The Physical Art of Sailing

Discussion in 'Hydrodynamics and Aerodynamics' started by Leo Lazauskas, Nov 24, 2014.

  1. CT249
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    CT249 Senior Member

    Sure, in the conditions in the vid, just flapping works. Those RSXs are pumping upwind in moderate winds, when there is significant apparent wind and therefore enough "fresh" air is coming on to the rig to create a flow over the sail.

    Downwind in light winds it can be a different story; it's easy for a well-pumped board to go dead downwind at the true windspeed or even faster. What seems to happen is that if you just use the same side-to-side pumping technique is that the flapping of the rig creates so much turbulence that the area around the rig becomes effectively a mass of eddies so there is no flow over the sail. Those at the back or middle of local fleets can sometimes be seen pumping frantically and sitting completely stationary, or going backwards.

    So when there is insufficient apparent wind, we try to ensure that the rig is tipped forward, into air that has not been disturbed by earlier pumps.

    As windsurfing coach Simon Bornhoft noted, in light to moderate winds upwind "There is very little fore and aft movement or fanning of the clew, it's a quick snap, with the rig kept virtually parallel to the board."

    However, downwind in light winds, "At the end of each pump the rig is guided forward again ready for the next one." In very light winds, "The rig is kept on one side of the board, with long fore to aft repetitions."

    In the old-style original Windsurfer One Designs (1980s updates of the original Windsurfer) which are still raced in Australia and Italy the fore-and-aft movement downwind in light winds is very exaggerated, possibly because of the tight leach and the fact that the rig is much lighter than modern rigs (despite what windsurfer sailmakers claim). We actually hold the rig so that the line from tack to clew is parallel to the board's centreline and then lean it forward as far as possible before "scooping" either the head or the clew back, as if the head or clew was a kayak's paddle; it's hard to describe and I can't find any videos of it.

    Our national championship fleet often includes two to four ex-Olympic team members who are veteran windsurfers with significant boat experience (2nd in the Tornado worlds, 470 Olympic team member) and others who have been champs in boats as well as boards, plus "visitors" such as Olympians and national-level Youth and Junior sailors who mainly sail dinghies; by coincidence the winner of the women's 49ers in the video you linked was one of those Youth. The fact that there's such as strong exchange between boat sailors and windsurfers in the class brings some insights; coaching someone like a Laser world champ/Olympian gives one an interesting viewpoint on the whole issue of pumping!
     
  2. CT249
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    CT249 Senior Member

    Yep; I've never sailed Finns but we do the same in Lasers although I always assumed the steering effect was due more to the hull developing asymmetric underwater shape when heeled. What I'm wondering is whether steering by heel also creates the extra benefit of projecting the head or clew into clear air that has not been disturbed by earlier pumps, which is more noticeable in windsurfing in light winds.
     
  3. Richard Woods
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    Richard Woods Woods Designs

    Agreed, and it is one thing that people new to fast boats take some time to learn. However in some light conditions "slow" multihulls, like Dragonfly or Farrier trimarans for example, are quicker sailing downwind. Same thing in skiffs, they can be slow in light winds compared to conventional dinghies.

    One might think that raising the board to make leeway (thus increasing speed to the lee mark even if you are pointing up) and reduce WSA (thus resistance) might be beneficial in light winds. As I said it seems to work on catamarans but not trimarans.

    For example the 35ft catamaran Bad Kitty has a similar speed to the F31R Blue Lightning (I've raced on both boats), the former raises its boards, the latter doesn't and it feels distinctly "odd" if you do (I tried it).

    I also wonder whether the rig flicking works better or worse with a carbon or metal mast, certainly a carbon mast has a "snappier" response

    Pumping upwind in a breeze you leave the "pumped" wind behind, but sailing downwind you are using the same wind for some time, so too frequent pumping won't work

    Richard Woods
     
  4. SukiSolo
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    SukiSolo Senior Member

    There is also a point where moving the board really far forward ie under the bow (and this has been done) results in rudder action reversing! So that to bear away you push the tiller down. It is obviously mighty surprising and so counter intuitive that it takes a lot of courage to persevere.

    In this case the CLR is between bow fin/board and rudder at a point where the hull will not 'pivot' in the expected central area. One very good helm and designer tried this layout in the 1980s' as an experiment (better volume at the ends) but ended up going back to a more conventional arrangement. It was also a pig to tack as you can imagine, more so as the rocker was straight from the bow for about 80% of the w/l length.
     
  5. johnhazel
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    johnhazel Senior Member

    Since the hull is so light there is probably a significant amount of keel flapping going on with this motion also.
     
  6. Mikko Brummer
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    Mikko Brummer Senior Member

    Sounds like paddling or rowing in the air, rather than "pumping". That said, the effect is recognised in Finns, too, where the French call it "l'effet palm", the palm leaf effect. The rule allows the mast top to lie max 120 mm from a straight line through the mast foot and the deck ring, and this is usually exploited to the max.
     
