Definition of Planing

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

  1. Yellowjacket
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    Yellowjacket Senior Member

    The reason that planing sailboats don't have much, if any trim change is that the driving force is applied quite high above the waterline, which pushes the nose down sooner. That doesn't mean that the hull wouldn't behave like a normal planing hull if it was powered, it just means that there is an outside force acting above the waterline that pushes the nose down earlier (and actually helps the boat plane earlier by increasing the planing area). For that reason a sail boat may not behave the same way as a powerboat, it just means that the outside force is changing the trim angle artificially.

    In addition, the crew will act to move their weight forward to get the boat on a plane, and then move aft to move the CG back as the speed increases, again basically changing the configuration so that the trim change can't really be observed.
     
  2. gonzo
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    gonzo Senior Member

    Long narrow hulls sink as speed increase. The early attempts at speed records show that some where powered to the point of submerging.
     
  3. tom28571
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    tom28571 Senior Member

    Gonz, I know you were not fully awake when you wrote that. Its a very common occurrence with displacement boats and is not related to long narrow hulls but to their bottom shape.
     
  4. tunnels

    tunnels Previous Member

    Why ids there no simple way of saying a hull is planing or not planing . as a hull increases in speed the tumbling water lapping at the transom gradually decreases to the point where the speed of the water flow coming out breaks free
    As a dumb boatie person this has always been understood as the planing speed at that moment the tumbing water leaves the transome !!,
    As a hull slowly comes off the plane the tumbling water behind the boat eventually finds its way back to the transom and eventually touchs !
    Trim tabs and power trim on outboards can change the angle of the hull and can raise or lower the speed at which all this takes place . Some big boats because the way they are made with everything stuffed in the engineroom aft dig a enormous hole as they try to get to that stage of loosing the tumbling water behind the transom !!, but they still not over the hump !!,over the hump is something else and takes lots power in big boats to struggle over that hump ! .:D
    But with lots power they literally jump out of the hole ,so thats another whole differant thing !. :idea:
    But what do i know ??
     
  5. daiquiri
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    daiquiri Engineering and Design

    Hey Tunnels, nobody is saying that you don't know your stuff. But... Just like you did in that thread about aerodynamics of powerboats, you are considering only one type of boats and disregard all the rest. And you probably haven't read all the posts in this thread before jumping in.

    Earlier in this thread the dry transom has been discussed, and the example of very slender hulls was brought up. Think about fast catamarans, which have slender (low length/displacement ratio) hulls. They can go pretty fast and have transoms dry, yet they do not plane because their hulls have too little waterplane area to get sufficient hydrodynamic lift force. They are moving in the so-called fast displacement regime. So the dry-transom criterion is not enough to cover all types of hull forms.
     
  6. tunnels

    tunnels Previous Member

    During my yachting days 1970s ans 80s Mr Laurie Davidson used to spend hours looking at the back of his yachts watching the water leaving hull Later designs changed as speeds increased became faster and they shed water out the back just like powerboats do now . But as for burying the bow never seen that happen ever . the bow is fuller and more bouyancy so as speed increases so bows lift rather than going down . The last 8 match racing yachts i was involved with these boats were so quick !!,we lost a couple of guys over the stern because of the boats acceletaion and they werent hanging on !!
    I wanted to Waterski behind boat one but no one had any water ski's !.
    Yeah understand the skinny hull thing with sailing cats !!. :D
     
  7. gonzo
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    gonzo Senior Member

    Tom: I know it is not just long narrow hulls. However, that was the shape the first attempts at high speed hulls had. They were attaining speeds way over what is now accepted as "displacement hull speed", but they would become submarines eventually.
     
  8. tspeer
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    tspeer Senior Member

    The drop in trim angle is typical of a powerboat when it gets on the plane, but it is not required to be planing. A sailboat may not exhibit this sort of trim change at all. I think we'd agree that this boat is planing, but it could hardly be said to have a low trim angle.
    [​IMG]
    It would have a much lower trim angle when not planing and sailing at displacement speeds.

