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

    Yes, that's the idea, but with two vertical pins, as the second axis has turned out to be a bad idea*. Just shape the ends of your two inserts into wedges, as seen from above, and then the rectangular tube needn't be as wide.

    EDIT:* but you could allow enough slackness in the holes for the vertical pins to give a few degrees of vertical movement.
     
  2. Avoid Rocks
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    Avoid Rocks Junior Member

    Ah yeah, now I get it, thanks. :)
     
  3. Avoid Rocks
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    Avoid Rocks Junior Member

    I'm going to use regular polytarp for the prototype. Given that I can get it for next to nothing, on a 2.7 meter wide roll, that it will last at least two summers and that it should be pretty easy to sew from mainly three lengths, I think it's a no-brainer. If the design proves successful I can invest more time and money later into longer lasting materials.

    So there will be air gaps, facing aft, where the battens are? I'm sure you've thought of this but my thinking is nonetheless that while it shouldn't hurt with the wind ahead of the beam, it will when running, as the wind will be blowing through these gaps into the sail. What are your thoughts on this? And why should they be wide? They of course have to allow for the battens to articulate freely but is there need for any more than this?

    I didn't know zip about zippers, but I watched a few Youtube videos and now I do! If I'm understanding this correctly, the best (or at least a good) way of doing it would be to get a "finished" zipper (for example one of these from Sailrite, with a locking slider) and cut it down to length leaving the finished end untouched. The finished end would then be at the bottom of the sail where assembly starts. At the top I'd just have to be careful not to pull the slider too far and off the zipper.

    Makes a lot of sense and better yet, it doesn't complicate my sewing process much and I can still do it using three lengths.

    Brilliant! Makes me feel like such a bonehead for not thinking about this. I've only been visualizing different options from the inside of the sail, but grommets make perfect sense. The fact that I'll have wooden jaws makes it even better.

    Hmm, I'll bounce back the question you asked me earlier; how are you going to get them around the mast if you're planning to bond them behind the mast? Or will you have a split luff former that you connect at the nose after assembling them around the mast?

    Another question. In the case where I'd be using a piece of square or rectangular aluminum profile for the hinge with two pivoting wedges, one in each end (10 degrees each to each side), why not just use a square or rectangular aft batten and make the single pivot 20 degrees to each side instead? Oh. Maybe because the width required to get 20 degrees of movement to each side would require such an unnecessarily wide (and thus heavy) profile? Sorry for asking questions instead of doing the math but I'm really tired right now. Work is the curse of the sailing class.
     
  4. David Tyler
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    David Tyler J. R. A. Committee Member

    Makes sense

    My battens are quite wide, and I have to avoid having too much of a change of angle in the sailcloth above and below each batten position, as this tightens the sail, vertically. This is bad, because it means the leech goes slacker. If you use, say, 2in tubes, and put your hinges well inside the doubled part of the sail, I suggest a vertical gap of 8in will be enough. Not wide enough to worry about the reversed airflow when running.

    I start the assembly at the top, and hoist as I go. It's not possible to assemble the sail to the bottom batten,with all the other battens lying on top of it. Therefore, the zip comes together at the top first, and you pull it downwards as you assemble. I could have bought three shorter, open-ended zips instead of one off-the-roll zip that I have to fiddle with to get it started, but I reckon that assembly only gets done at very infrequent intervals, the way I sail. Once the sail's on, it stays there until it need maintenance.

    I made the yard in one piece, for integrity, and then stepped the mast through it. So it can't now be removed without unstepping the mast, or using a skyhook to lift the yard off the top. I reason that the same might just as well apply to the forward battens as well. If I take the sail away, for maintenance or layup, all that need be left on the mast is the yard and the forward battens; the after battens can be removed.
    If I bond the luff former to the forward batten, in situ, the whole assembly can't be disassembled, but gets stronger, and won't break. If I simply screw the two together, the assembly is weaker, but can be disassembled for mending when it breaks. You pays your money and you takes your choice.

    Correct!
     
