Windsurf raked sail

Discussion in 'Hydrodynamics and Aerodynamics' started by Grunf, Nov 22, 2020.

  1. patzefran
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    patzefran patzefran

    Lift is the result of pressure difference across the sail (wing), as soon as the sail is raked from vertical , there is a vertical component !
     
  2. Grunf
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    Grunf Junior Member

    But sail chord line(boom) is almost allways parallel to board center line, so even sail is raked back doesnt mean it is reaked to windward ..
    When sail is raked to windward then produce vertical lift..

    No vertical lift

    no vertical lift.jpg
     
  3. Grunf
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    Grunf Junior Member

    Sail lean to windward produce vertical lift

    Sail lean to windward.jpg
     
  4. patzefran
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    patzefran patzefran

    When raked back there is a substancial amount of sail area, near the leading edge (were the maximum difference of pressure is !) whose normal is angled upward,
    so there is a vertical upward component of the lift ! if t was not the case the windsurfer in your first picture would fall his *** in the water !
     
  5. Grunf
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    Grunf Junior Member

    Yes that is correct, I didn't think about that..
    Only problem is when board goes +50knots than there is no need for additional vertical lift from sail..
    Hydrodinamic lift is huge...way too much,you can planing at one foot
     
    Last edited: Nov 23, 2020
  6. messabout
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    messabout Senior Member

    The fore and aft tilt of the sail is mainly used as a steering force. Fore and aft tilt does not cause vertical lift but it does shift the center of pressure of the sail with respect to the boat. Thus tilt is used as a steering force. Sideways tilt does generate some lift but it is also a means for the sailor to use his weight to keep the sail from capsizing to leeward. Your second picture illustrates that phenomena.
     
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  7. Grunf
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    Grunf Junior Member

    Planing board steer with foot steering,railing board on edges..

    Sail steering is used for old school long board ,when board is not planing
     
  8. Will Gilmore
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    Will Gilmore Senior Member

    The original windsurfers did not build in rake like that. It has come out of experimentation, as well as improved theory. The benefits include balance for steering, as well as spillage of excess lifting/heeling force. Sailboards don't sail like a sailboat. They can't spill air by heeling. The rider needs to be able to tilt the sail to windward. This helps lighten the board through lift as well as reducing driving force that would overpower the righting moment of board and sailor.

    Take a look at iceboats that are traveling at speeds in excess of 50 knots and their sails also rake heavily.

    The center of lateral resistance for a sailboard is very far aft and the rake helps move the CE to a more appropriate relationship to that.

    Bringing the mast aft and using less rake becomes awkward when trying to counter the lifting force and you can't get far enough out on the wishbone.

    -Will (Dragonfly)
     
  9. Grunf
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    Grunf Junior Member

    if you hold sail more upright you must lower the boom to get more out with body..


    Upright..


    upload_2020-11-24_22-55-6.png
     
  10. philSweet
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    philSweet Senior Member

    This is a rather complicated subject, but from an aerodynamic standpoint, the vertical extent of the sail is the main efficiency consideration, regardless of the planform shape, rake, or cant. Vertical span, and nothing other than vertical span, determines the lower limit for the induced drag. Getting to that lower limit means using a sail shape that is correct for the effective sweep angle of the mast, and the effective sweep angle is a function of apparent wind angle, rake, and cant (more on this later).

    So the starting point is how high do you want the top of the sail to be. Then you can choose the rake and cant you want, figure out how long the mast will be, and design a sail shape that has minimum induced drag.

    So why choose to use a raked mast, which adds length to the mast, but doesn't increase span or aerodynamic efficiency potential? It really comes down to load transfer at the mast base. With a raked mast, your arms won't get as tired. You want to be pulling at right angles to the load, and you don't get to choose the load vector - it is a state vector. So you craft the rig around the load vector such that the forces needed on the wishbone are minimized.

