Does steady flow exist in sailing conditions

Discussion in 'Boat Design' started by lunatic, Feb 12, 2008.

  1. lunatic
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    lunatic Senior Member

    Does the steady flow of powered flight or wind tunnels exist in sailing conditions? Can steady streamline flow form around a sail even if ambient air is turbulent. Might lift generated by turbulence be better? A low aspect slender wing develops, a leading edge vortex creating lift in sweptback delta wing high speed aircraft,but will it sail?
    Results of trying posted at http://proafile.com/view/weblog/entries/C12 under heading of Crab Claw Tests. Sailing these rigs is unlike any sailing I have done and is described in detail on above website. Could use some feedback as my local sailing community is not into experimentation
     
  2. Kay9
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    Kay9 1600T Master

    I think there is a thread or 2 here allready about the crab claw rig, might try doing a search.

    K9
     
  3. lunatic
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    lunatic Senior Member

    Thanks tough getting feedback on this subject
     
  4. Kay9
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    Kay9 1600T Master

  5. Petros
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    Petros Senior Member

    Unfortunately most sailors (even very experienced ones) do not have a clue about the aerodynamics of sailing rigs, there is a lot of outdated lore, mythology and misunderstanding about how sails work holding over from the 17th-18th centuries still passed among modern sailors. Much of the outdated terminology perpetrates these myths (ideas such as "center of effort" or "center of resistance"-for which I will get more than a few comments I am sure-but these are obsolete concepts). I can not hold an intelligent conversation about sail aerodynamics (or even keel hydrodynamics) with most sailors.

    What you suggest would work but it is not very efficient. If you design a rig to take advantage of vortex generated lift on the sail it will have a lot of drag (about twice as much or more) so your overall resultant force vector on the sail is less useful for forward motion. Drag is always defined as the force in the stream wise direction, not useful unless going dead down wind.

    The vortex generation of low aspect ratio delta wings on fighter aircraft has the advantage of going up to 45 deg. angle of attack without stalling, but it has a lot of drag. With a big powerful engine this is not as much an issue, especially since this would be for a transient maneuver as in a dog fight (giving you an advantage at that end of the flight envalope over an opponent who can not fly in that mode). The wings on a fighter generate lift by the engine driving it forward through the air, a sailboat generates lift to power the boat through the water (a very different goal requiring different demands from the "wing" or sail surface).

    On a sail that is powering a boat the overall L/D (lift to drag ratio) is more important than just lift alone since it is the wind that is powering the boat. If there is so much drag that the headway is hampered there is no performance advantage, indeed you end up with a lot of leeway and very sluggish headway as compared to a high aspect ratio sail. You can point it into the wind without stalling the sail, but your actual path through the water means you would lose ground to a more efficient sailing rig. That is why sail planes alway have such high aspect ratio wings, they are not powered except by the airflow, just like a sail boat. You will never see a sail plane use low aspect ratio vortex lift wings, the L/D would go from perhaps 23:1 (as high as 35:1) down to 4:1. That means for every foot of drop in still air, it moves forward 23 to 35 feet, vs. four forward feet to each foot of drop. Not very good for a sail plane.

    There is one place a low aspect ratio sail is useful, that is in a storm jib or a heavily reefed sail during high wind conditions. Power is not the issue here as much as maintaining control, by having a sail more resistant to stalling you will have better control of the boat, less flagging of the sails, less stress on the rigging, better able to avoid jibing, etc. Another place might be on a dead down wind leg where the lift vector and the drag vector are going the same direction, so the extra drag does not hurt your performance (this is likely why many spinnakers are lower aspect ratio that the jib and mainsail).

    Sailing does experience many steady state conditions, especially off shore in calm seas. Turbulent flow is not the issue, you get turbulent flow on all kinds of surfaces and conditions, the issue is steady state vs. transient flow. Do not confuse turbulent vs. laminar flow with attached flow vs. detached flow (as in a stalled wing or sail). You can have attach turbulent flow (steady state and useful to generate lift) and detached laminar flow (not steady nor useful, just as detached turbulent flow). Vortex generators, and vortex lift is a way for keeping the flow attached to the surface (and steady) where normally it would be detached and unsteady, but it is always at the cost of much higher drag (though lower drag than when the flow detaches from the surface!).

