drag, sail, induced and other.

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

  1. CT249
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    CT249 Senior Member

    Cheers; as noted the point wasn't aimed at you. :)

    Personally my impression is that decksweeping sails provide a small extra benefit. Julian Bethwaite says that a "cuff" like in a 49er provides a significant benefit, but some aerodynamic gurus disagree. However, the advantage of closing the gap is so small that if it causes handling issues, or sail sheeting issues, it's normally not worth it in my humble opinion. Just two cents worth....

    Re assys; I grew up on Sydney Harbour too, and we had assys on Manly Juniors, 16 Foot Skiffs etc. Andrew Buckland, who on the documentary evidence gets most of the credit for creating the modern assy, says that the difference between the old "flatty" shy kites and the modern assy is that the flatty was designed for reaching with attached flow, or running close to dead square with the pole back and semi-stalled or fully stalled flow.

    In contrast, the modern assy is designed to be sailed tacking downwind, with flow attached at all times.

    There were some really interesting articles on this in Australian Sailing in the mid/late '80s, firstly the article about the original modern assy (the one that Julian BEthwaite put on the 18 Foot Skiff Prime) and also from the 12 Foot Skiff and 14 Footer guys who picked them up pretty quickly; I think Andrew Crisp lead the way in 14s. Because of the lower boatspeed of the 12s and 14s, and the fact that the 12s sailed a lot on rivers, arguably they had to take the use and analysis of the assymetric another couple of steps further.

    I think it was about August 1989 when the first Australian offshore boat designed specifically for an assy (the David Payne Skiff 38) was launched. I sailed the first one in its first regatta; it was a strange experience getting to learn how to use an assy on a boat with a full interior, although the Skiff 38 had wings as well as two heads!

    BTW have you seen the occasional posts from the proa guru Russell Brown posts here and/or on Sailing Anarchy?
     
  2. tspeer
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    tspeer Senior Member

    You're looking at a wingsail (the world's largest) with a jib. The wingsail worked best when it had a substantial flap deflection.

    Qualitatively, what matters most in determining the amount of lift is the angle of the flap to the apparent wind. You can think of the forward half of the wing as being a leading edge device for the flap, even though most of the lift is actually carried by the forward element.
     
  3. Jamie Kennedy
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    Jamie Kennedy Senior Member

    Interesting conversation. Thought I would chime in.

    It is often useful to consider the lift/drag diagram of your sail, where you are on it when sailing in a given situation, and how you can alter the shape of it.

    When sailing to windward you are usually sailing somewhere on the small angle of attack side of the lift/drag diagram. If you are underpowered you would normally sail a higher angle of attack than the point of maximum lift/drag ratio. If you are overpowered you would normally sail at a lower angle of attack than the point of maximum lift/drag ratio. If you happen to have the ideal sail area and sail shape for conditions, you would be sailing at the point of maximum lift/drag ratio.

    Follow me so far?

