sail aerodynamics

Discussion in 'Hydrodynamics and Aerodynamics' started by Guest, Mar 21, 2002.

  1. Gary Baigent
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    Gary Baigent Senior Member

    I'll keep it simple for you CT: If a sailor enters a racing class that is not one design, then he knows that unless he is ahead of the play in skill, innovation and development then he is doomed to fail.
    He knows that.
    Repeat: he fargo trucking knows that is the situation when he gets invovlved.
    You sound like some nanny chook who has taken it upon yourself to protect the poor sensitive chickens who know no better.
    I think the "chickens" can sort things out themselves, can stand quite okay on their own, don't need you to spread your fluffy wing over them.
    In fact, to them, your attitude is degrading and ridiculous.
     
  2. CT249
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    CT249 Senior Member

    The only problem is selecting what pics to use as evidence.

    The most popular small junior dinghy in Oz for years was the local version of the Sabot, normally with full battens;

    [​IMG]

    The most popular junior/intermediate trainer in Australia until about 1970 was the 1930s era VJ. Look, full battens on the plan![​IMG]
    Click for Full Size Image


    The Moth was the most popular singlehander and it had full battens well before modern cats arrived. This is boat No 3, from 1928

    [​IMG]

    The most popular young adult's crewed class designed before the modern cat arrived (during WW2) was probably the Gwen 12 - full battens.[​IMG]

    The most popular widespread large dinghy, the Heavyweight Sharpie - with full battens.

    [​IMG]

    Perhaps the most popular dinghy in NZ before the modern cat arrived was the Idle Along - with full battens.

    [​IMG]

    Later I'll put up some pics showing that full battens were very common in some of the most popular classes in France and Germany well before the modern cat arrived.

    It is clear that full battens are NOT some new-fangled idea that mono sailors were too conservative to take on - generations of use have grown up with them, just as generations of people in some countries grew up with rotating masts. If it is implied that people do not use rotating masts or full battens because of conservatism then it is simply wrong.
     

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    Last edited by a moderator: Oct 3, 2015
  3. CT249
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    CT249 Senior Member

    Oh, those of us who sail development classes at championship level can stick up for ourselves, Gary. It's just that some of us don't like it when people like you
    abuse us for sticking up for our ideals by changing rules when it is necessary to keep classes alive.

    The strange thing is that when development class sailors sort things out themselves, by changing rules, some people who don't even sail the same sort of boat and weren't even there
    then throw abuse from the sidelines. It seems completely hypocritical to say that development class sailors can sort things out and then abuse them when they do so.

    Anyway, after yet another post from you with no information and content and nothing but abuse, I'll put you back on ignore. And you can go on and look at the tiny fleets that remain of
    once-strong and wonderful development classes like the Rs and Javelins, and see what destruction is wrought by those who fail to think about long-term class survival.
     
  4. brian eiland
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    brian eiland Senior Member

    Touche on the batten question
     
  5. tspeer
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    tspeer Senior Member

    That's not really true. It helps to improve the lift/drag ratio even when sailing off the wind. The fundamental sailing performance relationships are:

    Vb = Vt * sin(gamma - beta)/sin(beta)
    beta = arctan(D_aero/L_aero) + arctan(D_hydro/L_hydro)

    Vb = boat speed
    Vt = true wind speed
    gamma = point of sail (gamma = 0 is head to wind)
    beta = apparent wind angle (measured between the yacht's course and the apparent wind vector)

    Speed is improved when the apparent wind angle is reduced, and reducing the aerodynamic drag (at constant lift) will reduce the apparent wind angle at all points of sail.

    It is true that maximizing the aerodynamic L/D by itself does not always maximize performance because you need to take into account the corresponding effect on the hydrodynamic lift/drag ratio. This is because the hydrodynamic lift is entirely dependent on the applied load from the aerodynamics. For example, when sailing dead downwind, the hydrodynamic lift/drag ratio is zero because there's no aerodynamic side load, which means the hydrodynamic lift is zero.

    Especially for craft in which the added hydrodynamic drag due to additional side force is low, it can pay to operate at levels of lift above the point of maximum aerodynamic lift/drag ratio because the added side force improves the hydrodynamic lift/drag ratio, and the performance is better despite the increased aerodynamic and hydrodynamic drag.

    Interactions like this are why it really takes a velocity prediction program (VPP) to say what the sensitivity in performance is to any change in the boat.
     
