Dyneema String Theory

[maxr]

The Karver KJH handle mentioned above thoroughly disgraced itself at Rutland on Saturday by snapping in two under only moderate tension.

Perhaps you need a 'windsurf downhaul tensioner' ? This is simply a length of alloy rod with two holes drilled in it close together, and designed for tensioning smallish lines. You put you string through the holes, apply both feet to an immoveable object and pull mightily. There are also side entry varieties made of solid alloy with a ClamCleat style line grabber cast in. There's nothing to break unless you're Desperate Dan and capable of snapping the whole thing in two - £7-ish on Ebay

Max

[Graham W]

Perhaps you need a 'windsurf downhaul tensioner' ? This is simply a length of alloy rod with two holes drilled in it close together, and designed for tensioning smallish lines.

I've not heard of one of these before - perhaps it could also be used for braining Mackerel?

I still use my handy billy, which requires very little effort to exert a strong downward force on halyards.  But instead of the cheese-like KJH handle to grip the string, I use an icicle hitch in a second piece of string - cheap and easy to learn with the help of the Animated Knots website.

[Peter Taylor]

I've got Dyneema shrouds but a stainless forestay.  I recently adjusted the shrouds again since they had (again) got longer.  I specified Dyneema since, having to lower the mast to go under a bridge both going out and coming back, they are so much easier to handle than wire.  I tension the rig using a bottle-screw on the forestay and also using a winch to tighten the jib halyard.  I don't get the jib luff as tight as I would like but wonder if it's possible with a carbon fibre mast which doesn't have spreaders. 

I wouldn't go back to wire shrouds (because of the bridge!) but somewhere I read that you shouldn't use dyneema for standing rigging because of the creep and instead should use some other exotic (and no doubt expensive) synthetic rope.  Trouble is I can't remember where I read it, and riggers I've talked to say dyneema is what they use.

Peter

[Graham W]

I read that you shouldn't use dyneema for standing rigging because of the creep and instead should use some other exotic (and no doubt expensive) synthetic rope. 

Maybe Vectran?  Even more expensive than Dyneema.

Does Dyneema eventually stop creeping when it reaches a certain level or does it uncreep when you're not looking?

[Peter Taylor]

Vectran it was... I've found this quote from the "Rig Shop" web site...

"The abrasion resistance is 10-12 times better than that of Aramid, reducing internal rope damage. A unique property of Vectran it its resistance to creep. It doesn't! At loads up to 50% of break (far in excess of normal working loads), Vectran displays no creep, even at high temperatures. This is a feature that no rope making material can match including steel."

I haven't yet dare check the price but I think I will be saving my pennies for when I need to replace my shrouds!

Since the mast is permanently raised (except when under the bridge!) I don't know whether the dyneema would "uncreep", nor do I yet know whether it has reached a limit!

Peter

[Graham W]

Another topic raised from the dead.

Over on another forum, there's an account of a sudden and unexpected failure of 5mm Dyneema which was being used on a cascaded backstay arrangement on an average white yacht.  The consensus was that this was probably caused by heat generated by friction because of a seized-up block.

This then developed into a discussion about the physical properties of Dyneema, which is alleged to liquify quite readily if sufficient heat is applied.  Certainly true if cut with a heated rope blade but then all strings melt when faced with that.  What really caught my attention was the suggestion that small diameter Dyneema and "low" friction rings are not really the best combination in bits of rigging which can move under high stress.  That's got me thinking about places on my boat where this is the case.  So far, I've come up with my modified mizzen outhaul arrangement and my jib boom topping lift.  Probably something to be aware of rather than a major worry, as a boat with failures in either area can still sail perfectly well.  On the other hand, what about Dyneema soft shackles?

Dyneema used to be miracle string but as we use it more, its weaknesses become more apparent: it doesn't stretch but unless specially heat-treated it can creep, which can be irritating; it can degrade quite quickly under strong UV (not a problem in my part of the world at present) unless specially coated; even bog-standard untreated Dyneema string is expensive; and now this.