Semi-displacement boats don't generate lift?

Discussion in 'Hydrodynamics and Aerodynamics' started by ToMy, Feb 28, 2023.

  1. ToMy
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    ToMy Junior Member

    So I have been reading a book written in the early 1960's called "Naval Architecture of Planing Hulls" by Lindsay Lord. In this book it is often stated that surfaces that don't redirect water flow do not produce any lift. (Read the screenshotted texts below.)

    I am not an expert in naval architecture, but I can't see any reason why Lord's statements would not be true. But still there are many motorboat designs, even from well respected designers (Grand Banks for example) that include such surfaces and many of them seem to perform decently. Many of these designs are what most people would call a "semi-didplacement" or "semi-planing" hulls. Below is a picture of an Elling E6, a 60ft semi displacement yacht that reaches 22 knots WOT with single 900hp Volvo. Note how the bottom slopes up towards the stern.

    The statement that in order for a vessel to significantly exceed its hull speed, its hull must generate great hydrodynamic lift, seems to be generally agreed to be true. But is it really? If semi displacement hull's flat, upsloping buttocks do not generate lift, how does it exceed hull speed without extreme trim angle?

    Could it be, that the hull of such boat exceeds hull speed not because of the lift produced, but because of the flat and wide after body resisting immersion (caused by wave making resistance) effectively enough for the hull to ride bow wave rather than sinking between the bow and stern waves.

    What do you think?
     

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  2. jehardiman
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    jehardiman Senior Member

    Short answer...No. First of all, there is no "lift"...it is only a mental concept that separates out some portion of drag in the direction you want it to go.

    Short answer...Of course; but not the way you think. Secondly, there is also no such thing as "hull speed". A vessel will go as fast a power allows. The concept of hull speed is an archaic holdover from early theorists best left behind in the hubris of history.

    Finally, I have always felt that often the different sides in a hydrodynamic discussion are more about how an individual internalizes the concept rather than a difference in fact. Ask 3 Naval Architects to state the differences between displacement, semi-planing, and planing hulls and you will get 5 or more answers.

    All that said, all floating bodies are supported in the water by the dot-normal integral over the wetted surface of the fluid pressure, both static and dynamic. This interaction between the pressures and the shape produces both the buoyant force and the drag. Whether a vessel "lifts" or "squats" is just a physical manifestation of the interaction between the hydrodynamic pressures, the shape, and the vessels mass in the vertical plane. Generally a vessel is said to "plane" when the majority of the vessels weight is supported by dynamic pressure, it has nothing to do with the shape of the wake except as it relates to pressures on the wetted surface. So specifically, the shape and speed of the hull influences the dynamic pressure which then forms a dynamic force over the hull surface which retards the hull and causes it either to "lift", to "sink", or to stay neutral based on the static waterline. However, the integral of the pressures over the hull are always identical to the weight of the vessel.

    Now the shape of the hull for a given weight that causes the minimum drag at speed while meeting all other items in the SoR..... that's the rub.
     
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  3. Paul Scott
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    Paul Scott Senior Member

    2CF2A065-9B18-498E-B0A4-55A1FE5D193B.jpeg
    Notice what the stern is doing, and its depth in the water. And don’t fall for the idea that Robbie’s weight is being lifted much by the sail- try hanging from a lat bar on a weight machine & put your feet on a weight scale-
     
  4. jehardiman
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    jehardiman Senior Member

    Yeah.... and that brings up something that gets into the quaggy scrum of the "ancient interface". Rather than just look at the hull and the water which we can do with a CVN (which can go well beyond "hull speed" BTW), lightweight high-speed vessels need to be thought of as being fully submerged at the boundary between two fluids of different densities. So for that 3-point hydroplane it is just as important that the aerodynamics hold the hull down on the water as it is that the water lifts the hull up.
     
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  5. Paul Scott
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    Paul Scott Senior Member

    fun mind game- ~ D/L of the windsurfer hull as shown in the pic ~ 300, and the wave drag is probably off the charts- not bad for a narrow squash tail with a relatively small sail….:D Except for wetted surface (bow out) what’s changed? Stability?
     
  6. CarlosK2
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    CarlosK2 Senior Member

    "If semi displacement hull's flat, upsloping buttocks do not generate lift, how does it exceed hull speed without extreme trim angle?

    Could it be, that the hull of such boat exceeds hull speed not because of the lift produced, but because of the flat and wide after body resisting immersion (caused by wave making resistance) effectively enough for the hull to ride bow wave rather than sinking between the bow and stern waves."

    Could it be ... Yes
    Buttocks do not generate Lift ... !

