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

    Lift has the same meaning as for a wing, you need the Kutta condition. For a boat with squared trailing edge that just means that the flow leaves the bottom tangent to it, there's no longer backflow up the transom. If you look over the transom of your outboard rig as someone else drives then you'll see the sharp transition. Again, Newman in his Marine Hydrodynamics states that he saw the transition for the acceleration of a flat plate in his kitchen sink: he observed the shedding of 'the starting vortex'. Just think of the bottom of a boat on a plan as the bottom half of a vortex sheet, a velocity discontinuity.
     
  2. Barry
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    Barry Senior Member

    The reason that the bow comes up is because the center of lift begins closer to the bow when the hull begins to move then move further sternward, as speed increases.

    There are many papers that you can look at that give the pressure distribution and center of lift data for various deadrises.

    It does not matter if you tow a hull, push a hull with a propeller, a jet, a prop, high rake or not, the hull lifts because the center of lift is forward. (though they can make a difference in the amount of lift)

    As the speed increases the center of lift moves rearward and the nose drops,

    You are incorrect in thinking that there is not lift until the after transom ventilation. The second the boat begins to move and the bow begins to move up, there are lift forces.

    This is not a "kinda" sort of thing. Water must be moved out of the way, ie a cross section of water moves out of the way, a force is needed to do this, and the result is lift and drag (created from the angle of lift, I am not talking friction drag etc)
     
  3. daiquiri
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    daiquiri Engineering and Design

    A picture is worth thousand words, hence...

    1) Wet transom, separated flow:
    T-Wet.gif

    2) Dry (or ventilated) transom:
    T-Dry.gif

    4) A 3-D View of various levels of transom ventilation:
    Wet-Dry Transom.jpg

    Taken from this paper (worth reading): https://www.researchgate.net/public...g_Catamaran_Transom_Stern_Ventilation_Process

    As you can see, the flow separates along the transom edge at nearly every speed regime.
    When the transom runs wet or partially wet, there is a volume of recirculating water above the separation line, which considerably increases the hull resistance.
    When the transom runs dry (or fully ventilated), the water flow is continuous (no separation) and the resistance component due to recirculating vortex disappears.

    So what you and CT249 have called "separation" looks more like ventilation to me. :)
    I guess that your intention was probably to describe a situation in which the water flow "separates" cleanly from the transom edge. That situation is technically called "dry transom", meaning that the transom is 100% ventilated.

    The correct terminology is important in this kind of discussions, but I remember that we have already had a discussion in the past about that issue (it was about propeller blades geometry) - and the result was pretty much a draw. The terminology often depends on the context. ;)

    Cheers
     
  4. Barry
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    Barry Senior Member

    There was some discussion on a previous thread about transom ventilation and drag.
    I am curious as how water circulation above the separation line (wet transom) can increase the hull resistance and it creates higher drag than 3) a dry ventilated hull. Ventilation to me is a dry transom, ie no water against the transom

    The reason that I question this is that the back of the transom will be at a positive pressure, ie above atmospheric because if it dropped below atmospheric, the atmosphere pressure would be higher, and would then push into the transom and ventilate it.

    If somehow, 2) the pressure of the water behind the transom is below atmospheric, and creating a higher drag which disappears when fully ventilated 3) but still in contact with transom, then if you were to drill a hole in the transom, then there would not be any water flow into the hull.

    I know that with my tender when I am trying to get rainwater out of it, I can pull the plug but no water will go out the drain hole until the transom ventilates

    My thinking is that the water pressure on the transom cannot go below atmospheric psig = 0 when the transom is wet and a fully wet transom will not create more drag than a dry transom.

    ??
     
  5. sandhammaren05
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    sandhammaren05 Senior Member

    Correct, I understand center of pressure.
     
  6. sandhammaren05
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    sandhammaren05 Senior Member

    I understand center of pressure. The bow is up when the boat's plowing before lift develops. I'm always talking about form drag. I don't see that any ventilation is needed here, just normal plowing, lift onset, and later planing. It's very cumbersome to try to think directly about moving water out of the way. If you're labeling as 'ventilation' the lack of backflow then that's not good hydrodynamics. We know what 'ventilation' means for a surface piercing propeller. The transom of a planing boat is only trivially 'ventilated', use of the term masks the physics of lift.
     
