Sail camber

Discussion in 'Hydrodynamics and Aerodynamics' started by oatsandbeans, Jan 31, 2013.

  1. oatsandbeans
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    oatsandbeans Junior Member

    Mikko,
    It is a small world. I may be wrong on my exact dates- it was a long time ago. I know the story because in 1979 I started sailing with Jeremy Bickerton in the 470 who had the Cheret sails and won the nationals previously. He got the sails from Rodney Pattison who had them but had never tried them out. The sails finally ended up going around all the Uk sailmakers and Musto took them apart and copied them and Banks also copied them. The facts might be a bit rusty but my my point is that sailors get used to a certain rig and try get the best out of it. They dont always know intuitively what is the fastest.

    If you want another anecdote from 470 history. I remember Lawrie Smith telling me that he was at Kiel Week ( early 1980's) and saw a boat in the dinghy park. He couldnt beleive the position of the mast it was raked so far back. He didnt think much more about it he thought that it ws someone who didnt know how to set a 470 up. It was the boat of David Barnes and he won Kiel week -with some races leading by more than 5 minutes. He had developed a completely different rig and was blisteringly fast ( and went on to win 3 world championships - I think!) So the best sailors dont always know what is fast.
     
  2. brian eiland
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    brian eiland Senior Member

    I'm just reading back thru this subject thread, and I was particularly taken by this quote!...how true.
     
  3. brian eiland
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    brian eiland Senior Member

    Wanted to repeat these. Too bad this gentleman did not continue to contribute to the forums.
     
  4. brian eiland
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    brian eiland Senior Member

    A lot of daysailing/racing catamarans adopted this aft rake to their mast in the 70-80's
     
  5. Jamie Kennedy
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    Jamie Kennedy Senior Member

    The ideal camber changes depending on whether you are underpowered or overpowered, and depending on whether you are going for closer maximum efficiency because induced drag is dominant, or closer to maximum power because the other forms of drag are dominant. Also, whether you are going for VMG to windward, VMG downwind, or something else. Also, whether the wind and boat speed are steady state, or if there are alot of transients and turbulence. Some folks like highly theoretical performance in ideal conditions, and huge sails, but I like the messy stuff.

    This looks like a hoot...
    https://www.youtube.com/watch?v=B03L9lHXWKk
     
  6. Mikko Brummer
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    Mikko Brummer Senior Member

    Brian,

    While it's nice that oatsandbeans & my circles seem to have crossed on several occasions in the 70'ies, I don't think we have missed much in his contributions regarding sail shape vs performance. What you want to repeat is for the most part untrue. Like I've explained earlier in the thread, and like Jamie is stating above, sail performance cannot be expressed in terms of fixed camber & draft numbers, as oatsandbeans appears to wish. It all depends on sailing attitude, wind structure, waves, boat size & type etc., there is no "optimal shape". And it is surprising how very much different shapes can end up in an absolutely similar performance, when the sails are set correctly. The trick in sail design is "range", versatility, to create sails that can be adapted to as many different conditions as possible, without compromising performance too much in any particular conditions. I'm afraid in that process your so much disliked mast, and the mainsail attached to it, is a very invaluable tool ;-).
     
  7. brian eiland
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    brian eiland Senior Member

    I don't think the two quotes of his that I picked out in that 'repeat- posting' I made were suggestions of his as to an ultimate optimum camber. Granted he did seem to be looking for that initially, but I think he was coming around to the idea that there was not a set optimum.

    Then he went on to ask what optimum(S) there might be at various windspeeds, and then he questions some of the very knowledgeable source books as to what they had to say about the situation. He was pointing out conflicting statement/conditions right within some of those individual texts.


    Agreed, "there is no 'optimal shape'. And it is surprising how very much different shapes can end up in an absolutely similar performance...". I believe that is the conclusion that he was realizing after what appears to be extensive study of a good number of knowledgeable texts by himself. He appears to be just asking himself 'why?', for a second time, and doing it on this forum to solicit other opinions.


