Stand Up Paddleboard--on foils!

Discussion in 'Boat Design' started by Doug Lord, May 1, 2016.

  1. Rurudyne
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    Rurudyne Senior Member

    I believe he must be getting a fair amount of thrust from the wind coming from behind based on what was said in the video.
  2. Doug Lord
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    Doug Lord Flight Ready

    Flying SUP!

    He might be doing that for fun on the "videotape"!*

    *age may not be a factor in my comment.....
  3. Skyak
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    Skyak Senior Member

    IMO the sea state looks like wind speed is about 20kts and Vapp is about 0. The white caps do not shear off like they would in 30+. If you look closely you can see water drip off the board, straight down. If he was propelled by wind he would never do abrupt turns like a surfer to stay on the sweet spot of the wave. The board is almost always nose down but about the same distance from the water.

    He is surfing, Vapp=0.
  4. Skyak
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    Skyak Senior Member

    He said what he said because it is a commercial.

    "foiling SUP" ->Wow where do we place orders!?

    "foiling surfer" ->DUH! Kite surfers figured long ago that they don't even need the kite to surf foil! All they need is a wave of sufficient size/speed. New sales=0

    Look at the video more closely -does any spash drop go forward? No. Is he ever on a back face of a wave? No. Is he always on the front face of a wave? pretty much.
  5. portacruise
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    portacruise Senior Member

    What is the launch procedure/device, and how long does it take? How much time/ practice/ skill is required to master the balance of all forces involved for sustained flight? What minimum power to weight ratio is required? These were some considerations for pedal powered airplanes, as I recall...

  6. daiquiri
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    daiquiri Engineering and Design

    Ok, let me then put it this way:
    1. We don't know what is the proportion of various external contributions to the overall thrust, but
    2. I am noticing that he is pumping with his legs (and I can prove that this power input can help maintain or even gain some speed)
    3. You are noticing that he is going down a slight swell (and we all know very well that surfing works as a propulsion method)
    4. He is running downwind, in a wind of unknown intensity (and that obviously helps)
    5. He can occasionally use paddle for a further power input.
    All four of these propulsion methods combine together to help him fly on foils for apparently long periods of time.
  7. Tiny Turnip
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    Tiny Turnip Senior Member

    A google or youtube search yields quite a few videos of foiling SUPs and foil surfing.

    The most engaging one of the examples below, is, unsurprisingly, Laird Hamilton. (the last one)

    the first couple I think show getting the board onto the foil with vigorous help from the paddle, and the wave.

    They are all surfing to some degree.

    Laird Hamilton can be seen pumping the board towards the end of the run.

    You can also see the wake of his tow in jetski departing to the left of frame right at the beginning.

    I would expect jet ski support is commonplace.
  8. Skyak
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    Skyak Senior Member

    YES! and it is very interesting to me to explore the conditions needed and how often they occur. I am not just arguing out of vanity. I wish it were true that SUP could generate the thrust to foil but it does not. A tail wind can but it takes so much wind that it would be extremely rare to have enough wind without surfable waves (or not for long) and it would be common to have surfable waves without wind. My point is important because this board might not foil at all in slower waves -shallow water or fresh water.

    So, what is unique about your foil profile? What plan form did you have in mind and what span-wise lift profile do you expect? In the 'unpacking' video I was surprised to see the practice foil was a lower aspect trapazoid and the race foil was the high aspect ellipse. He notes the practice foil is more tolerant of bubbles. They all look too small and high velocity to even pretend to be human powered foils. I ask because I am trying to figure if these foils are within the properties of injection molded thermoplastic.
  9. daiquiri
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    daiquiri Engineering and Design

    If you are referring to the xflr5 file attached in one of my previous posts, the answer to both questions is - no goals at all. It is a very simple model made in no time, the only purpose of which was to let me (and the other interested folks, eventually) figure out the ballpark values of hydrodynamic characteristics of a hydrofoil with grossly similar dimensions like the one in the video. I actually don't know anything about the real geometry of that stuff in the video, the only reference I had when trying to figure out the dimensions of the foil was that guy's hand holding the board. :D

    I presume that guys who made the actual underwater "airplane" have done their job much more diligently, hence the actual values of drag, req. power etc. are surely much better than my estimate.

