Thursday, February 11, 2010

Soft Shackles - Modern Art & Craft

If you look at a modern high tech racing boat you'll find exotic fibres sprinkled all over it. Boats are even using exotics in place of things like chain plates and fibre rigging is making a come back. All that makes tremendous sense when you are in a high tech racing campaign where every gram of weight counts and strength per unit size is ultra important.
What's that got to do with traditional wooden boats though? Well the answer is "everything and nothing"! My boat displaces about 8 tonnes on a 29 foot waterline - a lightweight flyer she isn't. However our spinnaker block is an Equiplite spectra block - it's very light and what's more important it won't damage the mast varnish by banging on it as a normal block would. It also has massive reserves of strength.
The spinnaker is a great big 840 square foot asymmetric. The thing about a traditional style boat is that it comes, ready made, with the bowsprit and all the trimmings to fly an asymmetric. Our J measurement is 18 feet on that 29 foot waterline so we can carry a big spinnaker with ease. The real purpose of that sail is for light winds - from basically 0 to 15 knots apparent. After that we feel like it belongs below. The apparent wind angles range from 50 to 130 degrees in light winds through to 65 degrees to 160 degrees at the higher end. But what we really like it for is light winds. With an asymmetric in order to sail deep you need to ease the tack line and get the luff of the sail to rotate out to windward. Again the heavier the pressure the deeper you can sail.
However at lower pressures you can still get good outcomes if you can get the weight out of the running rigging. The less weight the sail has to lift - either from the sheets or from the tack line and shackle the more likely it is to fill and set. So that sets us on a path to get the weight out of the gear and that's where exotics come in. We want sheets that are light, strong and which don't absorb water. There's nothing worse than having the sail collapse, the sheets getting wet and then making it hard for the sail to set again. So that means spectra sheets with the covers stripped or single braid spectra with a non-wettable cover added in the way of the winches.
Elsewhere on the boat we still want "hardware" that isn't hard on the boat. Lots of manufacturers are now offering soft "shackles". These are great - they weigh nothing and are very strong - but they are also very expensive. USD$90 for a shackle is too much for me.
My first attempts are in the photo below:
Top to bottom: 1 & 2 use a brummel eye splice and a double figure of eight stopper
3 uses a slip eye and double figure of eight
4 uses a slip eye and a carrick bend stopper
Each of these shackles is incredibly light and potentially incredibly strong. The breaking strain of the line is about 5,300 daN or about 5.3 tonnes. Subtract say 30% for the knot and then halve that for a safe working load and you still come up with a working load of around 1.85 tonnes (and yes I know that daN doesn't quite convert to tonnes but it's near enough). I say potentially because I haven't tested these yet. I'm waiting till I perfect them.
The one I really like is option 4: Options 1 and 2 rely on a fixed eye and whilst I think it is secure it could possibly be induced to work loose. Option 3 is OK but the lengths and tensions are not quite right. Option 4 is the best because not only are the tensions right but the slip eye is highly secure as you can see in the two close ups:

This photo clearly shows the slip eye. The braid is formed into an eye, one side of the eye is passed through the centre of the other side of the eye. It's then passed back out and the two ends formed into the carrick bend to form the stopper.
Under pressure the outer braid grips the inner braid and even at very low tensions it is impossible to either open or close the eye. To open the eye the outer braid is bunched slightly and the eye slid open.

The cover bunched and the eye slid open
Time to make one of these is about 15 minutes, materials cost about AUD$8.00. It's a bit hard to find a new shackle when one breaks half way across the ocean but it's easy and simple to make one of these.
Just to clear up a couple of misconceptions: Spectra/Dyneema has extremely good abrasion resistance and extremely good UV resistance. The spinnaker turning blocks on our boat are attached to the bulwarks with some strops made of this stuff. They've been out in the sun for about 3 years with no signs of any degradation and the only thing that's being abraded is the bulwark.
All I have to do now is to find a better way of finishing the carrick bend so that the ends don't show (any suggestions?) and then put a couple on a test rig.


  1. Hello, I'm also making and useing these soft shackles. They're fun to make and very handy.

    To get rid of the ends you can use the diamond knot, that's a carrick bend with the ends going through the middle.

    I think this is the knot that is used by the liros shackles.
    After you've pulled the knot very (!!) tight you wind the ends with insulation tape just outside of the knot, and cut them with a hot knive so they're more or less flush with the knot.

    I hope this makes sense (sorry for my English).
    Good luck,


  2. Hi Hans,
    no problem with your English at all! Thanks for the comment. I have moved to the Diamond knot after I did the shackles shown here, it works well doesn't it?
    I am also experimenting with the Oysterman's Stopper. Perhaps you might like to try that, it's a very good stopper.

  3. Hello CA,

    The oysterman stopper knot might be safer because there's less change of slipping. It's not as symetrical as the diamond knot. Don't know if that's a problem. It's quicker to make. Thanks for the hint, I'm going to try !

    Do you happen to have the knot book from Geoffrey Budworth ? Your use of the double constrictor knot seizings and the oysterman stopper seems to point in that direction....

    Regards, Hans

  4. edit for my last post :

    I tried the oysterman stopper and it's unsuitable if you want to trim the ends short/flush. The knot will fall apart completely if the ends slip through.


  5. Hi Hans,
    I don't have that book - it sounds good.

    The thing about not being able to trim the ends is interesting. I'm not sure that I want to trim them. The main reason to trim them seems to be aesthetics and I don't know if that's a good enough reason. Also the whole issue of the stopper - whichever knot you choose - coming apart is interesting. I am working on setting the stopper by apply tension. This stuff is so slippery that I think there is real danger of the knot simply undoing. That's why I like the Oysterman's Stopper because I think it may be less likely to undo than the Diamond knot. However it does use a lot of line.
    Thanks for the interesting discussion.