sail aerodynamics

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

  1. markdrela
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    markdrela Senior Member

    Flying and sailing are the same in one way: The power extracted from either the engine or the air/water interface is exactly equal to the power dissipated in the boundary layers, trailing vortices, and (in the case of sailing) wavemaking.
    Reducing the dissipation coefficients through better aero and/or hydro design means you will go faster for a given extracted power.

    The main difference is that in flying the power is more or less fixed by the powerplant, while in sailing the power extraction can be increased by changing the sailboat (e.g. by more righting moment).
     
  2. Petros
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    Petros Senior Member

    the aerodynamics for a sail boat and an airplane work the same, though as you point out, the goal is different.

    the propeller on the aircraft converts mechanical energy to thrust by rotating a foil (or two or more foils) through the air in a circle, converting the flow over the foil in to thrust. the sail extracts thrust out of the moving air in a similar way.

    of course the size and speed is different, and that affects a number of detail differences, but it works the same way, uses similar equations.

    the moving flow over the wings provides lift that holds the aircraft in the air, the moving flow over a keel generated lift that counters the lateral wind force on the sail and hull, in the same manner.

    a proper understanding of aerodynamics in either aircraft design, or sailboat design, will give you useful knowledge of the other.
     
  3. daiquiri
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    daiquiri Engineering and Design

    Well, their basic physics is the same, of course. Aerodynamics is all about pressure and viscosity, after all.

    The important differences between airplane and sailboat physics are in details.

    Sailboat aerodynamics is much more coupled with the structural layout and mechanics of materials the rig is made of. In other words - aeroelasticity on steroids. There is no such thing like fixed-geometry in the world of sailing (with exclusion of wing sails, to some extent). Just think of the membrane-nature of the sail surface, and how its shape changes under the wind pressure (variable along the mast height), flow separation, heel, rig tension and age - each of these factors being independent from the others.
    For this reason, wind-tunnel test results for scaled sailboat rigs should always be taken with a handful of salt. The aeroelastic defomations are generally either unknown or non-quantified, non-controlled, or simply ignored by using rigid models.

    And then there is the influence of the environment in which a sailboat works. Waves and wind turbulence makes the steady flow condition (both aero and hydro) often an unrealistic hypothesis, although a useful one for ballpark engineering calcs. In other words, sailboat aero/hydrodynamics is inherently unsteady, but we need the steady-flow hypothesis in order to make our calculations look neat and linear.
    When the boat pitches on waves, the airflow may cyclically attach and separate over the sail sail surface. The sail surface will, in turn, cyclically change its shape due to this flow variability. That is one hell of a condition to analyse either numerically or empirically.
    In the aircraft case, the only vaguelly similar conditions to this can be found on helicopter rotors, but even then the airfoil shapes are fixed.
     
  4. Mikko Brummer
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    Mikko Brummer Senior Member

    CFD can nowadays handle pitching & motions... While drive & heel vary as much as ±50% during the pitch cycle, the influence of pitching on the average sail forces is small. The sim here is with rigid sails - working on soft sails right now.

     
  5. daiquiri
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    daiquiri Engineering and Design

    That's a terrific video, Mikko! As well as the rest of your work on YouTube. :)
    That is an interesting info, thanks. To some extent, considering that you have used a rigid sail model, it is also an expected one. Based on the rigid-airfoil data, the flow hysteresis generally presents a strong correlation with the chordwise position of the point of flow separation. In case of thin airfoils the separation always occurs at the leading edge. Hence, thin airfoils show nearly no hysteresis when AoA is increased/decreased. And a classic sail, when assumed rigid, is a thin airfoil - even when the mast is included. The flow always separates somewhere at the mast, just like it separates at the L.E. of a mast-less thin airfoil. Hence, small hysteresis.

    But I will wait to see the results for a flexible sail before jumping to a final conclusion. Real-life experience tells me that at low wind speeds and during the flow-detachment phase the forward portion of a cloth sail tends to collapse, meaning that in that flow condition it cannot sustain a pressure difference. Hence, I expect to see a much more pronounced discrepancy of average lift and drag forces, between pitch-up and pitch-down phases of the boat motion.

