drag, sail, induced and other.

Discussion in 'Hydrodynamics and Aerodynamics' started by Anatol, May 23, 2015.

  1. Anatol
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    Anatol Senior Member

    Hello All
    I am very pleased to join your conversations and learn. I have a number of general questions about drag but here will discuss only drag re sails. I'm not a aerodynamics engineer and so am looking for general answers like - insignificant (less than 5%), or 'not nearly a much of an issue as sailshape'... etc. My context is the design of a proa around 10m/30' - of which more in other posts. So the context for my questions is a level sailing, non-planing, non-foiling boat , with a high aspect, square topped two mast cat schooner rig. I'm interested in maximising windward performance in particular.

    1. Mast drag. How significant is it. ie, - given two more or less identical sails, one a jib on a forestay, the other a main on a mast (in this case, lets say a circular section). Is the knife edge luff of the jib a major contributor to drive? Is mast drag a major drag?

    2. I've looked at the discussion on deck sweepers - I understand that flow under the boom is a source of major losses - but how much? Why did the 'park avenue boom' go away. Was it theoretically sound?

    3. Winglets/endplates. I've seen sailcloth endplates on the jibs of Sydney 16 or 18' skiffs. It is often said that an endplate on a rudder increases the effectiveness equivalent to substantially extending the foil. Same I believe is true for sail peaks. So why don't we see endplates on squaretop mainsails?

    looking forward to reading your answers
    thanks!
     
  2. Petros
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    Petros Senior Member

    end plates on the top of a mast on a sailboat that heels over will add a lot of drag and weight way up high. It might improve performance on a catamaran that does not heel much, but the effect is to increase the effective aspect ratio, it is easier to increase the actual AR rather than the effective one with an end plate. If there are rules that limit mast height than there may be something to gain with an end plate, but not in most circumstances I suspect.

    Mast drag is significant at high speeds (and drag from the rigging and cables stays as well), but most sailboats, particularly monohull cruises move slow enough it is not as much an issue. foil shaped masts are used in high speed boats such as racing tris and cats, and there is also effort to either stream line the rigging or use streamlined struts rather than draggy cables. cantilevered masts without stays or spreaders would reduce drag, but would be significantly heavier and mast would be much larger dia, so there are costly tradeoffs to be made going this way. A far more significant aspect of the mast drag by the way, is that the disturbed flow behind the mast causes a large area over the main sail to not generate much drive until the flow reattaches further back, stream lining the mast also improve sail performance significantly due to this factor.

    From my studies and work in aerodynamics many years ago I would say that a large gap at the bottom of the mail sail would reduced the available lift by as much as 20-25 percent (as a guess), but this issue is more complicated, more drive or lift will also cause a mono hull to heel over further, spilling off the additional list and likely increasing the drag on the hull, so the extra drive is not as effective as you might think. Where it would be noticeable is in really light winds, you would make better headway, but putting up larger sails (as in spinnaker) would also have the same effect.
     
  3. Anatol
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    Anatol Senior Member

    Thankyou for your note.

    Not quite sure what you mean by cantilevered mast.

    "A far more significant aspect of the mast drag by the way, is that the disturbed flow behind the mast causes a large area over the main sail to not generate much drive until the flow reattaches further back,"

    I guess I was lumping this effect in with 'mast drag'. Thinking of A frame mast examples, how far away does the mast have to be - to windward or leeward - to negate detachment of flow?


    > From my studies and work in aerodynamics many years ago I would say that a large gap at the bottom of the mail sail would reduced the available lift by as much as 20-25 percent (as a guess), but this issue is more complicated, more drive or lift will also cause a mono hull to heel over further, spilling off the additional list and likely increasing the drag on the hull, so the extra drive is not as effective as you might think. Where it would be noticeable is in really light winds, you would make better headway, but putting up larger sails (as in spinnaker) would also have the same effect.

    Excellent and useful response, thankyou. So...on a level sailing platform, one can expect substantial performance improvement by closing the gap between deck and boom. Keeping mast height down while maintaining high aspect ratio. This has been my suspicion. So closing that gap is perhaps the most substantial single improvement one could make to a mainsail.
     
  4. DCockey
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    DCockey Senior Member

    An end plate on a sail or wing needs to be properly aligned with the sail/wing and flow to reduce drag. Otherwise it is likely to generate more drag than it reduces.
     
  5. CT249
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    CT249 Senior Member

    Anatol;

    I'm not an engineer or an aerodynamicist, but I've been around here long enough to see this discussed a lot. Luckily on BDF we have at least three people with outstanding expertise in these areas - you'd probably be best off by searching for their posts, where they have already discussed most of your questions.

