Understanding Porpoising

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

  1. CET
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    CET Senior Member

    I have a question for those of you who are less nautically challenged than I am, which is likely all of you. I would like to better understand the phenomenon of porpoising in smallish (18’ – 24’) planning hulls. My understanding is that porpoising is the result of a boat’s COG being too far aft and/or the wetted surface not being large enough to support the weight of the boat on plane. I believe I’ve also read that if the ratio of transverse waterline length to longitudinal waterline length (hope I stated that correctly) is too large it can also contribute to porpoising.

    Are the above statements accurate? Are there other factors that can cause or contribute to porpoising? I realize it is a complicated matter that is not easily understood, but any information would be greatly appreciated. Any recommended reading (books, articles, papers) would be appreciated as well. Thanks!
     
  2. Thunderhead19
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    Thunderhead19 Senior Member

    Yes, yes yes. Also porposing can be initiated with certain hull designs because there is a dynamic instability outside of certain angles of trim.
     
  3. jehardiman
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    jehardiman Senior Member

    Generally, it is the other way around; i.e. the CG is too far forward. Porpoising happens when the center of lift moves aft of the CG. In the normal way of things, the center of lift starts just a little aft of midships. As speed is gained, the lift increases and moves aft as the hull rises out of the water. This cause the classic pitch up seen in most planing hulls. As speed continues to increase, the center of lift moves aft with the decreasing wetted surface. When the lift moves aft of the CG, the boat experiences a rapid pitch down which leads to the wetted surface increasing and a huge increase in drag, which causes lift to be lost which whith the increase in wetted surface forward moves the lift forward of the CG and starts the whole cycle again.

    The major reason for jackplates now seen on modern high power OB boats is to get the CG aft enough (i.e. the engine weight) to allow the full power of the engines to be brought into play to achieve maximum speed.

    I have heard it said that on a high speed planing boat the CG can never be too far aft.
     
  4. CET
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    CET Senior Member

    Thank you for your reply. I'm not sure I understand what is meant by "dynamic instability outside of certain angles of trim". What does "dynamic instability" refer to here? Thanks, and I'm sorry for my ignorance.
     
  5. CET
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    CET Senior Member

    Thank you for this info. It’s a great explanation that makes it easy to visualize what happens when a boat porpoises. It seems a little counterintuitive at first but the more I think about it the more sense it makes. So, in a high speed planning boat, the further back the CG is, the better the boat will perform. Is that correct? It would seem then that the heavier the outboard used, the better the boat would perform (as long as the weight of the motor does not exceed the boats carrying capacity).
     
  6. cyclops
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    cyclops Senior Member

    The moving of the balance points rearward beyond what is needed for maximum speed creates the nasty conditions of blowovers if the bows upward angle is increased for any reason. Keep the weight in a neutral position. Too far rearward is worse then too far foward. Blowovers can not be sensed or stopped. Porpoising can be felt and stopped. All racers dread a blow over. Blowovers can occur at 20 mph when meeting a head on gust of wind that is doing 40 to 50 mph.-------------------------- I know . I have had almost blowovers under just such conditions on the St. Lawrence River.
     
  7. CET
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    CET Senior Member

    Thanks for the comments, cyclops. A blowover sounds nasty!

    So are there some rules of thumb for the ideal placement of the CG in a planing hull? What about the ratio of transverse waterline length to longitudinal waterline length?

    Thanks!
     
  8. cyclops
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    cyclops Senior Member

    I can not be a designer only tell you the good and not so good.
     
  9. nevd
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    nevd Junior Member

    No, this comment is NOT correct for porpoising - there is normally a trade off between efficiency (speed) and porpoising. Porpoising is caused by running too high a trim angle and moving weight forwards will decrease porpoising ie the opposite of running a heavier outboard.

    Porpoising is complex and is not easily covered in a few line explanation. If you want to get more information, you should go through research reports - some in the SNAME small craft CD or the earlier research reports primarily by Savitsky in the 60's and 70's.
     
  10. RThompson
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    RThompson Senior Member

    here's a good book that will put you in the picture - easy to read/follow, only 100 pages or so:

    Originally posted by mmd
    Introductory level:

    "Small Boat Design for Beginners" - Frank Bailey (1980) A.H. & A.W. Reed Pty Ltd, Sydney, Australia ISBN 0 589 50203 4

    It covers basic hydrostatics & planing theory, linesplans, powering, preliminary design. A good basic book.