    Now in terms of altering the shape of your lift/drag diagram, as a designer you can alter the amount of sail area and the aspect ratio and the amount of roach and size of headboard and detail of your end plates. You also have some control over the mast diameter and rigging. The mast diameter and rigging generally add some parasitic drag that can be included in you lift-drag diagram and generally shift the whole diagram to the right, adding some drag without affecting drag too much one way or the other. Higher aspect ratio for the same sail area tends to stretch out the lift and reduce drag at smaller angles of attack, but also stall sooner and, this is important, achieve a lower drag AND LIFT, in the region of maximum thrust near the angle of attack of stalling. This generally is of no consequence for upwind performance, unless you are seriously underpowered and are towing barge, or have an extremely high aspect ratio, higher than optimal for upwind performance. This will have a dramatic effect however when close reaching and beam reaching and for downwind performance, where lower aspect ratio rigs perform better. END PLATES tend to have the effect of increasing your effective aspect ratio. So when going upwind is generally when you want your endplates, but when going downwind, or beam reaching or even close reaching but not overpowered, you may want to reduce your end plate effects for more power for the same sail area. You are often doing other things at the same time, such as easing your outhaul, downhaul, mast bend, to increase draft. Rake is another interesting consideration you may have some control over while sailing and changing points of sail. Increasing rake for unwind, in addition to other things, can increase your effective aspect ratio by increasing the vertical height from clew to head. This is offset somewhat by reducing the vertical height of the luff, which might matter more in lower wind speeds. The other effect of increased rake is to help close the end plate, especially when driving more off the leech than the luff in stronger winds. Downwind, and even when beam reaching, you can reduce rake or rake forward, which can effectively reduce your aspect ratio and end plate effect and give your more total thrust. Particularly when sailing downwind you may want to rake forward if you can which can create some vortices over the top of the rig which can disturb more upper air flow and give you more thrust. There are some good diagrams out there on pacific proa sails that show different lift-drag ratios for different rakes, but it also applies to other cat rigs, such as Optimist, Laser Radial, Laser; where you want high aspect ratio upwind and lower effective aspect ratio reaching, and more disturbance in generally when going downwind. You can also heel the boat to windward when going downwind, which in addition to reducing drag on the hull and rudder can also raise your center of effort and reduce effective aspect ratio and end plate effect (from water) and increase forward rake, where easing the boom out farther rakes forward as well as changing angle of attack. This is particularly effective in an unstayed rig when sailing by-the-lee.

    For a cat schooner rig such as you have in mind it gets even more interesting. As a designer you can control the distance between the rigs. When sailing you can also control the relative rake, and may also want to control tilting the rigs to windward or to leeward. You might think of windsurfer rigs, particularly two person and four person windsurfer rigs, and how they used to tilt the rigs differently relative to one another, and whether or not that might be achieveable somehow with a 30 foot proa, by some practical means other than hiring some really tall crew. ;-)

    Hope this all helps.

    Jamie Kennedy
    p.s. Here is one of those links on proa rigs and rake, which can be applied to most cat rigs, and schooner or ketch cat rigs.

    http://www.multihull.de/technik/t-slotboom_gb.htm
     
    Last edited: Jun 2, 2015
  4. Jamie Kennedy
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    Jamie Kennedy Senior Member

    p.s. Regarding mast drag and mast diameter.
    If you can rotate the mast, even just partially for upwind, it can be more teardrop shaped for better performance. If you can't rotate it then it is often just as well to be round. If you can sneak the sail out to the leeward side of a round mast, such as with sail hoops or a sleeved sail, or sail ties or full length battens, or a lug or junk rig. Then the mast can be larger in diameter without hurting upwind performance too much. This is particularly important if you want an unstayed rig that can swig out past 90 degrees, and unstayed rigs tend to be heavier and fatter. You might consider a partially unstayed rig, think of a giant windsurfer rig but with stays going to some point half way or even just 1/3 of the way up the rig to help hold the rig up and allow you to control rake and tilt. This might be particularly interesting with a cat schooner or cat ketch rig, with or without wishbone booms.

    Consider Bruce Kirby's Norwalk Island Sharpies as a possible rig for a 30 foot proa, and then how you might have it partially stayed, or partially un-stayed depending on how you look at it. :)

    [​IMG]

    I think one of the things that makes Bruce Kirby a great designer is that he is also a great sailer, and he puts as much thought into his rigs, and how they might be used and adjusted on all points of sail as he does with the hull and how it interacts with the rig and balance on all points of sail. He raced the OK Dinghy before he designed the Laser, so he was intimately familiar with how unstayed rigs interact with the hull and the helm under different conditions and on all points of sail, as were the other two as well.
     
  5. Anatol
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    Anatol Senior Member

    Tom, thanks.
    "The wingsail worked best when it had a substantial flap deflection. "
    That kink down the middle looks so unaerodynamic. Doesn't it create all kinds of drag? Wouldn't it work better as a smooth curve and if so why didn't they do it?
     