  7. sandhammaren05
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    sandhammaren05 Senior Member

    Hi Daquiri, I've leaned over the transom of my Glastron v153 with my son driving. The flow doesn't separate below 8 mph, below that speed there is backflow up the transom. I expect that this is typical for all my classic boats, I can check it for my Power Cat 15t in late Apr. Again, check out Newman's Marine Hydrodynamics. Thanks, Joe
     
  8. sandhammaren05
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    sandhammaren05 Senior Member

    But Daq, in your (1) the flow has not separated from the transom, there's backflow! Also, I don't like artists' drawings or simulations. I would need flow photos.
    Best,
    Joe
     
  9. Barry
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    Barry Senior Member

    You said "The bow is up when the boat's plowing before lift develops" I am saying the bow is up because lift is developing. Bow up means lift is being developed.

    "Cumbersome to try to think directly about moving water out of the way" The reason that there is a force on the hull, which can be lift/drag in a planing hull is BECAUSE you are accelerating water to get it out of the cross sectional shape of the hull in the water. ( as well as buoyant forces throughout all speeds but diminishing at higher speeds)
     
  10. sandhammaren05
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    sandhammaren05 Senior Member

    Lift can't develop without the Kutta condition. But the idea of lift developing simultaneosuly with the trim angle increase is interesting. I'll check it again on my trusty Glastron later this spring.
     
  11. Rastapop
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    Rastapop Naval Architect

    I don't disagree with you daiquiri, because you could be completely right, and correct terminology does matter, but just in my personal experience it's pretty common for people to not use "separation" as a description until regime C occurs (transom completely ventilated), or at least certainly not "clean separation".

    As far as I can see that paper doesn't use "separation" to describe the shear line and wet transom regime either - it only uses it when the transom is ventilated.
     
  12. Joakim
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    Joakim Senior Member

    So you are saying a keel or a wing with thick trailing edge can't produce lift because there is backflow? Or a wing/sail that is stalling?

    Here is a flow field around a foil while stalling (note all the backflow): https://upload.wikimedia.org/wikipe...Flow_separation.jpg/310px-Flow_separation.jpg

    Here are the lift and drag coefficients beyond stall (note that lift is nowhere near zero and a lot of lift is generated even with the foil backwards):
    http://www.aerospaceweb.org/question/airfoils/q0150b.shtml

    Why do you think it is important to have another fluid phase (air) at the trailing edge before you can have lift? And why it is not needed for keel or propeller blade?

    Separation means that flow field is no longer following the shape of the body. It has nothing to do with transom being dry or not. Well, expect that with dry transom separation is obvious.
     
  13. Rastapop
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    Rastapop Naval Architect

    Yes, unequivocally true.

    It's just that when it comes to transoms I've noticed people apply the word "separation" differently (I even do it myself).

    I suppose I'll just have to remember common usage doesn't make it right, and try to catch myself when I do it.
     
  14. Joakim
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    Joakim Senior Member

    Since transom is vertical, only pressure on it can change the drag. Having water there will naturally increase the pressure and thus "push" of the transom. But that's not all. Energy is needed to keep up the backflow and turbulence at the wet transom. Thus it will change the pressure and flow field elsewhere as well. That change can be much bigger than the missing "push" of the transom.

    Displacement hulls with rocker or rounded hull at the stern try to keep the flow attached, thus not separated. Then the Bernoully equation is valid and there is high pressure at the stern pushing the vessel forward and reducing drag. Without separation and surface friction there would be no drag at all (well there would be "wave making drag" due to free surface).

    With a sharp edge between the bottom and the transom flow will always separate at the edge. This means that the Bernoully equation is not valid for the pressure. So even with a very low velocity close to transom the pressure is not high thus it is not "pushing" much.
     

  15. Barry
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    Barry Senior Member

    The trim angle is developing because of lift and the location of the center of dynamic lift.

    Simplified

    A boat will move a trough of water out of its way as it goes down a lake. The water within this trough is static, no movement. In order for the water to get out of the way of the boat it has to accelerate this mass. Any time you accelerate a mass, you get a resultant force.

    This force will give you two components, the vertical component, LIFT, and a horizontal component DRAG

    The Kutta condition as far as I can find, refers to a foil in a fluid, where the fluid is above and below the foil. This is not the case in a boat. And I believe in one of your earlier posts, you said that you believe that a hull act like half, either an air foil or hydrofoil.

    Most foil equations have the same fluid above and below and will have higher pressure underneath and lower above.
    Due to Euler equation, conservation of momentum considerations as well as Bernoulli, velocity and pressure are connected.

    You do not need Kutta conditions to get lift. I am assuming that we all agree that lift is the vertical force acting on the hull cause by dynamic forces????
     
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