Flat stern bottom extention on displacement power boat?

Discussion in 'Hydrodynamics and Aerodynamics' started by Jeff in Boston, Jul 9, 2021.

  1. Jeff in Boston
    Joined: Sep 2020
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    Location: Boston

    Jeff in Boston Junior Member

    Hi folks!

    Looking at various displacement power boat hulls I see flat stern bottom extensions on many of them. I assume these are designed to be near the waterline? Just under?

    Example:

    proxy.jpg

    I understand that immersed transom area increases displacement speed drag. When these are at the waterline they wouldn't have much immersed transom area. These flat areas at the stern add surface drag I would expect. But not that much?

    Won't this cause slapping when the boat rolls if it is near the waterline?

    Am I thinking about this correctly?

    Thanks!
     
  2. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    I'd say the idea there is to get some extra speed beyond the displacement "hull speed", that flat section would act to reduce squat.
     
  3. messabout
    Joined: Jan 2006
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    messabout Senior Member

    Roll velocity is small enough that the addition of "squat boards" is not likely to cause excessive noise or splashing.
     
  4. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    When the boat is powered up, it certainly would not be a problem, in some circumstances it might slap at rest
     

  5. Jeff in Boston
    Joined: Sep 2020
    Posts: 92
    Likes: 10, Points: 8
    Location: Boston

    Jeff in Boston Junior Member

    Thanks folks!
     
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