    Going back to the sweep - rake - cant thing. Rake and effective sweep are not the same thing. Imagine a non-raked mast that is canted 30 degrees to windward, and an apparent wind of 45 degrees. The wind hits the top of the sail first, and blows "down" the sail towards the trailing edge. This means that from the effective wind point of view, the sail has forward sweep, and that dramatically changes what the best span loading and sail shape looks like to get to minimum induced drag. So to get to neutral effective sweep, you have to rake the mast back 30 degrees. With 30 degrees of cant and 30 degrees of rake, you have zero sweep when the wind is at 45 degrees. So once you have a planform for the sail that is optimized for a particular sweep, you have to adjust rake if the cant changes, in order to maintain efficiency.

    So as you start to cant the sail windward as the wind speed increases, you have to rake the sail backwards to maintain the same load distributions and efficiencies. You can still design the whole thing to run at whatever effective sweep you want (choose one), but then you have to actually build a membrane that works on that planform and spar set.
     
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  11. Grunf
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    Grunf Junior Member

    I didn't understand first part about load transfer but your explanation about effective sweep,rake and cant is very interesting.
    Are you aeronautical engineer and do you think that wsurf sail designers knows your sweep,rake,cant explanation so they rake it on purpose or they end up with current concept by accident?

    But in windsurfing at constant planing speed sail is almost always fully closed/fully sheet in, parallel to center line ,sail stall at 18degress of AoA,so apparent wind in wsurf is very small maybe up to 25degrees..even true wind is 140 downwind ,apparent wind coming always from board nose direction

    So my oppinion is that most of things in windsurfing is made by accident, trial and error,designers didnt rake sail because of your rake,sweep,cant explanation..they were not people from aerodynamics background,they had sail sewing background..
     
    Last edited: Nov 25, 2020
  12. Doug Halsey
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    Doug Halsey Senior Member

    Don't forget the twist effects either. They can totally throw off your dreams about the ideal span loadings.
     
  13. philSweet
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    philSweet Senior Member

    My sailmaker of choice in the Florida Keys was married to a women's World Champion wind surfer. So testing concepts wasn't a huge problem for him, and his sails dominated Florida Wind surfing competitions 20 years ago. Wind surfer sailplans were largely optimized by feel and daily competition. Never underestimate how amazingly good the senses of an experienced athlete are. In the wild, they can capture data and process it at a speed and fidelity that can not be matched even today with machines - provided they can operate in the sweet spot of human performance. And that's exactly where windsurfers are at. (I'm not a windsurfer btw)

    I'm a mechanical engineer by training, but I got into the missile business, and being a sailor, have slowly pick up some aero theory. I took Drela's online aero course a few years ago, and I read a lot of papers. The crew here is pretty sharp as well. I don't really know how much canonical aero theory typical sailmakers know, but they have had sophisticated tools for membrane construction for 25 years or more, to get the shape, strength, and dynamic response from the sail. Mikko Brummer hangs out here, and he has Olympic medal credentials in sail design. Some sail makers have the chops for sure.

    We had a fun romp through sail aerodynamics here, and took exception to a bunch of promotional and educational spew from some sailmakers - About the induced drag of sails https://www.boatdesign.net/threads/about-the-induced-drag-of-sails.63887/

    Tom Speer, post 47 of the above thread - "When I first got involved with an America's Cup team, I was amazed that the sailmakers didn't speak the language of aerodynamics! "
     
    Last edited: Nov 26, 2020
  14. MushCreek
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    MushCreek Junior Member

    I used to do a lot of high wind sailing years ago. Pretty much everything is seat-of-the-pants when you're bouncing along at 30+ knots on a tiny board. I do know that raking the mast creates lift, as I've literally been blown out of the water on occasion. The next logical step, which it seems many have gone to is the kite board. Those things actually fly. In high wind, it always seem that the more you cranked in on the booms, the faster you went, at least on a beam reach. Light wind long-board windsurfing is totally different. I remember when they used to race displacement-type windsurfers. Those things were tricky, and more akin to ballet than waterskiing.
     

  15. garydierking
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    garydierking Senior Member

    What about the effect of closing the gap at the bottom of the sail? I'd always felt that that was what gave the boost in speed. Look at the new AC75's rig. The bottom of the sail actually drags across the deck so that no air can escape to the other side. They clearly thought that it contributed to their ability to do 30 knots in 8 knot of wind.
     
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