Definition of Planing

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

  1. idkfa
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    idkfa Senior Member

    I think the definition of planing should be split into planing and forced displacement versus trying to lump semi-planing into planing.

    This keeps the definition of planing is simple, "it is what a speedboat, windsurfer or skiff does", "it clearly can be seen to bouncy on the surface." Try claiming that with a keeled monohull or multihull.

    Also using semi implies an undefined other half, semi-planing is half planing, but half of what else?

    So we can have, by definition of Modes of Travel: Displacement -> Forced displacement -> Planing

    Then we are left easily observable defining characteristics for forced displacement
    1) speed (more than hull-speed, ie lots more force needed, like a spinnaker)
    2) ventilated transom (we are able to break the free of water closing back around us)

    These definitions can be for the benefit of the layman and marketing individuals as much as there are for us, and thus should not really have %lift or VCG or trim qualifications. Those can be used to define, the degree of forced displacement.

    sorry for the spanner, my most humble 2cents.
     
  2. DMacPherson
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    DMacPherson Senior Member

    First, I am not advocating acceptance of any particular definition. As I said, it is an arbitrary point in the noise.

    However, I do have experience in such definitions, and first establishing the process by which you find a suitable definition (whatever that's worth) is useful. Thus, my three initial requirements for a definition - quantifiable, measurable, and against an obvious criteria. I live with subjective terms (e.g., "U"- or "V"-shape sections, "cruiser" stern) and they often lead to less clarity. Such terms are not universal or well-behaved if your intent is to establish some measure of the "incidence" of performance. Stern shape description is a great example of a very subjective description, and to counter this, we have developed a numerical calculation for a stern shape "factor" based on geometric quantities (e.g., CVP, CWP) that are the same for anyone looking at a hull.

    Observations such as "it looks like planing" are not meaningful at all. The point is to quantify performance, not describe it.

    Daiquiri: I would not use the WL as the VCG criteria, but the static VCG. In other words, the criteria is a rise of zero units above static VCG, therefore removing any need for scaling.

    Similarly, the trim criteria is where it hits a peak, regardless if it dimensional value. You can define it as the point where the slope of the plot of trim versus speed is zero, if you wish. This trim metric is not affected by rocker (yet another descriptive term). There may be some initial trim "correction" (drift) that occurs due to the definition of baseline, but the dynamic change of trim after the boat is moving is the key. (In fact, we reset all trim calculations to a chine line ordinate, rather than a baseline ordinate for just this reason.)

    Don
     
  3. Ad Hoc
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    Ad Hoc Naval Architect

    In general I would agree with this. However, the problem is, is it important?

    For example:-
    If, during tank testing (as the only really effective means of obtaining all such data), a hull was say 5% short of satisfying said criteria yet meeting the required contract speed, would it concern you?..probably not. But a client who witnesses the tank test may then pluck out his/her smart phone and quote the “prescriptive” definition of planing from their "ittcwiki" app and point out the hull is deficient by 5% and the contract clearly states, the hull must be a planing hull and thus it fails to satisfy the contract and is rejected.

    I’ve sadly dealt with such clients before, not just once either!

    Yet why is the corollary not applicable too? By that I mean…when “we” talk about a displacement hull…how often does one quote Archimedes when defining a displacement hull form, to ensure the correct definition is used? Yes, we can define it exactly if required, but how many times is it quoted when discussing such terms to satisfy a query, even in the mathematical sense? It is generally "understood"...so, why not planing?

    These may not be meaningful, in terms of an exact quantitative prescriptive definition…but, is being “exact” in this case strictly necessary?

    I think that is half the problem..there is a lot of “noise”, which makes it rather tricky :(

    Exactly, thus the dilemma.
     
    Last edited: Nov 4, 2012
  4. DMacPherson
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    DMacPherson Senior Member

    I completely agree that a generic description of planing is useful. However, the original purpose of the thread was based on ITTC trying to quantify the term. As we have seen, this is a challenge.

    There are any number of ways that one might quantify planing performance, even beyond what has been discussed so far. One could use parameters of the bottom surface pressure field, or even when drag is reduced by shallow water effects (rather than increased as is the case for displacement hulls).

    Ad Hoc, your example about missing a criteria in a tank test is probably not a weakness of a criteria as much as it is failure on the part of the parties involved to properly establish what is the important metric. If a boat makes contract speed, does it really matter if it is "planing" based on some criteria? If for some reason the planing criteria was important by itself (and I cannot think of why it would be), then meeting the criteria would be proper as a contract metric. Otherwise, the parties should use the metric that defines expected performance (speed and/or trim) versus something that infers performance (planing).

    My world is not going to change whatever ITTC decides to do, but as someone that deals with parametric descriptions and their implications to a variety of audiences (our software customers), it would be a shame if a definition of an estimate for "incidence of planing" were to provide less clarity than more. Just think about "propeller slip" and "hull speed" as two examples of performance descriptions that in many cases cause more harm than good.

