planing theory

Discussion in 'Hydrodynamics and Aerodynamics' started by abohamza, Aug 22, 2011.

  1. abohamza
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    abohamza Junior Member

    HI
    I would like to have any information about Boat Planing Theory, because I used to deal with displacement ships.

    Regards

    Abohamza
     
  2. gonzo
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    gonzo Senior Member

    It's more of a definition than theory. The transition, or semi-displacement, is not well defined and a source of endless argument. It is generally accepted that a speed/length ratio of 3 is planing.
     
  3. DCockey
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    DCockey Senior Member

    There is some theory about planing, that is when a boat is mostly supported by hydrodynamic forces rather than hydrostatic. David Savitsky developed some of it. Do a search for Savitsky theory and similar.

    It's also true as Gonzo noted that there is not agreement on when planing starts.
     
  4. Ike
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    Ike Senior Member

    It is simply that planing starts at different speeds depending on the bottom design of the boat. A completely flat bottom will plane out sooner than a modified vee or a deep vee. It also depends on whether the boat has a constant deadrise or a dead rise that varies from bow to stern, and whether the chines are rounded or sharp corners. So if you have a group of boats that are the same length and beam, the bottom design determines at what speed each boat planes.
     
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  5. DCockey
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    DCockey Senior Member

    True enough that different designs plane at different speeds. But then there are the various definitions of when planing begins including:

    1) When the CG of the boat starts to rise. At slow speed the CG is lower than the static height relative to the surface of the water.

    2) When the CG has risen to the static height.

    3) When XX% of the weight of the boat is supported by hydrodynamic lift. XX% is typically given as 50% but I've heard higher percentages. Of course this definition is very difficult to determine experimentally.

    4) When the resistance starts to drop after passing though the hump. Does not happen with some boats so by this definition those boats do not plane.

    5) Froude number based on length above Y, with Y typically around 1.

    6) Froude number based on cube root of static displacement volume above Z, with various values given for Z, typically around 3.

    7) When the transom is no longer immersed. This definition does not work well for many boats. Recent published work shows that the speed water separates cleanly from the transom is closely related to Froude number based on static depth of transom immersion, and not related directly to length Froude number.

    8) When the water separates cleanly from the chines. What if the chines are at the static waterline?

    9) When the boat is riding on top of the water. Subjective and not particularly useful for engineering.
     
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  6. DCockey
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    DCockey Senior Member

    Saw an interesting theory in a book intended for the general boating public. It claims that a boat with a warped bottom with less deadrise at the stern will have higher resistance when planing than a boat with constant deadrise. The explaination offered was that the change in deadrise causes the water to rotate which takes extra energy.

    Not what I've understood from other sources.
     
  7. gonzo
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    gonzo Senior Member

    Warped bottoms are used on boats with low power. That theory does not stand to reality.
     
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  8. abohamza
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    abohamza Junior Member

    What is the Warped bottom?
     
  9. abohamza
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    abohamza Junior Member

    what is the optimum design for the bottom?
     
  10. abohamza
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    abohamza Junior Member

    I've found this fill on net
     

    Attached Files:

  11. Ike
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    Ike Senior Member

    Optimum design for what? speed, ride, economy, ....... etc.

    The simple answer is, there is no optimum design for the bottom. You design a bottom to do what you want it to do. Every hull design is a compromise of various factors to achieve the design goal. If you want sheer speed at expense of everything else, the bottom would be flat. If you want it to be a able to run flat out in a seaway you use a deep vee. Two extremes, and every thing in between is a compromise. Even the flat bottom, or the deep vee, is a compromise because you give up something to achieve something else.
     
  12. Leo Lazauskas
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    Leo Lazauskas Senior Member

    It's not all that helpful.
    The view of water as a sea of peas is very crude. Water molecules don't hit the plate and bounce off it.

    Admittedly, it is a very complicated difficult flow problem, but you should search for,
    for example, Savitsky, Tuck, Maruo, Vorus, L.J. Doctors, Troesch, and many others whose works
    can be found on the net.
     
    Last edited: Aug 23, 2011
  13. messabout
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    messabout Senior Member

    The claimant who says that warped bottoms have more resistance has been reading Lindsay Lord's Naval Architecture of Planing Hulls. L.Lord made a big deal out of "monohedron" bottoms versus suction at the afterplane if the bottom was warped. That proposition has been put aside along with with the buggy whip.

    Tis true that the fastest boats have constant section bottoms where it counts. They're flat. For garden variety boats, warped sections are likely to get on a plane more quickly with less power than constant vee types. And some of the hot rod type boats have a deliberate hook in the bottom aft.
     
  14. Mr Efficiency
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    Mr Efficiency Senior Member

    Monohedron might look better in theory, but in practice doesn't always work out that way.
     

  15. m3mm0s rib
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    m3mm0s rib Senior Member

    Everything is on a same hull Can be worked just as well if In this, the burdens are different; In this case the number of Freud and i changed; Or if we have more power we will have the same tack;
     
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