Dyneema and knots

[mrming]

Time to replace our jib sheets with something a little thinner as the current 12mm affairs are overkill.

Boat is a reasonably racy 23 footer with a 15 sq/m headsail.

Ordinarily I would choose dyneema, but...

The boat has a babystay. Any type of knot tends to get hung up on there, so we use one long jib sheet attached to the clew with a cow hitch.

Tying knots in dyneema apparently reduces it's strength, so should I stick with good old fashioned braid on braid? (Loops and soft shackles are out due to the aforementioned babystay).

 

[Judders]

Do you ever change your headsail?

 

[mrming]

Do you ever change your headsail?

Yep it's hanked on and we have a #1 and #3 so the sheets are taken off after every sail.

 

[lw395]

I've heard it said that a bit of stretch in the jib sheet improves gust response.
But if you have say 10mm dyneema, if the cowhitch weakens it by 75%, it should still be strong enough.
It might be more critical to choose rope that works well on the winches?

 

[Woodlouse]

All ropes are weakened by knots. Find the chap who told you it was a particular issue with dyneema and give him a slap for all our sakes.

 

[lw395]

All ropes are weakened by knots. Find the chap who told you it was a particular issue with dyneema and give him a slap for all our sakes.

it is more of an issue with Dyneema and other low stretch fibres.
Because the stretch is so low, a tightly bent rope does not share the load among all the fibres, those on the outside of the curve take all the load and break.

 

[DFL1010]

You could try a centre take-off:

(although sized up a bit from those diameters)


Or, a bit more heavy-duty:
http://www.chicagoyachtrigging.com/spinnaker-sheet-y-bridle/


I'm guessing you don't do headsail peels upwind, i.e. it's changed downwind when the kite's up, or between races (or before leaving the berth).

 

[PotatoNavigator]

Because the stretch is so low, a tightly bent rope does not share the load among all the fibres, those on the outside of the curve take all the load and break.

....or the those on the inside I would suspect.

 

[Judders]

I'm not sure what it is that you're concerned about. The rope is very unlikely to suddenly break. Yes, cow hitching it on will weaken it but not to such an extent that it's likely to break anytime soon.

We had new sheets again this year, we've gone thinner every year for the years now. We went dyneema and what as in the last two years I've cow hitched the bite onto a soft shackle, this year we are back to bowlines, simply because it's easier to explain to people and we don't need a separate change sheet.

 

[Woodlouse]

it is more of an issue with Dyneema and other low stretch fibres.
Because the stretch is so low, a tightly bent rope does not share the load among all the fibres, those on the outside of the curve take all the load and break.

My point is that all ropes are greatly weakened by tying them. Dyneema might take the largest reduction in strength, but it is still likely to be stronger than a more conventional line tied in the same manner.

When working with the riggers from THL from Falmouth many moons ago they said, from their own bench tests that a bowline weakened dyneema the most, whilst a cow hitch saw the least drop in strength so as far as the op is concerned I think he can cow hitch his new sheets on without fear of them snapping.

 

[Birdseye]

Time to replace our jib sheets with something a little thinner as the current 12mm affairs are overkill.

Boat is a reasonably racy 23 footer with a 15 sq/m headsail.

Ordinarily I would choose dyneema, but...

The boat has a babystay. Any type of knot tends to get hung up on there, so we use one long jib sheet attached to the clew with a cow hitch.

Tying knots in dyneema apparently reduces it's strength, so should I stick with good old fashioned braid on braid? (Loops and soft shackles are out due to the aforementioned babystay).

6mm dynema has a breaking strain of 2.2 tonnes. Even if you halve that by tying a knot its still way more than any pull from a 25 sqm sail. And 6mm would be a pita to handle anyway - go up to 10mm and the rope is string enough toi lift the boat easily.

 

[lw395]

My point is that all ropes are greatly weakened by tying them. Dyneema might take the largest reduction in strength, but it is still likely to be stronger than a more conventional line tied in the same manner.

When working with the riggers from THL from Falmouth many moons ago they said, from their own bench tests that a bowline weakened dyneema the most, whilst a cow hitch saw the least drop in strength so as far as the op is concerned I think he can cow hitch his new sheets on without fear of them snapping.

