Definition of Planing

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

  1. Leo Lazauskas
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    Leo Lazauskas Senior Member

    We have discussed this in several other threads, but this seems a good place
    to put the latest iterations.

    I recently approached some of the world's leading hydrodynamicists
    and naval architects for help with a definition of "planing", with a
    view to improving the present ITTC proposal, which is:

    "Planing hulls are those hulls that as the speed increases, under
    the effect of the dynamic pressure which develops on the bottom,
    undergo a remarkable reduction of the hull volume."

    Unhappy with that definition, I proposed, as a starting point:
    "Planing is a state where a vessel's weight is supported more
    by hydrodynamic forces than by buoyancy."

    Most respondents agreed with that simple definition, but added
    some interesting qualifications as shown below. (I have anonymized
    the responses, but thanks to AdHoc and others for their help.)

    I haven't combined the responses into something succinct enough for
    the ITTC dictionary yet, firstly because I'm not sure if it can be
    done and, secondly, because I'd like to get some more input.

    Now, read on...

    ====================== RESPONSES =========================
    I believe that your definition: "Planing is a state where a vessel's
    weight is supported more by hydrodynamic forces than by buoyancy."
    is the best that I can think of. One could add a few sentences
    noting that planing
    (a) Usually applies to a vessel with a relatively flat bottom,
    (b) Usually has a transom stern accompanied by clean separation, and
    (c) Is traveling at at a high speed.

    My thinking is that the word "planing" implies that the bottom of
    the hull is reasonably flat and this feature is hydrodynamically
    significant, in that is provides lift. All the other aspects are a
    secondary feature.

    Perhaps it would be appropriate to refine (a) above, replacing
    "relatively flat bottom" by a phrase which includes the hull shapes
    of boats which sail in the planing regime, and
    (c) replacing the words `high speed' by `Froude numbers above
    approximately 0.5'

    I would change (a) to say, "relatively flat rocker aft".

    There are a great number of boats that plane early and are round
    bottom. Most of the West Coast Sleds are round bottom narrow boats
    but have relativity flat rocker from the mast aft. Forward of the
    mast they have more rocker to keep the knuckle out of the water when
    planing, and aid in going up wind in chop.

    When you say 30' boat like Kincora I will assume 27.5' LWL.
    Kincora is up and planing at 7.8 kts of boat speed. At that point her
    wake goes flat and there is a clean run off. Also at that point she
    can accelerate quickly to higher speeds with little effort.

    It is difficult to compose a simple and precise mathematical
    definition for "planing". Experience has shown that the following
    definitions can be useful to identify if a craft is planing:

    1. If the length-based Froude number is greater than 0.4.
    F = U/sqrt(gL) where L is a the length of the vessel. There are many
    cases in which the length-based Froude number is greater than 0.4 and
    the vessel is not planing. But, it would be rare that a vessel that
    is planing is traveling at a speed of less than 0.4.

    2. The beam-based Froude number is greater than 1. F_B = U/sqrt(gB),
    where B is some measure of the beam of the vessel This can be observed
    in Figure 17 of the seminal paper by Savitsky. In this figure, the
    center of pressure is plotted as a function of beam Froude number for
    different values of the wetted-length ratio. If the vertical force is
    dominated by hydrostatic pressure, the prismatic hull form will have a
    center of pressure of 1/3 L from the transom. As the vertical force
    becomes dominated by hydrodynamic pressure, it will move closer to
    3/4 L from the transom. This is seen to occur in the range of
    1 < F_B < 3, depending on wetted length ratio.

    3. The volumetric Froude number is greater than 2.
    F_V = U/sqrt(g V^(1/3)), where V is the volumetric displacement (at
    zero speed). This quantity is widely used since it incorporates both
    of the previous length scales of L and B. The landmark experimental
    reference of Clement and Blount uses this to present their
    comprehensive data. It is observed that over a wide range of
    geometrical parameters such as L/B, lcg, and ratio of displacement to
    water-plane-area, that near F_V of 2 a hump in the specific resistance
    occurs, and for speeds greater than this the hull is planing.

    Your simple definition which is succinct, is ostensibly correct. Trouble
    is in trying to define it a bit more precisely or scientifically is
    open to endless caveats or disclaimers rendering the modified to the
    “try again” pile.