  5. OldYachtie
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    OldYachtie Junior Member

    Junk rig / wingsail

    Take a junk rig, make it hinging, and make the first half of the junk sail a foil shape with the mast in the center of the foil-shaped batten sleeve. Easily done, really. The result has all of the virtues of the junk sail with the additional handling improvement that the sail is centered on the mast, so it is easier to hoist. Reefing and furling couldn't be easier, and there is no reason at all to make the mast rotate. The junk rig is said to be unstayed, but this is not quite true - the mast is unstayed, but the sheet acts as a staying system that decreases the loading on the mast and partners. The idea has been put into effect as an independent invention a number of times. :eek:
     

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  6. Avoid Rocks
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    Avoid Rocks Junior Member

    Batten shopping!

    I will have access to a car (to transport the battens and other materials needed) in the beginning of February, so the clock is ticking and forcing me to make some decisions.

    For simplified construction, I'm hoping (or was, until I started writing this post) to be able to use a simple, single double-wedge for the hinge, along with both fore and aft square/rectangular battens. Like this:

    [​IMG]
    60 mm (wall 2.5 mm) wide batten, wedges going ~120 mm into the batten

    or this:

    [​IMG]
    50 mm (wall 2.5 mm) wide batten, wedges going ~90 mm into the batten

    I'm currently browsing the web for local stores that have suitable aluminium but I'm somewhat unsure about the dimensions.

    I should be able to get the following rectangular tubing:

    60x40x2.5 mm (grade unknown, will email them)
    50x30x2.5 mm 6062-T6 or 6063-T6

    and round:

    40/45/50x2 mm 6062-T6 or 6063-T6

    Been playing around with the following calculators to compare the different profiles:

    http://www.amesweb.info/SectionalPropertiesTabs/SectionalPropertiesHollowRectangle.aspx
    http://www.amesweb.info/SectionalPropertiesTabs/SectionalPropertiesHollowCircle.aspx

    Here are some numbers:

    60x40x2.5
    Sxx: 7825
    Syy: 6174
    Weight: 1.28 kg/m

    50x30x2.5
    Sxx: 4906
    Syy: 3593
    Weight: 1.02 kg/m

    I had initially thought about using round tubing, which has the following properties:

    50x2
    Sxx/Syy: 3480
    Weight: 0.81 kg/m

    Given these numbers I could strengthwise actually go for 40x40x2 mm square tubing with a section modulus of 3668 and 0.82 kg/m, but in that case the wedges wouldn't feel very sturdy anymore, with only ~65mm going into each batten. 60x40 seems very strong but also unnecessarily heavy.

    :!: There is of course also still the option of using David's proposed middle rectangular hinge tubing combined with round fore and aft battens. It's probably the most lightweight solution, but it would also be more work, especially creating the jaws, and if only viable I would like to use rectangular battens.

    Maybe the round battens might be best after all. In this case, what would be some good round batten dimensions (if we assume 60xx-T6)? Given the short lengths (2.3 and 1.3 meters), would something like this be enough?

    45x2
    Sxx/Syy: 2781
    Weight: 0.73 kg/m

    40x2
    Sxx/Syy: 2161
    Weight: 0.65 kg/m

    Or, square battens combined with the middle-part hinge?

    Or is square battens a bad idea simply because the load will not be purely on the horizontal plane since combined with the sheets it's more of a diagonal load and thus round battens would be better?

    More questions

    My boat's displacement is 3 tons, FWIW.

    The full chord of my sail will be around 4 meters, more or less, with the aft batten being ~2.3 meters and the front batten ~1.3 meters. Anyone more knowledgeable about structural engineering, or practical expertise (junk rig owners?), who could give me any pointers about which kinds of dimensions for the battens would be best (or even in the ballpark)? How much does the strength requirements change when the battens are divided into 2.3+1.3 compared to a junk rig's full 4 meter batten?

    How many battens should I use? David, you're using 9 + yard. How did you arrive at this? Cloth width, sail area per panel and/or something else? Would you recommend me just scaling down your sail to my proposed area of 30-35 and use the same 9 battens + yard or should I perhaps do something like 7 + yard? With a 35 sqm sail each panel would be on average 3.9 sqm using 9 panels compared to 5 sqm using 7. Given that the panel area decreases when deeply reefed, 5 sqm seems enough. How is the sheeting affected? And what about individual panel shape? I'm guessing I would need more luff formers between the battens if I go for 7+1.

