Stand Up Paddleboard--on foils!

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

  1. jakeeeef
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    jakeeeef Junior Member

    My understanding of leading edge turbercules is that they allow a relatively thinner foil to keep laminar flow at a higher angle of attack.
    So where a rudder foil would be usually a slightly relatively thicker foil section than a centreboard foil, so it can withstand a high angle of attack (so it stalls later when being used for turning), designers can get away with a thinner and less draggy foil when it has turbercules (they help keep attachment). This is why you are starting to see them a lot on rudders.

    I guess over millennia evolution may have equipped the humpback whale with them for similar reasons- to provide a higher stall angle and hence more 'user friendliness' to the 'foil'.

    To answer the earlier posters to this thread on the subject of SUP downwind foiling, as SUP racer, I've been following its development for a while. Firstly it is for downwind and down wave racing only. The initial video was not a fake of some sort, but was certainly edited to cut out the (probably quite frequent) episodes when he was not up on the foils. The rider in question is particularly fit and talented and the technology only suited to downwind racing- so this technology does not have much likelihood of entering popular usage outside of Hawaii.

    The big downwind races happen in Hawaii, and they are normally conducted on planing/ surfing boards- with riders managing surfs of many hundreds of yards interspersed with frantic paddling in between to catch the next wave. It's not a sport that ever can catch on worldwide as the combination of strong reliable winds and powerful, deep ocean waves, plus the safety and organizational infrastructure just do not exist in most other places.

    SUPers have been allowed to take part in downwind races on foil boards, but (and this is where the careful editing of the OP video comes in) they have found that the bits between waves of doing about 3 or 4 knots due to the massive appendage drag largely make up for the flying episodes in between. Better in terms of average speeds achieved most of the time to use a more conventional planing board and be able to catch every little bump as the planing board has a better off wave achievable speed due to its lower appendage drag.

    So, yes, downwind and down surf it is possible to get a foil board foiling without a jetski tow, and as shown in the foil surfing video it is also possible to head up wave for a few pumps of the foil, but this is not something you will see performed outside of a few professional athletes.

    I guess, downwind racers will start to have a couple of foiling boards in their quivers, but they will be reserved for special days when the conditions are suitable for them. Nobody would ever buy one as their only SUP as most days would be spent off the foils and going sloooow.

    Conversely foiling dinghies can be and regularly are owned as the owner's only boat.
     
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  2. Skyak
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    Skyak Senior Member

    Welcome Jakeee! Glad to see an intelligent post reaching the same conclusion I did -this is wave powered foiling.

    From all I have seen and read you are right-on with your understanding of how things work. My one correction would be to replace "laminar" in your first sentence with "attached". Turburcles break up laminar flow before it starts, but the resulting turbulent flow is much more tolerant of surface imperfections and high angles of attack. As a design feature it only offers tolerance and maximum force/area from a single foil. If you need lift/drag -and I say you do for this application -then turburcles have nothing to offer.

    There was some noise after the last video posted that this foil SUP was now capable of sustained flight with pumping action -I dispute that statement.

    Note in the video
    -He is riding a foiling surfboard -NOT SUP. It is a "sinker" incapable of floating under the weight of the surfer. This is important because the greater the mass that needs to be oscillated the lower the efficiency of the propulsion.
    -The conditions in the video are surfing. The depth is clearly declining. The waves are clearly steepening, proceeding to breaking, and most importantly the period is compressing, the distance between waves is reduced.

    Being able to swing off a wave and catch another is a cool trick that only a foiler can do, but it is not sustainable flight by any measure. He did not return to start position and his second ride would be short until the wave gets to the shallows. It's a q not a circle.

    Now lets go back to Jakeee's point about racing foiling SUP and take up the question "Foiling SUP -What is it good for?" The foiling SUP has lower drag at velocities over X (say 7 knots) but the thrust to maintain that flight can only be sustained for a short period in flat water (say 10 sec generously) and needs a wave to be initiated. The conventional SUP has higher drag at high speeds, but it has a significant advantage as long as the foilers hull is in the water. So the first chance for foil advantage would be in low swells where conventional boards would be left paddling, possibly to exhaustion and losing position on the wave train. Similarly, in slightly steeper waves that are at an angle to the race course can favor the foil. But this leads me to a question about downwind races -do they run or cancel races when conditions are not directly down wind and wave? In the case of an organized wave train of sufficient height the race will be a conveyor belt delivering all the racers in the order they got on. The foiler would have no advantage unless he could jump to a forward wave -no evidence of that yet. The last possible advantage would be confused waves where a skilled foiler might be better able to connect the waves with higher top speeds and longer glides.
     
  3. daiquiri
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    daiquiri Engineering and Design

    And what would be your measure?
     
  4. Devu De Goa
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    Devu De Goa Junior Member

  5. SVLB
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    SVLB Junior Member

    would a flexible 'self feathering' foil work for more leg pumping power?
     
