Sheet lead angle

Discussion in 'Multihulls' started by bax, Nov 10, 2008.

  1. bax
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    bax Junior Member

    I am planning a self tacking blade jib for my 9m cat for shorthanded sailing and want to make sure that I place the track in the correct spot.
    All I have been able to find regarding sheet lead angle is to bissect the angle of the leech and the foot. Is this the correct or best method? :confused:

    bax
     
  2. Stumble
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    Stumble Senior Member

    This is a pretty complicated question, based on a lot of parameters to get it exacally right. My advice is to get your local sail maker to come out and tell you where to put the leads based on the exact sail you order from him.

    But basically you first decide what the wind speed is you are planning on useing the boat it, and what sail you will have up in those conditions. Then draw a line from the max draft position through the clew, with the sail set to the proper alpha angle (angle of attack to the wind). Where that line hits the deck is the proper location for the track.

    So the variables are

    1) sail size
    2) sail draft and designed alpha angle
    3) clew height from the deck
    4) Nominal wind speed
    5) Other deck hardware that can't be moved :D

    Of course the sail size, draft, and alpha angle are also set based on the wind speed so these can all change each other rapidly. Which is why I normally just have the sailmaker do it, that and since they should be used to working with these numbers are much more familure with the process than most boat owners.
     
  3. Tcubed
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    Tcubed Boat Designer

    This is not at all complicated.

    First, since you mention self tacking and track in the same breath i presume you mean an arc of circle track on which rides a car with a pulley for the jib sheet and the fall of the sheet routed through a point near the forestay...

    Basic rule of thumb that seems to give a nice moderate amount of twist in every case i have done:

    Look at the sailplan from profile. Look at the luff length of the jib. 2/5 th of the way up the luff draw a point. Draw a straight line through this point and the clew and through the deck. That is where the sheet should go. This applies the same no matter the size or camber of the jib.

    There are two angles that completely describe the sheet position, let's call them azimuth and elevation. The first determines the sheeting angle and the second the amount of twist.

    Note that for twist to be independant of sheeting angle (where the car is on the track) the track must be a section of circle centered on the forestay and in a plane orthogonal to the forestay. This means the ends of the track will sweep upwards and need to be supported on little sculpted blocks.

    If you deviate from the 2/5 th rule aft you'll get more twist, further forward , less twist. If you just bisect the angle as you've read you'll end up with almost no twist, which is not so optimum as a sail should mimic the twist of the apparent wind.

    Another way is to sail with the jib sheet in one hand and a crayon in the other. Play around with it until the shape looks perfect and mark the spots on the deck. This method obviously for small boats only...
     
  4. PAR
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    PAR Yacht Designer/Builder

    The easiest way is to "read" your tell tails when on a close hauled course, which will tell you if you are "powered up" or not. If the fairlead is too far forward, then the upper leeward and lowest windward tell tail will dance. If the fair lead is too far aft the upper windward and lowest leeward tell tail will dance.
     
  5. catsketcher
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    catsketcher Senior Member

    Camberspar

    You may want to condider the Bierig camberspar. It is a half wishbone that Dick Newick used on Ocean surfer. Gets in the way a bit but could be tied to the forestay bridle at anchor - no furling. I have found it hard to be impressed with self tackers on cats. Usually the tack is too high to be able to get the self tacker to work well. If you have any space at all between the deck and tack you will have to move the leech forward or pull down too much on the leech. MOst cats do not have the deck sweeeping jibs that Solings, Cherubs, 49ers and other self tacking boats have. There is a problem with having a deck sweeper on a short handed boat. Also your cat may need to back the jib during choppy heavy weather so a self tacker may not be the best option there.

    A blade is not that much work and having a higher clew with better visibility would be well worth the trade off in any hassle. Wait a minute - What about a pretend self tacker? Use normal sheets on it for normal sailing but you can clip a pulley onto the tack and use a rope bridle, like a Laser uses, for short tacking. After the tack give the proper sheet a tug but coming into the tack ease the normal sheet off - the bridle takes the load and the sail goes a little fuller - you tack and the jib goes over to the new tack - not all the way down the bridle but drawing. At your leisure you pull on the new leeward sheet and get nice shape again. When you are out of the channel ease off the bridle and disconnect the pulley. It may suit your style of sailing.

    cheers

    Phil Thompson
     
  6. Tcubed
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    Tcubed Boat Designer

    Very good points there from Catsketcher. I would add though that, although the wishbone on a jib works very well (wishbone, 1/2 wishbone or flipping cambered spar in a pocket, concept is the same) , the sail must be specially cut for that, or an existing jib can be modified, but a normal unmodified jib will not work, and to avoid fatigue the forestay must be two pieces joined at the point where the 'boom' goes, which can create extra complications when dropping/raising the sail.
     
  7. bax
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    bax Junior Member

    Thanks for all the help folks.

    Stumble, I'm too cheap and would also like to work it out myself (with the help of all our friends out there), thanks for all the info though, it will certainly come in handy!