    Cheers
     
  6. SukiSolo
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    SukiSolo Senior Member

    Excellent video simulation, thanks Mikko. Intereseting how pressure slides under the boom from the lower mainsail on the run, and the 'bump' at the spinnaker head, on the two height particle stream simulation.

    I would agree daquiri, at low air speed the separation point moves all over the place when pitching. Which is what the hands on effect, of continual trimming over waves does to help maintain some pressure difference. It's quite easy to 'feel' in a una rig on the sheet - and of course the boat responds too. A little bit harder to get both sails working together well in the same conditions ie pitch on a dying sea in particular but rewarding. However it's not so much the cloth elasticity that's causing that but sheet tension, and to some extent kicker or leech down trim. It also matters how you sheet and how it influences such tension as well as obviously angle.
     
  7. Mikko Brummer
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    Mikko Brummer Senior Member

    I'm not sure what you mean by hysteresis, but there's a considerable difference in drive & heel, depending whether the bow is moving upwards or downwards. On the top of the wave, when the bow has come down to horisontal, the forces are typically only 50% of the forces at the bottom of the wave, when the bow has come up to horisontal (with the same, zero pitch angle).

    In the 470 sim, the wind is pretty strong and most of the separation due to pitching is happening on the windward side and from the luff. In less wind, with the sails loaded, separation will occur on the leeward side too, when the mast is pitching back. Also then mostly from the luff, but from the leech, too.

    While pitching happens seldom in light airs (no waves ;-), in very light winds the sailcloth stiffness in small boats (and full length battens in bigger boats) usually keeps the sail shape pretty much unaltered and the sail behaves as rigid. In heavier winds, there will be considerable deformation especially when the bow is pitching down, the luff is backwinding in the top part. I'm not sure how detrimental that is - yes, you get a negative pressure difference, but separation on the windward side is mostly suppressed by the flexing of the sail. The negative pressure at the top will reduce heeling moment, helping to keep the boat upright.

    In the simulations (and while sailing), there's always other motion associated to pitching - yaw (steering), roll (heel) variation, heave (important effect when the boat is considerably heeled), and also acceleration-deceleration in forward motion. Through the sim, we have been able to show that good steering will help a lot. It has been said earlier that pitching could even improve the aerodynamics of the sailboat - then sails would be extracting energy from the motion created by the sea. While I haven't seen that result yet, it could be that with a completely coupled method, flexible sails & rigging, and good steering, even that could be the case.
     
  8. brian eiland
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    brian eiland Senior Member

    Wind Tunnel And CFD Investigation Of Unconventional Rigs

    Hi Daiquiri,
    I see that you are Italian, so I thought you might have a particular interest in this aero/wind tunnel and CFD study that was carried out in Italy. I had posted it originally under the aftmast subject thread, but it never garnered a lot of critical review that I had anticipated. So I thought I would repeat it over here.

    WIND TUNNEL AND CFD INVESTIGATION OF UNCONVENTIONAL RIGS

    Would an aircraft aerodynamics guy have predicted this? ( heck, even most sailboat guys would disavow these results :p)
     

    Attached Files:

  9. Barra
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    Barra Junior Member

     
  10. CT249
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    CT249 Senior Member

    Scientists also regularly disavow test results that do not accord with reality.

    In this case, the reason many sailors would disavow such results is nothing to do with conservatism or anything that deserves anyone sticking tongues out. Sailors see reality regularly. Every time someone pulls a main down in an offshore race to repair it, we get to see what happens when you do your best to sail fast with just a jib. Every time cruisers sail home under just a roller furling headsail, we get to see what happens without a main. And for almost one hundred years, people have been experimenting with mast aft rigs that discard mainsails - and at the end of their experiments they have almost always discarded the jib-only rig instead.

    It's interesting to consider what would have to happen for good sailors to NOT notice if the test above was true. I'm sure that every experienced offshore racer has had to drop a main while racing. Sometimes they blow, sometimes they develop minor issues that need a quick repair, sometimes they are dropped so you can change to a trysail. Understandably, people get quite concerned when they have to drop a sail like this in a race. People change headsails to optimise power and trim without a main. Everyone gets anxious about losing ground. People monitor the speedo and VMG. Good trimmers will adjust jib trim and discuss it with the helm. Navigators will get involved in assessing VMG so that they can change the input into their predictions. Tacticians will watch relative speed and height like a hawk so they can work out the tactical options. And of course sometimes you have hours of trying to tweak the boat to be as fast as you can, while the mainsail is being rebuilt.

