Understanding Porpoising

Discussion in 'Boat Design' started by CET, Nov 22, 2005.

  1. wdnboatbuilder
    Joined: Nov 2005
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    Location: Cape Coral Fl

    wdnboatbuilder Senior Member

    how short ? how beamy? I was watching a show on a wooden speed boat once and it looked to that the boat was not only porpoising it chine walked also is that caused by too much beam or should this be a new thread altogether
  2. CDBarry
    Joined: Nov 2002
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    Location: Maryland

    CDBarry Senior Member

  3. yipster
    Joined: Oct 2002
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    Location: netherlands

    yipster designer

    just did, thanks!
    guess it was offline and i started searching for powersea
  4. bananas
    Joined: Jul 2005
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    Location: miami florida

    bananas Junior Member

    Hi CET

    The flatbottom boat racing classes have dealt with the porpoising/blowover problem by perhaps the best design technology known, natural selection. Since the price of a mistake is often the drivers life they carefully push the limits with empirical changes.

    Jerry Griffin is changeing his sample videos, but when they are back up I suggest you look a a superstock (600 HP) or K-boat (2000 HP) running.


    You also might want to visit


    to see the boat configurations. Over 30 years of racing one hull type has emerged dominant. The fastest and the safest.

    You might want to visit


    to see the speeds in the class records. The speed are spectacular.

    The technology:

    (1) Engine and driver placements are forward to provide aerodynamic stability when airborne.
    (2) Propellers are a couple of feet forward of the transom to push the hydrodynamic center forward when coupled with the water.
    (3) The aft sections are the hulls are rockered. At low angles of attack the rocker will move the hydrodynamic center forware of the bow!
    (4) The boats employ lifting strakes.
    (5) Turning fins are about at the center of gravity.

    A K-boat will do nearly 150-MPH on the straightaway and turn at speeds over 90 MPH. A superstock will do 120-MPH on the straightaway and turn a little faster than a K-boat.

    The physics of porpoising and blowover are well known. The mystery is in the component dynamic forces. Design by means of evolution has progressed far beyond the available component force data bases.

    There is a gentleman with the screen name gfinishline who posts at


    who makes his living making flat bottom boats go fast safely. If you have a real project you might want to contact him.

    Give me an e-mail addy and I will forward a good video sample to you.

    to see the boat configurations
  5. CET
    Joined: Sep 2005
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    Location: Utah

    CET Senior Member

    Hello Bananas,

    Thanks for your reply and offer to help with my questions. As you may have noted, I am not an experienced boat designer and thus don’t have much knowledge of hull design characteristics and terminology, but I’ve found the people here to be exceptionally knowledgeable and helpful, yourself included. Although my interests are more with slower semi-v bottom planning craft in the small to medium size range, I would think that the same principals would apply when it comes to porpoising and other stability problems. I would love to see the videos you referenced. My e-mail addy is cet1964@hotmail.com.

    In the past couple of years I’ve become very interested in designing planning fishing-type skiffs/boats in the 18’ – 28’ range and have been playing around with several software packages. Stability issues are the biggest blind spot for me. Any info you can provide would be greatly appreciated. Thanks.


  6. fede
    Joined: Sep 2003
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    Location: milano

    fede Senior Member

    does a demo of powersea exist?
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