transom edge sharp or not

Discussion in 'Hydrodynamics and Aerodynamics' started by fallguy, Jul 9, 2018.

  1. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    Does the transom edge need to be sharp on my boat?

    I calculate a Froude number of about 0.9, based on a 9m/s velocity and a 10 meter long hull. If the calculation is bad, forgive me, I can redo it.

    Someone told me the boat would go 2-3 mph faster with a sharp edge.
     
  2. HJS
    Joined: Nov 2008
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    HJS Member

    This is almost full planing speed, so the transom must be as sharp as possible.
    JS
     
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  3. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    "Sharp" needs to be defined, but losing 2-3 mph would require some kind of crazy radius.
     
  4. fallguy
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    fallguy Senior Member

    BFBAA246-C28D-4AC5-9842-C98015AAD297.jpeg How's this? Sorry bout the rotation.
     
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    Nothing to worry about, I'd say.
     
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  6. messabout
    Joined: Jan 2006
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    messabout Senior Member

    Mr E. is right.

    That radius will not cost you 2 to 3 MPH. If you make it into a sharp edge the corner will become vulnerable to chipping and breaking which may cost more speed that you might have gained
     
  7. sandhammaren05
    Joined: May 2009
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    sandhammaren05 Senior Member

    The sharp edge helps the boat to develop lift faster, to plane faster, and to avoid the transom running deeper in the water
    at high speeds (avoiding increased drag). A rounded trailing edge goes against all that. Race boats never have a rounded
    trailing edge unless someone got sloppy. The correct Froude nr. is the depth F-nr. at the transom.
     
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  8. fallguy
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    fallguy Senior Member

    To all. I did in fact run a bead of cabosil and milled glass there. Both will be finished sharp, but paintable sharp.

    Thanks.
     
  9. sandhammaren05
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    sandhammaren05 Senior Member

    True, but some chipping is better than a rounded edge for performance. All five of my classic runabouts have sharp trailing edges.
     
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  10. Michael Lambert
    Joined: Jan 2024
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    Michael Lambert New Member

    Hi everyone, I have a similar question for a different situation: wing and surf foil boards. Early foils were slower and lower aspect, so boards were designed with sort of displacement tails with cutaways to allow for the rider to pitch the board up and out of the water. Now foils are faster, higher aspect, and less tolerant of higher AOA, so full planning speed at takeoff. But many boards still have rounded tails, and when you touch down I think that makes them stick more than them need to. This is important because in many situations, like light wind winging and downwind SUP foiling, there is so little energy that a little friction fora moment can take you down. So my question is: how much of a difference would sharper release edges make in that situation? I’ll try to post some examples.
     
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  11. Michael Lambert
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    Location: Portland, ME

    Michael Lambert New Member

  12. Ad Hoc
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    Ad Hoc Naval Architect

    As sandhammer notes, sharp is always best.
    How much, ... simple trial and error will tell you the absolute amount - ie test with radius and after made sharp.... as too many variables to state categorically.
     
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  13. baeckmo
    Joined: Jun 2009
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    baeckmo Hydrodynamics

    In a hydrodynamic sense, a general definition would take the radius in relation to the actual boundary layer thickness.
     
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