    BTW this misunderstanding is a constant source of confusion by people who should know better (such as a lot of the navel architects on this list). It is very difficult to explain the terminology and correct understanding to someone who has been under the delusion they think they know how sail aerodynamics really works. The whole subject is very complex and is not intuitively obvious, that is why it has only been in the last 100 years of our existence that flying aircraft were ever built, despite at least 2500 years of attempts to build flying machines. Yet consider that sailing craft have been around a lot longer than that, giving us many centuries to develop very wrong ideas about how sails work. Mostly they discovered what works on sailboats by accident until only recent times. That is also why it is a terrible waste of time and effort to imitate obsolete sailing rigs on modern boats (unless you are trying to build a historic replica and are willing to suffer the consequences). It is also why it is mostly a waste of time to explain the aerodynamics of sails to most NA and experienced sailors, they will not listen and they will not learn because they have too many obsolete ideas built into their assumptions about sail design.

    That is not so say that the people on the leading edge of modern designs or world class racers do not know what is happening, but that is a class of people that is rare compared to most sailboat owners and designers.
     
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  6. masalai
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    masalai masalai

    I give you my "rep (brownie) point", and would appreciate more on what can be done to make sail more efficient and easier to handle on a cruising cat?

    What is your assessment of the John Hitch "hitchhiker" sail as a workable idea?

    How about something like the "para/wing/kite sails - used by the board surf crew - for passages?
     
  7. Guest625101138

    Guest625101138 Previous Member

    I agree with this apart from "one place".

    In the context of lunatics previous threads on this I consider there is merit in low aspect sails in certain applications. Land sailing could be well suited as wheels are used to react the sail loads and the overturning moment is quite low.

    I have posted the following link before and I believe the "sail" shape is the product of considerable insight and development.
    http://www.macquarie.com.au/speedsailing/design.htm
    You could never regard this sail as high aspect. With sailings boats the overturning moment of the sail is a key consideration and has a bearing on what will result in the optimum aspect ratio.

    Rick W.
     
  8. tspeer
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    tspeer Senior Member

    I don't think any landsailor would agree with this statement! Landyachts have long axles that give them a great deal of stability. Flying a wheel on a landyacht is much like flying a hull on a beach cat. The stability is limited only by the flexibility of the plank and class rules (in Europe).

    And the modern trend is toward higher aspect ratios, cleaner bodies, and rigid wings to reduce drag. When you're sailing at five times the speed of the wind, your apparent wind angle is only about 15 degrees, even off the wind. Low drag is everything.

    There are two aspects to setting a speed record. The first is having a low drag design that has a high top-end speed. The second is having the acceleration necessary to reach high speed when you have a suitable gust, especially if the venue is limited in area. Maquarie Innovation does have a somewhat low aspect ratio rig. There are two ways to look at this - either the span is short for the area, or the chord is large for the span. I think the latter is more meaningful.

    The span, and with it the height of the aerodynamic center, has to be matched to the size of the platform. Once the windward pod lifts off the water, the lift from the wing is essentially fixed. The rigid wing allows them to sheet out to maintain the proper level of lift at high speed without flogging. However, this also means the long chord has excessive wetted area and parasite drag at high speed.

    At low speed, however, the area helps them accelerate to planing speed. A key factor in setting a record is "shots at the goal" - the number of runs you can make. If you have an underpowered craft at low speeds, it takes just the right conditions to accelerate it to the point where it gets "hooked up" and starts "generating its own wind," and the probability of setting a record is low. I know from sailing landyachts (and analytically analyzing their performance) that they can have two stable speeds for the same conditions. One is plodding along at low speed. The other is sailing at high speed. On land, one often has to get out and push the yacht until you're running, then jump in and accelerate to high speed. That's not an option on the water, unless you use a tow boat. So Macquarie Innovation needs to have enough power at low speed to get over this performance hump, even if that means carrying excess area at high speed.

    I think they may have compromised somewhat on the maximum speed of the boat in order to maximize their probability of setting a record. That doesn't mean a rig optimized for their particular requirements can be generalized to other applications.

    Aspect ratio really a misleading criterion. It's much better to consider each drag contribution individually, according to the dominant factor for that contribution.
    - Skin friction increases with velocity squared and wetted area.
    - Windage due to bluff bodies (stays, hulls, people, running rigging) increases with velocity squared and frontal area.
    - Induced drag increases with the square of the lift, decreases with velocity squared, and is inversely proportional to span squared. it does not depend on area. It does not depend on aspect ratio.
    - Vortex lift has a drag component that is proportional to lift times the tangent of the angle of attack. Unfortunately, one only produces significant vortex lift at high angles of attack, so the drag due to vortex lift is high. And this drag-due-to-lift is in addition to the classical induced drag (which is a consequence of the wake shed by a finite span).

