The design of soft wing sails for cruising

Discussion in 'Boat Design' started by David Tyler, Jan 19, 2014.

  1. David Tyler
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    David Tyler J. R. A. Committee Member

    Tystie has had a single-masted junk rig for half of her sailing, 40,000 miles. For the other half, she had a ketch rig with soft wing-sails, very much based on junk rig, with chinese sheeting to stiff battens, but with articulating battens designed in the way that Tom Speer has described for designing a wing mast/ sail combination. I'll attach a cross sectional drawing. I used Wortmann fx77w153 section as the basis. This takes more work to make and assemble, but the performance is better than a pure junk rig. While it won't ever match the performance of a hard wing mast, because the soft sail is rougher and not held to a precise shape, when I've tested it by holding up a ribbon on the end of a long stick, the airflow was behaving very well around the luff and lee side. I'd rather like to go back to this rig, as it was actually even easier to handle than pure junk rig, on top of the performance advantage.

    So as a start to this thread, would the aerodynamicists care to comment on the selection of Wortmann fx77w153 as the section to choose? It works in practical terms, but is there a better one for this use? I've considered UI1720, and it's tricky to make a batten that will articulate reliably. Both of them, being thick, have the required tolerance to widely varying AoA and apparent wind strength that a cruising boat's sails experience when pitching over a steep, confused sea, and they are both good, in theory, as regards lift/drag and max Cl.
     

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  2. bpw
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    bpw Senior Member

    Thanks for starting this thread, I have lots of questions but I am leaving in a couple days for 3-4 months offshore, so just replying to subscribe and bookmark this thread for later....

    Would love to hear your thoughts on the wing vs junk, differences in sailing performance and handling.
     
  3. P Flados
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    P Flados Senior Member

    You are showing what I have previously called a reversible camber wing.

    I played around with free airfoil evaluation software exploring this concept a while back as described at http://www.boatdesign.net/forums/hydrodynamics-aerodynamics/what-significance-wing-thickness-45383-2.html

    I have attached some XFLR5 output & a txt file that go with where I ended up.

    Building a reversible camber wing is on my list of things I want to try, but I need a suitable platform & want to play around with foiling first.

    Looking at your photos, I doubt a profile similar to what I ended with would be optimum near the top. Probably want to taper the thickness down starting where the flap goes to less than 40% of total chord.

    Also the thinner I made the rear flap, the better. However, I needed some thickness for structural reasons. With a junk rig approach, a single layer rear flap could work out quite well.
     

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  4. P Flados
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    P Flados Senior Member

    I went out and grabbed the foil you mentioned (fx77w153) and ran a comparison.

    The foils evaluated pretty similar.

    However, this is just the actual fx77w153.

    I am rusty on how I generated DAT files. I did not bother to try to generate one to be similar to what was in the pdf.

    The curvature that matters (top, front 50%) of the FX looks to be not far off of the S1223 and the pressure distribution looks very similar.
     

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  5. David Tyler
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    David Tyler J. R. A. Committee Member

    Good, I'm glad that someone on this forum is an offshore mariner. You're my target audience - even if there's only one of you! :D

    Handling
    Junk rig One-Oh-One first of all.

    There are four main running lines:
    1. The halyard. The whole sail bundle is heavy, so it's usual to use a multi-part halyard. I find it most efficient to use a 3:1 halyard and an ARCO two speed self-tailer, using 14:1 most of the time, and 30:1 for shaking out a reef when running in a strong breeze (one of the advantages of JR - no need to round up to hoist or lower the sail).
    2. The sheet. This does the job of the clew pennants on a bermudan rig, as well as the trimming of the sail. A single sheet going to 6 points (6 battens) is common; but I use an upper and lower sheet on my present sail, for better control of twist.
    3. The yard hauling parrel (YHP). This pulls the yard into towards the mast, and is necessary because the yard changes its angle in the first part of the hoist. Otherwise, a traveller could be used. This line is heavily loaded on a big rig.
    4. The luff hauling parrel (LHP) or throat hauling parrel (THP). This pulls the luff back, peaking up the yard and taking the creases out of the sail.

    Additionally,
    5. downhauls for the lower battens are not generally needed.

