Definition of Planing

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

  1. latestarter
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    latestarter Senior Member

    Unfortunately "waterline length/speed ratio" is not dimensionless.
     
  2. gonzo
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    gonzo Senior Member

    It is a ratio, which makes it dimensionless.
     
  3. Leo Lazauskas
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    Leo Lazauskas Senior Member

    No. It has units of time.
    Froude number is non-dimensional: F = U/(sqrt(g*L))
    or
    Volumetric Froude number: F_V = U/sqrt(g*V^[1/3]).
     
    Last edited: Mar 23, 2013
  4. daiquiri
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    daiquiri Engineering and Design

    True. But, fortunately, time is measured with the same unit in both SI and British system. So you end up with the same numerical value. ;)
     
  5. latestarter
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    latestarter Senior Member

    length / speed in terms of dimensional analysis is L divided by L/T. the Ls cancel out which leaves you with T as Leo pointed out.

    http://en.wikipedia.org/wiki/Dimensional_analysis

    In practice if you are using feet and feet / sec or metres and metres / sec it would give the same result however on this thread speed is often quoted in knots and MPH.
     
  6. gonzo
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    gonzo Senior Member

    The original formula for the concept of "hull speed" gave a speed in knots and was the square root of the waterline length in feet times a number usually between 1.2 and 1.4.
     
  7. Kestrel
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    Kestrel Junior Member

    I guess dimensionless nr. should always be used, when applicable. I think the best dimentionless nr. to account for relative speed is the Froude Volumetric one based on Volume of displacement at rest, in planing hull matters (but obviously not only), v/sqrt(g*Vol^(1/3)). It's the one that can be used not considering variations with speed of wetted lenght (often also of wetted beam) which occurs with hydro-planing surfaces. Hulls of same size should be compared at same loading factor (Ap/Vol^(2/3)) too.
    K.
     
  8. Prismatic
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    Prismatic Junior Member

    When people are on those surfing simulators they appear to be planing but their speed is 0 ...the water has a speed for sure. It's amazing to see a 200# man supported stationary on 2 inches of water...is the board planing?

    http://m.youtube.com/#/watch?v=VPm6Lk0VFGU
     
  9. tunnels

    tunnels Previous Member

    Yes !!

    Yes the board is planing or it would not be able to support his weight !
    Its like a glider sitting in a steady air stream going up and over the high hill . In relation to the ground he is stationary but in relation to air speed he is moving same with the surf board its the waters speed that is making the differance !!:)
     
  10. Prismatic
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    Prismatic Junior Member

    So planing is relative to hull speed and water speed. So planing speed is when a specific point on a hull passes a specific point of water at a speed which causes lift whether hull or water or a combination of both is/are moving. Maybe?
     
  11. tunnels

    tunnels Previous Member

    The hull has to be moving in relation to the water to create lift from the water ! also hull shape is very important !! If a hull is stationary but there is a flow of water under the hull it will provide lift , faster the flow greater the lift !:)
     
  12. haribo
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    haribo Junior Member

    for the creation of lift not only speed of the water is nessesery, the hull must deflekt the waterflow downwards

    but under the water is the next deeper water, there is no place to flow down so the only possibility is to slow down the speed of some water and convert some of the kinetic(dynamic)-force into static-force !!!

    bernulli gave the formula for this conversions proces in th jear 1738: dynamic- into static-pressure,
    the sum of both is constant along every path of a flow.....