    Each of the sources of drag above must be attacked by going after the dominant factor for each component.
    - To reduce wetted area, reduce the sail area. My landyacht has two sails - one is 49 ft^2, the other 59 ft^2. The small sail is faster any time there's enough wind for the yacht to sail with it - which is most of the time.
    - To reduce windage, minimize the size of the rigging, and eliminate as much of everything exposed to the wind as possible. Round all sharp corners - a corner radius that is 20% of the width of a bluff body can reduce the drag by 80% compared to having all sharp corners, and there is very little more to be gained by rounding any further.
    - To reduce induced drag, make the span greater. A small increase in span can make a significant reduction in drag. This is equally true of keels as it is of rigs. Keel shape and appendages can make a marginal difference, but depth is by far the most powerful way to reduce the drag of a keel.
    - Strive for attached flow around leading edges. Leading edge suction is much more effective than adding vortex lift for high-performance sailcraft.
     
  9. lunatic
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    lunatic Senior Member

     
  10. lunatic
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    lunatic Senior Member

    The premise of a LEV speed sailer may be dubious but it has lead to some interesting sailing experiences. Reduced SA- planing Sunfish with LEV rig of 27 ft2 in 15-20 kts wind. Reduced windage- no rigging,sailed one rig with no sheet. Induced drag- reduced SA from 40 to 27ft2,AR.7 to 1.4 with little performance loss.Fascinating beyond original objective. Waiting for some tricky ice to firm up to run these rigs on an iceboat.
     
  11. Petros
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    Petros Senior Member

    Thanks,

    As for a simpler, more efficient rig I was thinking that a kind of a modified junk rig would not only be more efficient, it would be lighter and easy to control, eliminate flogging etc. If you use a contoured rotating mast (like a wing leading edge), full flexible battens, and multiple sheetlets from the trailing edge to control the sail shape you can eliminate the boom, and control both twist and camber at any point along the sail. No jib or other devices, perhaps an "end plate" at the top, and have the lower edge flush with deck/cabin top to act as the lower edge end plate. This would be simple and way more efficient than current technology. The fabric and rig can be lighter too since the loads are smaller. I would also try to eliminate as much rigging, spreaders and other stuff flapping in the breeze as much possible to lower drag and flow interference on the sail. I am going to build one for my 14 ft sloop to see how it works, or build a small cat day sailer to try it out in the next year or so.

    Hitch idea is interesting but it is only looking a one part of the issue. Tests have shown that most of the useful thrust off a sloop rig comes from the jib, so he just made a system with one large jib, and a minimal mail sail. Aircraft testing has proven that the best L/D ratios only come from a single high AR clean wing. LE slats and TE flaps as use on aircraft increase lift, but also increase drag, which is fine for landing a large aircraft, but not efficient for a sail. Hitch's idea only looks at lift, and ignores the idea of reducing drag of the rig. That is why I want to try a single sail with a real efficient shape.

    The kite sails are an interesting development for small water craft in open water, but I think is impractical for larger sailboats or in multi-leg races or navigating traffic or harbors, etc. It hold the promise for high speeds with small sail surfaces, but so far no one has managed to set any records with it.

    considering how fast landsailing and iceboats have gone as compared to water craft, there is still a lot of improvements to made in water craft it wood seem.
     
  12. FranklinRatliff

    FranklinRatliff Previous Member

  13. FranklinRatliff

    FranklinRatliff Previous Member

    Airfoils

    I would also investigate the airfoil section developed by Sir Alexander Graham Bell and now used in applications such as the kites made by Guildworks.

    www.guildworks.com
     
  14. lunatic
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    lunatic Senior Member

    Petros : rigs I am sailing have little to no camber or twist and thus no luffing, even sailed one without sheet- no standing or running rigging.Work posted at http://proafile.com/view/weblog/entries/C12 Hard to analyse for efficiency or potential praticality.
     

  15. lunatic
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    lunatic Senior Member

    Ratliff: good to know other uses of vortex lift. Have 1/12 model flow studies of rigs I've sailed on above site,and typtical sweptback sailboard flat planform produces an attached LEV on surface below tip. I remember being pulled off my board even with 4m2 sail before the larger sails with designed twist off. Model planform of gaff rig shows same and might be responsible for large heeling force which I thought due to area.Is drag reduction a priority in those kite designs?
     
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