    I assume you are referring to my mainsailess rig?
    Yes, perhaps my mainsailess rig might lack a lot of the traditional mainsail shaping technics such as rotating mast, bendy mast, boom adjustments, full battens, fat heads, etc, but it just might allow for enough good sail shape for all-around good and easy cruising with a WHOLE lot less complication.....just good roller furling gear, good modern sail cloth choices, and good adjustable fairleads for shaping those adjustable headsails.

    Perhaps even my mizzen sail could act as a 'aero flap' for the whole rig?
    ("think-one" as I believe you referred to the concept)
     
  8. tspeer
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    tspeer Senior Member

    When I said that sail designers don't really use the language of aerodynamics very much, this is a perfect example.

    Maximum camber, and the position of maximum camber, are geometric quantities that are easy to observe and measure. And, using thin airfoil theory, one can predict the lift and moment of a section from the camber line. Thin airfoil theory says you can get exactly same lift with different camber shapes, just at different angles of attack. But thin airfoil theory doesn't say anything about the profile drag of a section, and that's what makes the difference between different camber shapes.

    To understand the profile drag and issues like flow separation, you need to look into the boundary layer development on both sides. The boundary layer development is largely shaped by the pressure distribution. It's more useful to consider the windward and leeward sides separately when you're dealing with the boundary layers.

    As long as the flow is attached and fully turbulent, there is little difference in the profile drag of different section shapes. You can pretty much correlate the minimum profile drag of a fully turbulent section with the maximum thickness alone. The precise section shape starts to become important when you are trying to foster laminar flow or avoid separation.

    When the flow is attached, the profile drag is a comparatively minor contributor to the total drag. The induced drag is far and away the largest drag component. When you change the camber, you are effectively changing the twist because camber changes the zero-lift angle of attack. So, depending on the twist of the sail, a given camber change may move the sail closer or further away from the ideal twist for the sail's planform and the shear in the apparent wind.

    So if you're going to discuss the merits of different camber shapes, you need to do it in the context of the aerodynamic problems the sail has. It may be experiencing separation on either the leeward side (typically at the leech) or the windward side (behind the luff). The head may be twisted off too much so that additional camber for the same leech position would effectively reduce the twist. Point to the illness, and then consider the cure.
     
  9. Mikko Brummer
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    Mikko Brummer Senior Member

    Since I'm the only sail designer in the thread, I guess I need to defend myself ;)

    I've been trying to find an easy way to measure sail shape for about 25 years, and I am yet to find it. The best option is still to put a camera in the mast top, but it needs to be video since the sail shape is ever changing as the boat is pitching and wind pressure varies. From the mast top you cannot resolve mast bend, so if you are serious, you need to have a 2nd video in a coach boat on the side, and a 3rd behind, to be able to measure mast bend/fore stay sag. And a 4th video camera for the jib at the hounds. Handy also to place wind instrument & GPS displays in the view of the coach boat camera, to record the conditions.

    And all cameras need to be synchronized in time, so that you can reconstruct the scene later... Then you try to choose some representative stills from the video footage, for the average shape of the sails in those conditions... The analysis of 5 minutes of sailing usually happens only weeks later - not my view of easy measuring & observation. And the accuracy is maybe +-0,5% for camber and +-1 deg for twist, barely in the limits of acceptable.

    I can still remember how I was enthralled by thin airfoil theory as a young student in the 70'ies - apparently a perfect tool for the thin airfoils that sails are... but in reality useless for sail analysis. Firstly, it's 2D, while sails are highly cambered, twisted, raked (swept), heeled (dihedral) low aspect 3D and multi-element. Same applies to the lifting line, VLM is better but still very much inviscid, panel methods may handle separation somehow but not incorporate for instance mast effects. Only today with N-S can we predict flow separation on sails at some accuracy (?).

    BUT... even if we knew what should be changed on a particular sail, sails are not solid like wings... They cannot be sculpted at will, on the contrary, sails are flexible membranes that can only assume so many shapes. Even if I'm the first to promote an analytical approach, with the best existing tools, it still remains very much trial & error & experience, how a certain design change affects the sail's performance, over a wider wind range. Read more here, if you like.

