The great mystery of the bad tack

Discussion in 'Sailboats' started by laukejas, Aug 14, 2016.

  1. gggGuest
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    gggGuest ...

    Or failing that someone standing on the end of a pier or pontoon while you sail down to it then away. Even still pics would be better than nothing. You need to adjust your position so photos are precisely from astern to get the best results.

    Agree with Richard that behaviour on a close reach could be rather interesting too. As he says, 30% is a huge difference, and my experience is that such very big differences are quite likely to be caused by technique rather than boat design.
     
  2. JRD
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    JRD Senior Member

    It's a bit hard to tell as there seems to be more wind when you tack onto starboard each time. The shape looks somehow better on the bad tack. To me it almost looks like the leech is hooked to windward on the good tack.
    As a kid I once swapped my Flying Ant for a friends opti for the afternoon. I set the sprit to give what looked to me like a nice full sail. It was dog slow, too much twist and the "full" sail failed to catch any wind. The optis with tight sprits and flat sails disappeared into the distance while I wallowed.
    Is it possible you are getting better sprit tension and less sail twist due to the sail being held against the mast. Just guessing really, it's a bit of a mystery.

    You might try some lighter tell tales or woollies on the sail. They don't seem to indicate much even in the gusts.
     
  3. sharpii2
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    sharpii2 Senior Member

    Watching your little video, it appeared that you were pushing the tiller end away from you while you were on the 'good tack'. If this is the case, you may have a slight lee helm, on the 'good tack', but not on the 'bad tack'.

    This would explain the relatively large difference in performance bet ween the two tacks.

    Your boat originally had a Standing Lug.

    When you changed to a Balanced Lug, did you move the mast aft a bit?

    If not, the Center of Area (CA) of the new sail is probably further forward than the CA of the old one.

    If I am correct, the amount of lee helm, on the 'good tack', is probably quite slight.

    If at all practical, you might consider shifting both the Boom and Yard aft, in relation to the Mast, about 15 cm, then seeing if the difference either decreases or goes away.
     
  4. laukejas
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    laukejas Senior Member

    When reaching, boat is slightly faster on "bad" tack, but not too noticeable. On run, there is no difference. I thought that one ought to make sail fuller in light winds. I'll try flattening it out as much as I can.

    Yes, I'm aware of different sheeting angles on different tacks. I use horizontal distance between boom and transom corners as a reference for sheeting. Basically, I sheet to 10-12° off the centerline, and then steer the course along with windshifts for best VMG, using boat speed as a reference. I know exact boom-transom distances for 10-12° angle from boat drawings, they are a bit different on each tack due to boom offset.

    Hmmm, I will try to get someone to take pictures, but it's quite difficult to organize around here. We have no powerboat available. And with extremely shifty and unsteady wind, it might be difficult to get in position to take pictures from peer.


    CA/CE shift is an interesting idea, but you might have gotten a wrong impression from the video - on both tacks, I have slight weather helm, so I have to pull the tiller towards me. When I changed my Standing Lug to a Balanced Lug, I made sure that the new CA/CE remained in the same place - actually, I moved it forward a few inches, because the old sail had too much weather helm. Current Balanced Lug has less weather helm, but still felt on any course, any tack. If I had to design this sail again, I would move CA/CE even more forward for less weather helm (sometimes boat gets caught in irons). I can increase weather helm a little by moving the boom aft, or reduce it by moving it forward. Tried that already. Doesn't seem to influence performance on different tacks, though it influences other things (like sail twist).

    Yesterday, I sailed some more, making sure I pointed the same angle to the true wind on both tacks. The performance difference was huge. On "good" tack, I could barely move the boat. On "bad" tack, the boat accelerated so fast after tacking, my butt almost started sliding on the rail... Whatever it is, the "bad" tack has at least twice as much power. It pulls like a train, while on "good" tack, the boat is crawling, even with quite a lot of wind. I can see I'm on a good angle, all telltales are horizontal, the sheeting angle is good, but the boat isn't moving... Mysterious stuff.
     
  5. sharpii2
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    sharpii2 Senior Member

    I see.

    OK, I looked at the video again.

    The sail was definitely more full on 'good tack' than on the 'bad tack'.

    I also noticed the Leech was straighter on the 'good tack' than the bad one.

    On the 'bad tack', the Leech appears to curve in towards the Mast.

    This may do two things:

    1.) it flattens the camber aft the mast, and
    2.) it straightens the run to the Leech.

    Normally, a cambered sail is a good thing, as it has an airfoil effect, like that of a wing on a plan.

    But an absolutely flat sail is better than a cambered one which has the wrong camber shape.

    The wrong camber shape is too much camber near the Leech. This tends to create more drag than lift.