    I have missed that part, but it is not that surprising thing if you think it over a bit. A low-AR trapezoidal planform is more forgiving that a high-AR elliptic one. It has softer, more gentle behavior and is less prone to stalling. Plus, it is probably more robust and less detailed than the racing version. I guess it costs less too (for all the previous reasons) which is a desirable feature for practice equipment. One can concentrate on practicing the technique without worrying excessively about smashing the gear against an underwater rock or whatever. ;)
  10. daiquiri
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    daiquiri Engineering and Design

    I guess it will depend on the type of thermoplastic you would use. They have pretty scattered mechanical properties and, more important, some of them become brittle in wet ambients. But I guess you know that already, since you seem to be into IMT.

    How to evaluate the blade stress... Well, the simplest yet efficient calculation which will put you on the safe side would go as follows.
    Say that you have a hydrofoil with a span b, root chord c and airfoil thickness t:
    1. Assume a working load on the hydrofoil, which would be the total weight Wtot of the person+board+paddle multiplied by some safety factor which accounts for people doing strange things like... pumping the board. :p
      I would use a SF=1.5, so the load on the foil is F = 1.5 Wtot.
    2. Each blade carries half that load, hence Fblade = 0.75 Wtot
    3. An elliptic lift distribution over a wing has a point of application at y_ac=0.21*b from the wing centreline. A rectangular lift distribution has a point of application at y_ac=0.25*b from the centreline. The latter case is more conservative from the point of view of the wing-root bending moment, but it is not realistic. Hence, assume a still conservative mid-value of y_ac=0.23*b
    4. The root-bending moment will be Mb = (0.75 Wtot)(0.23 b) = 0.173 Wtot b
    5. Now you need the Section modulus Sy of the airfoil at the root of the wing (hydrofoil). If you don't know the exact value of Sy (Autocad can help you there), then you can approximate the airfoil with a very elongated ellipse with the same value of c and t (see the explanation at the beginning of the parpagraph). The Section modulus will then be Sy = pi/32*c*(t^2)
    6. The maximum bending stress at the root of the hydrofoil will then be simply sigma_max=Mb/Sy.
    7. The maximum admissible stress is usually the yield stress divided by the material safety factor. You can again use SF=1.5 . There is a small problem there - plastics doesn't have a yield stress. You are left on your own here, it's up to you to decide which point of stress-strain curve of your plastic material you want to chose as the reference maximum stress. Beware of the material fatigue, so stay on the low side. In any case, don't forget to divide the chosen reference stress with the safety factor SF, so: sigma_adm = sigma_ref/SF.
    8. Finally, compare the bending stress and the admissible stress and check that the former one is smaller than the latter one:
      sigma_bend < sigma_adm.
    9. If the above condition has not been met, then you have to either increase thicknes t of the root airfoil, or to increase the overall size of the root foil, or the whole wing.
    Hope it helps.
  11. Jeandupont
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    Jeandupont Designer/Boatbuilder

    Foils are the future in other level of the performance water sports :mad:)

    I like this....

    Jean Dupont
    Instagram @supcatbr
  12. The Q
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    The Q Senior Member

    I not sure why but to me the video just looks fake. It maybe there just doesn't seem to be enough down slope to get the propulsion to fly...
  13. Westview
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    Westview Junior Member

    I agree .... the video doesn't show the boarder getting up to speed . I think they were towed up to planing speed then the tow rope released and the video edited to look like a longer ride than it really was .
  14. CT249
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    CT249 Senior Member

    Windsurfers do abrupt turns in that way, so certainly some people (including Kai Lenny himself, when windsurfing) do.

  15. Skyak
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    Skyak Senior Member


    I don't think the part will be close to breaking but I do fear deflection and instability. The 'trick' I have in mind is to use 3D printed molds to greatly lower the break even price. That limits the material selection to low pressure. Liquid crystal polymers have super properties -at a price- but they don't have the common plastic yield curve. It would be nice if a simple filled material did the trick but mold wear might be a problem. It may be better to just insert mold carbon fiber tow.

    Back to the failure modes -I would prefer that the lift be centered or close to the moment of inertia so alpha does not change with lift. This make take some screwing around with the profile. If alpha is stable I think the foil will ride stable even with span deflection.
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