Jumping Foilboard Windsurfers : Technical Question

Discussion in 'Sailboats' started by Doug Lord, May 4, 2007.

  1. Doug Lord

    Doug Lord Guest

    There have been several incarnations of foiling windsurfers but the one that intrigues me most is the Rush Foilboard Windsurfer:

    http://www.neilprydemaui.com/items.php?id=48
    It seems to me that one of the keys to this thing being able to jump and successfully re-enter is the delta shape of the mainfoil. I'm going to test the idea in a while but why couldn't a high lift ,early take off, type of foil-like those used on the Moth do the same thing provided a system was devised to by-pass the wand momentarily. Any thoughts on this now would be most helpful.
     
  2. frosh
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    frosh Senior Member

    Same old, same old!

    The storyline of some soapies on daytime TV can be picked up immediately even you have been absent for a few years. So it is with the foil mania. Doug, I suggest that you ride a modern shortboard in a rough sea in 20 knots of wind (if you are indeed capable) and you will realize that the last thing you would need is lifting foils. Indeed travelling over 20 knots most of the time, the board is jumping at every wave ramp in the water, whether you like it or not. There is a very good reason why the foiled sailboard has not captured any noticable market share. Simply that the sensory experience is very likely less intense when the board is above the waves. This is ignoring the cost, unecessary complication, and the significant extra control problems, when a sailboard is fitted with foils. Try giving it a rest for a while!
     
  3. CT 249
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    CT 249 Senior Member

    Windsurfers without foils can jump.
    Moths without foils cannot jump in the same way.

    Therefore it is obvious that what works in jumping a board does not necessarily work in jumping a Moth.
     
  4. Doug Lord

    Doug Lord Guest

    Re-entry

    I'm really interested in why the air chair(foilboard) type of foil re-enters so well and any thoughts regarding the behaviour of a higher aspect-longer span Moth type foil in a similar circumstance. For the purposes of this question forget structural considerations. Assume that the wand does not deflect the back end of the main foil flap up as it would in normal circumstances if the boat jumped. In an intentionally jumped boat the wand would be bypassed.....
     
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  5. frosh
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    frosh Senior Member

    OK Let us assume that somehow a high performance dinghy with hydrofoils could be made to jump. ( I cannot think how this could intentionally be done BTW). While completely in the air the disconnection between boat and its appendages with the water means that any potential rotational inertia already present on the boat and crew longitidinally, or for that matter laterally as well will have virtually nil resistance now that we are airborne. Therefore in the seconds that we are airborne there will almost certainly be an attitude change on the craft. Due to gravitational forces the craft drops back into the water still changing attitude all the way to touchdown. It is a folly to assume that because a skilled sailboarder can manuver the board's attitude while in the air the same can apply to a dinghy which does not have a hand held free-sail system, and never will have.
    The probability of the lifting foil now being laterally fairly parallel to the water surface is very small. More importantly the angle of attack of the foil is almost certainly not going to be in the normal range of 0 to 3 degrees positive. If it is negative then the craft will somersault forwards with catastrophic consequences to boat and crew at the speed it will be travelling, say 20 knots plus. This argument I believe can not be refuted by any reasonable person with experience jumping a sailboard, and with an understanding of the simple behavior of wings moving through air and then a far denser medium.
    The whole jumping a foiler idea reminds me of Icarus who hoped to fly to the Sun (in Greek mythology), with wings attached to his arms with wax.
     
  6. sigurd
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    sigurd Pompuous Pangolin

    I think that short, swept foils stall at higher angles. That is why I think they are used, it is not so critical which direction they enter the water. Also, maybe they shed air faster when swept?
     
  7. Doug Lord

    Doug Lord Guest

    63412

    It appears to me from looking in Theory of Wing Sections that the 63412 foil has lift from -2° to around +8° particularly if the flap is down a bit.
    Seems like a fairly wide range to me...
     
  8. water addict
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    water addict Naval Architect

    gotta agree with Frosh on this one.

    Foils on a boardsailor makes no sense. Try a short board in a breeze, you'll see that foils on such a craft is borderline retarded.

    There have been attempts at foil windsurfers, I've seen the videos a couple decades ago. It's been tried, with poor performance.
     
  9. Doug Lord

    Doug Lord Guest

    foilboards

    WA, did you even look at the Rush Randle(neilpryde) website?? I've seen a foilboard similar to those up close sailing with a fleet of other windsurfers and it was slightly faster but it was capable of jumping quite high in flat water-something none of the other boards could come close to matching. Being slightly faster than a windsurfer but 2' off the water is EXCELLENT performance. Being able to jump in flat water is icing on the cake.
    The Miller foilboard has excellent speed from what I've been told but Rush's 'air chair' version seems pretty damn cool. I talked to the guy that was sailing here at Kelly Park and he'd only been at it- since he built it- for three hours and was capable of remarkable performance for such a short time on the thing. I'd say it has a lot of potential and it was obvious that it worked really well.
    Any foiler has the potential to jump quite high in any condition as long as it is foiling-the ONLY question is can it safely re-enter from a structural and foiling(lift) standpoint. I think its highly likely that by manualy by-passing the wand on a boat designed to jump that re-entry can be successfull every time or close to every time. And that sounds like it could be a lot of fun.
    ===============================
    But the topic of this thread is what makes the air chair foil so successful at jumping and what would be theoretically possible to enhance the ability of a relatively high aspect foil to re-enter successfully. Thanks for your response, Sigurd!
     