  6. Petros
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    Petros Senior Member

    it works the same way a flap or aerion on an aircraft wing. Yes the gap can incraese drag vs. a surface without a gap, but the idea is to defelect the air flow to generate high lift. when a large aircraft is coming in for a landing it has both leading and trailing edge devices depolyed to generate the most amount of lift (to keep the landing speed as low as possible), but it also generated a lot more drag in that configuration. You will not be at the best L/D with large flap defelctions, but you will be at max total lift. As pointed out above, there are times when you you will be sailing off of best L/D, and there are times, like when sailing down wind, that the drag also contributes to the forward motion of the hull. consider that the maximum lift never occurs at best L/D.

    Best lift to drag ratio (L/D) is beneficial in some sailing conditions or points of sail, and max lift (not best L/D) will be beneficial in others.
     
  7. tspeer
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    tspeer Senior Member

    What looks like a kink was actually two symmetrical sections in tandem, with a slot between them. The attached plot shows a typical cross section shape, the predicted pressure distribution, stagnation point streamlines, and the thicknesses of the boundary layers and wakes. The flow is coming from the left, parallel to the main element. The flap is deflected 24 deg from the chord of the main element.

    A smoothly curved single-element section would not work as well. The slotted flap section has a much higher maximum lift, and the profile drag is not much more, if any, than for a single element section designed to similar requirements. The structure and control system needed to get a smoothly curved single element section to change its curvature for the opposite tack would weigh more than the simple hinge used by this two-element design.
     

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  8. Jamie Kennedy
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    Jamie Kennedy Senior Member

    oops. posted to wrong thread
     
  9. Anatol
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    Anatol Senior Member

    Jamie Kennedy,
    sorry for delayed reply. I had to wait a day or two before I had time to mull over your posts.
    Thanks so much for the link to the NIS sharpie pages. Very interesting, because my plan is to use not only the cat schooner sail plan and I have also been looking into the sharpie style flat bottom hull shape.

    to your other very useful points:

    "END PLATES tend to have the effect of increasing your effective aspect ratio. So when going upwind is generally when you want your endplates, but when going downwind, or beam reaching or even close reaching but not overpowered, you may want to reduce your end plate effects for more power for the same sail area. "

    windward performance is the goal here, so end plates and the cheat on higher aspect ratio is a win. Happily, in my design, lower end plate effect is reduced sailing downwind, so win/win :)

    "Rake is another interesting consideration you may have some control over while sailing and changing points of sail. Increasing rake for unwind, in addition to other things, can increase your effective aspect ratio by increasing the vertical height from clew to head. "

    "increasing the vertical height from clew to head" this can only be in the 1-2% range, no? Maybe not worth the trouble in a fast cruiser, as opposed to an all out racing machine?

    "There are some good diagrams out there on pacific proa sails that show different lift-drag ratios for different rakes," where? can you point me? Do you mean the classic Marchaj diagrams?

    "You can also heel the boat to windward when going downwind, "

    not on a proa :)

    "For a cat schooner rig such as you have in mind it gets even more interesting. As a designer you can control the distance between the rigs. "

    right. I have to say I'm a bit baffled by differing opinions about the rig. Some say its inefficient because the aft sail is always in the dirty air of the fwd. But the closer they are, the more they look like a bermudan sloop and thus slot effect comes into play (?). In the extreme case you have the hinged sail Tom Speer discusses in #22. Can someone unpack this for me? With some ideas of 'ideal' (or not so ideal) arrangements. What about, say, two high aspect balanced lug sails with the almost vertical leech of the fwd brushing the luff of the aft? (I am also trying to solve the shunting proa CE problem without masts on tracks, hydraulic pumps, etc. Quick and easy shunting is a priority.)

    In a proa, of course, you could have booms longer than the mast-mast distance, as long as you had solution to the going aback/capsise problem, such as a fuse or other release on the outhaul of a loose foot sail...or simply a hinge in the boom. This is all OTOH, I'd have to actually *think* about it :)

    "Slotboom"
    is that a joke name? :) No offense, but its funny.
    That link is great - I hadn't found it before. But its specific to crabclaw rigs.