  6. Doug Halsey
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    Doug Halsey Senior Member

    Tom : I beg to differ ...

    Since it's getting late here, I'll confine my comments to one simple counterexample & save my longer explanation for later.

    Consider a boat sailing dead downwind. The driving force from the sails consists of their aerodynamic drag only. There is no aerodynamic lift or hydrodynamic sideforce. The larger the aerodynamic drag, the faster the boat goes. That's why symmetric spinnakers were invented & why low-aspect ratio sails can be very fast downwind.

    I'm sure you'll agree that increasing the drag helps in this case, but you might be tempted to think that it's a very special case & it wouldn't happen at any other angle. But if more drag helps at gamma = 180 degrees, is it so implausible to think that it might also help at 179 degrees, or maybe even somewhat smaller angles?

    Let's not confuse the issue by suggesting that the boat could have a better VMG by sailing at a higher angle. We're discussing heavy displacement boats, where it isn't uncommon to have totally convex speed polars.
     
  7. brian eiland
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    brian eiland Senior Member

    Isn't this the situation where the aircraft aerodynamicist experiences some problems interpreting the aerodynamics of sailing?

    Or perhaps I have worded this incorrectly.

    What I am trying to get at is the fact that in sailing we are operating at such low velocities that many aircraft foils & designers
    really never need to contend with....on the very low fringes of flying aerodynamics. Then you add in the concurrent hydrodynamics
    of our sailing situations, that the aircraft guys do not have to contend with, then I begin to question some of their axioms,...such as recently proposed...

    ....that its the trailing edges of the foils(sails) that control almost all the lift of the foil, with little or no contribution by the leading portions or interactions of the sails??

    Additional edit:
    While the trailing portions of the airfoil very well might be controlling the substantial portion of the lift generated by the airfoil, BUT is it really controlling that
    relatively small portion of the lift that pushes our sailboats FORWARD?....that portion of aerodynamic lift that the airplane guys have no concern for?
    Is our airfoil on the sailing boat oriented in the correct direction (sail trim and incidence angle), verses what the aircraft designer might be seeking out with their wing orientations?
     
    Last edited: Sep 27, 2015
  8. Doug Halsey
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    Doug Halsey Senior Member

    Before I write anything else, let me repeat what I wrote on this subject earlier (Post #585, Page 39) :

     
  9. Doug Halsey
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    Doug Halsey Senior Member

    Rather than attempt yet another long explanation, I decided to dust off an old VPP code & get some numerical results. Unlike the special cases referred to earlier, these results are not limited to heavy displacement boats or cases with no drag due to side-force.

    The calculation is for a lite-weight dinghy approximating an International 10-Square-Meter Canoe. The blue curve shows results with aerodynamic characteristics typical of a an isolated sail, while the red curve adds an arbitrary constant to the drag coefficient (& calls it parasite drag).

    The curves cross at about Gamma=137 degrees. At angles smaller than that, the extra drag is detrimental, but at angles larger than that, it is definitely beneficial. It's not shown in the graph, but that's also the crossover point where the apparent wind angle becomes greater than 90 degrees.

    I hope this will convince everyone that increasing the aerodynamic drag can sometimes be beneficial.
     

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  10. brian eiland
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    brian eiland Senior Member

    Wing Mast on J 90

    There were a number of postings back on this page about wing mast, and specifically one experiment by Hall Spars,...their J-90 Blackwing experiment
    http://www.boatdesign.net/forums/hydrodynamics-aerodynamics/sail-aerodynamics-457-43.html

    I have a response to a note I wrote to Eric Hall:
    1. First to say is in my comments below, the “una rig” was a fully rotating airfoil shaped mast. Everything should be understood in that context.

    2. It was inferior going to windward.
    a) A lower frontal area airfoil section would have improved it (but would have had sideways stiffness problems).
    b) Any rotation past in-line with the apparent wind slowed down the boat where added drag trumped added power (as flaps do on a plane)
    c) There is no substitute for a jib to improve upwind performance
    d) The jib seems to add lift while improving the flow around the main​

    3. Beginning with a close reach the Blackwing rig was superior.
    a) With the mast able to rotate, the wind “sees” a much better mast shape with less turbulence that a conventional non-rotating mast.
    b) As soon as a jib is eased, its positive effect on the mainsail reduces quickly
    c) Rotating past apparent wind adds substantial power this time trumping added drag
    d) Adding a standard J90 A-sail made this system a real weapon!​
     
    Last edited: Oct 3, 2015
  11. powerabout
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    powerabout Senior Member

    seems like that was findings from 1960, was it?
     