  10. sigurd
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    sigurd Pompuous Pangolin

    It looks so very fun! Keeping in mind the recent discussion in foiler design thread about ventilation on foilboards, I wonder if they have that problem, or how they solved it?
    It does not look like they heel a lot to ww. maybe they just have enough vertical area?

    I am trying to imagine how to control flight attitude in the air. as was pointed out, there is no air steering mechanism and limited cog manipulation possibilities! Maybe short hangtimes at least are possible without?
     
  11. frosh
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    frosh Senior Member

    the ONLY question is can it safely re-enter from a structural and foiling(lift) standpoint. I think its highly likely that by manualy by-passing the wand on a boat designed to jump that re-entry can be successfull every time or close to every time. And that sounds like it could be a lot of fun.
    (QUOTE- DOUG LORD)

    Imagine if a airplane glider had it's rear flaps disabled (=manualy by-passing the wand) and had to make a landing over water. Is it likely every time or close to every time it would land flat on the water suffering little or no damage? I very much doubt it.
    A close friend of mine who I would consider an accomplished athlete and an expert sailboarder was catapulted forward while sailing in 25 knots of wind in choppy water. His neck impacted the mast and he broke a cervical vertabra. The loose vertabral condyle impinged on his spinal cord caused temporary paralysis of arms and all body parts below the neck. He could not rescue himself and was sailing alone at the time. He was actually in danger of not surviving. He was finally rescued by two young boys sailing a surfcat who were inexperienced, but not before they capsized the cat virtually on top of the stricken sailor in their efforts to come alongside.
    This person gave up sailboards fairly soon after this event, and has fortunately eventually recovered all normal body movement.
    It goes to show that it is easy to be frivolous regarding jumping a sailboard or other high performancve craft if you have not had the experience of danger or injury, accompanying high speed falls. And yes, you do hit parts of the boat with your body at speed quite often, rather than the water!
    Have I made myself clear yet?
     
  12. Doug Lord

    Doug Lord Guest

    stuff happens

    Too bad about your friend; hope he is doing well. I've been sailing high performance boats for 45 years and haven't been hurt(seriously) yet. There are a lot of activities like walking across the street where one may be hurt. Some people base their decisions on what they are going to do on
    how they can stay the safest; others attempt things that may well be beyond them. In my experience there is a happy medium where one can have exciting new experiences after carefully preparing and getting the best information possible.
    Once the technical issues are solved jumping a foiler will never be 100% safe but it could be 100% fun for most people who approach it in an intelligent manner.
    It is an untapped extraordinary potential for a foiler that may or may not work out-way too early in the game now to say for sure. One thing that is sure is that I will try to find out every thing I can about it from a theoretical standpoint, and from a practical standpoint from people that have tried it on foilboards or what ever. Surely worth looking into very seriously....serious fun is the payoff .
    ++++++++++++++++++++++++++
    By the way, you completely misunderstand the concept of by-passing the wand. The wand is held 'against' the water by a shock cord . In an inadvertant jump in rough conditions the foil can come out of the water with the wand/shock cord moving the back end of the flap to max up. Then when the thing re-enters-depending on a number of factors- the wand/flap position can result in a dive by the foil which is sucked down by negative lift . The system I'm still working on allows the skipper to by-pass the wand by twisting the hiking stick to initiate the jump and to keep the flap down in a positive lifting position for re-entry.
     
  13. water addict
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    water addict Naval Architect

    Doug,
    So the purpose of the foils is to jump better in flat water?
     
  14. Doug Lord

    Doug Lord Guest

    foils

    ====================
    The Rush Randle promo stuff talks about being able to jump in flat water; I witnessed a guy do just that with a similar board.
    The point of this thread was to discuss what would make a high lift foil(63412; 7/1 aspect ratio-for example) function as well as the foilboard in re-entering. The point of hydrofoils is to fly the boat and to me is to do that in the lightest wind possible. The speed is nice too...
    And jumping -if it can be worked out- would be a lot of fun.
     

  15. water addict
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    water addict Naval Architect

    I was an avid windsurfer in my younger days, did the pilgrimages to Maui, Barbados, the Gorge, Fla, Hatteras, Bonaire and others. Foils on a fast board wouldn't appeal to me, I'd be removed from what makes it interesting- the water surface. If there is enough wind to have fun, ie 12 knots to plane, then there is enough chop to jump regardless. I think I have a pretty typical attitude of most boardsailors. I doubt you'd get much interest from windsurfers in 6 knots of wind to loaf around on some foil born gizmo, and if there's more wind, the foils have no purpose. But I could be wrong.
     
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