    Rob
     
  11. tspeer
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    tspeer Senior Member

    I don't find the explanations above very satisfying. They don't answer some very essential quesitons, like,
    "How does one design the shape of the hull to ensure it will not porpoise in its design speed range?"
    "What is the range of center of gravity locations for stable operation of a given design?"
    "What is the critical trim angle for a given design?"

    I think this paper gives a much more satisfying explanation of porpoising. What the author is saying is, porpoising starts out as an instability for small motions that grows in amplitude until limited by nonlinear effects. So if you want to avoid porpoising, it's sufficient to look at the linear conditions for stability - you don't need to be able to predict the amplitude of the oscillations that ensue if you get it wrong.

    Starting from a trimmed, steady running condition, if the craft is disturbed in heave or pitch, it will be stable if the following four conditions are met:
    - increasing depth (heave) creates a force that wants to accelerate the craft back up (C33 is positive in his equation),
    - increasing pitch (bow up) creates a moment about the c.g. that wants to pitch the bow down (C55 is positive),
    - increasing the pitch tends to accelerate the craft upwards (C35 positive),
    - increasing the depth tends to pitch the craft bow down (C53 positive) [this one actually sounds backwards to me, so maybe I'm misinterpreting the paper]

    The craft will be unstable when C53 is negative, meaning the that for an increase in depth, it tends to pitch the craft up. So the question becomes, "What conditions will drive C53 negative?"

    Here's my application of Ikeda and Katamaya's princples to the situations cited in previous messages:

    The pressure distribution on a planing surface is heaviest just behind the leading edge. As the trim angle is reduced without changing the center of gravity, the leading edge moves forward.
    Eventually the heavily loaded portion gets to be far enough ahead of the center of gravity that it causes a bow-up pitching moment when the depth increases. This causes the craft to be unstable, and it reaches the onset of porpoising.

    Moving the center of gravity forward without changing the trim angle will reduce the bow-up pitching moment from the planing surface and make the craft more stable.

    But in general, moving the c.g. forward will also reduce the trim. So if the leading edge of the planing region moves forward slower than the c.g. moves forward, the c.g. shift will be stabilizing. If it moves forward faster than the c.g. shift, it will be destabilizing.

    The same calculations that one does to estimate the wetted area and running trim for performance could be used to get the variation in the forces and moments about the c.g. for small changes in the heave and pitch angle. Then you can apply Ikeda and Katamaya's static stability criteria (C35 and C53 have the same sign) to find out if the craft is stable or not.

    Better yet, one could estimate all the matrix elements in the equation, including inertias and damping coefficients, and get the four eigenvalues to see if their real parts are all negative.
     
  12. wdnboatbuilder
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    wdnboatbuilder Senior Member

    I'm no designer........yet. But the old timers that I have worked for, say a running angle of 3 degrees and the CB aft of admidship. But this is what i learned in school. When I designed my boat the CB was aft of station 4 in a 6 station boat. Ofcourse I had them to fall back on to look at my design that was very handy. And I am sorry to say the one who held records has passed on. He was tough to get information out of but if you watched and asked the right questions he may or may not answer according to how bad he hurt that day. I follow simple rules that I picked up from him, But it seems that the question was answered in the reason that if your boat porpuses then move some wieght forward which is very simple and more than likely it will not take much wieght to do so. I'm sorry i have not answered your question and these gentlemen have a lot more schooling than I have, So their opinion is much more valuble than mine.
     
  13. yipster
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    yipster designer

    yes it seems simple never the less thanks Tom, here the short porpoising pdf from that site

    edit: put working porpoising pdf link in
     
    Last edited: Nov 28, 2005
  14. Thunderhead19
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    Thunderhead19 Senior Member

    Let's not forget about the natural frequency of the boat here. Shifting wieghts around in the boat can actually make the problem catastrophically worse if you don't watch it. If you push weights just far enough forward to hit a node, bye-bye boat.
     

  15. cyclops
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    cyclops Senior Member

    Obviously we do not know what causes Porpoising in general. There are too many conflicting statements on the subject. Poor design for given loads and speed range. I have read of old woodies that had enough speed to pass thru the Porpoise range.
     
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