Froude and planing

Discussion in 'Hydrodynamics and Aerodynamics' started by sandhammaren05, Feb 26, 2017.

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  1. sandhammaren05
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    sandhammaren05 Senior Member

     
  2. Peaky
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    Peaky Junior Member

    Thanks Tom. Right , let's call the hydrodynamic vertical up force that is dependent upon the Kutta condition existing Lift, and any hydrodynamic vertical up force that is not dependent upon the Kutta condition existing "lift".
    I think SH05 is arguing there is no such thing as "lift", or at least it plays no part in planing. To deny the existence of hydrodynamic forces until separation at the transom is observed is crazy.
     
  3. sandhammaren05
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    sandhammaren05 Senior Member

    Ok. Try to account for the pressure required to carry the weight without using the bottom as vortex sheet. A 14' Allison R14 v-bottom runs 67 mph (GPS 2 way avg.) with a Johnson 75 (68 hp at the propshaft, 75 at the crank). The total weight of the rig is 840 lb. Air lift provides only about 5 lb lift. I await your calculation to account for carrying the total weight. Attached is an earlier empirical study. My semi-empirical lift coefficient is c=(alpha) cos(beta) where alpha is the boat's trim angle (3-6 degrees) and beta is the deadrise angle near the transom.
     

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  4. Peaky
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    Peaky Junior Member

    This isn't relevant to the discussion. But obviously the total ability to carry weight is the sum of the dynamic and static forces. You have chosen to use lift coefficients to calculate the dynamic part.
     
  5. sandhammaren05
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    sandhammaren05 Senior Member

    I observe that at high speeds there is no buoyancy, there is only lift. In this context, Faltensin follows Newman on lift and the Kutta condition but goes on to show in a model that the lift goes to zero if the transom width goes to zero. Newman also calculates the lift in a simple model with a sharp trailing edge in a later chapter. My model for a v-bottom is half of a delta wing with deadrise beta. That lift coefficient is c=3(alpha)cos(beta)/2 where alpha is the boat's trim angle and is 330% too large due to side spray and waves created at the air-water interface. The correct semi-empirical lift coeff. is c=(alpha)cos(beta), which accounts for carrying the weight of both v-bottoms and tunnel boats (excepting airflow between the sponsons where the lift coeff. is for 2 dim. flow and is much larger) for both water and air flow. I.e., my lift coeff. accurately takes into account the 3D effect of lateral flow, which flow only creates drag. By the way. It also summarizes Clement's empirical data pretty well if I include the effect of aspect ratio linearly, which I have in a generalization.
     
    Last edited: Mar 7, 2018
  6. sandhammaren05
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  7. sandhammaren05
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    There is nothing but the dynamic part on a race boat. Buoyancy plays no role at high enough speeds. With my 940 lb 15' Glastron powered by a Johnson 70 (shaft hp) buoyancy plays no role above about 20 mph. The top speed is 46 mph. Lift due to the vortex sheet carries all the weight.
     
  8. sandhammaren05
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    sandhammaren05 Senior Member

    With my 15' Glastron the transom is completely dry at about 8 mph with the motor trimmed completely under. That's the onset of lift: buoyancy no longer can carry all of the weight once the transom is dry. The next stage is that the boat plows with a large trim angle and comes up on the water, begins to plan. Buoyancy still carries some weight. At about 18 mph the boat breaks over to its normal trim angle of a few degrees. At that point lift overwhelmingly dominates buoyancy. In the case of an old PT Boat or the water taxis that we sometimes take from RĂ¼gen to Hiddensee the boat never breaks over and so buoyancy continues to carry some of the weight. I define full planing as when the boat breaks over to a low trim angle of a few degrees (less than 6 degrees). At that stage buoyancy can be neglected in the weight calculation.
     
  9. sandhammaren05
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    Let's go to high enough speed that lift dominates buoyancy. For my Glastron that's about 18 mph. Try to calculate the force that carries the boat's weight without using lift, but don't forget D'Alembert's Theorem when you start.
     
  10. sandhammaren05
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  11. TANSL
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    TANSL Senior Member

    As long as the ship is in contact with the water, the buoyancy will always, fortunately, play its role, even though it is less important than in the displacement regime.
     
  12. sandhammaren05
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    sandhammaren05 Senior Member

    The side spray and waves at the air-water interface are not calculated, that's hard. Getting the right lift coefficient is easy. You imagine that there's some force other than lift (due to the bottom as vortex sheet after flow separation at the transom) at high enough speed that the boat breaks over to a small trim angle (about 18-20 mph for my 15' Glastron, but there are hulls that plow and never break over). Here's why you're wrong: Let w=rig's total weight, r=density, U=boat speed, A=effective wet area (taking deadrise into account). I can calculate w/rAU^2 for three entirely different rigs, e.g., and the results depend only on the dimensionless parameter trim angle, deadrise, and aspect ratio. That's what tells you that I'm talking about lift and not Oscar. The rigs are a 14' v-bottom race boat, a 15' v-bottom pleasure boat, and a 13' tunnel boat.
     
  13. sandhammaren05
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    Bouyancy is negligible compared with lift for a 930 lb 15' Glastron with Johnson 70 above 20 mph. Etc.
     
  14. TANSL
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    TANSL Senior Member

    No, it will be very small compared to the lift generated by the air but you always have to take it into account. When the boat jumps in the air and falls back into the water, it is the buoyancy that prevents it from sinking.
     

  15. sandhammaren05
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    sandhammaren05 Senior Member

    Good joke but my boats rarely jump, they tail-ride, buoyancy is negligible there. But consider an offshore race boat. Not only buoyancy but also dynamic lift will prevent it from sinking too deeply after launching off a big wave.
     
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