Bungee And Blocks

Discussion in 'Sailboats' started by SuperPiper, Dec 28, 2009.

  1. SuperPiper
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    SuperPiper Men With Little Boats . .

    What do I need to know about shock cord and turning blocks?

    The rudder on my pocket keelboat uses a bungee cord to keep the blade from kicking up. The cord snakes up out of the rudder head and tethers at the tiller. I'm not familiar with anyone turning bungee around a block. Is it done? Has anyone made a multi-part tackle with bungee? Are there design rules? What is the ratio of relaxed length to loaded length?

    Jello and bungee: the wonder materials of the future.
     
  2. Doug Lord
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    Doug Lord Flight Ready

    SP, bungee stretches to about 1.5 times its original length. You can buy polyurethane cord(and fittings) that stretches 2 times original length. I've seen Rave foils(wand) rigged with bungee thru a block two to one.
     
  3. ancient kayaker
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    ancient kayaker aka Terry Haines

    The stiffness would vary with length between the blocks, stiffest when hauled in and springier when let out. If that is likely to be a problem then a fixed length bungy bundle in series with a conventional block system would be advisable.
     
  4. PAR
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    PAR Yacht Designer/Builder

    I have a few setups that require blocks and bungee cords, but it's usually just a turning block or cheek block to redirect the energy fairly and not completely necessary, as a fair lead will likely serve the same.

    The concept of using a bungee to hold down a kickup rudder is very common and involves an "over the center" approach. If you understand that concept, they you're on your way to resolving the issue(s). Tackle on a bungee makes little sense. If you want more leverage, just employ a thicker cord.
     
  5. SuperPiper
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    SuperPiper Men With Little Boats . .

    Actually PAR, the top of the rudder acts like a quadrant and the bungee pulls it against a stop to keep the rudder in the full-down position. The shock cord pulls the top of the rudder aft to drive the bottom of the blade forward. The bungee comes out at the back of the rudderhead and slides over a radiused cut in the wooden fabrication towards the tiller. I sometimes find myself tugging and pulling to get the shock cord to apply its load to the rudder, rather than to the friction of the wooden slide. I would like to install a block at the back of the rudderhead to ease the friction.

    I didn't know if the shock cord would deform and bind going over the block. I can imagine in my noggin that the bungee will flatten going over the sheave. I was wondering if there was a multiplier for cord diameter vs sheave groove width.

    Doug, I appreciate the 1.5:1 info. That helps me a ton.

    More later as this exciting project progresses.
     
  6. gonzo
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    gonzo Senior Member

    Pole lunchers use bungees on blocks, sometimes multiples, with no problem.
     
  7. gggGuest
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    gggGuest ...

    The flatter the profile of the sheave then the better, but it really makes very little difference.
    Shock cord downhauls tend to be inappropriate on fast boats because at speed the drag stretches the shock cord more and the blade tips aft, greatly increasing the steering loads. If you're really unlucky this can break pintles etc. If the boat isn't fast enough for this to happen then no worries, but "fast" is a very variable term in this context.
     
  8. PAR
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    PAR Yacht Designer/Builder

    Personally, I wouldn't bother with bungee cords, but use a small diameter double braid and a "break away" cleat. The cleat will release if the line receives a shock load, plus it will hold the board down or can be easily cast off, such as getting up the ramp.
    [​IMG]
     
  9. TollyWally
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    TollyWally Senior Member

    That's a cool little deck ornament! Does the line stay locked?
     
  10. SuperPiper
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    SuperPiper Men With Little Boats . .

    Shock cord or break-away cleat. A fundamental question.

    Instead of tethering the shock cord to the tiller, I was considering turning it through a fairlead and attaching it to something in the cockpit directly under the tiller. Then, the shock cord would act (a little) to pull the tiller toward centre when unattended, like when tied to the dock or during the 1.038 seconds in the middle of a tack that I am looking for the windward sheet.

    This scheme would fit the 1.5:1 ratio that Doug quoted. And NO, it would not be a self-steering rig.

    The rudder blade would get a little DOWN bump with every tack or by pumping the tiller up away from the bungee. I think it's kinda clever, myself.

    Is this similar to anything commercially available? Can I learn from the catalog?
     
  11. wet feet
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    wet feet Senior Member

    PAR has it right.A CL257 will do the job and the makers could do more to market them as it seems that very few people are aware of their existence.Its hard to believe that there are still people using lumps of lead to hold rudders down because they have no idea about the use of shock cord.Its even more difficult to understand why those who are aware of the way to rig shock cord would do so when a rope downhaul terminating at a CL257 is so much more positive.
    As for bumping the rudder up and down with every tack,it makes no sense to me.Just set the system up with the blade in the most efficient position and go sailing.
     
  12. SuperPiper
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    SuperPiper Men With Little Boats . .

    Shock cord or CL257? Analog or digital? I'll have to let this one percolate for a while. Good discussion.
     
  13. PAR
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    PAR Yacht Designer/Builder

    Wet feet, is correct, in that you want your blade at the proper location, below the boat, without potential for moving around as it stretches shock cord (or other wise). There's one place, below the hull where the helm will balance the way you (the skipper) wants it, if the blade moves, the balance is going to be off and you have two choices, move your butt to rebalance the helm or move the blade to rebalance the helm. This is a pretty half assed method.

    A solid line down haul solves the issue easily and securely with the break away cleat. They're offered by a few different suppliers, some times called "auto releasing" cleats. They hold the line fast as a regular "jam" cleat does, but under a shock load they release and spill the line from the jaws. It's still attached, particularly if a stopper knot is employed and can be quickly reset. The model shown above is adjustable. By this I mean the amount of tension it needs to release is adjustable, which is a handy feature.

    You can order one here http://www.duckworksbbs.com/hardware/cleats/sd002570/index.htm and tell Chuck (the guy who owns the site) that PAR sent you.

    Or you can continue playing with bungee cords.
     
  14. SuperPiper
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    SuperPiper Men With Little Boats . .

    Yeah, But I Tried It Anyway

    And it failed miserably.

    The bungee did a good job of keeping the tiller on centre. It would have provided excellent feedback for trimming the sails rather than sailing with weather helm. But the bungee did not do a good job of keeping the rudder down. No matter how much tension on the shock cord, a little finger could push the bottom of the rudder away from its full down postion.

    It was as if the shock cord did not do a good job of transmitting force. The bungee could be taut at the tiller but still somewhat slack at the rudder. I deduced that the length of cord from the turning block to the rudder had its own dynamics independent of the tension in the length of cord at the tiller.

    It was a 5mm bungee and seemed to provide the right load for centring the tiller. Therefore, I did not think to go to a larger bungee for keeping the rudder in its down position.

    CL257?
     

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  15. wet feet
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    wet feet Senior Member

    There have been tens of thousands of racing dinghies built to use shock cord rudder downhauls.Those that I know of include Enterprises,GP 14's and Wayfarers.Those which I have encountered used 3/8 inch shock cord (10 mm) and no more than a foot of it,using rope to make up the length.With such a short length and using most of the stretch to hold the rudder blade in place,the resistance to moving goes up pretty rapidly.The system works reasonably well at modest speeds but less well at planing speeds and this has a lot to do with the development of the CL257.They just work very well,and if you use Spectra/Dyneema rope you have most of the advantages of a fixed rudder combined with the safety feature of knowing that your transom should still be there if you hit something.
     
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