    "If you can rotate the mast, even just partially for upwind, it can be more teardrop shaped for better performance. If you can't rotate it then it is often just as well to be round.

    right, I'd mulled over this. Trying to find the simplicity/cost/performance sweet spot. Rotating wing makes sense, but accomodating the rotation is custom fabrication to beat complex forces. A round mast with a 'wharram pocket' seems like a nice solution, but reefing is a problem I couldn't resolve yet.

    "If you can sneak the sail out to the leeward side of a round mast, such as with sail hoops or a sleeved sail, or sail ties or full length battens, ""

    on a proa, a round mast with a track up the leeward side achieves this, for free :) and reefing is accomodated.


    "or a lug or junk rig. Then the mast can be larger in diameter without hurting upwind performance too much."

    This was the thrust of my original question about jib vs main, ie how much does a thin or thick leading edge effect performance. The reply I got said the thin edge is no advantage.

    " unstayed rigs tend to be heavier and fatter"

    clearly. I'm for wires. lighter and safer.

    "You might consider a partially unstayed rig, think of a giant windsurfer rig but with stays going to some point half way or even just 1/3 of the way up the rig to help hold the rig up and allow you to control rake and tilt. This might be particularly interesting with a cat schooner or cat ketch rig, with or without wishbone booms."

    nice idea

    " partially un-stayed"

    I have seen reference to using windsurfer masts as a flexy top to a stayed bottom half - interesting.
     
  10. Jamie Kennedy
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    Jamie Kennedy Senior Member

    thanks for the reply and comments. My last post about posting in the wrong thread wasn't meant to be cheeky. I literally posted something in this thread that was meant for another, and couldn't seem to delete it so I replaced it with that comment.

    I like the idea of having a track on the leeward side or a proa. I also like balanced lugs, but not so much that they are balanced but they do have a nice entry. The lift drag diagrams I was referring to was Marchaz but more specifically that link to Slotboom. Yeah I thought that was funny also, but I don't think it is too specific to crab claw sails. My daughter sailed Optimist last two years and I think it helped us understand things like rake and sailing downwind. I agree you can't really heal a proa to windward, but maybe tilting the rigs to and fro might be worth looking into. The lug sails on a proa wouldn't have to be dipped as they would always be on the right side I suppose, unless you want to go wing on wing. I am not sure you would need to though as the speed might be good enough to keep the angle of attack forward, and you could also play with the rake and tilt and maybe the point of balance of the balanced lugs. I love the look of lug sails, especially the ones like on the Goat Island Skiff, and on Roxanne and Romily by Nigel Irens. I would love to see how these feel when sailing upwind, and also downwind and by-the-lee.

    As for working two of these higher aspect ratio lug sails, that could be very interesting like you say. I think there would be times when you want to reduce the separation and other times when you would want to increase the separation. In addition to rake and tilt you might be able to slide the mast step, athwartship as well as fore-and-aft. Might be complicated to do, but might be practical for experimenting to figure out what is worth doing, and then find an arrangement to do it practically.

    http://www.roxane-romilly.co.uk/
     
  11. Anatol
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    Anatol Senior Member

    "I like the idea of having a track on the leeward side or a proa."

    which track? BTW - I'm thinking *atlantic* proa here

    "I also like balanced lugs, but not so much that they are balanced but they do have a nice entry."

    back to my luff/mast drag question - how far away does the mast have to maintain attached flow across sail ?

    "The lug sails on a proa wouldn't have to be dipped as they would always be on the right side I suppose,"

    no, one or other sail would always have to be dipped when shunting.

    "you might be able to slide the mast step, athwartship as well as fore-and-aft".

    I'd like to avoid that kind of thing if possible :) Fine on a 12 foot skiff, but we're talking 30' masts here.

