Understanding Porpoising

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

  1. wdnboatbuilder
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    wdnboatbuilder Senior Member

    This is only some thing else thrown out ther but if we increase beam aft would that not help in deminishing the porpoising effects? I don't know just throwing that out there. and if pile most of the weight on top of the CB/CG this also will help no/yes?
     
  2. CET
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    CET Senior Member

    Thanks to everyone who has responded to my questions about porpoising. All the responses have given me a great deal of food for thought. As the last post by cyclops points out, it seems there is a great deal of disagreement about what causes porpoising. I had a hunch that would be the case as the information I had read prior to posting my questions has been fairly contradictory. Nevertheless, I thought it an interesting topic that would make for fun and informative discussion. It has certainly been interesting for me. Any additional input, both from those who have already responded, and others, is welcome. Thanks.

    CET
     
  3. tspeer
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    tspeer Senior Member

    It seems like neither one of us can generate the URL direct to the paper. The reference is:

    Ikeda, Y. and Katayama, T., "Porpoising oscillations of very-high-speed marine craft", Phil. Trans. R. Soc. Lond. A (200) 358, 1905-1915.

    Google the title and you can get to the PDF from that page.
     
  4. tspeer
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    tspeer Senior Member

    I disagree. Porpoising is an issue of dynamic stability. There's a well understood process for finding the stability of any dynamic system: write the equations of motion; assume small perturbations from a trimmed condition to linearize the equations of motion about that equilibrium; estimate the corresponding stability derivatives based on the geometry, hydrodynamic forces and mass properties; calculate the eigenvalues (natural frequencies and damping ratios) to determine the stability; calculate the eigenvectors to determine the "mode shape" (motion components) corresponding to each modal frequency; and look at the sensitivity of the eigenvalues to the stability derivatives to determine how to improve the design.

    The linearized equations of motion can be found in "Principles of Naval Architecture, Vol III", among other places. Commercial software like Matlab or free software like Octave and Scilab can do all the math. The hard part is coming up with the values of the stability derivatives.

    The Ikeda and Katayama paper goes through this process. They show very clearly that it's the cross-coupling between pitch and heave that is the culprit. I think this answers the question, "What causes porpoising?" Damping and mass properties play a role, but by far the dominant contributors are the way the normal forces vary with heave and pitch. Which is good news because these should be amenable to being estimated with a reasonable degree of accuracy.

    The problem is most designers are not versed in dynamics or how to go through the steps above. So they fall back on empirical approaches that may only be relevant for a limited range of designs or may have an indirect connection with planing stability.

    For example, consider the criteria, "running angle of 3 degrees and the CB aft of admidship." CB aft of amidship establishes a forward limit to the center of gravity. A running angle of 3 degrees establishes a minimum loading for the planing surface. Together with the c.g. limit, and a typical bottom geometry (V-shaped sections, straight butt lines), may constrain the cross-coupling derivatives (especially heave-to-pitch) to be in the stable range.

    This may well work for a given class of boats, but doesn't really say much about the cause of porpoising or how to cure the problem if a given design doesn't fall into the same class that determined the empirical criteria. A different set of correlations can be created for each class of designs, which is where all the confusion comes from because there isn't necessarily any overlap between the various criteria.

    What's really needed is either a program akin to SMP to calculate the derivatives based on the hull geometry, or design charts showing the derivatives based on systematic experiements or calculations (ala DATCOM). Then designers could be given a handbook for determining the dynamic stability of a particular design, loading, and operating condition.
     
  5. CET
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    CET Senior Member

    Tom, thank you for your replies! They are very informative, although I have A LOT of homework to do before I'll completely understand them. Much appreciated.
     
  6. yipster
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    yipster designer

    nah, whatdoiknow, placed the pdf link right i my previous post ( checked and it worked for a few hours )
    and now its down again but glad it wasnt me beeing dumb breaking things up :D
    as attachment here the porpoising fast craft pdf i downloaded before
     

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  7. tom kane
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    tom kane Senior Member

    Understanding Porporising

    Thanks for pdf file yipster.This is what a well designed fast planing hull with the right dynamic and and aerodynamic design should look like.
     

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  8. cyclops
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    cyclops Senior Member

    Tom. Are those full length skid fins or secondary keels under the blue boat?
     
  9. tom kane
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    tom kane Senior Member

    Boats are from 2005 World Jet Boat Marathon.I would call the hull additions,sponsons,not just fins.There are many different configerations on racing hulls.It is amazing how little of the hull is in the water with these boats,and they remain very stable.But add a little wind,ahead or side,then look out.
     
  10. CDBarry
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    CDBarry Senior Member

    Martin, Journal of Ship Research, about 1985, has a definitive solution of both porpoising and motion in head seas. Zarnick has done similar work, but I don't have a cite for his papers. POWERSEA (www.shipmotion.com) is one implementation of it.
     
  11. yipster
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    yipster designer

  12. cyclops
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    cyclops Senior Member

    I had watched your boats on TV and thought it was in fast foward. The Vacum Cleaners can corner!!! I automaticly thought they could do anything after that. Proves, one style can not do everything. Thanks for the wind effects information.
     
  13. wdnboatbuilder
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    wdnboatbuilder Senior Member

    Is the same for the BASS boats, a wedge, with more horse power the hull will continue to rise? chines and butt lines never run flat. is that the theory ? I worked for a builder/designer of sport fish and he beat that into my head.
     
  14. tom kane
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    tom kane Senior Member

    I have watched a lot of boats in my 73 years,but have seen very few porporsing and the ones that did were short and beamy.
     

  15. cyclops
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    cyclops Senior Member

    I totaly agree on a short and beamy, high speed 3 point hydros, always being the Porpoise Kings.
     
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