Canoe Sprints

Discussion in 'Hydrodynamics and Aerodynamics' started by DCockey, Aug 10, 2012.

  1. Ad Hoc
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    Ad Hoc Naval Architect

    Eerr..no, you asked about the oscillating weight, no reference to the boat oscillating per se.

    But since I now assume you mean pitching of the boat, not the oscillations of the weight.

    If there is a significance difference in weight (ratio of weigh to boat to cause significant pitching) then the amplitude shall change, but not the period. Just like with any damping device.
     
  2. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Although developed for rowing, the equations in Chapter 2 and 3 of the attached thesis should give you an idea of how to proceed.

    Good luck!
    Leo.
     
    Last edited: Aug 12, 2015
  3. DCockey
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    DCockey Senior Member

    Let's try again.

    Will be any difference in the amplitude of the pitching of a boat with a weight oscillating fore/aft (with the weight and amplitude of the weight's oscillation large enough to cause significant pitching) as the average velocity of the boat through the water changes if the boat, the moving weight and the amplitude of the oscillations of the weight does not change?
     
  4. Ad Hoc
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    Ad Hoc Naval Architect

    As interesting as all that looks Leo, :rolleyes:
    Im off to the flix now :D
     
  5. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Hope it's something interesting! Total Recall?

    I'd love to get involved in this discussion at some stage, but I'm stuck on
    creating wave pattern animations of SES while listening to Gibbon's "Rise and Fall of the Roman Empire".
     
    Last edited: Aug 31, 2012
  6. DCockey
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    DCockey Senior Member

    Thanks Leo! My questions are more related to the topics of Chapters 4 and later. How does mean linear velocity affect the "unsteady" hydrodynamic forces? For a given unsteady motion of the hull are they independent of mean linear velocity?
     
  7. Ad Hoc
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    Ad Hoc Naval Architect

    Struggling to follow the sentence’s meaning owing to poor structure to be honest; it doesn’t flow properly. However, I think you’re trying to do an Einstein and get a one equation does all.

    You really need to address what is affected by the swinging mass, and as such, the interaction is not so straight forward.

    Looking again at the factors that affect pitching:
    1) Inertial moment
    2) Damping moment
    3) Restoring moment
    4) Exciting moment

    So briefly addressing:
    1) This has the term mass and added mass, which is not going to be influenced by the swinging mass, since it is already included in the ship. The radius of gyration however shall be affected.

    2) As noted above, the swinging effects the damping coeff’s. If the coeff’s are significantly changed it can alter the period. You also have to be careful in what way too, since if the decay constant is very much greater than the natural frequency of pitch, this becomes aperiodic. This is not studied by NAs only mathematicians, as it is of no interest to NAs.

    You could ask Leo if he knows any decent mathematicians that wish to explore this :D

    3) Restoring, again see above.

    4) The hydrostatic pressure distribution can be argued to change owing to the trimming moment created, but that again depends upon many factors.

    No. Went to see Prometheus, it has only just come out here. Was..hmm..so-so. Could have been better, and the meaty parts were over too quick and left unanswered questions. Too much “action” and not enough plot driven for me. Not cerebral enough. If I want a “visual” film, I’d go for Star Ship Troopers :p
     
  8. DCockey
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    DCockey Senior Member

    Your thinking is mistaken. Nothing about equations.

    It's clear you are struggling but the reason may be other than the sentence structure.

    Are the damping moment or the restoring moment different when the vessel has forward velocity versus not no forward velocity?

    My understanding is a very common assumption in boat and ship dynamics is to assume that the damping moment and restoring moment are independent of forward velocity and depend only on the heaving and pitching. When this assumption is no longer valid may be worth further discussion but probably in a separate thread.
     
  9. Leo Lazauskas
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    Leo Lazauskas Senior Member

    David, I can't answer your question because (1) I have not analysed canoes
    and (2) I haven't calculated the quantities you would like to see for
    rowing or kayaks.

    However, the following might give you some insights, remembering that
    in rowing there is a large variation in speed during the stroke.

    For rowing there is a significant increase in pitch amplitude with
    increased stroke rate. See, for example, Kleshnev:
    http://www.biorow.com/RBN_en_2012_files/2012RowBiomNews03.pdf

    Experiments I have collated suggest a similar trend: pitch amplitude
    increases as stroke rate increases.
    The first attached graph shows measured pitch amplitude as a
    function of stroke rate for 13 classes of rowing. (There are about
    11,000 strokes in the set.)

    Unfortunately, matters are complicated by the fact that rowers exert
    higher forces at higher stroke rates, boat speed is faster than at
    lower stroke rates, and there is a larger vertical component to the
    force exerted at the footstretchers, which increases pitching.

    At higher stroke rates, the rower will tend to push the bow down
    further than at low stroke rates as they slide backwards on their
    seats towards the bow. This means that the LCB tends to move a little
    more bow-wards than it would at lower stroke rates. Rowing shells
    (and many other hulls) tend to have a larger sinkage force when the
    LCB is further bow-wards, either because of design, or because of
    "induced" trim due to the location of the CG.

    Rowers will also induce larger bow-up moments at higher stroke rates
    as they move back towards the stern.

    The second graph shows the position of the bow and stern during a
    typical stroke for a men's rowing single.

    The pink and light blue curves are for the case where the rower
    moves forwards and backwards in the shell, but no squat forces are
    used in the calculations. It can be seen that the bow is up at the
    start of the stroke (when the rower is farthest sternwards) and down
    at the middle of the stroke when the rower is farthest bow-wards.
    Similarly, the stern is down at the start, and up at the middle of
    the stroke. The red and blue curves include dynamic forces and moments
    in the calculations.