    One of our experts is Tom Speer, who has a proven record in the field designing for Boeing and the Oracle America's Cup team. He is very generous with his expertise. You may also want to search the posts here from Mark Drela - like Tom Speer he has helped to design America's Cup rigs. Mark is an aerodyamics professor from MIT who has designed other world record foils. Another person who has done many very interesting relevant posts on BDF is sailmaker Mikko Brummer.

    If I can condense what the above may say (judging on their earlier posts), the knife edge luff of the jib is NOT a major contributor to drive, nor is the mast drag a major drag. Tom Speer put me right years ago about this, when he said "The thin leading edge of the headsail is actually a disadvantage because there's no forward-facing area upon which leading edge suction can act". He also noted that "The theoretical effectiveness of the headsail is not due to its cleaner leading edge. It's due to its interaction with the mainsail."

    I suspect that a lot of the belief that masts are disastrous to flow over the main came from tests that used vastly over-sized mast sections. I understand why they did so, but the result is that the tests can be seen as completely unrealistic. Some well-known tests used masts of 7.5 to 12.5% of the main's chord length - that's like pretending a J/24 has a round mast section a foot wide! That sounds a bit like using a chainsaw to do brain surgery and then claiming that it's proof that you shouldn't use a scalpel.

    Another way of seeing that the mast cannot be an enormous problem is to sail wing-masted boats - they are often a tiny bit faster but very, very rarely are they as much faster as some theories would claim. This is pretty easy to test in a fashion by de-rotating a wing mast during a race - in my experience you may drop a few places but it's no worse than have the mainsheet a few inches too tight or two loose, or having the jib leads a few inches too far out or too close in.
     
  6. CT249
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    CT249 Senior Member

    There's already a current thread on this. There have also been many craft in which the gap has been closed as much as practical, and time and time again this has NOT caused the expected gains in performance.

    I sail one class where sometimes the gap is so closed that the mainsail foot presses on the deck. If you lift the mainsail foot up say 15-20cm it makes a bit of a difference, but at a rough guess I'd estimate it would be less important than (say) adjusting the vang correctly on a Laser.

    I've done some calculations about how much of a performance increase you need to make a real impact in racing. It turns out that on the racecourse, a 1-2% improvement in speed is often considered a massive breakthrough and is picked up widely and in a hurry. That makes sense, since even a tiny advantage in speed (say 5-6 lengths in a leg) is enough to give you an easy win.

    Given that a 1-2% improvement in speed makes such a difference, and that (despite claims to the contrary) many sailors will very quickly adopt such a breakthrough design, it seems that it's impossible that closing the gap makes as much difference as claimed. If it did make much difference, it would be almost universally done where allowed.
     
  7. Petros
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    Petros Senior Member

    The issue of loss of thrust off the sail from the mast interference and the gap under the boom are explored in "elements of Yacht Design" by ellefson and larson. Authors site a number of studies, and have charts and comparisons. It is a well documented issue.

    But for most recreational sailing absolute performance is not the goal, if you need to get somewhere faster you do not use a sailboat! In most racing classes rules limit you to "conventional" sails and rigs for each class.

    In unlimited racing classes you will see end plates, deck sweeping jibs and mainsails, foil shaped masts and even rigid wing sails if the rules allow it. Most of these have not made its way into recreational sailing because of cost and other practical reasons as noted above. Sailing is either sport or recreation, there is no benefit to complicating the rigging for most uses. Most recreational sailor want their boats to look "traditional", not like half of an aircraft sticking out of the water, that will not sell well, even if it performs better.
     
  8. Anatol
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    Anatol Senior Member

    CT249 -
    thankyou for pointers to threads and thinkers. I know Tom Speers' work on the bidirectional foil, and will seek out the others you mention.

    " the knife edge luff of the jib is NOT a major contributor to drive"

    that is really useful information, thanks.

    "Given that a 1-2% improvement in speed makes such a difference, and that (despite claims to the contrary) many sailors will very quickly adopt such a breakthrough design, it seems that it's impossible that closing the gap makes as much difference as claimed. If it did make much difference, it would be almost universally done where allowed."

    The idea that reducing to zero the induced drag caused by flow under the boom would amount to less than 1-2% advantage seems counter intuitive, but your argument seems irrefutable.

    Petros -
    "Most recreational sailor want their boats to look "traditional", not like half of an aircraft sticking out of the water, that will not sell well, even if it performs better."

    Happily I'm not encumbered by these concerns with this boat. In any case, a Proa is already weird. Why stop at half measures :)
     
  9. Petros
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    Petros Senior Member

    That is exactly correct, there are those who continue to try and push the performance envelope on inventive and creative designs, they satisfy the designers need for performance, and perhaps a few others, but I do not think very many were sold.

