Square top mains?

Discussion in 'Sailboats' started by kenwstr, Dec 6, 2005.

  1. kenwstr
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    kenwstr Junior Member

    Hi

    Square top mains are very popular, you see them on the Americas cup boats, 18 foot skiffs, F18s and others.

    Are they really any better and why.

    According to standard aero theory on lift distribution, induced incidence and induced drag, I think they should be a bad idea. Lift distribution diagrames in any aero text indicate that for untwisted lifting surfaces, the local load is a proportion of the real chord to an eliptical chord of the same surface area. Therefore if the real chord is larger than the eliptical, the surface unloads due to a local reduction in angle of attack (induced incidence). This rotates the whole reaction vector rearward (induced drag). Also in the very best lifting planform (eliptical), there is added pressure leakage around the tip which causes a further very local reduction to zero lift right at the tip. The tip however still produces form drag and skin friction but contributes little to no lift even on the best plan form and the situation becomes much worse the broarder the tip is.

    So does anybody know any theory, dynamic priniples or engineering studies that indicate these square top mains are better than a full eliptical roach?

    I sail an MG14 which has an upwind sail area limit of 9.3 m^2, same as NS14.
    Some boats have the square top mains but these are not the champion boats. The placing boats in National and state champs all have full or exagerated eliptical roaches.

    I'm wondering if the square top mains sometimes appear better because they are newer sails or where sail area is unrestricted. Is this a fad or is there a good and proven basis for them?


    Regards,
    Ken
     
  2. Doug Lord

    Doug Lord Guest

    square top mains/Wing Tip Rig™

    Ken, there are solid aerodynamic reasons for square tops but I'm sure Tom Speer could do a better(more thorough) job of explaining that than I can.
    I had Johnson Sails in St.Pete make my first square top in 1975 for use on an experimental 16 footer. Since then I have used them and variations of them on many boats including all of the radio control sailboats my company(http://www.microsail.com) builds. I've been experimenting with a square top main and jib on a 16' hydrofoil(http://www.monofoiler.com).
    One thing that stands out is that if the ratio of head length to foot length is correct the sail can have excellent gust response w/o having a complicated rig and it can be depowered easily.Needs a more powerfull than normal vang. Most of the "peaked up " square tops(Wing Tip Rig) I've used also have a form of rotating gaff supporting the upper part of the sail as well as a boom fixed to a rotating mast. The design of this carbon gaff allows control of the "peak" of the sail in a fore and aft direction creating an upper outhaul. A little complicated to rig on full size boats but excellent for models. In my own testing with two identical models one with a fully roached rig and one with a square top the square top was always faster upwind with the margin declining downwind.
    In looking at full size sailing all the AC boats and most of the Volvo's are using them as well as many ,many other classes as you know-not a fad...
     
  3. LP
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    LP Flying Boatman

    planform

    Do the square top sails typically a fair amount of roach also? Is a square top sail that much different than an elliptical? In either case, it seems that additional square footage at the top of a mast is going to increase the power of the rig. i.e. more efficient rig. So, before the vipers attack, some study of the planform should be made. Without knowing the intricacies(sic) of elliptal vs. square top, I suspect that both have a better spanwise distribution lift than a standard bermudian/sloop rig. (See my apology under the "sail aerodynamics" thread.)

    Any good links to square top sails?
     
  4. usa2
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    usa2 Senior Member

    well, the squaretops do go faster, so somewhere in there someone is doing something right....
     
  5. kenwstr
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    kenwstr Junior Member


    Hi

    Area distribution and lift distribution do not equate.
    In aero texts, if you have a rectangular (constant chord wing), then the changes in induced incidence along the span produce an eliptical Cl distribution. Now lift force distribution is a product of local Cl and local chord which is constant in this case so the lift force distribution is still pretty much eliptical.

    Now take an eliptical wing, induced incidence along the span is constant. Therefore Cl is constant but the chord is eliptical so the lift force distribution is again eliptical.

    local lift is proportional to eliptical chord / local real chord (close enough for practicality) So any planform that is not geometrically twisted and is not stalled anywhere should produce pretty much the same eliptical lift distribution with the force centre close to 40% of the span.

    Now all this is rather simplified but illistrates the point.

    Sails are twisted though and the twist largely follows the apparent wind gradient up the mast. But the point here is that the broarder the tip, the more the apparent wind comes from ahead (induced incidence) and the more the reaction vector rotates rearward. Increasing incidence at the tip to recover load increases this induced incidence effect.

    It is generally accepted that an eliptical wing gives the most even induced incidence and therefore lift distribution and minimum induced drag.
    In terms of a sail, provided the sail twist is adjusted to match the apparent wing gradient, an eliptical sail would give the lease disturbance to the natural gradient and might therefore be expected to minimise induced drag.

    Now in the MG14 experience with our very small sail area restriction, efficency (minimum drag) is most important and the square top mains do not seem to be working out from what I can see. The idea seems to violate standard aero theory. Given the fact other classes seem to use them to some advantage I have to assume someone hase made a study of this and found good reason but I can't find anything that shows why they should work and the MG/NS results would so far seem to confirm they do not.