    Note that at about t/tstroke = 0.2 and 0.8 the bow and stern are at
    the same level, and they are both at z=0 (the undisturbed waterplane).

    When squat is included, the bow and stern are at the same level at
    about t/tstroke = 0.3 and 0.65, and that they are both less than zero:
    that is due to dynamic sinkage.
    I suspect that the curves will look very similar at stroke rates not
    too different to the 36 strokes per minute I used here. For very low or
    very high rates there will probably be some differences.

    Lawrence J Doctors and his colleagues have examined some unsteady
    effects in their papers that might be of interest. See:

    Alexander Day, Ian Campbell, David Clelland, Lawrence J. Doctors and
    Jakub Cichowicz,
    "Realistic evaluation of hull performance for rowing shells, canoes,
    and kayaks in unsteady flow",
    Journal of Sports Sciences, July 2011, pp. 1059-1069.

    and

    Lawrence J. Doctors, Alexander H. Day, and David Clelland,
    "Unsteady Effects During Resistance Tests on a Ship Model in a
    Towing Tank",
    Journal of Ship Research, Vol. 52, No. 4, December 2008, pp. 263–273.

    I really do feel for Mola ( the author of the thesis I attached previously)
    and his research colleagues. They tried to solve the complete problem
    using CFD, but found that it took too long, and failed to converge for a
    Wigley hull. They also tried simpler Euler methods (IIRC) but they too were
    computationally expensive. The last I saw, some of the group used
    thin-ship methods to get the wave resistance and the ITTC line for skin
    friction, but they didn't give comparisons of predicted speed and experiments.
    Compare with my model which gives pretty good estimates of the speed and
    acceleration (if I am allowed to choose, within reason, the motion of the
    rower in the shell).
    http://www.cyberiad.net/library/pdf/rsn07sep10.pdf
    And I can simulate an entire 400 second race (with 100Hz experimental data)
    in less than 1 minute.

    I agree with Ad Hoc that:
    1. You might be asking a little too much at this stage. Either nobody is
    interested, and/or they haven't calculated what you want.
    2. Prometheus is a mediocre sci-fi movie. :)

    Leo.
     

    Attached Files:

  10. Leo Lazauskas
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    Leo Lazauskas Senior Member

    That is my understanding too. When I get some time I'll look into the motions codes I have to see how they are implemented.
     
  11. DCockey
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    DCockey Senior Member

    That is much more complicated than what I was asking about. The reason I asked about a boat with an weight with the weight oscillating at constant frequency and amplitude was to simplify my question, eliminate all the other effects of rowers and paddlers such as stroke rate, etc, and concentrate on the effects of velocity on the damping and restoring forces and moments.

    Perhaps the assumption that the damping and restoring forces and moments are independent of forward velocity is so deeply imbedded in naval architecture that it is very difficult to consider other possibilities.
     
    Last edited: Aug 31, 2012
  12. Leo Lazauskas
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    Leo Lazauskas Senior Member

    IIRC, the US Navy's Ship Motion Code (SMP) includes several routines to
    calculate speed-dependent added mass and damping. The program also
    outputs coefficients for zero-forward speed and at speed so, in principle,
    you could estimate the effect of forward speed.
    But I haven't looked into it for years, so my memory might be faulty.
     
  13. Ad Hoc
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    Ad Hoc Naval Architect

    I see irony still does over the head of most Americans :eek:

    Your sentence structure and grammar is often shocking. It is very hard to determine what is your asking as the “subject” constantly moves or is left hanging on its own! Perhaps it is an American ‘thing’ in the way you write. There are many prepositions and articles missing in common American text which I find terribly confusing to actually comprehend what is being said or asked. But you are of course free to make your own conclusions, doesn't bother me one bit.

    Perhaps you are failing to grasp the individual parameters that are being influenced in ship motions by such a statement and as such looking for another meaning somewhere else.

    The damping coefficients in heave/pitch are influence by the encounter frequency of oscillation. Ergo forward speed makes a difference. How much depends upon several factors. I've no idea where you get your deeply embedded assumption from?

    As for the restoring moment for heave, for example, it is just the additional buoyancy from having an increase in draft. In pitch it is similar and can be considered to be linear proportional to the angle of pitch. This is dictated by the hull shape and waterplane area at that angle and ultimately the GML. Thus it is still questionable whether speed is influencing these factors, as it depends upon the nature of the hull geometry and the wave profile upon the hull. There may well be, analytically, but reality, requires large changes not endless decimal places. But these are ostensibly dependent upon hull geometry and centres of weights, and 'generally ' considered independent of speed.

    As a side note we have tested many different type of bows, from the normal to the ridiculous to establish the differences in pitch/heave. One can note very small differences. But in reality they are negligible. It is all about "perception".

    May I suggest that you look at how the damping coefficients and restoring moments are actually calculated for a vessel for heave and pitch, it is not straight forward. If that doesn't answer your question, then I‘m afraid you’ll be searching a while for the answers you seek.
     
  14. Ad Hoc
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    Ad Hoc Naval Architect

    Concur.
     

  15. NoEyeDeer
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    NoEyeDeer Senior Member

    I think you mean "goes".


    Quite honestly, yours isn't exactly brilliant either, and I am no American. I can grammar Nazi the rest of your post if you think it will advance the thread, but I'm inclined to think that it would not. Perhaps it would be better for everyone to concentrate on making genuine attempts to understand each other.


    There is no encounter frequency, since the example for this thought experiment is a boat moving in calm water. D Cockey has already stated this, in an attempt to clarify the problem.

    His current question seems clear to me. In the absence of any other changes in the proposed craft, will changing the forward velocity from zero to non-zero have any effect on pitching?

    To further clarify, this is assuming that when the average forward velocity is non-zero it is also constant.

    Hope this helps. :)
     
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