The great mystery of the bad tack

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

  1. laukejas
    Joined: Feb 2012
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    laukejas Senior Member

    Hello everybody,

    Last summer, I have built myself a small dinghy, Melatelia. Some of the folk here might remember me crying for advice regarding it's design for nearly a year. The boat was a success, but after a season of sailing, I found there's room for improvement. So, this summer, I have sewn her a new sail (this time, of more proper material), replacing standing lug with a balanced lug.

    The new sail performs better than the old one. However, there's a puzzling performance issue I just cannot seem to solve. Everybody knows that the lug sail, same as sprit, has this "bad tack", where the sail presses against the mast, and that supposedly ruins it's shape, disturbs the airflow, increases drag, and reduces lift. That's what I expected from my sail too. However, I was soon to discover that actually, this "bad tack" is WAY better when beating to windward. In moderate breeze, Melatelia is 1-2 knots faster and points around 5 degrees higher on the "bad" tack in comparison to the "good" tack. At first, I thought it was just a wrong impression based on poor judgement and coincidence of wind shifts, but after 2 weeks of sailing and 1 regatta, I am now completely sure that my boat performs a lot better on the so called "bad" tack.

    I am now trying to figure out whenever my "good" tack is somehow bad - maybe the sail shape didn't come out as it was supposed to, or maybe the mast windage plays a huge part when mast is exposed. It may be of some significance that my spars are box section, with corners rounded off somewhat.

    I have a video of my boat performing on the water, though not in enough breeze to make the speed difference between tacks noticeable, still you can see how the sail looks. In the first part, I'm sailing alone in a very, very light wind, and later on, I'm sailing with my brother in a little more breeze.

    https://youtu.be/3GKneNJIvfQ

    Some basic information on my boat:

    LOA: 3.2m / 10' 6"
    LWL: 3.0m / 9' 10"
    Beam: 1.2m / 3' 11"
    Displacement: 130kg / 286lbs single handed, 210kg / 463lbs two handed;
    Hull weight: 32kg / 70.5lbs
    Rigged weight: 50kg / 110lbs
    Sail Area: 7.4m^2 / 80sqf
    Cp: 0.525
    Mast height: 4m / 13' 1 1/2"
    Boom&yard length: 3.0m / 9' 10"


    I hope some of you might have an idea to solve this mystery. As I said, 1-2 knots faster on "bad" tack, when beating to windward. Definitely not a marginal difference...
     
  2. heikki
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    heikki Junior Member

    How do you estimate true wind direction? Does the heeling force from the sail feel the same on both tacks? If not perhaps you are not pointing as high on both tacks.
     
  3. laukejas
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    laukejas Senior Member

    Well, for true wind direction, I use flagpoles on shore as an indicator. When in regatta, I use other boats as reference. I also sheet to the same angle (10-12° off the centerline) on both tacks. I'm dead sure I point at same degrees on both tacks.

    Heeling force... I think it is a bit higher on the "good" tack. Not a lot, though.
     
  4. Richard Woods
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    Richard Woods Woods Designs

    I suggest you let someone else sail the boat so you can watch and see what happens. Also it might be your technique

    I would also try the sail on the other side of the mast to check the problem is not with the hull or foils

    RW
     
  5. gggGuest
    Joined: Feb 2005
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    gggGuest ...

    1-2knots faster and 5 degrees higher is an awful lot... Richard's right, try the sail the other way round and see if the behaviour swaps sides.

    Also, if you can, take a smart phone on board which has a gps trace capability and take GPS tracks. I've found sometimes having real numbers identifies behaviour that I hadn't spotted from "feel". GPS traces coupled with the swapping of sides might be very interesting.

    One thing that occurs to me, with the sail against the mast one of the side effects is that the draft is effectively reduced. Might it be your sail is setting over full, and if so might changing sheeting arrangements help?
     
  6. laukejas
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    laukejas Senior Member

    Good ideas. I have nobody else to sail the boat for me, but I did try the trick with GPS. I'm not sure how accurate it is, but over half an hour of sailing, it showed an average of around 2 knots on "good" tack, and 3 knots on "bad" one - a significant difference. I used several landmarks on land to make sure I pointed the same amount of TWD on both tacks.

    I also tried shifting the boom and yard to the other side of the mast - and the "bad" and "good" tacks shifted too. So I guess the hull and appendages are not to blame here.