    Tcubed, from what I have looked at since, it looks like your 2/5 rule is the way to go. I do have a couple of friends who either were, or still are in the sailmaking business and am trying to get ahold of them for input as well.

    Catsketcher, I don't think I am too keen on a jib boom, though I can certainly see the merits of using one. The pretend self tacker is an interesting idea... I'll have to ponder that one!

    Thanks again to All,

    bax
     
  8. Howaya
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    Howaya Junior Member

    Hey Lincoln,

    I created a false self-tacker of sorts that may work for you. I used two pad eyes with a bridle between them. One block rides on the bridle; attached to it is another block through which I lead a single jib sheet. Essentially, the bridle and first block function like a track and car on which the sheet block rides. As I didn't have much choice as to where the pad eyes would go, I made certain the length of bridle itself was adjustable; if I needed to lead the sheet lower (to simulate moving the pad eyes more aft), I just shortened the bridle a bit. With a little trial and error I learned where to set the bridle and, for the most part, leave it. This arrangement still allows full range for adjusting the single sheet regardless of which tack I am on. Admittedly, it isn't a perfect solution but the full-battened jib I use with this setup doesn't seem to mind much.

    Mike
     
  9. Spiv
    Joined: Jun 2007
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    Spiv Ancient Mariner

    Hi Mike,
    this seems a good arrangement for my beach cat, can you shoot a couple of pictures to make it more clear?
     
  10. farjoe
    Joined: Oct 2003
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    farjoe Senior Member

    I am thinking of fitting a boom to the jib to reach the same aim.

    The plan, which is as yet not fully thought out, is to fit a windsurfer universal joint as close as I can to the jib tack point and use a 2" al tube which just clears the mast and gets the jib clew.

    the rope system needed to control this seems simpler than what is being proposed above.
     
  11. bax
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    bax Junior Member

    Hi Mike,
    Sounds like a fairly simple and effective arrangement. where does the sheet lead to after it goes from the clew to the block on the bridle? Do you lead it forward to a block below the tack?

    Lincoln
     
  12. Howaya
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    Howaya Junior Member

    I knew I forgot to mention a couple of details after my last post. The sheet is led to a third block mounted to port on the main cross beam, where it then can turn straight aft to the port genoa lead block and continue on to the port cockpit winch. I chose a spring-loaded stand up block so as to permit a fair turn nomatter where the bridle "car" is and to keep the sheet clear of other things. A cheek block will not suffice. It is important to make certain you locate the third block wisely so the sheet runs free through it on both tacks; my first attempt resulted in the sheet rubbing against the central cabin while on a port tack.

    As for my earlier comments about the adjustable bridle, it works because the clew is fairly high (about the same height as the boom) and the bridle is just a few inches above the cross beam. Catsketcher was correct when he mentioned a deck sweeping jib will not serve you well. Overall, this arrangement is simple, cheap, and fairly effective; I would never call it ideal as I have noticed the jib isn't trimmed perfectly on all points of sail. Fortunately, the full-battened jib mitigates some of the imperfection and, if I ever decide to try another approach I've really only committed two pad eyes and one turning block.

    Mike
     
  13. bax
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    bax Junior Member

    Mike,
    Does the fact that the sheet leads off center to the third block have much effect on the sail shape and trim on stbd vs port tack? Is the third block forward or aft of the clew?

    Lincoln
     
  14. catsketcher
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    catsketcher Senior Member

    Jib boom idea

    Farjoe and others

    I have drawn a system that I was going to use on my little cat that uses a jib boom and a furler. It also has a vang (yes a vang) that attaches to the crossbeam. As the wind increases you pull more on the vang and you tension the forestay which is good.

    It needs a block of nylon underneath the furler which is attached to the bridle with a stainless tube through the block - the block must extend past the furler drum at rear
    the jib boom attaches to the block
    the jib clew is attached to the boom with a slippery sleeve so that it can slide up the boom
    the vang is attached to the boom about a quarter of the way back and is also attached to an end pulley which pulls on the sleeve.
    You pull on the sheet (leads centrally for angle of attack) and then vang to unfurl the jib and it slides back along the boom and then when it reaches a stopper the vang tightens the leech.

    If anyone is really interested I can draw it in CAD. At the mo it is on sketches in my many sketchbooks. I started one but I think I threw it out when I went cold on it.

    Cheers

    Phil Thompson
     

  15. Howaya
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    Howaya Junior Member

    Hi Lincoln,

    The third (turning) block has no effect on the sail shape. Sometimes the "car" fails to come all the way across on the new tack and this certainly has an effect on sail shape. This has happened to me in light air and I've found that either releasing or tugging on the sheet from the helm position can cause it to move across. At times I've had to go forward to correct the problem due to a legitimate snag.

    As for the third block's position, I have it just aft of the clew; I may try and move it a bit further aft but there isn't much room on account of the central cabin. You made mention of leading the sheet to a forward location; if you've seen this in practice and it is successfull please share that with us as I do certainly not claim to have found the perfect solution. More trial and error may yield a more refined result but my cat is now hauled for the winter, about 35 miles from home.

    Mike
     
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