    The change in trim angle with planing for a powerboat is due to the relationship between the thrust line, center of gravity, and the aft shift in vertical force on the boat as it starts planing. The sailboat above has an entirely different thrust line, so it has a different change in trim with speed.

    If you're trying to define a hydrodynamic phenomenon like planing, it shouldn't depend on the configuration of the boat. It should be applicable to all boats.

    Other observations that have been proposed, such as trim change or rising above the resting height, are indirect aspects that are the result of planing on specific configurations. They may be useful in a specific context, but they can't be used as a general definition.

    Say you have a V-shaped prismatic model in a tow tank, and the model is fixed in pitch and heave and equipped with a 6-degree-of-freedom force balance. Will there be a change in character of the hydrodynamic flow and in the trend in the forces as the speed is slowly ramped up? I think a definition that describes what happens in this case would be the surest way of coming up with a valid definition.
     
  9. Yellowjacket
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    Yellowjacket Senior Member

    It would have a lot lower trim angle if it wasn't jumping off a wave too... It doesn't matter if it is a sail boat or a power boat, or if it is simply towed, the relationship is still the same.

    Unfortunately you are looking at it backwards, the effect of thrust line or moving ballast all obscure the FUNDAMENTAL relationship between trim angle and speed that defines planing. You can push the trim angle up or down by changing things like thrust line or CG, but for any given configuration, the trim angle must to go down if the speed increases (see below as to why). Sailboats have a thrust line well above the surface of the water and therefore, higher thrust will push the nose down UNLESS the center of gravity is moved aft (which, if you have ever sailed a planing hull you realize you have to do to keep the nose from diving into the water as speed increases).

    What happens in case of the test you described is that, for each speed, the hull creates a specific amount of lift. The amount of lift created can be less than, equal to or greater than the weight of the boat and match the moments about the center of gravity. Only at one speed will the test be valid, that being the speed where lift created matches the weight of the boat and the moments about the center of gravity are balanced.

    The fixed model would would simply show increasing lift with increasing speed, since it is fixed in height, the waterline length isn't going to change, and the moment of the lift vector won't change. With increasing speed, only the amount of lift would change, but that's not at all what happens in the real world.

    For a given configuration and speed, there is a trim angle that will supply the correct amount of lift and correct combination of moments. As speed increases, to remain in equilibrium, the trim angle has to go down.

    If the trim angle didn't go down, the hull would rise further out of the water, to reduce the surface area and keep the lift equal to the weight of the hull. But the rise would reduce the length of the hull in the water, and the average lift vector would also move aft. But then the CG would be ahead of the lift vector, which would create a nose down pitch moment, and that would result in decreasing trim angle. So you see, there is really only one specific combination of lift and moments that satisfy the requirements for stable operation, and that's the trim angle for that speed.

    As I explained in another post, the lift created by a planing surface is not unlike an airplane wing. For a given speed and given angle of incidence, there is an associated amount of lift. But a boat is different from an airplane, in that it can't climb above the surface of the water. So what happens is that it climbs to the surface, and then as you go faster, the trim angle comes down for everything to remain in equilibrium.
     
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  10. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Beautifully argued, Yellowjacket!

    There are some interesting mathematical paradoxes that arise too,
    similar to d'Alembert's paradox in inviscid flow.
    For example there is no upper surface for planing surfaces, so
    there is no leading-edge suction. Instead, there is a planing splash.

    In thin wing theory, a wing can rise higher, but every part of the
    sky is the same, so it doesn't make any difference. In planing there
    is "von Karman's paradox" which essentially says that there is formally
    no solution to the planing equation for infinite Froude numbers in
    infinite depth water because the vessel would be riding infinitely far
    above the undisturbed free-surface. The way around the paradox is to
    notice that it holds only at fixed angle of attack, and the vessel height
    is proportional to its angle of attack. For flat plates in 2D flow, the
    angle of attack tends to zero like 1/FrX^2 as FrX tends to infinity.
    Here, FrX is the Froude number based on the distance from the trailing
    edge to the centre of pressure.
    (Thanks go to E.O. Tuck for explaining this to me in words and in his
    papers!)

