Transom pressure recovery or negative lift?

Discussion in 'Hydrodynamics and Aerodynamics' started by InetRoadkill, Jul 28, 2019.

  1. InetRoadkill
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    InetRoadkill Junior Member

    I'm a little confused. I have a transom sketch that is flattish and tapers upward to the waterline as it moves aft. The upward slope starts around 2/3 of the way back and slopes up linearly for the remaining 1/3. My confusion is whether this form will produce pressure recovery or will it produce a low pressure area and lift the bow. Just looking for general guidance since I know there's going to be factors that might throw the answer way or another.
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    Do you have a sketch of the boat that defines the shape ? I know you have given a description, but you know what they say, a picture is worth a thousand words.
     
  3. InetRoadkill
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    InetRoadkill Junior Member

    I don't know if these pics will help. I'm using Blender 3D to sketch a hull and it doesn't know what stations, butt lines, and water lines are. Those will have to be done by hand later on. This is a trimaran concept so the narrow hull below the waterline is not a stability problem. transom1.jpg transom2.png
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    Naturally waterlines and buttocks are needed to complete the picture, but I doubt "pressure recovery" is that much of a concern, with hulls with a high L/B ratio, and light displacement. Those are the two main things that will affect wave-making resistance once pushed beyond the "hull speed" dictated by hull length.
     
  5. InetRoadkill
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    InetRoadkill Junior Member

    Yes, I know waterlines and stations would have been more helpful. But as mentioned, the modeling program won't generate those. They have to be done by hand. Anyway, it's difficult to see in the pictures but the transom becomes more U-shaped and narrows as it moves aft. I read somewhere that the sloping form can generate negative lift drawing the stern down and causing the bow to lift -- something that might be desirable in a tri when pressed hard. The concern is that if there is a low pressure area being developed whether that's going to represent significant form drag. I'm tempted to believe that the hull is too narrow aft to develop a significant pressure drop since the developed flow would quickly fill the area.
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    A little bit of squat won't matter much to resistance of a narrow, light hull.
     
  7. DCockey
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    DCockey Senior Member

    InetRoadkill: By "transom" do you mean a more or less vertical, transverse surface at the very aft end of the boat which is the conventional definition of "transom", or are you referring to the aft portion of the bottom.
     
  8. InetRoadkill
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    InetRoadkill Junior Member

    I'm probably misusing the term transom. I'm referring to the aft section of the hull below the waterline leading up to the vertical portion above the waterline. I'm not real clear on the exact definition of transom. I've seen it used to describe both the above water portion and the shape of the hull leading up to it.
     
  9. waikikin
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    waikikin Senior Member

    That may be the tuck InetRoadkill or the aft buttocks a avast behind.. Word List: Definitions of Nautical Terms and Ship Parts http://phrontistery.info/nautical.html

    upload_2019-7-29_18-10-38.png
     
  10. jehardiman
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    jehardiman Senior Member

    You need to make sure that as the streamline moves aft from the aft shoulder point, there is no pressure deficit. There will never be a pressure recovery, because real water is viscous. If you expect enough power, the transom doesn't have to be continuous to the waterline (i.e. use a submerged transom). Otherwise, extend the hull to the effective waterline while maintaining the nominal pressure streamline.
     
  11. DCockey
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    DCockey Senior Member

    How does viscosity prevent "pressure recovery"?
     
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  12. jehardiman
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    jehardiman Senior Member

    Viscosity always "removes freestream energy" from the water, meaning that the further along the hull, the less the Bernoulli Equation is correct. The absolute best you can do is to maintain the pressure at the aft shoulder. You can never recover pressure...this is why "wake waves" are formed. <SOAPBOX> The water only had the static pressure energy before being touched by the hull. By viscosity, the hull added velocity to the water in the direction of motion. Therefore, by the Bernoulli Equation you cannot ever return to the original static condition and pressure will always be less. This is what most inviscid CFD gets wrong by using a "freestream velocity"...there was none to begin with. </SOAPBOX>
     
  13. messabout
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    messabout Senior Member

    The profile, as well as your description, indicates that the lowest part of the bottom is at or near two thirds aft of the stem. That is characteristic of boats that do well at lower speeds. Uffa Fox and others discovered, long ago, that the lowest part of the bottom is usually more appropriate for speed at one, third more or less, aft of the bow.

    The object was to reduce the half beam buttock angle as it transitions toward the aft end of the immersed parts. To make the graph of the section areas more gently curved, the maximum beam was moved aft.

    I suspect that we could get into a spirited argument here but the basis of the argument involves the boat speed to which your design is aimed .
     
  14. DCockey
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    DCockey Senior Member

    So you are claiming that there cannot be complete recovery of freestream pressure which I agree with. However there can be partial recovery of the pressure.
    By "wake waves" do you mean the transverse system of waves? If so those do not require viscosity to occur.
    A frame of reference moving with the hull can also be used with full viscous analysis, and in that frame of reference the water away from the vessel has a velocity.
     

  15. jehardiman
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    jehardiman Senior Member

    Correct, and that just adds drag, which is why the form aft of the after shoulder is so important for a sailing vessel but not so much for a propelled vessel (assuming an aft proplusor).

    True that the transverse waves are developed by the pressures, however, without viscosity, the transverse waves would be larger. Inviscid solutions don't correctly represent this.

    "All models are wrong, some models are useful" (The aphorism is generally attributed to the statistician George Box, although the underlying concept predates Box's writings. )
     
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