    [This is a side bar comment. For a rough estimate of the speed of incidence of planing, we use a very simple formula that relates LCG (from transom) to beam of the bottom planing surface. The idea is roughly that LCG and beam are parameters that crudely describe a hull shape. It presumes hard chines and transom. We worked it up years ago, and it has been shown to do a pretty fair job: Vkts = 7.2 x LCGm / Sqrt(BCHm).]

    Don
     
  5. gonzo
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    gonzo Senior Member

    I think that at the end, it is a matter of semantics and not physics or engineering. It would be better to say, for example, "this vessel at a speed of 54 knots has 93% dynamic lift to displacement ratio", rather than argue whether it is planing or not.
     
  6. Leo Lazauskas
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    Leo Lazauskas Senior Member

    The difficulty is in measuring that displacement ratio.
     
  7. Leo Lazauskas
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    Leo Lazauskas Senior Member

    The ITTC is NOT involved in this in any official capacity, except that it is the
    owner of the wiki.
    I took it upon myself to edit some pages, noticed there was no definition of
    planing, and so I asked some old acquaintances and experts for their opinions.
     
  8. BMcF
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    BMcF Senior Member

    I agree with that to the extent that the "lift fraction" (what I've always called it) is a fundamental design parameter for so many vessels we deal with..SES, hydrofoil-assisted, planing..and hybrid combinations of those. Ring around the old "sustention triangle" again.
     
  9. Ad Hoc
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    Ad Hoc Naval Architect

    I agree that bottom surface pressure, perhaps a function of its WSA v speed must be more than unity or a given fraction of unity. The unity Being related to vessel weight.

    Unfortunately as for the other, again this is where being too prescriptive falls foul. We have designed many displacement hulled vessels whose Fn is above 1.0 where their drag is reduced in shallow water, not increased. It also shoots holes in the rise in VCG and trim aspect too....thus bringing in shallow water effects makes matters worse, not better:

    speed v trim-1.jpg speed v trim-2.jpg speed v trim-3.jpg

    I think we may end up like Einstein in looking for his theory of everything...almost there, but just not quite.
     
  10. markdrela
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    markdrela Senior Member

    Out of curiosity... Why is it important to have a precise definition?
    Surely it will be arbitrary to some extent.
     
  11. gonzo
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    gonzo Senior Member

    There are unending arguments over whether a boat is a planing, displacement or semi-displacement. From the legal point of view, it could be a failure to comply with a contract if it was written with one of those terms as a condition. Otherwise, it may not be as important. Most of us use the terms loosely in casual conversation. Almost no-one argues that a 25 foot boat running at 40 knots is planing. Or that a 25 foot boat running at 3 knots is in displacement mode. The heated discussions are about the transitions.
     
  12. Ad Hoc
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    Ad Hoc Naval Architect

    Certainly not "heated" on this thread. It is emotiveless on this one, thankfully.

    Indeed. But, wouldn't it be "nice" to have a definition that everyone could agree on, that could be used to help those unfamiliar wit the concept? And as Leo pointed out once before, to stop charlatans. There are plenty of snake oil salesmen out there...I've meet a few of them; very unsavoury characters!. They capitalise on this lack of any definition etc to further themselves and pull the wool over the eyes of those less informed. Which i suspect is the main motivation for Leo behind this. Which I full concur with....not saying it shall be easy though :(
     
  13. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Thanks, Mark.
    I doubt that a precise definition is possible, but I thought we could improve on the ITTC's proposed one, namely:
    "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."

    Maybe something can be cobbled together from the responses I got from my acquaintances, and from some of the more astute comments here.
     
  14. Leo Lazauskas
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    Leo Lazauskas Senior Member

    I agree that most of us know whether a given vessel is planing or not, or
    whether it is in a transition phase, but that is not good enough for a
    "dictionary".
    I'm sure we can nail down some conditions that are clear indications that a
    vessel is planing, and others (like a wet transom) that mean it is definitely
    not.
     

  15. Perm. Stress
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    Perm. Stress Junior Member

    To me, "50% dynamic lift" definition seems to be the most consistent.

    *There is no place for different interpretations.
    *The only difficulty is in measuring the parts; however:
    1 we can measure raise of CG and change in trim of running vessel,
    2 from those, we can easily calculate the part of volume below the undisturbed water surface,
    3 that volume gives us weight, supported by hydrostatics,
    4 the rest is dynamic lift,
    5 when dynamic part is just over 50% of weight, we are planing.

    So, we have "easily" (or "just" :D ) measurable criteria

    *The "50%" is of course arbitrary, but intuitively "half-and-half" fits pretty well. A few % to this or that side do not change clear and hard principle.

    If we need a hard definition, my vote is for "50% dynamic lift".
     
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