    There are happily a few exceptions, the original Hobie Cat I think was one such revolution in design, and changed the whole sport of recreational sailing. Created new class of boat, not often one sees that, but it could happen.
     
  10. CT249
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    CT249 Senior Member

    You won't see many of those in most unlimited classes, and you won't see all the boats that win in unlimited classes using all of those features. No one uses end plates (if by that you mean mast-tip end plates) with success, for good reasons backed up by theory. Deck sweeping mains have been used in some successful boats, but also in some unsuccessful boats. Surely if the benefits were as great as often claimed, they would be a necessity on any winning boat. Foil shaped masts have been tried and rejected in 12 Foot Skiffs, 18 Foot Skiffs, R Class, Merlins, Canoes, Moths, Supermaxis, speed windsurfers, Raceboard windsurfers and other classes. If the benefits were as great as often claimed, they would have worked and they would be popular. They didn't, and they aren't.

    Wingmasts and other leading edge devices do work in other classes (which is why some of us use them in those classes) but it's not a situation where there is a bunch of hidebound classes and a bunch of open-minded ones, as some people (and I'm not pointing the finger at you) on the net would have us believe - it's a situation where the specific benefits of each type of mast have to relate to the specific design of the boat. The mystery is not why some classes don't use wingmasts, but why some people on the net continue to make exaggerated claims about them.

    A lot of people on the net claim that conservatism prevents rig improvements. There is also objective evidence that (1) many of those who make the claim have their facts wrong; and (2) people in many classes and places are actually very quick to pick up developments that make craft go faster, if they are practical.

    As an example, I can recall when leading-edge devices (RAF battens and camber inducers) came into windsurfing, and when assymetrics came into yachts. They were generally greeted with open minds and keenly watched in a quite sophisticated way. For example, those of us who raced against the Gaastra Windsurfing World Cup sailors noticed that the early very rigid camber induced race sails encountered handling issues above a certain windspeed and speed issues below a certain windspeed. However, when you watched the way the sails performed, it was obvious how good they were overall and how much potential they had. The introduction of assys into offshore boats in the '80s was a similar process - it was a calm and evidence-based analysis of the strengths and issues of the performance and practical aspects, not a battle against conservatism as such things are often portrayed.

    What seems to be interesting, and very heartening, is that the more modern and sophisticated information that people like Tom and Mikko are showing us is a really good match with reality - dramatically better than the information we get from older books and wind tunnel tests. It's just a pity that the old stuff has such a strong grip.

    As noted, this post is not aimed at anyone in this thread, but more a comment about why such rig features are not more common, and about the mismatch between some theory and reality.
     
  11. philSweet
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    philSweet Senior Member

    There was an excellent thread a while back called "wake wash". One of the posts by TSpeer showed the AC trimaran with lots of telltales. If you zoom in, you can see that the stagnation point on the spar is quite far aft, about halfway into the black. Which raises the question of how much drag a mast creates if the sails are trimmed such that the stagnation point is dead on the beam.

    http://www.boatdesign.net/forums/at...70d1387775296-wake-wash-dogwing-jibhighup.jpg
     
    Last edited: May 28, 2015
  12. Anatol
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    Anatol Senior Member

    Last point taken - happily. Nor am I trying to make an argument.

    Regarding your first point about boat classes. My interest is in the wide open field of proas - where class rules do not apply. So my question is very open - I could build a boat with decksweeping sails - should I bother?

    As for asymmetricals, when I was a kid on Sydney harbor they were called shy kites. Very effective on the skiffs. Why did it take the big boys 10, 15, 20 ? years to notice them?
     
  13. Anatol
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    Anatol Senior Member

  14. gonzo
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    gonzo Senior Member

    Asymetricals were also called drifters. The new sails are an evolution of previous light air sails. New materials and textile manufacturing techniques allow for sail design that wasn't possible previously. Similarly, there are theoretical advantages to some designs that can't be built yet because there are no materials available that would perform adequately.
     

  15. CT249
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    CT249 Senior Member

    ????????

    Where were assys called drifters? The first articles I can find about them (the earliest such articles by a long time) were written when an assy designed by Andrew Buckland was put onto the 2nd (I think) "Prime" 18 Foot Skiff, designed and sailed by Julian Bethwaite. Julian had been thinking along similar lines himself, because by that time (mid '80s) the 18 Foot Skiffs were going downwind so fast that they were tacking downwind with the pole on the forestay all of the time.

    The development of the assy as we know it seems to pretty clearly stem from the 18s, according to all published sources I can find.
     
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