    Too. Much. Coffee. Help!
     
  7. Avoid Rocks
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    Avoid Rocks Junior Member

    Improved wedge design, which should actually work in practice:

    [​IMG]

    In a 60x2.5 mm tube the wedge goes roughly 14 cm into the tube before tapering to 10 mm width.
    In a 50x2.5 mm tube it goes roughly 11 cm in.

    Don't really know what conclusions I should draw from this. Is 11 cm enough? No idea.

    If 14 cm is preferred I think I'd rather go with the "middle hinge box" so that my battens could be a bit lighter. 60x40x2.5 mm seems overkill.
     
  8. David Tyler
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    David Tyler J. R. A. Committee Member

    There's a lot to reply to here!

    I initially used square section GRP tubular battens in my previous wing sails (chosen to make the hinging easier), and over many ocean miles, they were chafing the sail. I'm only using a rectangular section now because I'm moulding it myself, on a mandrel with radiused edges. If I were using commercially available sections, whether GRP, CFRP or aluminium, I would choose a round tube over a square tube with sharp edges. A square tube is less efficient than a round tube in terms of strength per unit weight, as well, since it is weaker when loaded on its diagonal than when loaded perpendicular to a face. We cannot guarantee the loading will be perpendicular to a face, so we must take the lower value. A round tube has equal properties in all directions, and I would prefer it for this reason.

    I changed to round aluminium tubes in New Zealand, where I found it difficult to buy T6 temper, and had to settle for T5. In my mainsail with 4.5m chord, 38dia x 1.6 6xxx T5 round tube was definitely not enough, but 50dia x 1.6 6xxx T5 was. I didn't dare risk 45dia x 1.6 6xxx T5, but it might have been enough, and you might want to consider it for a sail of 4m chord. However, when using these double wedges (or double cones, as it will be with round tubes), it has been found essential to sleeve the end of the tube in way of the hinge, to carry the bursting load that the hinge exerts and to reduce the stresses on the metal in way of a hinge pin (if used).

    I like your improved double wedge/cone design. This has been suggested by others working in this field, but I'm not sure whether it was ever tried in practice. However, a hinge pin through the batten tubes has not generally been used; the battens are allowed to butt against a raised area or axis of some kind at the centre of the double wedge/cone. My rule of thumb is that the working length from pivot to tip of wedge/cone should be 3 times the tube width/diameter, at least. [the hinge pin was the better way when the hinge was a rectangular tube and the battens had solid wedge-shaped ends, but not, I believe, in the present case]

    I suggest that you use 45dia x 1.6 or 50dia x 1.6 aluminium battens, T6 if you can get it, with a thicker sleeve that is bonded to the batten tube and extends beyond the end of it, so that the hinge has a larger diameter to work within. I suggest that you mate these with a double-cone hinge, which you might well turn in a lathe, out of solid PVC bar of the same diameter as the tube, leaving a central flange against which the battens will butt. I suggest you use a lashing to keep the hinge from separating, rather than a mechanical hinge pin.

    This way, you have a batten construction that is able to take a load equally well in all orientations, with no tendency to capsize to a "bad" orientation, and that's a good thing. I think I'd go that way, if I were starting over again; but I have some serviceable rectangular after battens that I'll need to re-use.
     
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  9. David Tyler
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    David Tyler J. R. A. Committee Member

    You'll need to consider the drift needed, between clew and deck blocks, for chinese sheeting. The minimum is 1.25 x the panel width, for the simplest sheeting arrangement (read Practical Junk Rig for more on this, and on chord /batten pitch ratio).

    I use an upper and lower sheet, for better twist control, both 3:1 with each sheet controlling four points on the sail via two single spans. Then I have one short, unsheeted batten. If you would prefer the simplicity of a single sheet, then six sheeted points would be better, with three single spans and again with one short unsheeted batten. This is the main factor that will govern your choice of number of battens.

    My sail panels are made from a single cloth, and that's a consideration, too, but not an important one. You have said that you'll used vertical cloths, so it doesn't apply.

    Sensible reduction in sail area for each reef is also a strong consideration, and with just one sail, I like the smaller size of each of my panels, with nine battens. With a two sail rig, the panels can be relatively deeper.