    Any more ideas? As for the sail camber amount, it is pretty much the same on both tacks, I don't think mast flattens the sail too much... I tried flattening it more on "good" tack, but that didn't fix performance at all.
     
  7. gggGuest
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    gggGuest ...

    Fascinating... I must admit I'm not long on ideas here. If you can manage gopro type video from a camera on the rudder post that might tell us a bit, especially about whether you're managing to be precisely even on both tacks.

    I do vaguely incline to your exposed mast/mast in contact with sail speculation, but on exactly no evidence, and only because I don't have any other ideas. Strange though if what people had believed to be true for years turned out to be wrong, but I suppose stranger things have happened and as you say your heavily squared off mast might be a bit of a special case.
    Be interesting to construct a round one and see what happened, but easy for me to say, much more work for you to do it. Could you find some alloy tube to construct a quick and dirty round mast to test the theory? For a test piece you wouldn't need high quality alloy tube, especially in those light winds.

    She looks pretty sweet in your video doesn't she? Good job. Maybe trim a little further forward without the passenger?
     
  8. messabout
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    messabout Senior Member

    Notice the difference in camber distribution between good tack and bad tack. Bad tack appears to have a straight area from luff to mast. After that point camber appears to be just fine. On good tack no such thing occurs and the camber chord is longer and perhaps a little deeper.

    Aaah Yes, we can speculate at length without definitive solution.
     
  9. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    I tried a half wishbone sprit boom a few years back, to test if the bad tack thing was real, compaired to a conventional sprit boom. I found in very light air, there was a decided difference from good to bad tack side. In zephyrs, I found tacking from the bad side about 40% of the time wasn't successful, but had no issues from the good side. Once wind strengths got to 4 - 5 knots there was no discernible difference from either side. I speculated the flow was more than sufficient, at these higher wind strengths, to overcome any bad tack flow shortfalls.

    Tests were all done in various wind strengths, across several hours underway. Once I noticed the light wind results, I focused in them toward the end of the sessions. FWIW, I found the half wishbone superior in these low wind strengths, but as winds rose over 13 knots, compression on the half wishbone issues started to crop up This was because of the laminate orientation I used, which was devised for ease of making the curved boom. Since, I've made a new boom, with a horizontal laminate and this one suffers much less compression length deviation, but is a much more difficult laminate to build, with edge setting and the resulting cupping, being the biggest problems.
     
  10. sharpii2
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    sharpii2 Senior Member

    This is not the first time I've heard of this happening with a balanced lug.

    My guess is that on the 'bad tack' The sail lays against the mast, which tends to flatten it out.

    On the 'good tack' this doesn't happen.

    I had a problem with sails setting too full with my first boat.

    see attachment.

    The foot of the two sails had sleeves, which the two booms slid into.

    Mind you, both sails were cut absolutely flat. But they came to a point at their heads, and to this the halyards had to attach. this was done by extending a strip of doubled over duck tape out each head, folding it over the halyard loop, then spiraling duct tape around this bit, in hopes that it would hold.

    It held fine. In fact it held fine enough to capsize the boat.

    But what it also did was put a lot of camber in the sails.

    Try as I could, I couldn't get that pig to sail to windward.

    I later came up with the notion that I needed to flatten the sails, so I taped in a head stick at the top of each, a piece of quarter round I found laying around the garage (I was 16 at the time). It looked ugly as hell, but after that there was no problem getting it to sail to windward.

    If your sail is too full, it may be because the material they are made of stretches, making it fuller than you had in mind. The aggravating part of this is, once relieved of the wind's pressure (which puts the material in tension), the sail will revert right back to the original shape you had planned, erasing all the evidence.
     

    Attached Files:

  11. laukejas
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    laukejas Senior Member

    Yeah, well, the GoPro hasn't exactly arrived in our country in abundance - I don't know anyone who owns such a camera... Though it would be an interesting experiment.

    I am on holiday now with my boat, far away from anywhere I could get a round substitute for a mast. Not a lot of tools with me either to make one. I sold my previous round mast, because it was too lean and too tapered for this new sail.

    Thanks for the tip about the trim, I am trying, but due to internal arrangements (the daggerboard and the last sheet block) it is really uncomfortable to be so far forward in the boat - in the light winds, I can't sit on the rail without capsizing the boat, and inside, there's no place to be where the daggerboard is. I should have though of this when I made this boat...