    Perhaps the best approach may be first to list down certain conditions
    that must exist, which everyone can agree on, that would only exist
    under conditions of what even a layman would call “planning”.
    For example;

    1) More weight supported by hydrodynamic forces that hydrostatic.
    2) A virtual rise in the VCG, from its static datum.
    3) A ventilated transom
    4) The flow of water on the hull bottom is generally transverse rather
    than longitudinal.
    5) A changing in waterline length from static to running

    Etc etc…and so on.

    Then may be try to remove those which could also be considered to be
    definitions of semi-planing and/or confusing and see what is left.
    Then try and form some single definition based upon the remaining list
    of behaviour characteristics of planing hulls.

    Other than that, I think your definition is as best as could be written
    without causing any confusion or misdirection. It may not be perfect,
    but it is correct.
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  2. DMacPherson
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    DMacPherson Senior Member


    My two cents...

    First, my personal opinion is that such a definition needs to be completely quantifiable. We struggle when we try to define something like this with a subjective shape description or performance attribute.

    a. If you are looking for a "snapshot" definition of whether a boat is fully planing, I use the vertical position of CG. It is planing if the dynamic VCG is higher than the static VCG. This corresponds to your comparison of buoyant and dynamic forces, but is easily measurable.

    b. If a range of speed data is available, I then tend to add a "comparative" measure with trim, in that if the VCG test is confirmed, it really is fully planing when the trim begins to decrease after reaching a peak. Again, fully measurable and quantifiable.

    Attached is a graph I made up for an IBEX presentation some years ago on "Hull Form Hydrodynamics".


    Don MacPherson
    HydroComp, Inc.

    Attached Files:

  3. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Thanks, Don. Your opinion is worth at least 500% more than 2 cents! :)

    I agree that having something measurable is very useful.
    That's why I started with the 50% dynamic support idea, but it is not
    (as I think you imply) an easy quantity to measure.

    How do you measure the location of the VCG on the water?
    Is is relative to the undisturbed free surface?
    I ask that because it is not easy to measure squat and the location of
    the free-surface because of, among other factors, the "moving dish"
    effect you have referred to previously.

    Dr. Tim Gourlay and his colleagues recently used GPS to measure
    the sinkage, trim and roll of a large ship in Hong Kong. I doubt
    that the method is accurate enough for small vessels, but the
    attached paper might be of interest to you and other "practical types".

    All the best,
    Last edited: Aug 12, 2015
  4. DMacPherson
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    DMacPherson Senior Member


    We were peripherally involved in some RTK (real-time kinematic) GPS studies for underkeel clearance some years ago, and the results were impressive. A pain to set up and calibrate, but still very useful.

    I guess I start with the idea of "proving" the test in the tank, where VCG from original static position is easy to measure. Then proceed to in-field tests as measuring capability becomes available. I currently use photographs from some distance to get a reasonable profile and free surface. Then I use target points on the hull to derive VCG position. I bit crude, and not something that anyone really needs all that much, but it works.

    (We now have a prediction of VCG rise in our newest version of NavCad. It is simply based on the geometry of predicted wetted keel and chine lengths, with corrections for real geometry. Our validation is showing that it is pretty good, but we are still looking for more test data for me to be fully satisfied.)

    Regarding the "moving dish", I think this is an attribute of large ships, and not something that would be of any big concern in planing. I have never seen or read anything that suggests there is enough of a low pressure zone under a planing hull to develop a significant reduction in the surface profile.

  5. latestarter
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    latestarter Senior Member

    Speaking as a layman I am apprehensive about joining in with the experts but a couple of thoughts struck me.

    Is there anything special about >50% of the weight being supported by hydrodynamic forces.
    The responses often try to indicate how it is achieved rather than what it is.
    The comments also give me the impression that people know planing when they see it but trying to define it other than in general terms may not be possible.

    Surely the dynamic VCG will start rising when only 1% of the weight is being supported by hydrodynamic forces.

    Leo, what do you see the benefits of an agreed definition will be?

    Anyway the last time I planed was in the 1980s :D
  6. Doug Lord
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    Doug Lord Flight Ready

    I have seen many posters(particularly on SA) refer to a hydrofoil that is lifting as being "on the plane". Perhaps, to not feed such nonsense, the rise in VCG from static to "planing" should always be accompanied by some verbiage that is only true of a boat that is planing, not flying on foils?
  7. sottorf
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    sottorf member

    my comments as follows:

    I am not sure I would agree with comment (a). Planing hulls come in so many shapes and it is possible to make a round bilge hull plane (according to your basic definition). I would therefore be cautious to use (a). I think the only universal characteritic of a planing hull is that they all have a transom stern which allows flow to break off cleanly.

