Porpoising

Discussion in 'Powerboats' started by Ernie Travers, May 12, 2009.

  1. Ernie Travers
    Joined: Feb 2009
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    Location: Auckland, New Zealand

    Ernie Travers Junior Member

    Can anyone tell me what causes porpoising on a planing powerboat
     
  2. Guest625101138

    Guest625101138 Previous Member

    Porpoising is a dynamic pitch instability. It is caused by the interaction of two force couples - the centre of lift moving fore and aft of the CoG while the line of thrust moves up and down relative to the centre of drag.

    I have not experienced severe porpoising but I have experienced chine walking, which is similar dynamic instability but primarily involves yaw couples rather than pitch couples.

    This linked calculator has a porpoising predictor included. If you enter data close to your hull then it will give you an indication of the tendency to porpoise at a particular speed:
    http://illustrations.marin.ntnu.no/hydrodynamics/resistance/planing/index.html

    On most planing hulls shifting the CofG forward will reduce the tendency to porpoise. The images below compare the same hull but with the lcg from 2m to 0.8m. The sort of difference you could get with a small boat by moving the crew for and aft.

    Rick W
     

    Attached Files:

  3. Jimboat
    Joined: Feb 2002
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    Location: Canada

    Jimboat Senior Member

    ernie - Porpoising is a common trait of performance hulls - power catamarans, vee hulls and vee-pad hulls in particular are susceptible. Each boat has it's own unique speed at which is can experience the transition through phases of planing performance. This transition speed is called the "hump zone".

    All such hulls are planing on running surfaces and get their performance from a combination of hydrodynamic (water) and aerodynamic (air) Lift. The transition of the Vee-Pad hull to "running on the pad" can cause dynamic instability. Here, the hump zone represents the speed at which the amount of Lift from the highly efficient "pad" section of the hull (aftward, center located, flat planing surface) becomes significant compared to the Lift generated by the veed (full length, higher deadrise) portion of the hull.

    Porpoising comes from a rapid change in the location of the center of Lift as the boat accelerates. The location of static weights is one way of dampening the rate of change of the CofL...so it's not always obvious whether to move weight fore or aft in order to cause the "dampening". The solution can be calculated, but we use software for that. It's not too difficult for you to find out through testing, whether moving weight fore or aft will help your particular problem.

    Propeller selection can often change the dynamic balance of the hull/setup. For example, a change to a prop that provides more aft-Lift can alter the dynamic balance of the hull, and similarly change the speed and range of the "hump zone" – often eliminating porpoising. Weight distribution changes can also have a positive effect on "where" the "hump zone" will occur, as can change in power application. Changing "trim" angle while driving through the "hump zone" even if less efficient, will also provide a better experience, and when well controlled, can "close up" the range of "hump zone" substantially – eliminating porpoising.

    Check out the article on "
    Hump Zone/Why does my Boat Porpoise?".
    And this article on “Chine Walk” if you’re interested.
     
  4. Ad Hoc
    Joined: Oct 2008
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    Location: Japan

    Ad Hoc Naval Architect

    Well, rather than use my own words, i'll paraphrase the 'guru' himself.

    "Porpoising is defined as the combined oscillations of a boat in pitch and in heave, or sustained or increasing amplitude, occurring while planing on smooth water....The results of a porpoising study showed that for a given deadrise angle, there was a specific relationship between trim angle and lift coefficient which defined the inception of porpoising.."

    See attached graph.
     

    Attached Files:

  5. Ernie Travers
    Joined: Feb 2009
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    Location: Auckland, New Zealand

    Ernie Travers Junior Member

    Thanks Rick Willoughby, Jimboat and Ad Hoc.
    It will take me a while to study your replies.
    Coicidentally, I am reading some research by Daniel Savitsky.
    It is all so interesting.
     
  6. Guest625101138

    Guest625101138 Previous Member

    Ernie
    The calculator that I provided the link to is based on Savitsky's work. The formulae he developed are the result of extensive test data but not the fundamental physics.

    There are analytical methods used to determine the forces involved in planing but none of these are used as widely as the Savitsky relationships.

    There are threads covering Savitsky and a couple here have developed their own spreadsheets using his results - a lot of hard work.

    Rick W
     
  7. Frosty

    Frosty Previous Member

    Big long words to say that the boat is too short or the weight distribution needs moving.

    Stick a sand bag in the front and see what happens.
     
  8. Boston

    Boston Previous Member

    not to belittle the excellent responses from the technical end
    but Frosty your response is almost word for word what my old grandad said one day to a guy who came with that exact problem
    he had talked to a bunch of engineers and architects and then came down to the shop
    I think we moved the fuel tank about two ribs forward and that was it
    was a long time ago but still
    the issue was identical

    cheers
    and good luck with it Ernie
    B
     
  9. Guest625101138

    Guest625101138 Previous Member

    Frosty this reply does not answer the original question. You are providing possible solutions; not providing any understanding of the cause.

    There are many other possible solutions that do not involve adding extra weight or rebuilding the boat.

    Rick W
     
  10. Ernie Travers
    Joined: Feb 2009
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    Location: Auckland, New Zealand

    Ernie Travers Junior Member

    Thanks Frosty, Boston and Rick. All your comments are very much appreciated.

    I am mathematical enough to follow the theories and practical enough to appreciate the solutions.

    It was a minor problem on one boat I built and just thought I would ask you guys for the answer.
    Am learning more than anticipated.

    Great fun.
     
  11. Boston

    Boston Previous Member

    might be fair to say
    we went overboard :D
     
  12. edjunior
    Joined: Feb 2008
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    Location: Texas

    edjunior Senior Member

    Boy I'll say! It was an interesting education though. I think I got through most of it. Understanding it all is a different story! :confused:
     

  13. keysdisease
    Joined: Mar 2006
    Posts: 794
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    Location: South Florida USA

    keysdisease Senior Member

    Trim Tabs

    An appropriately sized set of trim tabs will stop porpoising with the touch of a button with added benefits.
     
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