    HydroDynamic Lift occurs where the bow wave spray is (Savitsky, 1964)
     

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    Last edited: Jul 21, 2023
  7. CarlosK2
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    CarlosK2 Senior Member

    In the limit ...

    hydroDynamic Lift ...

    would be on the transom because the bow wave would be there.
     

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  8. CarlosK2
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    CarlosK2 Senior Member

    General ideas

    We divert Fluids, and by diverting Fluids High Pressures and Low Pressures are produced

    High Pressure depends on the Angle of Attack.

    Low Pressure depends on the Curve


    and

    => 0.50 Froude the High Pressure is in the Bow, the Low Pressure is in the Stern

    Semi-Displacement vs. Displacement

     
    Last edited: Jul 21, 2023
  9. CarlosK2
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    CarlosK2 Senior Member

    Then

    In this type of hull (semi-Displacement) what is sought and desired is to destroy the low pressure (B = Low) at the stern.

    In the video we can see how the stern of a Displacement hull is sucked by the Low pressure of the stern.

    And in the foreground we see a semi-displacement hull
     

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  10. jehardiman
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    jehardiman Senior Member


    Yeah....but....different horses, different courses....
    Gearing class DD, semi-displacement, 390.5 LOA, 3460 LT FL, 60,000 shp, 36 knts. Transport capacity 96.1 ton/knots or 0.001 ton/knot/hp
    C3 Cargo ship, displacement, 492 LOA, 12,000 LT FL, 8,500 shp, 16.5 knts, Transport capacity 727 ton/knot or 0.085 ton/knot/shp

    It all depends on the SoR....
     
  11. CarlosK2
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    CarlosK2 Senior Member

    E x a c t l y

    Depends on what we want

    (1a) if we want a Hull for speeds of 0.20 Froude it is different if (1b) we want a Hull for speeds between 0.30 and 0.40 Froude.

    (2) If we want speeds higher than 0.80 Froude we need large quantities of hidroDynamic Lift

    (3) If we want Speeds between 0.45 and 0.60 Froude (Speeds usually called "Semi-Displacement") then we have to watch carefully the Hull Attitude, namely: chase and destroy the Low Pressure, the Suction, which could create the Stern

    And

    (4) if we want a small sailboat to Surf big Waves then we want a 'Stern Down, Bow Up' Hull Attitude which we can achieve with

    (A) High Pressure (hydroDynamic Lift) at the Bow
    (B) Low Pressure (hydroDynamic Suction) at the Stern and
    (C) Combinations of A and B
     
    Last edited: Jul 22, 2023
  12. CT249
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    CT249 Senior Member

    Wow, I really do have to get my act together, get that board some new centreboard case gaskets, and put it on the water. She hasn't seen her natural element in well over a decade.

    EDIT - Sorry for the hijack guys, but I've owned the board in the pic since about 1984
     
  13. Barry
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    Barry Senior Member


    Your comment " the hull is sucked by the Low pressure of the stern" is misleading. Are you saying that the pressure here is less than atmospheric??
    Unlikely as well as this part of the hull is still "moving/accelerating" water out of the way to make room for the hull. The graph in Post 6 does not show any lower than atmospheric pressure
    at the transom of the hull (of course as the water rolls around the final portion of the hull at the transom, there would be some Coenda effect but negligible)

    The reason for the bow up is due to the center of lift being further forward at lower speeds and moves rearwards as speed increases
     
    Last edited: Aug 7, 2023
  14. Dolfiman
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    Dolfiman Senior Member

    This paper attached reports on model tests with 3 types of stern body : hook, straight and rocker , and various speeds covering semi to full planing. And the results are ... it depends of the speed ! Also, interesting considerations on the dynamic stability associated.
     

  15. Stephen Ditmore
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    Stephen Ditmore Senior Member

    Cool paper from Donald Blount, Thanks!

    I referenced a paper from Daniel Savitsky at Definition of Planing https://www.boatdesign.net/threads/definition-of-planing.45248/page-22, but I see my link no longer works. Regardless, in that paper it was clear semi-displacement was a speed range considered anything between Length Fn=0.4 to 0.9 (speed length ratio 1.34 to 3.0).

    I would think "lift" refers to the vertical component of the bottom pressure above what it is in the static condition. In the lower part of the semi-displacement speed range the bow of most boats tends to rise because lift tends to develop at the bow. If the boat continues to accelerate, the lift starts to move aft, so the boat starts to level off (depending some on the stern shape). What happens from there is very dependent on bottom shape including whether there are trim tabs or other flap or hook at the stern, and the boat's longitudinal center of gravity.

    I'm very interested in the topic of improving hull performance at semi-displacement speeds. Should we get into that in this thread?
     
    Last edited: Aug 11, 2023
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