junk rigs in light air

  • 27 Jun 2012 01:59
    Reply # 983592 on 981388
    Deleted user
    thank you every one for pitching in. the news letter idea is a very good one and Arn's works are enlightening. Again David thank you for a very clear explanation (I have been trying to understand why everyone has been trying to get me to use stiff battens while the ones i have have been perfect for our little boat) What we see with the very light cloth and bendy battens is lots of power, as you say this may be because our boat is so easily driven. The Cat will be a different story (well kind of; the size will be much greater and the need for a good rig essential.) We would like to get it right the first time and have leisure time to play with sail geometry. 

    I have too much of my own research to do that i dont want another ten things on my list of subjects.  thank you all again and please keep the conversation coming, I am having allot of fun learning this stuff. 
    Ernie
  • 26 Jun 2012 04:56
    Reply # 982732 on 981388
    I'd like to add the reason why bendy battens, though they may work moderately well on a light daysailer, used on flat water in a limited range of wind speeds, are not so good when you take them offshore. As the boat pitches and rolls over the seas, the whole rig "pants", cycling between too much camber and too little camber. This not only does nothing for the performance, it also wears the rig out faster, through extra fatigue and chafe. So however the camber is added, it ought to be added in a stable form - one that results in the sail blowing out easily into the designed shape (whatever you think that ought to be), but no more (and no less). As Arne said, bendy battens give too little camber in light winds, too much in strong winds: it's the opposite of what we would like to see.

    A slim, light, easily driven boat can get away with less camber than a wide, deep, heavy boat, but they both benefit from having some. The difference is that it is desirable in the former, verging on essential in the latter. Ernie, your good-looking Tariand your proposed big cat will both fall into the former category, and flattish sails with little camber will be fine; but it's a different story for a heavy offshore monohull, where the right amount of camber, in the right position, becomes more of an issue.

    It is quite often seen that where a boat has a "good tack" and a "bad tack", the "good tack" is when the mast is behind the sail. If the sail is flat, there is a separation bubble behind the luff, and the airflow should re-attach further downstream; if there is some camber in each panel, the mast gets buried in the sail, and the airflow doesn't see it; if the batten has a slight horizontal 'S' bend, again, the air flow tends to leap over the mast without seeing it, to re-attach on the curve of the cloth further back. If the mast is on the windward side, in all cases the sail works better, with a clean curve on the lee side - but there is a lot of parasitic drag from the exposed mast, which more than negates that extra performance. 
  • 26 Jun 2012 03:21
    Reply # 982636 on 981388
    Deleted user
    There have been a few complicated explanations for junk sails over the decades, and have eventually been discredited (Bunny's bumblebee fibulator being a notable one).
    It comes down to the same aerodynamics that explain bermudian rigs and aircraft wings - foil shape creates lift. 

    I think what your boat is experiencing is shape in the sail, any kind of depression in the panels is preferable to no depression at all*. Just general bagginess is preferable to flat. But a perfect NACA foil shape depression is best. 

    Problem with bendy battens is that they need stronger wind to work.
    The whole history of this subject is in the newsletter archive, I highly recommend downloading the whole lot.

    * Except, maybe if you are a cruiser, when good sail handling is more important.
    Last modified: 26 Jun 2012 03:28 | Deleted user
  • 24 Jun 2012 23:44
    Reply # 981519 on 981388
    Deleted user
    nice reading Arn!
    Thank you.

    However it seems you misunderstand the S curve i was speaking of. Since Tari has bamboo battens she does show the upward curve you explain so well. 
    What i was referring to is the S shape that develops in the other direction. the batten curves around the mast then tends to curve back a little. this curve in the lateral plane seems to be of good benefit to the sails performance. wind slides off the luff (rear) of the sail (I get luff and leach confused since i only started laundry sailing a few years ago). very well and the sail seems to develop a good deal of force in the after part of the S. To stiff of battens tends to stop this form from developing and On Tari this has the effect of less power. Stick battens that do not tend to curve up in the vertical section and are free to develop the S in the horizontal section seem to be a good answer (we have slowly replaced the bamboo with cedar 1"X2" battens in the stress points. 

    Our sails do make a nice cambered pattern between battens with the flexible cloth without the work. Even in light airs we see this. our biggest problem was no dagger board so i installed one in front of the sail and this seems to work well. for into the wind. Our next project will be much bigger than Tari Tari so i am doing some small experiments now for that project.
  • 24 Jun 2012 21:21
    Reply # 981426 on 981388
    Anonymous member (Administrator)

    Ernie

    Yes, some camber is important. Could i suggest you have a look on my page here to see how I cut each panel to get the desired camber? Also have a look at the photos in my personal album to see the resulting sails.

    The problem with bendy battens is that they leave the sail flat in light winds and with too much camber in strong winds.

    Cheers, Arne

    Last modified: 25 Jun 2012 08:21 | Anonymous member (Administrator)
  • 24 Jun 2012 20:23
    Message # 981388
    Deleted user
    Since starting my study of this rig several years ago I have repeatedly heard that junks do badly in light airs. 

    here are a few things i noticed and some things i have done to make this a myth in my own sailing. 

    While researching i got ahold of lots and lots and lots of old pictures that show several types of junk rigged boats and ships, as i looked i came to see some differences in approach to sails that kinda stood out for me. 

    1: longer masts; this stood out when i was looking at junks in calm water and i noticed the sails where lower on the masts than usual. seems moving the sail up or down the mast allows you to deal with different conditions. we have found this to be true on the TariTari
    sometimes the wind is low and sometimes high. We move the sail up or down the mast to take advantage of the wind in very light conditions. 1 to 5 knts and we make at least half that in these airs. 

    Tari Tari has no keel and is a traditional hull form so we do tend to be very easy to drive anyway. 

    2: an extra panel at the foot; if you look close at many of the older pic's you can see extra fabric lashed to the lower batten. this is another section of sail to make your sail area larger. It works a treat in very light airs where you have a uniform distribution or when you dont want to chase the wind up the mast.

    3: Very light fabrics; Our sail is made from 2 OZ rip-stop nylon it balloons between the battens and shapes them to nice S curves under sail. the 8 oz canvas sail we had didn't do this and its performance was very poor in light river winds. Older pictures also show this same thing if looked at close enough the battens on a hard driven junk warp to an  S curve and the lighter sails are those used in the harbor on the smaller boats. 

    All of this leads me to believe that proper flex in the battens and lighter sail fabrics will be more effective on a junk rig than the "new" camber sails in stiff fabric. An extra panel will end the light airs argument and that we need to study our sails much much closer if we are to get the hang of this rig. 
       " ...there is nothing - absolutely nothing - half so much worth doing as simply messing about in junk-rigged boats" 
                                                               - the Chinese Water Rat

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