    The only way the test above can be right is if at no time in the decades that the modern boat has existed has any one of the thousands of professional and amateur sailors (including sailmakers and designers with degrees in engineering) who have had to drop a main noticed that the boat is faster without the main. At no time has anyone watching the speedo seen the numbers climb with the main down. At no time have other boats noticed that their opposition has dropped the main and started gaining. At no time has the navigator, trying to predict where the boat would be, noticed that it would get there faster under one sail. None of the owners and pros seeing a multi-million dollar campaign have seen the boat gain instead of slowing down.

    There is no logical way that thousands of people would suffer such a major collective attack of stupidity and blindness over such a long period. This isn't a subjective matter, it's a matter of numbers blinking on speedos and nav screens, and relative bearings on other boats. The more logical explanation is that this test is wrong.
     
  11. brian eiland
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    brian eiland Senior Member

    Pardon me guys but most sloops these days, particularly racing ones, are VERY dependent (design wise) on their mainsails and their bendy mast to keep a tight forestay. So yes, if you just take their mainsails away they will likely end up with lousy forestay tensions.

    Lets examine this test in the view in which it was conducted. Almost all wind tunnel testing DOES NOT try to account for forestay sag. I'm sure the racing monohull they utilized in this testing was not burdened with forestay sag either. They sought to compare apples with apples,...perhaps idealized, but that is a nature of testing sometimes. So no headsatay sag for any of the rig configurations was considered.

    Can someone find other faults with the actual testing, including the RANS & CFD portions of verification?
    BTW the fellows name, Fabio Fossati, that appears at the top of this report was a very well respected name in the field of sailboat aerodynamics.
    http://www.sailingscuttlebutt.com/2015/12/03/eight-bells-fabio-fossati/
    His book “Aero-Hydrodynamics and the Performance of Sailing Yachts: The Science Behind Sailboats and Their Design” is a worldwide reference text for sailing yacht theory.

    So if we assume that we can design a rig with very little sag, then it appears as though this double headed rig, with overlap, out performed the others. Then why not try to design a rig with very little forestay sag,...just as many already try for. Its not impossible.

    So please look at the test results, and get off this old 'forestay sag' excuse/observation/whatever to denigrate a new idea.

    I wonder how some you fellows felt about the new 'sailboats-on-foils idea' that's not that old. Bet you found lots to criticize there before it really started to materialize and evolve.
     
  12. brian eiland
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    brian eiland Senior Member

    I can find you hundreds of cruiser testimonials of folks that no longer even raise their mainsails, particularly among the older crowd of cruisers.

    Have you bothered to read it, and tell me where they made their 'obvious' mistakes to make it so wrong.?
     
  13. daiquiri
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    daiquiri Engineering and Design

    Hi Brian,
    I was looking at those plots of drive force vs. heeling moment, in particular the most representative one (Fig. 17, pg.97):
    Polars.jpg
    It shows that measurements (assumed numerically reliable) indicate that the A-mast with overlapping sails does seem to have some advantage over a traditional sloop, but we are talking about 5-10% difference at most.
    For example, at 32° AWA and Cx_area=0.6, the CMx_area=1.1 for classic sloop and 1.0 for A-mast setup. That is 9% difference. Same values and same percentage difference for 22° AWA and for Cx_area=0.4.
    For smaller values of Cx, the difference even gets much smaller, according to that graph.

    Next, I have noticed these two particulars in the scale models tested:
    Diff.jpg
    which imo show apples (on the left side) vs. oranges (on the right side).
    That portion of naked mast at the head of the sloop rig seems to have pretty important dimensions. Considering that:
    - it is situated in the area where the AWS is highest, and with the highest AWA along the mast length (due to wind shear);
    - a naked quasi-circular mast will give mostly drag, and drag is proportional to AWS squared;
    makes me conclude that this small portion of naked mast might be one of factors responsible for the higher heeling moment measured on the sloop rig.