    With seven battens, I agree that more luff formers might be needed, but I found that the law of diminishing returns applies, and I wouldn't add more than two per panel. My previous wing- mainsail had seven battens, and was of a similar size to your proposed sail.
     
  10. Avoid Rocks
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    Avoid Rocks Junior Member

    I've put some thought into the sail plan now according to your advice and using PJR, and have come up with this rough sketch so far:

    [​IMG]

    The mast is located just aft of the main bulkhead, with 2 degree rake. I was initially going to put it in front of the bulkhead but after playing around with Dmin and lead I think this could be better. Panels were increased from 7 to 9 to achieve proper Dmin. I also had to cut the chord a bit to achieve the desired 2.25P (at deck level) with the mast aft the bulkhead so the SA is now on the low side of 31 sqm (would've wished for closer to 35) and AR is 2.34 (PJR method) or 2.20 (luff/chord).

    The problem I have with this is that it definitely looks like the sail is too far aft. I assumed that the CE would be around 35% of the chord and as such, the lead is around 9%. Some better way to calculate the CE and lead would help as my current method isn't very scientific and PJR is based on flat sails.

    A separate upper/lower sheet sounds like the way to go, to have some method of controlling twist.

    I'm confused (once again :D). It sounds like you are suggesting round battens with cone hinges, which would create equal horizontal and vertical articulation that I thought was unwanted? If not, then round battens with the rectangular middle hinge with wedges sounds like the best option.
     
  11. Avoid Rocks
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    Avoid Rocks Junior Member

    Looks like I messed up a bit with my calculations. Disregard my previous sail plan.

    [​IMG]

    Back to having the mast in front of the bulkhead. The circle on the right is the CE of the bermuda rig (21.7% lead(!)). The circle on the left is PJR's suggested CE (6% aft compared to bermudan). This one looks more like it should, but I think it might need to be placed even further forward, the PJR-suggested CE is now at ~33% of the chord.

    I've only positioned the circles longitudinally correct.
     
  12. David Tyler
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    David Tyler J. R. A. Committee Member

    I've consulted "Principles of Yacht Design", Larsson, Eliasson & Orych. They recommend using geometric centres only and the empirical rules of thumb that have been used for centuries, not calculations based on aero/hydrodynamic centres, for a long keeled boat. Then they recommend a lead of 10% - 14% for a fractional rig on a long keeled boat. It looks like you used the aerodynamic centre for the sail. If you used the geometric centre, you probably come close to 10%, by eye, and my feeling is that a little more wouldn't hurt, though your second sailplan is not much too far aft, as the first seemed to be. Certainly, my sail is a long way forward, and the balance is good (though the hull, with bilgeboards, is more akin to a fin keel boat).

    My point about the cones vs wedges is that my hinges boxes allowed large and uncontrolled vertical movement, and there was a definite "flip-flop" action between correct orientation and trying to stand on edge. Once the battens had flipped to near vertical, it was impossible to get them right again, until the wind pressure had been taken off. With double cones, there is an equal ability to rotate to a fixed angle in all directions, and no tendency to seek a "wrong" orientation. My current feeling is that it's OK to allow an angle of vertical movement up to or equal to the angle of horizontal movement, but not greater. However, until this is proven on a soft wing in practice (it's perfectly OK with standard junk rig), you might consider it safer to use double wedge hinges. I'm pushed into going the safer way (probably a good thing :D), because I'm already using rectangular section battens. With access to a lathe, double cones would be the easier way.
     
  13. Avoid Rocks
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    Avoid Rocks Junior Member

    Good estimate, it's around 8% in the second one. Here's ~11.5%:

    [​IMG]


    I do have access to a lathe so it looks like I might be the guinea pig. :)
     
  14. David Tyler
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    David Tyler J. R. A. Committee Member

    Looks good. Another thing to bear in mind is that with an unstayed mast, the actual mast line is further aft than the one you draw. No mast is perfectly stiff. You'll need more mast length, to get the halyard coming more vertically from the centre of the yard.

    The 933mm batten pitch will give very convenient reefing steps, and you have enough drift for any variation of sheeting, so all's well there.
     

  15. mfly
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    mfly New Member

    (Deleted post.)
     
    Last edited: Nov 22, 2015
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