    Is it any wonder that, before the advent of low stretch fabrics, such as Egyptian cotton and later Dacron, sail makers did their level best to keep their sails as flat as possible?

    I know that when I endeavored to flatten the sails on my first boat, it turned from a complete pig to a hot rod. And the Super Snark I once owned had absolutely flat sails. Though it was clearly inferior to other boats with correctly cambered sails, it never failed to go, and went up wind quite well. I probably have more experience sailing with flat sails than with cambered ones, due to the amount of time I spent sailing the Super Snark as opposed to the other boats I owned.

    So, camber is good, but only if its the right shape. If its the wrong shape, well, its a good down wind sail.

    Now the question becomes how to turn the suspected bad camber of your sail into a good one.

    The best method I can think of trying is to tape or sew battens to the aft end of your sail. These can be made of harvested sticks that are maybe 1.2 cm in diameter and maybe 121 cm long. I suggest you try just two, evenly spaced along the Leech, with the top one parallel to the Yard and the bottom one parallel to the Boom. Be sure to have their fattest ends aft.

    This experiment is easily reversible if it doesn't work. Just simply cut the tape or thread to remove the battens.

    If it does work, you can make more permanent ones once you get home.

    One question before I go:

    Can you let your sheet out far enough for the sail to flutter, when sailing cross wind?

    Try this on both the 'bad tack' and the 'good tack'.
     
    Last edited: Aug 17, 2016
  6. Petros
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    Petros Senior Member

    almost certainly that kind of performance change has to be due to the change in shape of the sail from one take to the other. The it almost sounds like when on the "good" tack, the sail is not generating any lift (or forward thrust), so perhaps it is stalled. Too much camber can cause the air to separate off the surface and you loose all lift. You have to accelerate (or curve) the air around curve of the surface to get lift off the sail. For some reason on the bad tack you are getting more attached flow accelerating across the surface to give more lift than on the good or "clean" tack.

    It is the sail shape that is giving you the difference. One simple test you might try is put a "dummy" mast on the other side of the sail so it creates the same "spoiled" shape on the sail and see if it performance the same on both tacks. If the answer it that it does, than you now have a good starting point to try and reshape the sail.
     
  7. laukejas
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    laukejas Senior Member

    Very good points. Thank you. I also suspect that the maximum camber might be too aft on the "good" tack, generating more drag than lift, separating airflow. If I understood correctly, you suggested make-shift battens on the aft-end of the sail. But I already have fiberglass battens inserted there! I thought it was evident from the video. Three pockets and three 50cm (20") battens, spaced evenly along the leech. I made them specifically for keeping the leech flat, even though there's no roach.

    I had a very foul weather these past few days, didn't get the chance to sail. But next time I do, I'll try to flatten the sail out as much as humanly possible, and see if it makes any difference. The fabric I have is a pig: though I've been using this new sail for only a little more than 2 weeks, I already see some permanent elongation, and it's getting really difficult to flatten the ******* thing.


    Do you mean shifting the sail to the other side of the mast? If so, I have already tried that - the good and bad tacks also shifted (making it clear that the issue is with the sail). If it's not what you meant by "dummy mast", could you please clarify?
     
  8. sharpii2
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    sharpii2 Senior Member

    Sorry. I never noticed the battens.

    My guess is that they're too short. My guess is they should be around 40% of the sail Chord in length.

    As for Petros's idea, you could run a line from the top of the mast to the bottom, on the 'good tack' side, which could act at least somewhat like a 'dummy mast'. Once the sail is up, you could tighten the line. This would take a lot of camber out, but not quite as much as the mast does on the 'bad tack'. This experiment is so easy and cheap to do, I'd be inclined to try it first.
     
  9. SukiSolo
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    SukiSolo Senior Member

    Aside from the lug issues, if you have different pointing/height on the tacks, very carefully check the alignement of the dagger board box and the foil itself. It's amazing what a degree or so can do there.
     
  10. JRD
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    JRD Senior Member

    So maybe concluding from some points above the unsupported luff below your sprit/gaff is allowing the cloth to deform into an over full shape, on the bad tack your mast is preventing that. The bad tack sail shape looks quite good to me and it seems quite a large sail for the size of the boat. Big sails (relative) don't need as much camber to give the same power.
    Comparing the unsupported luff with a jib sail, you would not want even half the amount of camber that it has on the 'good' tack.

    Petros Dummy mast idea/retainer line is good idea to try. A partial pocket sewn or tied down part of the mast area would give a similar result.
     
