New design for a self tacking catamaran jib

Discussion in 'Boat Design' started by MichaelRoberts, Feb 3, 2016.

  1. MichaelRoberts
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    MichaelRoberts Junior Member

    Good morning all,

    A self tacking jib seems to be a good idea for the 60 foot catamaran I am building - especially because I'll be really ancient by the time it's finished.

    I'm doing everything possible to minimise upwind air resistance. Here are some of ideas I've been considering:
    1. The jib needs to overlap the wing mast a little bit, so the track should arc around the front of the mast and end a bit behind the mast
    2. The sheet should not go up the wing mast because it will disturb the flow through the slot between jib and main and possibly cause trouble with rotation
    3. Why does curvature at middle of the jib track matter, the sail will be fluttering as it goes across
    4. Can the jib sheet tension be eliminated? Maybe a jam cleat on the traveller which can be tripped from the cockpit. That way there will be less load on the traveller
    5. When tacking, the jib needs to "back" for a few seconds then fling across. So I'm thinking the traveller should have a latching system which releases when the back wind load changes the direction of the sheet to the clew.
    6. The traveller assembly will be made of carbon/epoxy, the bearings will be lots of little nylon wheels, the track will be laminated too with the inner
    bearing surfaces moulded over something smooth like aluminium extrusion.

    I would be very grateful for your opinions and experiences with self tacking systems. And by the way here is a picture of the Rhino rendered design to put things into perpective.

    Thanks

    Michael
     

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  2. gonzo
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    gonzo Senior Member

    Why do you require the jib to overlap? It complicates the system enormously. So does the requirement of an automatic jib backing mechanism. It would have to self adjust for windspeed and other sailing conditions. A traditional jib boom can't be beat for simplicity and efficiency.
     
  3. Skyak
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    Skyak Senior Member

    I see the value of the slight overlap -it does wonders for sail interaction aerodynamics- but I think you are overestimating the aerodynamic interference by the sheet to the mast. I agree that the middle of the track could bump forward inboard of useful sheeting angles as long as it is back on the arc for sheeting.

    About your sheet concerns/desires, I have always wondered about not fixing the track to the boat. I always want a method to tune the tension vs position and it seems possible if the track is not fixed. I will give it more thought but I think the paradox is that it would be for sailors demanding optimal shape but too lazy to tack a jib -the null set.
     
  4. philSweet
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    philSweet Senior Member

    Funny, but not necessarily the case. Ones concerns depend on the circumstances. Tacking up a narrow channel with heavy traffic, selftacking is more than nice. Sailing off shore on one tack for a week, sailshape matters. But you can tweak things off shore with improvised sticks and string and bubblegum. Then clear it all away to regain selftacking. Integrated controls for tacking are available, such as coordinated sheet release and power furling.

    But I agree with Gonzo - the first thing you do when wanting to self tack is add a boom (or a long club). It solves the power problem. You have to keep both the leach and the foot tight, and there really isn't any other good way to keep the foot tight during self tacking.
     
  5. The Q
    Joined: Feb 2014
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    The Q Senior Member

    This link shows several commercial systems that are available,
    http://www.harken.co.uk/content.aspx?id=3911
    But on the Norfolk broads on somewhat more traditional boats we use
    http://www.huntersyard.co.uk/wp-content/uploads/cache/2014/01/2-Berth-Hustler/3337215917.jpg tracks are not generally used traditionally as a boom and sheet are quite sufficient and less complicated.
    A variety of the Norfolk system is when running the leech / clew is pulled tight into the centre of the boat and the luff /tack released so the sail is boomed out and provides a more efficient running sail by almost closing the gap in the middle.
     
  6. Skyak
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    Skyak Senior Member


    Phil, I guess I oversimplified in the interest of humor.

    Today the overlap and precise trim are given up for convenience with a boom (baseline). I can design a system that has the convenience of the self tack and the performance of the overlap and precise trim -but how much is performance-complication worth? My feeling is 'not enough' because the circumstances where the self tacking is highly valued are distinct from the circumstances where the extra performance are valued.

    I can do it, but I can't 'sell' it. The obvious compromise is to furl away the overlap and use a simple boom in the rare circumstance self tacking is preferred.
     
  7. MichaelRoberts
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    MichaelRoberts Junior Member

    Thanks for the collective advice.

    Seems like it's a boom then, for simplicity, reliability and control of sail shape.

    Still need a track I suppose, probably about two thirds along the boom.

    Maybe the jib could have a stiffened or battened flap extending a bit past the clew so it could scrape across the mast and give a bit of overlap.

    Maybe since we now have a boom, we could eliminate roller furling around the forestay and roll the jib around the boom instead. Come to think of it, this means we could now use battens to extend the sail past the clew.

    The advantages of would be
    1. A better shape of blade when reefed
    2. A lot less windage on anchor with less tendency to sail back and forth when on anchor.

    Anybody want to talk about how to attach the shrouds up to the wing mast?