  7. markdrela
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    markdrela Senior Member

    This doesn't follow. Even if the sail is pumped in "bad air", this should still increase the net aft momentum transfer to the airmass and thus increase the force on the sail. You don't have to be optimally efficient in pumping the sail. You only have to be more efficient than everyone else.
     
  8. markdrela
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    markdrela Senior Member

    Whatever you call it, sail pumping is pretty much equivalent to wing flapping.

    One feature of wing flapping is that if the wing airfoil has a narrow drag bucket, then the twist angles during the motion must be such as to always point the nose into the local flow to avoid separation off the leading edge on either side. Conversely, if the wing airfoil has a wide drag bucket then the twisting is less critical -- you can always count on leading edge suction for thrust as long as the flow doesn't separate. The keel on a pumped sailboat wants to have a wide drag bucket for this reason.

    I suspect that ineffective sail pumpers have poor phasing between the flapping and twisting motions. If I was trying to learn proper pumping technique, I would put small tufts on the whole front 30% of the sail to detect LE separation and give feedback on the proper amounts of twist.
     
  9. Mikko Brummer
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    Mikko Brummer Senior Member

    Interesting idea with the tufts, I will forward it to our olympic hopefuls.

    While windsurfers have more control over the whole wing twist & flapping, sailors with rigs attached to the boat have less. Birds have muscles to control/vary twist along the span, while for sailboats the twist is load induced - there is some control over it, but you can enforce it only through loading. In that respect sails are more like insect wings (?).

    Some serious pumping of a bat

     
  10. Mikko Brummer
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    Mikko Brummer Senior Member

    What would a wide drag bucket mean in terms of a symmetrical keel profile? Max thickness forward and large leading edge radius (blunter LE)?

    What about a sail preceeded by the mast - would a large radius, more rounded, pear shape profile perform better for pumping than a slimmer one, NACA 6-series style? See illustration for mast shapes.
     

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  11. tspeer
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    tspeer Senior Member

    I think the effect of pumping would be to increase the circulation around the sail and thus the leading edge velocity. You'd want to design the section to avoid too large a leading edge suction peak. A shape like the attached might be suitable.
     

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  12. Mikko Brummer
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    Mikko Brummer Senior Member

    Makes sence and the profile you submit looks good, thank you.

    Yes, I believe that at least upwind the pumping action heds a vortex from the leech, increasing circulation & lift. Several rapid pumping actions could raise the lift momentarily very high, resulting in stall and recovery with hystreresis involved. One would think there is an optimal frequency depending on apparent wind speed - definitely reserch needed to do there.

    Running downwind, in "parachute mode", it could be different. The rolling of the boat could be associated with trying to achieve lift from the sail, instead of merely drag, judging from the windex switching from side to side.
     

  13. CT249
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    CT249 Senior Member

    Yes, in boards the optimal pumping frequency does seem to change radically depending on apparent windspeed.

    The info about leading edge profiles is interesting; when downwind pumping on the original Windsurfer One Design, which has a narrow luff tube and a soft sail, you normally use more variation in the sheeting and more fore-and-aft movement than in RSXs and Raceboards, which have camber inducers and therefore much wider luff pockets. However, the modern rigs are also much heavier and therefore much harder to swing through the range of fore-and-aft and rotational angles.

    Many years ago I tried tufts on a Windsurfer rig but found that the sail was very slow when the leeward tufts streamed. Greg Hyde (Cherub dinghy champion and windsurfer Olympian) told me then that they'd tried them and found that they did not work effectively on a windsurfer; "they" could well have been fairly clued-up people like Kevin Wadham (skiff champ/sailmaker), Andrew Buckland (ditto, and main inventor of the modern assymetric spinnaker despite PR spin from a certain quarter), Bobby Wilmot (Soling Olympian, etc) who were the sort of people in the class at that time and place.

    Although I'm addicted to tufts on boats, in boards there's a lot of feedback coming through the hands and arms when pumping, and of course many hundreds of sailors have spent many hundreds of hours doing two-board training in pumping technique. It would be very a interesting experiment though; I may chuck some on an old sail or two.

    I haven't heard it from a sailmaker's mouth but it has been said that rigs like the old Olympic IMCO 7.4 were designed around their pumping ability so much that they were hard to handle at the top end of the range; pumping is certainly no free lunch. We've found that it can dramatically reduce fleet numbers because most people don't really enjoy the level of effort required, and the gaps open up so much that many people have no chance no matter how good they are at tactics. It also makes a second-row start even harder to come back from - you haven't had dirty wind unless you've been behind a row of Olympic windsurfers pumping hard off the line!

    When unrestricted pumping started out in boards we didn't really foresee where it would end up, if I recall correctly. Opponents of pumping may not realise how much technique can be involved but many proponents of free pumping have not bothered to learn from the experience of classes that went down that route years ago.
     
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