    So, maybe sail designers don't speak so much in terms of aerodynamics (especially not in terms of airplane aerodynamics ;-), since the real design finesse still lies in the intricacy of the behavior of the soft sail and the various adjustments like mast bend, stay sag, foot & luff tension, twist control, rather than in the state of the boundary layer or interaction of the separation vortex behind the luff with the trailing vorticity at the head...
     
  10. oldsailor7
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    oldsailor7 Senior Member

    +1. After many years of sailing and racing both monos and multis I have come to the realisation that the NO.1 eyeball is the best judge of how the sail is acting. Use of tell-tales is manditory. How many times have I seen sails which have what I call a dog leg shape. The lower half of the sail over sheeted and stalled with the sail head at zero AOA and doing nothing.
    Many skippers are averse to sheeting the outhaul OUT and the mainsheet DOWN, to get the sail leech straight from top to bottom,thus getting max drive and least drag from the whole of the sail.
     
  11. brian eiland
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    brian eiland Senior Member

    Hey Mikko, how about some of those GoPro cameras ;)
    I understand they are pretty good, and getting even smaller.
    http://shop.gopro.com/hero4/hero4-session/CHDHS-101.html

    Now we just need lots of 'detection stripes' on the sails?

    BTW, I applaud a lot of your work and experimentation on these questions of shape, etc
     
  12. Erwan
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    Erwan Senior Member

    Section Drag (2D)

    Hi Everybody,

    Thank you for this interesting discussion.
    Mr Speer's post has triggered a question, but I am a bit embarrased to state clearly the concern as I am not familiar with BL theories & equations.

    It is about 2D drag with full turbulent BL:
    If the 2D drag is almost only proportional to section thickness, does it mean:
    Shape of rooftop gradient, Momentum thickness & displacement thickness, type of recovery distribution, have no significant influence on section drag ?

    The context could be the new A-Cat rigs which addresse at least partially the induced drag issue.
    Typical conditions could be 10 m/s AWS and 400k<Chord Reynolds< 1200k
    with of course the assumption of unseparated flows.

    Hope it is not off topic & thanks in advance

    Regards

    EK
     
  13. oldsailor7
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    oldsailor7 Senior Member

    Quote:-
    It is about 2D drag with full turbulent BL:
    If the 2D drag is almost only proportional to section thickness, does it mean:
    Shape of rooftop gradient, Momentum thickness & displacement thickness, type of recovery distribution, have no significant influence on section drag ?
    and
    Typical conditions could be 10 m/s AWS and 400k<Chord Reynolds< 1200k
    with of course the assumption of unseparated flows.

    Jeez, you've lost me. :confused:
     
  14. Erwan
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    Erwan Senior Member

    Sorry OldSailor7,
    It was not intentional, I just try to make an effort to achieve as much as I could, the good "academic formulation", with reference to the appropriate concepts which are well known by CFD gurus.
    The rooftop is on the upper front part of a wing or leeward front side of a sail
    It is the low pressure side where velocity increase well above wind velocity

    Momentum & displacement thickness definitions are available on wiki

    Recovery distribution is the part of the pressure (or velocity) distribution which is just after the rooftop on the low pressure side of the lifting surface (wing or sail)

    You can find many attached files on this thread, with wing/sail section and their XFOIL analysis with velocities / pressure distribution graph.

    Sorry again

    Wish you a good week

    EK
     

  15. markdrela
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    markdrela Senior Member

    What you call "2D drag" is more commonly called profile drag.

    If there is no flow separation, and the flow is either fully turbulent or the transition location is fixed, then the profile drag coefficient is rather insensitive to section thickness or the details of the section shape.

    The profile drag times the freestream speed is equal to the overall viscous dissipation in the airfoils boundary layers and wakes. The local dissipation scales as the local velocity^3. So thickness and lift do tend to increase profile drag, mainly because they increase the average velocity^3 on the surface.

    What's interesting is that although pressure drag contributes to profile drag, pressure drag is more of a symptom rather than the cause. Viscous dissipation is the ultimate cause of profile drag. This video talks about this a bit:

    https://www.youtube.com/watch?v=2IbYyvvBQeg

    It's intentionally "slow", so you may want to speed it up.
     
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