    There are good reasons to call the last paradox "Wagner's paradox" because
    it first arose during Wagner's lecture that von Karman attended. See:
    Fridman, Grigory,
    "Planing plate with stagnation zone in the spoiler vicinity"
    J. Engineering Mathematics, 2011, pp. 225-237.

    /end hyper-nerdity
     
  11. daiquiri
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    daiquiri Engineering and Design

    I agree with Yellowjacket about the relationship between trim angle and speed in planing regime, and it does sound like a good criterion for planing of boats and models free to trim.

    Thinking it over again, even the infamous ball example can be explained in a similar manner if we admit the flexibility of ball's surface, which can cause the local flattening of the impact area. In that way a ball can assume a form which is much more similar to a flat planing surface than to a spherical cup. The spherical hull (ball) surface flexibility could also explain the difference between behavior of non-skipping towed buoys (Mikko's observation) and my example of violently swung skipping waterpolo balls. The surface roughness is imo not relevant in the process, because I have made skip shots even (and much more easily) with smooth plastic beach balls for kids.

    Why am I sticking to the ball example (and please excuse me for that)? Because until we find a valid explanation to how can a ball skip off the water surface, we won't have a universal theory of planing and hence cannot be able to formulate a universal criterion applicable to all types of hulls.

    However, I realize that there is probably too little scientific knowledge and experience relative to the ball case, compared to the well-documented conventional hull forms, so for now it's imo ok to just stick to the practical hulls and try to elaborate a criterion. My vote goes to Yellowjacket's trim criterion, which is imo very simple and hence elegant. Paul Dirac would have loved it. :)

    Cheers
     
  12. Joakim
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    Joakim Senior Member

    If you use trim angle, how would you use that in practice for sail boats? By towing it?

    What about hydroplanes? They do plane and they can show different kind of changes in trim depending on the design.

    I can also imagine that you could play with rocker etc. to make a monohull that would plane without showing the typical trim change.
     
  13. tom28571
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    tom28571 Senior Member

    When can a boat be called planing?

    Its a risk to present your own work for scrutiny by this august group but, I will do it anyway. I don't think that the attitude and performance of this boat fits any of the "definitions" of planing offered. I still maintain that no definition other than an arbitrarily agreed upon one will be of any use. The fact that it may not be possible to always determine just where the 50% displacement/dynamic lift point occurs, is of no concern since it is only a hypothetical talking point anyway. Knowing the point of planing helps no designer or operator anyway. Its is clear when a boat is displacement and when it is clearly planing, but, between these regimes in the transition range is highly variable between different boats.

    www.bluejacketboats.com

    There is also a grainy video on the right side of the page that shows the same things. Can anyone detect where the inflection point of displacement to planing mode occurs by trim or any of the other proposals? I doubt it.
     
  14. Kestrel
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    Kestrel Junior Member

    That's right Joakim, we deal with stepped hulls, specially single-step one, and one of the targets is to match, for example, low overall trim changing from displacement to planing regime, what one can reach designing each local planing surface with its local trim, in a certain range of velocity, and we experimented such a behavior in a prothoype, which, also, had a 18 deg deadrise, so not so flat, in order to have a good performance in rough waters. In this case it would be difficult to assess planing regime just on (overall) trim, cause, once reached (early) a dynamic attitude it tends to mantain it costant, while increasing speed, hull (CG) heigth naturally increases too, as hydrodynamic lift becomes preponderant respect to hydrostatic (residual) force. So I remain on my guess, perhaps a bit of conventionally stated is needed, but a certain % of dynamic lift versus residual hydrostatic, may be 50% at least (or 75%, why not?) could be stated to identify the planing condition of a planing hull, cause I think we must limit to this type of craft the evaluation (hard chine, transom stern, stepped hull,...), and a Vol. Froude n. of 2 its a good indicator normally, not related to lenght or beam, but only to the displacement volume at rest.
    K.
     

  15. gonzo
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    gonzo Senior Member

    I think the ball is bouncing off the water and not planing. Planing is when a body travels through water, which the ball doesn't.
     
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