    "Roxanne and Romily by Nigel Irens".

    Those high aspect lugs are pretty, but they are almost high aspect Bermudan in shape. What is the advantage?
     
  12. Jamie Kennedy
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    Jamie Kennedy Senior Member

    - which track? BTW - I'm thinking *atlantic* proa here
    - back to my luff/mast drag question - how far away does the mast have to maintain attached flow across sail ?

    If I remember from Marchaz the mast still hurts, but hurts a lot less if it is on the windward side of the sail. and I suppose the best location would be at the stagnation point on the sail, which upwind might be 10% to 30% back from the leading edge I suppose, but it depends on angle of attack and interaction with other sails. Another fun thing to experiment with. :)

    - no, one or other sail would always have to be dipped when shunting.

    yes that does make sense now.

    - I'd like to avoid that kind of thing if possible (sliding the mast step) Fine on a 12 foot skiff, but we're talking 30' masts here.

    yes, even for experimenting it would be difficult to do.


    - (Roxanne and Romily by Nigel Irens) Those high aspect lugs are pretty, but they are almost high aspect Bermudan in shape. What is the advantage?

    In general I think the idea is to use high aspect rigs for better performance upwind where the lift-drag ratio is more critical and you don't need to maximize lift because you are limited by things like righting moment in high winds and induced drag in light and medium winds. As you bear off to a beam reach and beyond you can increase the angle of attack to get more power as you can carry more power because is pushing more forwards and less sideways. Off the wind a lower aspect rig would still be better, but you are still going fast enough usually, or if you really want to you could pop a drifter in light to medium winds, or I guess you might call it an assymetrical chute in heavy winds, if your rig is strong enough for it. With two lug sails and no head sails you might be able to do something where the sails are sometimes separated and sometimes interacting but I can't recall how that might play out. Normally you want them separated upwind, but maybe not if they are high aspect ratio and can work like a main and jib. Advantage to main and jib is the main works like a flap, which gives you more lift for the same sail area, but also more drag. I think the idea is you can have more range, with more power when the winds are light, and you can depower more easily when the winds are heavy. You can still do this with a cat rig like a laser or finn but you have even more range with a rig like a soling, and even more with a Star. With a lug rig you will want to be able to power up and depower down by flattening the sail with the sheet and outhaul, but also by bending the lug and tightening the downhaul. You might also be able to depower by racking the aft sail back and the forward sail forward to separate the two sails some, especially up high. You can also reef of course, but I think it is always good to have some good range of powering up and depowering before you go to the reef. I think the other advantage of a high aspect lug like Nigel Irens Roxanne and Romily is that lug really works well for powering up and depowering by changing the amount of bend in it as the wind increases, and bending the mast as well, like you would with a conventional bendy Bermudan or unstayed cat rig. The lug also twists off sideways nicely, for more twist and less power in high winds and gusts, but more straight up for better pointing in lighter winds and lulls.

    Hope that helps.
    How fast is this designed for, relative to wind speed? I think for a cruising proa you are more likely to want fuller sails except in high winds and for a racing proa you are more likely to want flatter sails even in lighter winds and downwind. I think it depends on how fast you are going relative to the wind speed.
     
  13. Anatol
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    Anatol Senior Member

    One of the great things about boat design is that people have contrary opinions which they hold to be true. I didn't feel my question about leading edges/mast drag got as much attention as I hope it would so I went looking for other opinions and came across this interesting exchange in sailing anarchy on a thread about Nigel Irens' Farfarer - a big blue water cat schooner. The authors are Beau Volkyry ? (sorry I mangled your name) and someone called DDW,both of whom might also be here. The argument about mast sections in oft-cited tests is important. In any case, this is what they said, I'd be glad to hear opinions - thx.