    I would propose to remove (a) completely. One is not really saying anything by using the words: "the hull shapes of boats which sail in the planing regime"

    Regarding (c), it would be better to use volumetric Froude number instead length Froude number as waterline length is not a constant value for a planing boat and has little meaning. That being said, I dont belive it is possible to define a specific Froude number above which all hulls will be planing. A light weight flat bottomed hull would plane easily at FnD = 2.5 but a heavy hard chine catamaran might only start planing at FnD =3.5

    I do not think this adds value to the definition: flat rocker relative to what? Also there are a number of planing hulls that use rocker in their design and stepped hulls sometimes have quite large angles aft of the step. See attached paper for some information on rocker of planing hulls.
    In general I agree. I however believe that nearly all added conditions can be distilled down to the original definition that weight is supported more by hydrodynamic forces than hydrostatic ones. The main problem is this is not easily measurable (as mentioned already). Any additions to the definition that make things more measurable would add value. I can think of the following which would hold universally true:

    - rise in the CG
    - ventilated transom

    Attached Files:

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  8. Leo Lazauskas
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    Leo Lazauskas Senior Member

    I'm not happy with the proposed ITTC definition. The ITTC dictionary is a
    terrific resource, but it needs a few rough edges knocked off.

    In part it's an exercise to see if there is any consensus among some old
    colleagues, and also to see what a wide variety of people think.

    Also, I'm sick of seeing charlatans, salesmen, and fools claiming almost
    magical powers for their boats because they "get on the plane early" when
    they are doing nothing of the sort.
  9. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Thanks, Gunther.
    Personally, I'm not really wedded to anything except the "50% rule".
    I agree that anything measurable would be a great bonus.

    Incidentally, anyone can edit the ITTC wiki, so if you or others here see something
    barbaric, or that needs fixing, please have a go. (I'm a bit of a tragic in that if
    I see an editing mistake, I have an urge to fix it.)
  10. DMacPherson
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    DMacPherson Senior Member

    The best analogy I have found in describing the transition of a boat to planing for the "cocktail party" crowd is an airplane flying upside down.

    1. At low speed, the boat is fully supported in its vertical position by buoyancy.
    2. As a boat increases speed, curvature of flow actually creates a downward force - much like the lift on an airplane wing, but upside down.
    3. This downward force (inverted lift), which is predominately toward the stern, causes stern down (bow up) trimming of the hull.
    4. This trim can be treated as an angle of attack that causes an upward lift of the hull (the dynamic force that is being discussed here). This mode is just like the inverted airplane - while lift from the wing's camber wants to pull it down, it is offset by the lift from angle of attack that wants to pull it up. If you have seen an inverted airplane fly, you will recognize the very steep angle of attack that is required to keep it airborne.

    So, any rise in CG comes from sufficient dynamic lift (and remaining buoyancy) to overcome craft weight and the downward stern trimming force. This, to me, is as suitable a criteria as any other to define an arbitrary point in a rather noisy physical system.

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  11. sottorf
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    sottorf member

    I would only agree with Don's comments for a prismatic planing hull If the hull has rocker, hook or variable deadrise the tendencies can be quite different. I therefore think this is not a universal measure of planing.
  12. BMcF
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    BMcF Senior Member

    As just a personal observation...the transoms on many non-planing hull designs can be and often are fully ventilated. That criteria does not seem to me to be a very strict differentiator therefore..
  13. Mikko Brummer
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    Mikko Brummer Senior Member

    With ref. to your illustration, would your criteria for planing be when the VCG has risen sharply over it's static value (after the initial drop), and starts its linear, slow rise? Which is whem trim starts to go down? Does the trim always start to go down when "real" planing has started? Wouldn't that be much easier to measure than VCG movement?
  14. DMacPherson
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    DMacPherson Senior Member

    I think we all first need to agree that there is no one binary point of planing. It is not either on or off. It is a transitional regime, and therefore, any attempt to define a point of planing is somewhat nonsensical.

    Therefore, the pragmatist in me looks to something that is a) quantifiable, b) measurable, and c) with a clearly defined criteria. For example, characteristics of hull shape are largely subjective and are quantifiable only to geometry, not performance. VCG and trim are quantifiable and measurable, while forces and pressures less so. The big hurdle is how to handle the criteria of VCG and trim, thus the two points that I use - when VCG is above its static position, and trim hits a peak.


  15. sottorf
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    sottorf member

    very good point. a ventilated transom does not imply planing but a non-ventilated transom precludes planing.
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