    Another factor is highlighted at the mast base, where the sloop rig is missing a part of the area which was included in the A-mast wing, thus decreasing its drive force.

    In order to have a more fair comparison (if the goal of the comparison was to demonstrate that the rear A-mast is aerodynamically better than the classic mast) the sloop rig should have been made in this way:
    Diff2.jpg
    where the shaded area shows the sail area which was "forgotten".
    Either that, or the A-mast sail should have been shaped with the same form and area of the sloop rig.

    Apart that, remains the problem of the weight of the stern-mounted A-mast. By shifting the rig weight aft (will it weight the same or, imo more likely, more?), one ends up with the boat CoG shifted towards aft. That can be beneficial for sailboats designed for speed, but only if the new rig brings no weight penalty. Otherwise, any aerodynamic advantage will be at least partly eaten by the increased hydrodynamic resistance due to non-optimum CoG position (in case of small and weight-sensitive boats) and/or higher displacement.
    So, the question is - have these factors been taken into account for the VPP analysis mentioned in the paper? It has not been explained by the authors.

    And finally, I really do not see how the A-mast rig delivers the claimed advantage of "simpler sail handling while reefing and dropping the sails", and how is the "elimination of the boom and of the mainsail sheet track" advantageous in terms of sail trimming and optimization. I am probably missing some info about the handling of this type of sails.
    Hopefully, someone more knowledgeable and with more practical sailing experience will jump in to explain these. Mr. Markstrimaran, perhaps? (http://www.boatdesign.net/forums/sailboats/aftmast-rigs-623-52.html#post753327)

    Cheers
     
  14. CT249
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    CT249 Senior Member

    For one, the testimonials of people who don't bother to raise their mains when cruising isn't relevant if they are not chasing speed, if they are not recording relative and actual speeds closely, and if they are unsure that they are trimming their boat to its optimum.

    Secondly, if you're going to accept such apocryphal tales, how can you reject the tales of the many, many racing yachtsmen who find their boats go more slowly when the main comes down? The racing yachties are actually checking their speed closely, unlike the vast majority of cruisers.

    Thirdly, I did NOT say that there were 'obvious' mistakes in the wind tunnel test - I said that the results were obviously wrong since they did not replicate well-proven real-world reality. The mistakes could be caused by a variety of things, such as the way the forces were measured in the test rig, the trim of the test sails, the wind shear in the tunnel or lack of it, the lack of heel, etc. It could even be that gust and wave response is the missing factor.

    Like Daiquiri, I'm puzzled about why there's a lot of bare mast sticking up above the sloop rig and I can find no justification for it. It's a bit odd to make a comment about the "problem" of mast drag and then to create a test model with spare bit of stick waving around doing nothing.

    You still haven't answered the basic point - if mains are slow (to use the terms loosely) when mainsails are so often dropped for repairs during races, how come no one ever noticed that dropping the main is faster?
     

  15. CT249
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    CT249 Senior Member

    Re "BTW the fellows name, Fabio Fossati, that appears at the top of this report was a very well respected name in the field of sailboat aerodynamics."

    Mr Fossati may well be very well known - but so are the names of many other highly-respected people in the field of sailboat aerodynamics.

    Why should we believe the knowledge of one such respected person and reject the knowledge of all the other equally respected people in the field? OK, so Fabio is an expert, and the guys who design the rigs for the AC boats, IMS/ORCi boats, offshore multis etc are all idiots? Really? I always thought guys like Vincent P, Nigel Irens and Farr were pretty damn smart.


    Re "Pardon me guys but most sloops these days, particularly racing ones, are VERY dependent (design wise) on their mainsails and their bendy mast to keep a tight forestay. So yes, if you just take their mainsails away they will likely end up with lousy forestay tensions."

    Actually, many of the boats I was specifically recalling had runners or were masthead riggers. I can assure you that last time I dropped a mains to repair it in an offsore race, we used runners to maintain forestay tension. I can easily check pics of other boats that did the same thing in the same race, so that's not relevant.
     
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