Porpoising

Discussion in 'Hydrodynamics and Aerodynamics' started by TobiasJohan, Oct 21, 2014.

  1. TANSL
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    TANSL Senior Member

    PAR, all these explanations are very valuable to understand the porpoising and I sincerely appreciate them. For me, a phenomenon that repeats without a certain cadence, is not cyclical but, as I say, here I find my difficulties in explaining without talking nonsense.
    In any case, the OP did not want a physical description of what is the porpoising but above all, to know why it occurs. I tried, in post # 4, give a little hint, before entering this absurd discussion if it is a circumstance or a situation, if cyclic or periodic.
    Seeking to explain why porpoising occurs, if anyone does not know, the best thing to do is shut up.
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    The OP has gone AWOL, apparently. Not much use talking about it in theoretical terms, he may have a particular problem peculiar to his boat because it is stern-heavy, or whatever, but we may never hear from him to know !
     
  3. gonzo
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    gonzo Senior Member

    TANSL follow your own advice then. Understanding of cyclical phenomena is critical to understanding porpoising. There is a moment created by the vertical difference between the lift, resistance and impulse. In ideal conditions, flat water and no weights shifting onboard, the frequency will not change. However, it is possible to observe that the motion of boats repeats within a range that is consistent with the variation of the conditions. That is, within the limits of standard deviation.
     
  4. powerabout
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    powerabout Senior Member

    plenty of boats that porpoise will do it in perfect rhythm
    its just a lift and drag thing
     
  5. gonzo
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    gonzo Senior Member

    It's the kind of thing anybody that has operated boats experiences. The math to explain it can get complicated, but the concept is simple.
     
  6. Joakim
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    Joakim Senior Member

    I would say purely longitudinal porpoising is very much cyclic and the period is quite constant, if nothing is changed (e.g. waves, power etc).

    Here's a simulation of porpoising: http://www.youtube.com/watch?v=z0h-oLPt8Gw

    Here's a real boat: http://www.youtube.com/watch?v=KBnqzjttnlo

    If you also or only have transversal instability, often called chine walking, then the instability is typically not cyclic in a similar way. Chine walking can start from longitudinal porpoising, waves or just by itself. Here's an example: http://www.youtube.com/watch?v=WKROibULq1U

    Longitudinal porpoising can be predicted. It depends on hull form, trim angle (keel vs. water, not motor trim), speed and loading. At each operating point there is a maximum stable trim. If that is exceeded, porpoising starts.

    The higher the speed the lower this maximum stable trim will be. That is why most boats porpoise only at high speeds. But also trim angle gets naturally lower with speeds. Thus there are boats that porpoise at some speed range and then became stable.

    I had a small boat, which porpoised quite violently from 15 to 35 knots, but was then very stable from 35 to 48 knots (top speed). The few seconds to accelerate thourgh the porpoising range were very uncomfortable untill you got used to it. Sometimes it started to chine walk as well, but you learn to deal with that be steering and controlling power.

    So if the OP has similar porpoising to the first two videos, the solution is to lower the trim angle or to modify the hull to cope with the current trim. The first one can be achieved by moving the center of gravity forward or chaging the engine trim (or sail configuration).

    The latter by going to narrower hull(s) or deeper V, but that will also increase the trim angle. Typically the maximum stable trim increases more than the actual trim.

    Here's paper about porpoising: http://www.dtic.mil/dtic/tr/fulltext/u2/b240994.pdf
     
  7. TANSL
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    TANSL Senior Member

    I do not want to stress my mistake, but exchange views. Is it then correct to think that in reality (not in experimental conditions) porpoising is not a cyclical phenomenon?. Thanks.

    I think it's also a question of the relative position of the pressure centers, centers of gravity and center of rotation of the boat. The position of these points is so close that it is impossible to maintain equilibrium because any slight disturbance breaks it.
     