    Now as this applies to the soft wing sail:
    1. The halyard. No difference.
    2. The sheet. The use of an upper and lower sheet becomes more important, as the thicker sail will develop diagonal creases around the luff if allowed to twist. Also, the soft wing sail must be sheeted closer than JR, because of the camber. Yet the upper parts of the sheet are pulling steeply downwards; there is an argument for putting the upper sheet onto a track car and hauling it to weather, when on the wind.
    3. If there is a short yard, at the same angle as the battens to the horizontal (my proposal for a new rig), or if there is some form of headboard that dispenses with the yard (my old rig), then the YHP is not needed.
    4. The LHP is actually a good way of shaping up the sail. In my old rig, I attempted to make each batten butt against the mast, with a semicircular plastic bearing. This reasoning is foiled by a) the taper in the mast and b) the inevitable bend in the mast. The plastic bearings in the lower part of the sail could be observed to float clear of the mast, contributing nothing. The leech would then go slack. In contrast, a LHP could be used to haul the luff back and straighten the leech, independent of whether the mast was bending; or whether the head of the sail was on the upper, smaller diameter of the mast, or the lower, larger diameter of the mast, when reefed.
    5. Downhauls are needed for the lower battens, to keep the luff in shape. It is convenient to fit separate downhauls for reefs 1, 2, and 3, and then to span downhauls together for the less frequently used reefs 4 & 5. The LHP will haul down reefs 6, 7 & 8.

    Soft wing sail operation
    To make sail: let go the sheet and all the downhauls. Haul on the halyard (very little tension required in the luff, so don't pull too hard). Trim the sheet. Put a little tension onto the LHP to shape up the sail.
    To furl the sail: let go the halyard. The whole sail will clatter down into the lazyjacks. Take in the sheet(s).
    To take a reef: ease the halyard a little more than will bring the first batten down to the "boom" (which in reality is just another batten). Then take in and secure the downhaul, and set up the halyard against it (very little tension required). Put a little tension onto the LHP to shape up the sail. Then re-trim the sheet(s). A 15 second procedure for the first reef on the mizzen of my old rig.
    To shake out a reef: ease the sheet(s), cast off the downhaul, haul on the halyard, put a little tension onto the LHP to shape up the sail, re-trim the sheet.
    To tack: put the helm down. The rig is self-tacking.
    To gybe: put the helm up until the boat is nearly on a beam reach on the other gybe, and let the sail whirl across. Yes, really. It's easy and safe that way. You can of course haul the sheet in if you need the exercise, or are in confined waters.

    Performance
    That's a difficult question to give a straight answer to. My boat is not like any other, so no trial horse is available. She's a heavy, shoal draught cruiser with bilge-boards, and windward performance is sacrificed to other more worthwhile attributes such as being able to go onto the beach to repair, scrub and paint, and to go into the gunkholes that other cruisers can't. I can say, though, that she's faster and safer downwind than she would be with an external keel, and faster downwind with any of my rigs than any similar bermudan rigged boat.
    I changed from a single JR to ketch rig with wing sails, and back to single JR, and that's comparing oranges to apples.
    A two masted JR is not so effective as a single JR, so I was reluctant to go that way. I reckon the ketch with wing sails to be equivalent in performance to my current single JR, which is not as fast as my first JR, but is easier to manage. I reckon that a single soft wing sail would be rather more effective to windward than the ketch wingsail rig, but would suffer from weather helm off the wind, like many cat boats. Nonetheless, I'd go it a go, if only I didn't have to get a new, taller mast, which would cost more than I'm prepared to spend (a new hybrid mizzen mast wouldn't cost too much).
    So the choice of number of masts is for the owner to make.
     
    Last edited: Jan 25, 2014
  6. David Tyler
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    David Tyler J. R. A. Committee Member

    Thank you. In my implementation of fx77w153, I can't get the lower (windward) surface as convex as it should be, anyway, so my shape tends towards yours.
    In my old full sized sail, with the single skinned after section, there was a large turbulent or stagnant area on the windward side, at the point of articulation. I thought that this would produce unnecessary drag, even if the lift was better - hence my move to a double skinned after section, with less lift and less drag.
    With the single skinned after section, there needs to be a greater angle of articulation, which is a problem in light airs, with the sail failing to flip across.
    Maybe there's a compromise somewhere in the middle, as with your section, but I must admit that I can't think of a practical way to achieve it.
     