  12. powerabout
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    powerabout Senior Member

    I was just trying to get my head around the success and failure is not the aerodynamics but other factors.
    In beach cats if you have the rotation wrong the fleet goes past you.
    Seeing how they work with the cunningham etc its a whole package of optimisation including rigging and sail cut that took years to get to where they all are today
     
  13. CT249
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    CT249 Senior Member

    Sources like Bob Fisher's early book on cats indicate that it didn't take very long at all for wing masts to show their performance. There was very little regular racing in the C Class when wingmasts were adopted there and a tremendous amount of other development to do, yet wingmasts showed lots of potential very early on and quickly realised that potential.

    That is quite a different situation to the one in mono dinghies, where wingmasts have been tried repeatedly but almost never won a series and have shown so many issues that the many champions who have tried to develop them have normally binned them. We also know that it's NOT a question of being closed-minded, as in Australia for example wingmasted dinghies are more popular than skiffs and very influential in skiff design, so it's not as in wingmasted monos are seen as freaks here. Here they are quite mainstream (the two most popular two-adult dinghies in the country use wingmasts, so they are as mainstream as Snipe and Vanguard 15 rigs are in the USA) and very well understood in many ways - and yet they still don't work in most monos, because of issues like gust response.

    It doesn't seem odd that wingmasts work better on some types of craft than others - lots of rigs work better on some craft than on others. And in beach cats if you have the rotation "wrong" you still only go a fraction slower than the others, which underlines how over-blown some (not all!) estimates of the extra efficiency of wingmasts is.

    The fact that there are such different answers to the under-rotate/over-rotate issue in heavy air sailing, and that they get such similar responses, also seems to indicate that some theories about the "superior" efficiency of wingmasts are overblown.

    Just to repeat, this is NOT claiming that wingmasts don't work well in some boats, but the fact is that they have repeatedly been shown not to work in other types and surely we should be accepting that reality and adjusting the theories to fit, rather than saying that reality is wrong as some people do.

    Cheers
     
  14. CT249
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    CT249 Senior Member

    Cheers.

    In order to save thread space, I won't put up more pics. However, in France the 9m2 Sharpie (one of the most popular of all boats from about 1930 up till about 1955) had full battens and a big roach. In Germany, full battens were standard on the popular J-Jolle 22 sq m dinghy and in all of the high-performance Rennjolle classes which were very popular from about 1920 to WW2.

    Uffa Fox wrote about the superb Z Class Rennjolle (perhaps the fastest of all dinghies for decades) and the hassles the full battens caused.

    In England, the most popular of all designers (Holt) had full battens in the popular Hornet class in the '50s, but they were abandoned because they made it hard to adjust sail shape for different conditions. However, for some decades many or most of the popular classes had one full batten, and very popular boats like many Moths and Holt's Solo were all full batten.

    Of course the Canoes, the first popular small-boat racing class, had full battens from about 1860 for decades.

    So the evidence is clear - in Europe, Australia, NZ and (in the very early days) America, full battens were completely accepted. However, they didn't always work well for various logical reasons, and they were therefore not universal and still aren't.

    Since full battens (and rotating masts, for that matter) have been completely accepted in most countries for eons, "conservatism" can't be the reason they are not more widely used. Many of us grew up with them (for example, my father, my two brothers, my wife and I all had full battens on our first boats) and therefore if we took the conservative route, we'd stick blindly with them. If we don't use them, it's normally because they are not suitable for that particular application.
     

  15. tspeer
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    tspeer Senior Member

    Yes, you're right that if drag is all that's driving the boat, then the more drag the better. In that case the apparent wind angle is 180 deg, and the lift/drag ratios of both the hydrodynamics and aerodynamics are zero.

    As the boat heads up from DDW, lift does start to come into it, and there will be a point of sail at which an incremental addition of drag has no effect on the performance. Above that point, any additional drag will be detrimental. Where that crossover point of sail is has to be determined by a VPP. It can't be determined by aerodynamics alone, because the lift/drag ratio of the hull is involved.

    I suspect the crossover point of sail is pretty deep, even for a heavy keel boat. It would be interesting to see if it's as wide as the course for best downwind velocity made good (Vmg). Only for the case where the crossover point exceeded the best Vmg angle would there be a regime where additional aerodynamic drag would be beneficial.
     
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