    Ken
     
  6. kenwstr
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    kenwstr Junior Member

    Hi

    I'd like to add that sail area or at least adding sail area is not the same as improving efficience. Efficiency is about lift drag ratio so if you add area increasing both lift and drag, that boat will not go faster or point higher upwind unless the % increase in lift is greater than the % increase in drag. In the skiff classes, the boats only sail upwind, that is they always sail into the apparent wind. Even on the down wind legs, the flow is still from luff to leach because we sail an angle faster than the wind. In this type of sailing, L/D efficency is the only thing that counts.

    MG14s are like a small low powered skiff, they just have the power ratios for true skiff style sailing. The sail area is limited so sail plan is all about how to distribute that limited area for greatest efficency.

    Sail area only counts with the apparent wind astern but then you would not be sailing faster than the wind.


    Regards,
    Ken
     
  7. Doug Lord

    Doug Lord Guest

    Square top uselessness

    Ken, I've seen theoreticians that are a little low on practical experience but for you to deduce that square top mains don't work with the limited exposure you've had to them takes the cake. But maybe you're right and everybody else isn't. I'm not trying to be a smart *** and I'm very interested in this field ;I will continue to research it. Maybe Tom will jump in and help us both out....
     
  8. kenwstr
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    kenwstr Junior Member

    Hi Doug

    I think you misunderstand my position. I am aware that these mains are used in classes where a great deal of reasearch is invested So I have to assume somebody knows something. I just havn't been able to find a technical reason for it.

    I'm just pointing out that according well accepted aero theory, at least in my understand of it they should not provide an advantage and that so far no square top MG has achieved championship standard. So without a solid explanation, there remains a question in my mind that I want to settle.

    That is why I posted the topic, to find an explanation. I had included aero theory to indicate my understanding and set a level for debate.

    Sorry if it sounds like I have a definate opinion, its not the case, just a question. I may debate this pretty hard though because that brings out more detail.


    Regards,
    Ken
     
  9. Doug Lord

    Doug Lord Guest

    Tom Speer on planforms

    Ken-check this out:
    Minimum Induced Drag of Sail Rigs and Hydrofoils
    http://www.tspeer.com/Planforms/Planar.htm
    ------------------
    edit: Much of Toms writing is way beyond me but he did mention"modern fat head sails" . I was disappointed that there were no illustrations of these low drag sail planforms.Seems like sketches of such sails would help in understanding the theory.
     
  10. gggGuest
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    gggGuest ...

    AUIU according to standard aerodynamics textbooks the difference between a true elliptical wing - eg Spitfire, and a tapered square tipped wing - eg Messerschmidt bf109 is very small. Shouldn't be a suprise - elliptical wings are very rare and Mitchell had other reasons for adopting it.

    With so very many other complexities in the much more complex aerodynamics of sailboat rigs I doubt it makes a blind bit of difference compared to getting the dynamics right.
     
  11. water addict
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    water addict Naval Architect

    I wonder if it is a rating thing, that the sail area is rated as the triangular dimensions. Then having a lot of roach and a square top is the most aero- efficient way to get unrated sail area? Just thinkin out loud.
     
  12. FAST FRED
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    FAST FRED Senior Member

    "I wonder if it is a rating thing, that the sail area is rated as the triangular dimensions. Then having a lot of roach and a square top is the most aero- efficient way to get unrated sail area? Just thinkin out loud"

    No rating rulles for the early Dutch botters ect , yet they all used a nicly curved yard to get the max efficency with the minimal work or cost.

    Perhaps they figured out what the "modern" fluid dynamics computer driven crowd is just discovering?



    FAST FRED
     
  13. usa2
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    usa2 Senior Member

    You get more drive up there, and the wind is typically stronger 60-120 feet above the deck (depending on the size of your boat).

    It may be more drag, but it puts driving force where the wind is, and by the time the wind increases as to make the square top inefficient, then the rest of the sail has become efficient, and the top depowers.
     
  14. RCSail
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    RCSail Junior Member

    Another note that I have seen from RC gliders is that an elliptical planform tends to tip stall, especially at high AoA. An optimized quadruple-taper wing is very similar to a square top rig and is the prefered glider wing for use in high CL conditions.
     

  15. kenwstr
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    kenwstr Junior Member

    Thanks Doug, that is an interesting paper.
    I haven't read it all yet but so far it seems to say that when the lifting surface is constrained by the lift moment at the root as is the case in sailing, then deloading toward the tip allows a taller rig and that the higher aspect ratio of the taller rig reduces induced drag more than the efficiency loss of departing from the ideal eliptical plan form.

    So this poses some more questions in that the sail can be deloaded by either an exagerated roach or by twist. So far the optimum approach appears to be roach. That may well be due to the linear velocity gradient in light air. The other suggestion here is that where class rules limit the height of the rig as in NS/MG, there can be no gain in aspect ratio and that is going to significantly impact the optimum plan form I would think. It may at least in some part explain why our experience of square tops so far is negative while less restricted designs are positive.


    Regards,
    Ken
     
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