    Thanks again for your valuable advice

    Michael
     
  8. CT249
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    CT249 Senior Member

    Why not copy the way the shorthanded racers do it? I don't have any recent pics but I think from very, very vague memory that they just have one vast shackle hanging off a laminated terminal on the leading edge, with all the shrouds and forestay coming off that.

    Years ago when I went over the ORMA 60s at the end of the Transpac I was very impressed by the rigging. These guys have been evolving it for eons, shouldn't you just go to them?
     
  9. MichaelRoberts
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    MichaelRoberts Junior Member

    Dear CT249

    one vast shackle

    Is this called a jesus shackle? The good thing about the one shackle solution is it does not fight back when you rotate the mast. The bad things are wear, eccentric compression loading (we are talking 13 tonnes compression in a 30kn wind), and no redundancy.

    Last summer in Greece there was a new French cat and I think I could see through the binoculars that they just had webbing straps to each shroud from the sides of the mast. Does this seem reasonable, there could be quite some counter torque fighting rotation.

    I'm thinking about using Dyneema Max for the shrouds - main ones and intermediate ones - about 13 mm diameter. Lanyards and splices and all that good traditional stuff.

    Trying to make a boat that is uncluttered, light, hydro and aerodynamically efficient and fast enough upwind in light conditions so you don't have to motor. Yes I know "gentlemen don't sail upwind", but sometimes you have to.

    So many design problems! You haven't heard the one about the brushless 25 kW DC motors yet. Probably should start another thread? The picture shows the battery banks (red) in the engine rooms.

    Best wishes all, keep up the advice.

    Michael
     

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  10. FAST FRED
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    FAST FRED Senior Member

    2 pad eyes with a line between is lighter and cheaper than a track, and more versatale.

    The line is run thru a rope loop attached to the foot of the sail above..

    On tacking the line&loop will work to tension the luff so the boat does not loose way ,

    and when the tack is completer the sheet can tension the part of the sail that overlaps the mast.

    Almost effortless efficient tacking.

    If your wallet is big, using screw in removable pad eyes could save a toe stubber.
     
  11. Turnpoint
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    Turnpoint Junior Member

    Hi Michael,
    I wouldnt do the boom on a modern catamaran. You will be a lot hapier with a non overlapping blade jib on a track in front of the mast base. The track gives you a lot of sail control options...and you can get a lot of power from that blade jib on a reach by dropping the traveler while maintaining sheet tension.
     
  12. gonzo
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    gonzo Senior Member

    The sheets could be connected to the boom. A simple line traveller allows the jib to change sides. Then, when the boom crosses, a line attached to it will snub or trim the jib sheet.
     
  13. Turnpoint
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    Turnpoint Junior Member

    I am sure the boom would work ok and is a great option on a displacement offshore cruiser/mono. but the traveler will give more twist control between pointing and reaching on a modern blade jib.... And why have the ankle breaking liabilty of the boom up forward when the track will be lighter, simpler, and give more control/power. There were a few big tris that were designed with club footed jibs in the 70's. (I owned one) and the ones that are still sailing got rid of their booms decades ago.
     
  14. philSweet
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    philSweet Senior Member

    Fred, I have used rope travelers on small boats, but it isn't what I call self tacking unless you have a lot of height to work with. You need about as much height as travel in my experience.

    Possible problems with a 60' sized version -

    Flogging during a tack.

    8 tons or so of traveler tension if you catch a wave wrong and throw a couple barrels of seawater into a 500sqft jib.

    Strumming.

    High cut sail. If you want it to self tack you need height. If it is low, you lose too much energy and have to either use traveler control lines or you have to adjust the sheets after tacking.​

    The idea of using arm power to trim a shorthanded 60' cat is a nonstarter in my opinion. The rig should be designed to take full advantage of the strength of modern lines, minimize their exposed length, and pull on them really hard. I'd probably opt for hydraulics on a 60'er. Sequencing the valves gets you push button tacks. Better to have a straight pull of 3 tons on a single part line than to try to rig a purchase system that will end up with 50% efficiency when its all done. You need power outfeed and controlled damping during maneuvers. Otherwise, tacks will begin by starting the lee diesel and furling the headsails, and will take several minutes to complete.
     

  15. FAST FRED
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    FAST FRED Senior Member

    Possible problems with a 60' sized version -

    Flogging during a tack.

    ****Not more than any style of headsail when passing thru the wind

    8 tons or so of traveler tension if you catch a wave wrong and throw a couple barrels of seawater into a 500sqft jib.

    ****Use thinner line , a far cheaper/easier replacement than a track ripped up off the deck

    Strumming.

    *** As the luff begins to work sooner there is less stress and flailing canvas


    High cut sail. If you want it to self tack you need height. If it is low, you lose too much energy and have to either use traveler control lines or you have to adjust the sheets after tacking.

    Not from illustrations I have seen on older Dutch commercial boats for the past 150 years. Of course they were not into deck sweepers as radar to see fwd had not been devised.
     
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