What knots for Dyneema?

[JesseLoynes]

Hi folks,

Hoping someone here can help. I have a dyneema bobstay, which seems to loosen however I tie it. The line is only the core of the dyneema, with none of the grippy outer weave.

There is a loop spliced through an eye in the bow, and then a single line from the end of the bowsprit. Last time I tied it, I preloaded the bowsprit with some downward pressure, then tied off the bobstay through the loop with a series of half hitches. It seemed fine to start with, but has gone loose now, without even flying a sail from the end. I think it's just that the dyneema is so slippy.

Any ideas?

I really want to try playing with the big sails for the first time this weekend, but need to make sure this will take the tension first!

Thanks in advance,

Jess

 

[emsworthy]

Hi Jess,

I was about to point you in the direction of this picture as it is the only other dyneema bobstay I know of.... then I read your profile!

Is it Collective Spirit's bobstay you're on about? If not, maybe ask Mark Covell how it was rigged?
View attachment 33912

 

[knuterikt]

Hi folks,

Hoping someone here can help. I have a dyneema bobstay, which seems to loosen however I tie it. The line is only the core of the dyneema, with none of the grippy outer weave.

There is a loop spliced through an eye in the bow, and then a single line from the end of the bowsprit. Last time I tied it, I preloaded the bowsprit with some downward pressure, then tied off the bobstay through the loop with a series of half hitches. It seemed fine to start with, but has gone loose now, without even flying a sail from the end. I think it's just that the dyneema is so slippy.

Any ideas?

I really want to try playing with the big sails for the first time this weekend, but need to make sure this will take the tension first!

Thanks in advance,

Jess

On my bowsprit (before making the bobstay adjustable) i had an eye spice in both ends.
The stay where made a little bit short (on purpose) and I had a lashing made with thin (2 mm) Dyneema between the end of the bobstay and the bowsprit.

If you make enough rounds in the lashing the friction will keep it from slipping,

Make the bobstay so short that you can adjust for creep i the bobstay/lashing

 

[lw395]

Dyneema is not only slippery, but also creeps, i.e. stretches if left under tension.
It can then shrink over time if exposed to UV.
It's easy to splice, I would splice it with a lashing as suggested.
I would guess that it is OK for it to stretch a little, so long as it takes the load before the bend in the bowsprit exceeds a sensible amount?

 

[lpdsn]

Also beware that knots like a bowline which involve tight curves will be much weaker than the line strength. Dyneema doesn't give like polyester so the strands on the outside of a curve take proportionally much more of the load.

 

[knuterikt]

Dyneema is not only slippery, but also creeps, i.e. stretches if left under tension.
It can then shrink over time if exposed to UV.
It's easy to splice, I would splice it with a lashing as suggested.
I would guess that it is OK for it to stretch a little, so long as it takes the load before the bend in the bowsprit exceeds a sensible amount?

My bobstay is in fact made of Vectran which has slightly better creep properties than Dyneema.
Use of lashing will also make it easy to adjust for creep after some use.

 

[JesseLoynes]

Hi Jess,

I was about to point you in the direction of this picture as it is the only other dyneema bobstay I know of.... then I read your profile!

Is it Collective Spirit's bobstay you're on about? If not, maybe ask Mark Covell how it was rigged?
View attachment 33912

Nope, it's for the new boat developed from CS. The bob stay on CS had the outer cover on the line so it gripped much better.

I had another look at my knots, and actually I'm pretty sure it's creep I am dealing with here, not slippage in the knots. I'll re-tie it with load again, and see what happens.

Thanks to all for suggestions, I am seriously looking forward to flying the kite, but will look for a light wind day for first trials... 60 square metres sounds quite large to me. Still, we've had 8.5 knots from jib and reefer main, so she should have some serious get up and go with a spinnaker!