I can believe that, the cow hitch is a fairly gentle knot in that the rope is bent through a larger radius than a bowline where it's wrapped around one part of its own diameter.
When you tie dyneema or kevlar to solid objects it can be more unreliable than ordinary polyester.
I see this with dinghy halyards, they fail close to the knot after a few months use, despite being at a small fraction of their breaking strain. Polyester will stretch a lot before breaking, dyneema not much.
That allows the polyester to adapt or from itself to what it is tied to, while the dyneema gets stressed.
Jib sheets I'd be happy to use knots, but some other things I'd splice.

 

[vyv_cox]

6mm dynema has a breaking strain of 2.2 tonnes. Even if you halve that by tying a knot its still way more than any pull from a 25 sqm sail. And 6mm would be a pita to handle anyway - go up to 10mm and the rope is string enough toi lift the boat easily.

Exactly. I was tensile testing Dyneema earlier in the week. The load it will take is incredible, there is no way the force on a jib sheet on a small boat is going to break it, even with the world's most inefficient knot (whatever that may be).

The best way to attach the sheets is to use a simple splice, just inserting the Dyneema through itself for a distance of 100 - 150 mm. All the samples we tested were made up in this way and the splices were always stronger than the single length of line between them. I realise this is not the fastest way to change headsails if the sheets are always the same ones but it is the strongest method.

 

[lw395]

Exactly. I was tensile testing Dyneema earlier in the week. The load it will take is incredible, there is no way the force on a jib sheet on a small boat is going to break it, even with the world's most inefficient knot (whatever that may be).

The best way to attach the sheets is to use a simple splice, just inserting the Dyneema through itself for a distance of 100 - 150 mm. All the samples we tested were made up in this way and the splices were always stronger than the single length of line between them. I realise this is not the fastest way to change headsails if the sheets are always the same ones but it is the strongest method.

150mm is a bit of a short splice for anything bigger than 5mm dyneema. I found this out the hard way! Might be OK on something that never goes slack.

 

[mrming]

Thanks all - sounds like the dyneema in a cow hitch will be just fine.

 

[Keen_Ed]

Lots of good stuff on this thread about knots in dyneema.

http://forums.sailinganarchy.com/index.php?showtopic=154025

 

[Ingwe]

If you want to stick to using a similar attachment method to a cow hitch but want to be able to change sheets more quickly have a look at Tylaska spool shackles, we have been using a P4 size one (they are less than £20) on our 8-10mm continuous jib sheets for about 5 years they have all the advantages of using a cow hitch but crucially you can remove and the re-attach the sheets without having to re-thread them, they also work very well on continuous asymmetric spinnaker sheets (but obviously in a smaller size) . I suspect Tylaska don't advertise them because they would rather you use 2 x £120 snap shackles to attach your sheets with!

 

[Keen_Ed]

Why have any hardware at all to bash in the mast? Go with a soft shackle.

E.g. here: http://www.bethandevans.com/pdf/improvedsoftshackle.pdf
or: http://l-36.com/soft_shackle_9.php

 

[Woodlouse]

If you want to stick to using a similar attachment method to a cow hitch but want to be able to change sheets more quickly have a look at Tylaska spool shackles, we have been using a P4 size one (they are less than £20) on our 8-10mm continuous jib sheets for about 5 years they have all the advantages of using a cow hitch but crucially you can remove and the re-attach the sheets without having to re-thread them, they also work very well on continuous asymmetric spinnaker sheets (but obviously in a smaller size) . I suspect Tylaska don't advertise them because they would rather you use 2 x £120 snap shackles to attach your sheets with!

They occupy the same amount of the tylasker catalogue as the snap shackles. I was going to mention them but don't have practice experience of them so can't really endorse for jib sheets.

 

[vyv_cox]

Why have any hardware at all to bash in the mast? Go with a soft shackle.

E.g. here: http://www.bethandevans.com/pdf/improvedsoftshackle.pdf
or: http://l-36.com/soft_shackle_9.php

Yes, by far the best solution. Cow hitch tightens so much that it becomes almost impossible to undo and the whole length of both sheets needs to be unthreaded and rethreaded. A soft shackle can be undone in seconds, doesn't do any damage to the mast, stays or you and, crucially for us, passes the baby stay easily when tacking.