    BV: DDW, you frequently claim that a Cat Rig is more efficient, you have at the end of the paragraph above. I disagree with this for one very simple reason. Unless the leading edge of the sail is fared into the mast, any sail that is hung from a mast is much less efficient than one that is hung from a head-foil. As I'm sure you know, the issue is attached air flow across the sail and a mast causes a tremendous amount of disturbance in that airflow. There is plenty of wind tunnel data to show that a headsail that is flown from a aerodynamic foil is much more efficient than any sail flown from a mast.


    DDW: What wind tunnel data are you referring to? I believe in my library I have all of the widely published tests, and probably most of the obscure ones. Probably the most frequently quoted in this misconception is the series done at the Gottingen tunnel (see for example Marchaj "Sail Performance" pg. 103) comparing a membrane sail to one with a round section mast at the leading edge. With a mast of 7.5% chord (the smallest tested) the max lift coefficient is reduced by 16%, but the efficiency as measured by L/D or drag angle is identical to the mastless case. My mast is less than half that big, at about 3% of chord. From this we can guess that the max CL on my rig might be 5 or 10% less than the ideal membrane, and the efficiency just as good. A sloop mast is typically an oval section and presents more of a disturbance than a round section, as has been demonstrated in the wind tunnel. Additionally, the chord on the mainsail of a sloop is perhaps one half of the same size cat, so the mast represents twice the disturbance as a percentage of chord. It is actually quite easy to make the mast on a modern cat airfoil shaped, but there is not much evidence of a dramatic performance increase, and they are problematic operationally. A cruising jib is rarely as good as the ideal membrane either, being set on a round furler. When detailed studies are made of airflow around real sails, there is almost always a separation bubble at the leading edge, followed by flow reattachment, then tripping to turbulent flow a short way back on the chord. A mast ensures this separation bubble, but it normally exists anyway, even on a jib. One weakness that a round mast cat does have is in very light wind it is easier to get laminar flow separation around the mast which does not reattach. A jib is better under those conditions, if you can keep it full. However having all sails fully battened and on spars seems to more than make up for this. In Lake Ontario in near drifting conditions I came upon a group of 5 sloops, I believe they were racing to somewhere. We had 2.5 - 3.5 knots true wind, and could maintain around 1.7 -2 knots boat speed. We sailed over to this group, sailed a circle around them, then sailed away. I put this either to having a lot more sail area/wetter area than they, or that my sail stayed trimmed and shaped even in no wind at all, while they were having the devil of a time keeping things flying.

    In any case you are talking here about form drag, and a larger component of drag on a sailing rig is induced drag. The induced drag on a sloop is very bad, because the lift distribution is poor. Induced drag is much more difficult to test for, requiring large scale wind tunnels.
     
  14. Jamie Kennedy
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    Jamie Kennedy Senior Member

    Very interesting. Some masts really disturb the air, but the flow does reattach, in most cases, so it is not so bad as we might think. Very thin carbon masts, like the new mast on the Finn Dinghy, make a big difference. As far as Cat vs Sloop, well it depends on what you are trying to achieve I think. Ultimately a single foil is most efficient, but when you need more push to overcome all the other drag on the hull etc, in less wind, or to get up on foils, or in a boat that is heavier than the modern ideal, or when reaching, often two or three sails are better to get the job done. Biplanes and Triplanes had their day, then gave way to monoplanes. In sailing, biplanes and triplanes still rule the day, but monoplanes do have their place here and there.
     

  15. Anatol
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    Anatol Senior Member

    "In sailing, biplanes and triplanes still rule the day, but monoplanes do have their place here and there"

    yeah but...as has been observed, rules of high speed airplanes don't necessarily apply to low speed foils be they hang gliders, kites or sails.

    Did you see pics of an eccentric monohull with a stern-mounted fwd-raked mast with three jibs hung from it? The argument, I guess, was that jib-luffed sails have less drag, and getting the mast disturbance out of the way, into the dirty air.

    So my question remains - given two otherwise identical sails, one on a 30', 4" round mast, one on a 30' forestay or jib foil - is the mast drag a major negative factor in lift vs drag when close hauled?
     
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