  11. laukejas
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    laukejas Senior Member

    All right, I tried Petros idea with a rope. Stretched it tight from the mast top to the centerline of the boat, inside yard and boom to maximize it's effect on the sail shape. The shape became pretty much the same as on the "bad" tack. Surprisingly, it didn't make any difference in speed - well, maybe 5-10% at most, but I definitely didn't feel it that much. I tried this numerous times while underway (for example, releasing/tensioning that rope while beating upwind), and I'm sure there's no significant difference in performance.

    This is getting more mysterious by the hour... If the sail shape is not to blame, and the hull isn't either (it's perfectly symmetrical), then what's left? I want to blame myself, as I am not a professional sailor, but then again, I know enough basics not to oversheet or steer to high...
     
  12. JRD
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    JRD Senior Member

    Laukjas,
    I agree that does still seem mysterious. But there has to be a rational explanation somewhere.
    Have you been sailing in a variety of wind strengths and directions? Is the difference consistent between these?

    Sail shape can vary from good to bad with some quite subtle adjustments. I would try Petros idea with a straight batten of wood. Without seeing it with the rope, I imagine it would still allow a certain amount of extra camber to develop once you have some pressure on the sail..
     
  13. laukejas
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    laukejas Senior Member

    I have been sailing in various conditions, yes. But I saw no change in performance difference between tacks in high or low winds. It's pretty much constant.

    Well, wooden batten might flex too. I actually tensioned that rope so much, that it didn't bent more than an inch. As I said, it "ruined" the sail shape pretty much as much as on the "bad" tack... I don't think that inch of bend would contribute to 30% performance difference.


    Anyway, I just returned from another sailing session. This time, I flattened the sail out as much as I could - tensioned the outhaul, and pulled brutally on the downhaul, as much as I dared without risking the mast crashing through the floor. The wind was light to moderate. The "good" tack performance improved somewhat. Maybe to around 20% difference from the "bad" tack. Difficult to tell exactly with so many variables going on...
     
  14. Skyak
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    Skyak Senior Member

    Lug sails are the artsy-fartsy brothers of junk rigs in my mind.

    Lug
    On the "good" tack the sail pivots on the halyard and tension and position depend on the catenary curve which depends on the wind. On the "Bad" tack the sail pivot around it's contact with the mast and tension equals downhaul. Because of the different pivots you will have different twists. You have a huge sail area for such a small boat so compared to common experience you should expect to want that sail FLAT. The differences you describe seem too severe to be completely explained by sail camber so I suspect that there is a significant difference in twist on the two tacks.

    Junk rig is the no nonsense brother. The sail pivots around the mast, even on the "good" tack because the full length battens are tied to the mast. The sail tension is the same on both tacks because the wind can not raise tension by blowing the sail away from the mast. Because the pivot is the same the twist is the same -but the junk does not leave anything for the wind to decide -it has sheets to the ends of each batten to set the twist exactly.

    If you sail again 'as-is' please note the difference in twist -the angle difference between the boom and the yard and how the leach responds (straight, curved, more curve high or low).

    For experimental modification I would love to see you add a full length "batten" to the sail, on the side opposite the mast, tied through to the mast. If you add a sheet to the end of the batten that you can pull to either transom corner you will essentially taken the first step to conversion to a junk rig. With this setup you should have all you need to figure out what your boat wants for a sail. Lugs have questions, junks have answers.
     

  15. laukejas
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    laukejas Senior Member

    Thank you for your suggestion, Skyak. I can see that you are much inclined towards junk rig. I haven't tried it yet, but I should sometime.

    Anyway, I have just returned from one of the biggest annual regatta's in Lithuania, where I scored a 2nd place in an "open" class (7 boats in total) over 3 races. Even better, I scored 5th position in the whole 30 boat fleet (all classes sailed together, no handicap) - which consistent of Lasers, 420's, 470's, catamarans, a trimaran, several cruising yachts, a 36 feet Dutch racing monster, and a bunch of other, race-oriented dinghies.

    So, after leaving most of these fancy, factory-built, racing-oriented sailboats in my wake, I can't continue complaining about the performance of a little, lug-rigged, home-made boat in good conscience anymore. :D Really, the boat performed great. After flattening the sail out and reducing twist to a minimum, the performance difference between "good" and "bad" tacks is pretty much negligible. Maybe the "bad" tack is still a little better by a few percent, but that's just being picky now.

    Here's one photo after regatta which captures the sail at a perfect angle. Looks pretty fine to me now. (Before you say it, the passenger on the boat is from my support team, a girlfriend of my good old school friend).

    [​IMG]

    So, thank you all very much for your ideas and advice to solve this mystery. It helped a lot, and the improvements I made based on your suggestions made this huge victory possible. Yay for lugs and Boat Design Forum community! :)
     
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