  7. David Tyler
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    David Tyler J. R. A. Committee Member

    There is a way that I've been considering to do this without reducing the width at the forward end, which I also need for structural reasons. Instead of making the sides of the after batten straight, they can be concave, with the sail not fixed to it, or only loosely fixed, but tensioned fore and aft. The sail will then billow in to a concave shape on the weather side. It does this anyway, in between the battens.
     
    Last edited: Jan 20, 2014
  8. capt vimes
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    capt vimes Senior Member

    just a few questions to your downhaul and reefing configuration:
    am i right that the mast is freestanding but NOT rotating?
    it looks very much like that your wing and battens will rotate around the mast but not the mast itself...
    where then would you "downhaul" any reef or the foot of your wing to without preventing rotation?
     
  9. frank smith
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    frank smith Senior Member

    Great, I will follow with interest.
     
  10. David Tyler
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    David Tyler J. R. A. Committee Member

    Correct, the mast is fixed. I have a platform clamped around the mast, just above the mast coat, onto which are mounted six single blocks on deckplates. Because very little luff tension (in fact, the luff is self-tensioning by the weight of the battens, except for the bottom panel) is needed with this kind of rig, it's perfectly OK to rotate the sail on the mast, relying on the elasticity of the sail and downhaul line to cater for the extra length of downhaul that is required when the sail is squared off.
     
  11. David Tyler
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    David Tyler J. R. A. Committee Member

    e420 and e422

    Eppler e420 and e422 are strong contenders:

    Eppler E420 high lift airfoil
    Max thickness 14.3% at 22.8% chord
    Max camber 10.6% at 40.5% chord

    Eppler E422 high lift airfoil
    Max thickness 14% at 24.1% chord
    Max camber 7.1% at 34.8% chord


    The Omer wing sail uses e420:
    http://www.zinio.com/pages/YachtingMonthly/October2013/416278343/pg-84

    but I tend to the view that 10.6% camber is OK for a wing sail in which the camber can be reduced, but 7.1% camber would be safer for a wing sail in which it can't ( and I find 7.1% camber to be quite sufficient for my kind of usage). I stick to KISS in this case, having tried to devise a way of reducing camber in my previous wing sails.

    I have found that I can design a batten that achieves the profile of e422 almost exactly, so this is my favoured section for the time being, unless anyone comes up with a better one. I also think that it will be too greedy to try to design a batten that achieves the profile of e420; I foresee articulation problems.
     

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  12. David Tyler
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    David Tyler J. R. A. Committee Member

  13. daiquiri
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    daiquiri Engineering and Design

    Just a note about your graphs. I see that you have used the value of Ncrit=9 for your analysis, which is imo not very conservative. It would be a value to use for simulations of very calm upwind air and very smooth airfoil. Some comparisons with the published data on NACA sections, which I've made quite some time ago, had led me to think that Ncrit=5-6 is more realistic for our imperfect world. Depending on the Reynolds number, the choice of Ncrit might considerably influence the calculated value of Clmax and the shape of the drag polar.
    Cheers.
     
  14. David Tyler
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    David Tyler J. R. A. Committee Member

    Thank you. I've added Ncrit = 5, and lift and drag get a little worse above alpha = 12 with Re = 1,000,000, but not much. Airfoil Tools does not permit me to go above Re =1,000,000 but I don't think the situation will change much.
     

  15. P Flados
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    P Flados Senior Member

    I see the "slightly concave" lines. If you can get the sail to push against this line on one side and billow out on the other, the back half should do very well indeed.

    The overall curvature front to back looks good with this assumption.

    However, I have one suggestion. Consider making the very front more rounded. A more pointed nose can be good if you are at the perfect camber and AOA for a condition. However, it is less tolerant of non optimum AOA. If you were using a self controlling rig (big flap sticking out to the rear that controls AOA) you could probably stay more optimum. Otherwise your AOA is constantly shifting around with each gust. Also the there is the apparent wind angle changes top to bottom that is the basis for the effort people put into controlling wing twist.
     
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