    the biggest possible lift under a planing boat exsist at the point were the flow comes to zero, all dynamic force is translated in static pressure

    so we call the lift of a planing boat dynamic lift, but the force that lifts the boat out the water is only a converted dynamic force, converted into static force, the same static force that lift a boat in displacment mode, working rectangular to the surface of the underwater hull

    thats why we can not decide on a boat where the lift under each place is coming from, we can messure the sum of both parts (dynamic- and static-) pressure-force under each place, and we can messure the pressure-force if we slow down the flow to zero, the so called ram-pressure, but it is not possible to deside if in the first pressure this part is from static and that part is from converted dynamic pressure

    we can compare the pressure at high boatspeed with the pressure at displacment (laying for anker), but until the boat at speed create a new water surface with the bow waves and the holes behind, with spray and so on.... there is no way to decide exact what part has which history of his static pressure at the moment

    and for a ilustration that we should not try it, to compare in this way, think at the pressure of a part of the boat that is out of the water in planing mode, (sum of pressure is zero), in displacment mode there was a pressure, if we compere both pressures and say the dynamic-lift is just the difference, than we must say the dynamic lift of this flying part is negativ, a negativ dynamic force on a place without contact to the flow ???....IMO impossible


    at the end we need the exact speed of the flow around a hull to deside how much force is converted from dynamic,
     
  13. tunnels

    tunnels Previous Member

    If the water is moving in the same dirrection as the boat is moving its the speed of the boat on the surface of the water thats important . if you are traveling against the flow also is the speed of the boat on the surface of the water thats creating lift/planing the speed of the moving water has to be related to the speed of the boat to create lift .
    In a fast flowing river a boat could actually be moving backwards but still planing in relation to the land !! but going down stream then add the speed of the water flow +the boat speed on the water surface the boat is still only moving at the same planing speed as it was when it was going backwards down the river in relation to the land !:confused:
     
  14. terhohalme
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    terhohalme BEng Boat Technology

    As a boater I can clearly feel when the boat starts to plane. As a graduated boat engineer, it is not so easy to explain, but... let's have a try:

    1. In this world, a wave has a speed of V = (g * lambda / (2 * pi))^0.5, where g is acceleration of gravity and lambda is the wave length.

    2. Every boat makes tranverse waves formulated above.

    3. When the wave length is similar to boat's waterline length, the boat is at "hull speed" or it's Froude number Fn= 0.4. Fn = (V / (g * LWL)^0.5

    4. When the boat still accelerates, the wave length increases and the boat must glimb over it's own bow wave. If there is enough power to weigth -ratio to do that, the boat will actually reach the highest point of its own wave. The boat starts also to trim when glimbing.

    5. When the center of gravity of the boat is over the highest point of the bow wave, the boat will go downhill. At this speed the wave length of the boat is somewhat 2 - 2.5 times the length waterline. So the Froude number is somewhere 0.8 - 1.0.

    6. Now we have a downhill to accelerate (resistance will decrease) and lot of trim to greate hydrodynamic lift and suddenly the boat will plane, if the bottom shape is a proper one.

    In my point of view, for planing we always need
    1. a proper planing hull shape (flat to deep V-bottom, straight aft lines, clear flow separation at side and aft)
    2. decent power to weight ratio
    3. some trim (CG aft enough) and
    4. speed above Fn > 1.

    Well, planing is something we can calculate with Savitsky method....

    And yes, the kajaks or other light slim hulls can easily go fast, but they seldom plane. They don't need to.
     

  15. haribo
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    haribo Junior Member

    mr. savitsky himself pointed his hydrodynamicsist-fingers of the full development of the stagnationsline in front of the wetted surface as one part of his definition for planing, and not so much of a speed definition compared to humpspeed or fn(whatever), thats for average yachtsman......

    as I found in the boatdesign history in a letter to jamesflett, date 2006

    http://www.boatdesign.net/forums/boat-design/calculating-planing-velocity-14289.html#post110601




    ""To the researcher dealing with the hydrodynamics of
    planing craft, the inception of planing is associated with complete
    separation of flow from the chines and transom and full development of
    the stagnation line at the forward edge of the wetted bottom area. This
    will occur prior to the so-called hump speed when the trim angle is
    maximum. My 1964 paper in Marine Technology discusses this.
    To the average yachtsman, planing is taken to occur at
    speeds higher than the hump speed when the craft runs at trim angles
    smaller than the hump trim. They refer to this as "getting over the
    hump and running on plane".
    As a hydrodynamicist, I use the first definition.
    Good luck with your studies,
    Daniel Savitsky ""
     
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