  6. jakeeeef
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    jakeeeef Junior Member

    Thanks Skyak,
    My memory of Turbercules was a little hazy. They were explained for me by a naval architect some years ago. I still maintain that whales have probably evolved them to have more user friendly 'foils' that can handle a high angle of attack. And what you say about them making the rest of the foil less sensitive to surface imperfections now makes sense too. Whale flukes often get barnacles, and in the context of the toothed whales- chunks bitten out of them by other whales. I'd want turbercules on my leading edges if I treated my foils like they do.

    Yes, sorry, I should have been clearer in my post, the surfing footage where he pumps back to the next wave- this is a shortboard, not a SUP as you say. Totally different thing. They are mostly used for tow-in surfing on big waves (jet ski tow to get them on foils and fast enough to get on the wave.
    You can actually see in the footage that his pumps are getting distinctly more laboured and the board getting lower to the water by the time he picks up the next wave- this would be seriously hard work and unsustainable for more than you can see here- if that second wave had been about a second later he wouldn't have made it and I bet they had to shoot all day to get him doing it that time.

    Pumping foils is difficult without something to pump against (another foil with set depth forward usually), and even then it's seriously hard work- which is why my Aquaskipper is in the loft waiting while I think of a better use for its foils. But for 'sustainable' pumping foils please see the size of an Aquaskipper's main foil. There is no way on earth that either of the foils shown on the SUP or the Surfboard would be large enough for sustained flatwater usage. If they were larger though then they would probably be too draggy to get foilbourne in the first place without a tow boat or a wave.
     
  7. SVLB
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    SVLB Junior Member

    pumping on a foil board is a different sensation, I find you are trying to 'jump' off the board allowing the foil to unload and the board to rise momentarily before reapplying your body weight and diving, gaining a bit of speed in the process. the hard part here is keeping your balance while 'jumping' this is where a front foot strap comes in handy
     
  8. Skyak
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    Skyak Senior Member

    To call it human powered I would like to see someone get up on the foil in flat water and sustain foiling for a distance that is not easy to swim.

    To consider it a breakthrough in SUP I would (per my note above) like to see foils used to move forward on a wave train. Foiling is efficient enough to ride some pretty small swells -maybe even angle a bit in the sweet spot of a wave then shoot for the next one pumping and paddling. It looks like they are close to this now.

    It would also be useful to able to jump chop.
     
  9. Skyak
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    Skyak Senior Member

    Yep! Whales have more use for max force/area -high angle of attack.

    So you have an Aquaskipper. That saves me a few paragraphs describing what it takes to design for leg pump propulsion. The key parameter I would play with is aspect ratio. I am pretty sure that induced drag is the majority of the problem. It is a quite a challenge though because speed is crucial. The foils I see in the original post video look a bit too much like kite board foils to be optimal for SUP (which I would expect to be significantly greater span and area). I was also thinking that if the main foil AOA was sprung it would make it much easier to pump a SUP foiler. Is the Aquaskipper foil sprung? I have been thinking that there may even be a way to do both the AOA and a change in sweep and span -like the way some birds fly -on the down stroke their wings are extended to full span, then they sweep them back as they glide and move back for another stroke. Maybe glide is not the word, they just flap then bring their wings in to minimize drag.

    So SVLB, yes I think feathering is definitely worth exploring.
     
  10. daiquiri
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    daiquiri Engineering and Design

    With all due respect, this is just your personal opinion and arbitrarily chosen criteria. I disagree with using them to define a "sustainable flight". They can be desireable practical goals, but THAT thing in the video was foiling. In an arguably impractical way, but still foiling.

    And, regarding the bold-letter part of the above quote - they are not "close to this now". They are exactly there now.

    I personally was only interested to verify the opinion which I wrote in the post #4 of this thread:
    It was written well before the second video was made and posted on YouTube, and my prediction about the possibility of leg pumping-foiling turned out to be correct: http://www.boatdesign.net/forums/at...tand-up-paddleboard-foils-legpump-foiling.gif
    So leg pumping can indeed allow foiling over short distances - the limit being the methabolic and cardiocirculatory system of the athlete. IMO, any "measure" of what can be called "sustainable foiling" has to take this human factor and limitations into account, because of the obvious fact that the power for this type of flight comes from the human body. The other obvious fact, that this type of foilng needs to a certain forward speed before becoming effective, is another pair of shoes - not relevant for my considerations.

    The second foil (like in Aquaskipper) can help but is not strictly necessary if the athlete is able to coordinate the body movement (Longitudinal-vertical CoG shift and down-pushing leg force) in such a way to make the foil perform correct (sinusoidal-like) alternating movements through water. Again, the proof is in the video, but it can be demonstrated mathematically too.
     
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  11. Doug Lord
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    Doug Lord Flight Ready


  12. Ara
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    Ara Junior Member

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