Skeg box drag

Discussion in 'General Paddling Discussions' started by rider, Oct 29, 2018.

  1. Tangler

    Tangler Paddler

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    I have noticed, during the many hours that I have spent sitting on and standing in rivers, that they seem to have a noticeable pulse with a period of less than a minute. Might make for noisy data...
     
  2. mick_allen

    mick_allen Paddler & Moderator

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    [referring to Pascal's last post]
    I'm with you on the first paragraph - but we have to accept what you say in any case.

    The tough part of the 2nd paragraph is having the kayak in both cases exactly aligned with the current. And then making sure there is zero slop in rudder deployment - which I would say is virtually impossible with typical pinhead rudders [ as they are almost the definition of sloppy deployment and usage.] And then on top of that, our typical deployment [sliding or rotating footpegs] are also so sloppy and crude and unable to be finely tuned - that any deployment measurements would be useless as it would be difficult to control and especially duplicate.

    And the 3rd paragraph, I don't know . . . do you use a slab skeg to reduce skegbox water ingress while retracted?; do you use a thin foiled shape that's sloppy in the box that is not rigid when it is deployed?; is there a large box water volume when the skeg is deployed? It's one of the reason that vertical skegs or drum rudders are so interesting - unfortunately not applicable to yaks. It'd also be interesting to test the difference if the rigid skeg was deployed behind the kayak [like a typical rudder]

    I don't know about pulse, but I get mesmerized by the interesting variations that happen in smooth current.
     
  3. Pascal

    Pascal Paddler

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    Mike:

    I'm with you on the first paragraph - but we have to accept what you say in any case.
    I suppose/hope(!) you meant “they” instead of the second “you”

    The tough part of the 2nd paragraph is having the kayak in both cases exactly aligned with the current.

    Maybe, but my experience lining boats in rapids makes me think it might not be that difficult to have the kayak relatively stable and aligned with current lignes in a smooth section of river or tidal rapid instead of swaying widely around from side to side. It would strongly depend on the method used to attach the line to the kayak. I have an idea how to stabilze yaw without afecting the drag but it remains to be seen in the field. This is just guessing game from intuition and experience.

    And then making sure there is zero slop in rudder deployment - which I would say is virtually impossible with typical pinhead rudders [ as they are almost the definition of sloppy deployment and usage.] And then on top of that, our typical deployment [sliding or rotating footpegs] are also so sloppy and crude and unable to be finely tuned - that any deployment measurements would be useless as it would be difficult to control and especially duplicate.

    Not sure I understand what you are pointing at here (or why). Variations in deployment and angle is part of the things you want to measure and average over many readings, since this is precisely how the rudder is going to be used. It would be pointless to derive a rudder drag independent of the boat it is affixed to, unless you are the manufacturer of the blade and want to publish its hydrodynamic characteristics for marketing purpose. Don’t think that’s the case here. So the trial aims at testing the system as a whole, not the rudder blade alone in total isolation. If the rudder mechanism often produces angle error (in both planes), it’s good that the trial can account for it and allow to measure the total average drag that will include the slight increase in drag resulting from that imperfect/sloppy control system.

    Left and right angle errors induce the same drag increase (by symmetry), so it does not matter if it happens that the rudder systematically drops slightly to the right (or to the left) on the day of trial. The slight increase in drag due to the sloppy mechanism will be taken into account. That’s all one needs. A small pair of wise-grips applied on both rudder control mechanisms (the foot step ramps for instance) could be easily used to freeze them under tension and at “neutral” if one does not want to include the sloppiness of the control.

    Similarly, the consistency of the vertical deployment is pretty easy to improve, if one really wants to, by having a visual mark on the rudder control cord. That would not produce 100% consistency, but I doubt any small variation would affect the results drastically enough to void the trial concept. But again, I think it might be better to just average the rudder drag readings with the control mechanism working as sloppishly as it does in normal/real operation.



    And the 3rd paragraph, I don't know . . . do you use a slab skeg to reduce skegbox water ingress while retracted?; do you use a thin foiled shape that's sloppy in the box that is not rigid when it is deployed?; is there a large box water volume when the skeg is deployed? It's one of the reason that vertical skegs or drum rudders are so interesting - unfortunately not applicable to yaks. It'd also be interesting to test the difference if the rigid skeg was deployed behind the kayak [like a typical rudder]

    For testing the skeg box drag with something similar to the rudder set up I have proposed (and still think about thanks to your inputs), I see no other option than averaging out the drag reading with no cover over say, 5 minutes, then cover the box with whatever (thin waterproof tape?), averaging again, then switching back and so on, hoping for the hydro and wind conditions to remain relatively unchanged between 2 successive series (used to devive one delta).
    It would be harder and takes longer than the rudder test. Plus the drag increment will be a lot less than with the rudder, so maybe undetectable.


    I get mesmerized by the interesting variations that happen in smooth current.


    You probably think of relatively slow currents (2-3 km/h) with surface eddies. The test I am thinking about would be better done in a fast current where surface eddies become negligeable and for which drag differential would be max and easier to measure. I don't think it's very difficult to test whether visible “variations” are too large and affect the kayak drag too rapidly to render the test impossible. That’s part of site selection, and yes, it won't be easy, but no impossible (I think), to find a suitable enough spot to perform this test.

    Tangler:

    I don’t know what a river “pulse” is, especially a river pulse with a period less than a minute. First time I hear about that and found nothing online. Can you describe what it is in more details, what is changing/pulsing exactly -current speed, level,..the amplitude, etc. on which river you saw it, etc.?, and how you think it is generated, the mechanisme behind it? I have witnessed river levels changing slowly with snow melt/freeze or rain events during the course of the day, but apart from flash-flood events, never seen a river going up and down within a minute. Anyway, for the test, I’ll try and be careful to choose a spot on a river showing no pulse (would that be a dead river?).

     
  4. Tangler

    Tangler Paddler

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    "dead river", lol. :)
    If you watch (or try to surf) a wave in a river you will notice that it's intensity waxes and wanes.
    Sit in an eddy and watch the water coming over the edge of the rock and you will see pulse like variations. Even the eddy level can rise and fall.
    If you stand in a river (for example when fly fishing) you can often feel the intensity of the force on your legs varying with time.
    All of this is on a sub-minute time scale and is likely due to the chaotic nature of rivers.
    My guess is that you will notice this variation on your scale as your boat dangles off the end of it.
    Just a guess of course.
    Looking forward to seeing your results as we are bound to learn something. :)
     
  5. mick_allen

    mick_allen Paddler & Moderator

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    Pascal - Yes, I meant 'they' instead of 'you', sorry. For the rest of it, I've poured too much rain on your thoughts or inclination to try it out - just don't be discouraged if there's maddening influences that keep happening or think that we would have expectations that you'd have to meet: it's all the journey and we'd/I'd like to know how easy of difficult that it was.

    I previously said "I do not see any mention of rudders in that first article.", but obviously did not read the bottom of the first image I put up:

    ruddertest.jpg
    In that first article, they tried a first order imitation of paddling difficulty by placing the tow point below the kayak to simulate where the average paddle pull would come from. They also had a little difficulty in directional control of one kayak.

    And yes the smooth current variations that captivate me are in slow current where I can see the variety of tiny changes. As far as pulse is concerned, I have experienced 3-5 sec surges because of high volume/quick current where the water level changed by about 3ft or 1m every change.