces by hydrodynamic pressure on the bottom. So we, also conventionally, assume that FnV value of "2" as indicator of planing condition. "Fully planing" could be when "the residual hydrostatic lift becomes negligible with respect to the Hydrodynamic lift", and it occurs normally when FnV>4.5-5. Sure a rise in CG position accompains the penomenon, and it is an indicator too.
    More difficult is to synthetize all this in simple mnemonic definition....


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  4. daiquiri
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    daiquiri Engineering and Design

    Have you noticed that they didn't define neither the term "Displacement" yet? :)
  5. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Well spotted!

    They do use displacement volume in, for example, the definitions of prismatic
    coefficients, and several other places.
  6. Richard Woods
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    Richard Woods Woods Designs

    And as far as I can see they haven't defined Stability or Buoyancy either, or is that too obvious, or am I missing the whole point of the wiki?

    Richard Woods
  7. Leo Lazauskas
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    Leo Lazauskas Senior Member

    It is a work in progress.
    One day they will get around to adding those. Unless you want to do it :)
  8. Petros
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    Petros Senior Member

    I see no reason for any of the qualifiers at all. many shapes can plane to one degree or another, no reason to put those conditions on it. I also do not see why 50 percent is the magic number either, if any portion of the weight is lifted out of the water by hydrodynamic lift you will reduce the drag and increse speed.

    I would say than planing occures if any of the weight is suppored by the dynamic forces on the hull from the water. A little or a lot of the weight being supported is illrelevant, the fact that the hull is not fully in displacement mode means that lift forces on the hull are partially supporting the weight of the boat.

    I think the real problem is the word "planing" itself is obsolete and based on archaic ideas about fluid mechanics. The origin of the word assumes it is even possible to be above or "on" the plane of the surface. We know a lot more about the process and forces involved but are stuck with these obsolete terms.

    how much of the displacement volumn must be above the surface to be "on plane"? all of it, most of it, half of it, any of it? since planing begins to occur when hydraulic forces lift the hull, how much until it is actualy "planing"? Any lift replaces displacement forces with dynamic ones, so the begining of planing occures when there is any lift not associated with displacement.
  9. Ad Hoc
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    Ad Hoc Naval Architect


    Other "observations" to throw more into the mix, is the clearly obvious change in wash. The classic Kelvin wave pattern and its associated angle of occurrence is changed.

    I've searched through endless research papers, Journals and books I have in my library. Not one is conclusive nor consistent either; save for the opening one liner you have given. It seems this 'simple definition has eluded many for many years.. :eek:
  10. Yellowjacket
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    Yellowjacket Senior Member

    As per this paper by Kukner and Yasa

    they are describing that planing occurrs at Frode numbers above something like 1.0 and 1.2 and that semi planing occurrs between .4 and 1.0.

    In reality when the hull obeys Savistkys' calculations for drag, it's planing. While the hull may be heavy enough that it still has a significant displacement boyancy, at that point it's planing. Savistkys' calculations tend to fall apart in the semi-planing regime so to my mind if the basic Savistky analysis predicts the performance, or at least the change in performance as the speed increases, then that's planing.
  11. Leo Lazauskas
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    Leo Lazauskas Senior Member

    And I would contend it is of interest to know at what Froude numbers that
    lift becomes positive. At low Froude numbers the force is downwards, and so
    it is also of interest to know when the force reaches a minimum. Finally, it is
    also of interest to know when dynamic forces dominate buoyancy, hence
    why the 50% criterion comes up in some books and papers.

    I guess you could argue that they are just arbitrary points on a continuous
    curve, but they can also be useful landmarks if they can be related to hull
    geometry and speed.
  12. daiquiri
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    daiquiri Engineering and Design

    As DMacPherson has noted, the problem here is that "planing regime" is not something that happens all of a sudden. It's more of a psychological fact that, at some point, folks on the deck realize that they are not moving through the water the same way they moved at lover speeds. It feels more like flying over the water at that point. But when exactly did they start to feel like flying, nobody will ever be able to say with sufficient precision. Different hull types will cause different sensations to the people on board.

    The only solution I can envisage is to define planing regime by convention. Like a definition of a meter or of a degree Centigrade or Fahrenheit. The DMacPherson's VCG criterion looks like the best candidate to me, because it is more easy to measure than the ratio of dynamic to hydrostatic lift, yet is related to the latter quantity. It just requires a precise definition of the point above the WL where we want to place the beginning of the planing regime. Of course, the VCG heights would have to be normalized to some reference length - be it the LWL, BWL, or sq.root of some reference area - because 0.3 meters is not the same stuff when talking about a 6 m powerboat or a 50 meters long ship.

    Getting back to the subjective feeling of the persons on board, I believe that anyone who has been on board a planing vessel can witness that it's movements became "stiffer" and more jerky when encountering waves at high speeds than it was during low-speed navigation. A mathematical explanation for this behavior is that the perturbation of hydrostatic lift component is in linear relationship to the vertical speed of the incoming wave disturbance, while the dynamic lift component increases as a square function of the vertical speed of the perturbation. Hence, the vertical accelerations become much more severe in high-speed regime.
    It makes me think that perhaps it can lead us to a completely different criterion for the definition (by convention) of the planing, based on the vertical acceleration response of the vessel to incoming waves, or to a single standardized perturbation which could be reproduced in towing tanks. The planing regime would then be indicated during sea trials by the on-board accelerometers, rather than through GPS or VCG measurements.

    Just some food for you guy's creative minds. ;)

  13. tom28571
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    tom28571 Senior Member

    What is the fascination with a pining down a definition of planing as a point where the hull is no longer a displacement one? Is such a definition necessary to design boats, whether displacement, planing or in between the two regimes. The need for an in-between regime should be a hint that no such inflection point definition means anything of importance. I doubt that any of the proposals will properly suit even a majority of all possible hull shapes and other vessel characteristics.

    I think only an arbitrarily agreed upon definition can be useful in discussions between designers and, as such, I prefer the 50% displacement/dynamic ratio.
  14. TeddyDiver
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    TeddyDiver Gollywobbler


    To avoid the problem of transition point btw displacement and planing modes maybe use more quantitive description of planning so it could include different forms of semiplaning, like 60% planning??

  15. Leo Lazauskas
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    Leo Lazauskas Senior Member

    As I said earlier, this just started as an attempt to fix the ITTC definition.
    In some ways it is like trying to pin down the difference between laminar flow
    and turbulent. (Thanks to AdHoc for the analogy). We all know the
    difference, but there are times when it is important to make the distinction,
    and othertmes when it is irrelevant.
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