  8. TANSL
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    TANSL Senior Member

    Gonzo Frankly, with all due respect, what you say makes no sense to me. I will analyze your statements:
    Understanding of cyclical phenomena ...: there are many cyclical phenomena that have nothing to do with porpoising and therefore understanding them can not help understand the porpoising.
    a moment created by the Vertical difference ...: I do not understand. A moment is a force times a distance (perpendicular to the force) and do not understand how a vertical difference can lead to a moment. That means nothing.
    In perfect conditions, flat water and no shifting weights onboard, the frequency will not change: that's not correct at all. Only by changing the speed porpoising change or even disappears.
    Within a range repeats that is consistent with the variation of the conditions: Probably you must have sailed a lot to understand this. I do not understand.
    Within the limits of standard deviation:'ll think for a while to see if I can know what the standard deviation is and its limits. Wish me luck.
     
  9. gonzo
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    gonzo Senior Member

    Cyclical phenomena are all based on the same principles of physics. As a naval architect that should be easy for you. I can only think you are being facetious.
    A moment is a force times the distance. If you had quoted the rest of the sentence, it clearly indicates that the forces are not acting on a point but there is a distance between them. Ergo, a moment is created
    I said that the frequency will not change if the conditions do not change. You argue that I am wrong because porpoising disappears with a change of speed. Exactly what I said, only a change in conditions will generate a change in behavior. Once more, I can only think you are being facetious.
    Boat movement is a consequence of the conditions it is in. I find hard to believe an expert like you doesn't get that.
    Standard deviation is a term from really basic statistics.
     
  10. TANSL
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    TANSL Senior Member

    Gonzo my friend, thank you very much for your kind (although not clear) explanations. Unfortunately with them I am not able to understand anything. Fortunately I do not need them. But the effort, huge, you have used is highly appreciated.
    I wish you the best.
    Ignacio
     
  11. PAR
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    PAR Yacht Designer/Builder

    Porpoising can be cyclic and usually is, but the very nature of the conditions that generate this dynamic state, suggest the period may fluctuate to some degree. If the boat is screaming along in dead flat, undisturbed water, the undulations will be very rhythmic and quite cyclic.

    The problems with this description is as I mentioned in my first post, which is the sea state is rarely dead calm and more importantly, there are other influences that can affect and/or cause a longitudinal instability, including hull shape, drive type, prop choices, etc. As TANSL mentioned pressure wave variances, can effect the cause and period. I had a very difficult time many years ago, trying to diagnose a boat with this condition. It wasn't completely speed related, but a combination of issues all contributing to some degree to it's instability. The boat was warp bottom, but also had these extensions that stuck down vertically, along the aft portion of the chine. The idea with these was to help this straight shaft equipped boat turn a little better, which all reports suggested it did after installation. The boat also had a noticeable hook just above and aft of the prop location. It almost looked like the rudder had struck something an shoved the bottom up.

    Ultimately, the pressure wave variances under the hull increased exponentially with the chine extensions, which cause the aft chine to dry out occasionally, promoting a mild walk. If speed was increased and the water fairly flat the transom would stay wet (enough) to drive though this subtle walking, just to begin porpoising. It was clear she was at the upper end of her abilities. The fix was to remove the hook and chine extensions, which decreased the progressive pressure buildup, as flow moved aft, typical of warped bottom hull forms, however the porpoising continued, just later in her speed range. After I move it's four 8D's aft several feet, she was able to stay at WOT without undue dynamic situations coming to play.

    Simply put, there's lots of things that can cause, affect and make variable this type of instability, including a few I haven't mentioned, but there's no doubt it can be cyclic in nature - not always, but most of the time in the usual conditions and that figuring it out, could be as simple as adjusting trim or much a more complex set of problems, though all affected by sea state conditions.
     
  12. Mr Efficiency
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    Mr Efficiency Senior Member

    Funny the way people ask questions, then disappear !
     
  13. whitepointer23

    whitepointer23 Previous Member

    I got my boat to lift and drop the nose with the leg trimmed out but I didn't do it on porpoise.
     
  14. Mr Efficiency
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    Mr Efficiency Senior Member

    It was only a matter of time ! :D
     

  15. PAR
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    PAR Yacht Designer/Builder

    I think he found what he was after, an explanation of what it is and/or was turned off by the syntax debate that followed, some of which was indeed on porpoise.
     
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