    Yes, the camber amount has crossed my mind too. But if anything, I think the camber to be too little for my boat and the usual conditions I sail in. During the last regatta, I looked at the other boats (Lasers, 470, etc.) for reference on how much camber is needed for these conditions. Even in what I would call moderate to high winds, these boats had a lot of camber, much more than I can get out of my sail. This was a problem I have forseen when designing and sewing my lug sail, but could do little about it - since the lug has only the head edge attached to a spar, it is the only edge that can benefit from edge round induced camber - and up high, sail is better off flatter anyway. I tried compensating with broadseaming to induce more camber in the middle of the sail, where it's needed most, but to no avail.
    In the video I posted earlier, if you go to 1:10, you can see one of the problems with my sail - there is a flat spot that runs from the "throat" corner of the sail horizontally all the way to the leech, as if it was stressed. I guess lug sail is very resilient against perfectly honest intentions to put decent camber in it. I wish I could get some depth in that flat spot, but with no spars or sail edges nearby, I'm out of ideas.

    TLDR - there's too little camber in the sail anyway, at least the way I see it. The only thing I find suspicious here is that the max camber depth may be too far aft - not in the 40% of the chord, but maybe 50% or maybe even 55%. I can't tell exactly, but maybe you can spot it from the video?
    If that were the case, then on the "bad" tack, the forward part of the sail gets "cut off" from the main part, and this main part now has camber in the right place - around 40%, producing more lift as a result. This is a wild speculation on my part, however, as I don't really think that a shifted camber location alone could be enough to overcome the drawbacks of the bad tack.

    But anyway, the sail material is quite stretchy, compared to the real Dacron. Yet, I would say that in higher winds, the pressure actually does some good for the sail shape - it smooths it out, makes it more uniform and induces some good camber, as opposed to light winds, where the "bags" and flat spots in the sail are much more prominent. Still, as I mentioned before, it might be possible that maximum camber depth is too far aft.
     
  12. LP
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    LP Flying Boatman

    How much down haul tension are you carrying? There is quite a pocket of loose sail at the boom and a definite pucker at the yard/head. If I recall correctly, Micheal Storer and his Goat Island Skiff lace the sail to the yard along its entire length and maybe the boom too. With a flexible yard and a laced head and/or boom, you will be able exert variable control of the camber by means of down haul tension.

    Limber yards and/or booms could actually be detrimental to sail shape if the sail is only attached at the corners. More down haul tension could "bag" the sail even more.

    Here are a couple of links to the Storer website. You may already know of them. There is an exceptional amount of information on his website regarding lug sails.

    http://www.storerboatplans.com/GIS/GISplan.html

    http://www.storerboatplans.com/wp/d...ster-2013-setting-up-sails-spars-and-rigging/
     
  13. laukejas
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    laukejas Senior Member

    Thanks for the info. I am well aware of the GIS, and even used it as a role model for my own sail, with obvious modifications. The head of my sail is, of course, laced to the yard. The pucker up there is because I cannot tension the sail head enough - it turns out that the yard is a bit too short. I thought that making spar 5 inches longer than the sail edge would be enough for tensioning, knots and whatnot, but with material this stretchy, I was wrong. I see no easy way of shortening the head of the sail...

    My downhaul tension is pretty high. I have 1:2 tension system in place. I tried various tensions, from super high to none, but it doesn't seem to affect camber that much. My spars are not very flexible - I may have overbuilt them (tried to be on the safe side since the last season, when I broke my boom in a storm).

    Anyway, there are obvious shortcomings on my boat, but I see nothing that would explain such a huge performance difference on different tacks...
     
  14. Petros
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    Petros Senior Member

    this is just a thought after watching your video. it looks like the center of effort on the sail must shift back on the "bad" tack (I am assuming the area ahead of the mast is not generating much lift), than on the good tack when the sail is full and holding the proper shape.

    It might be possible there is less trim drag with the center of effort further back? Perhaps you can test this by raking the sail back so the center of effort is moved back, see if it behaves different on the "good" tack.
     

  15. Richard Woods
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    Richard Woods Woods Designs

    are you faster/slower when reaching and running? or is it just to windward. You are taking about a 30% speed change which is huge. Most people call a 2% change in performance a "breakthrough". Certainly it is more than just the drag from an offset mast

    In very light winds a flat sail is better than a full one. Your sail is offset from the boat CL so your sheeting angle relative to the transom will be different on each tack Also twist might vary from one tack to the other

    If you cannot find someone to sail your boat while you watch you need to have someone follow and video you in a powerboat as you sail on each tack.

    RW
     
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