next »

[simon_jones]

We plan to use SK78 for caps, D2's and lowers and leave the main shrouds wire. Does anyone have any experiance with this, as one point of veiw that I heard suggested that SK78 creeps or needs a while to bed in then is fine. while someone else said to stick with wire. HELP

[Born Slippy]

We use 4mm D12 dyneema from an eBay shop in Germany (~87p a m). The braid did bed in under load and need a tighten after a while. This is also called elastic-creep by someone making up semi technical words (i.e. I'm fairly sure he is not a material scientist or a Cherub sailor). We overcome this by never letting the uppers off and so after the initial stretch it really is not a problem.

Will's tests suggest this is similar to using a 2mm ish wire in stiffness, but weighs less. Although more wind drag. For the lowers I would suggest 6mm or 4mm doubled at the very least.

Disclaimer
After talking with people over the weekend and crawling around several rigs I wouyld describe BGM's knowledge on the art of rig tuning as "infant". Over the next few weeks we hope to learn to crawl.

[john_hamilton]

is this the supplier you were talking about at the nationals???

http://cgi.ebay.co.uk/DYNEEMA-ROPE-4-MM-100-MTS-Available-5diff-Coloures-/330377180599?cmd=ViewItem&pt=UK_Sporting_Goods_Sailing&hash=item4cec03b5b7#ht_514wt_911

[Born Slippy]

Thats the guy we used before.  Price has gone up

Hayley found it was too slippy for spini halyard. Our uppers, trap lines, kicker and cunno use it.

[john_hamilton]

awesome
may buy some

[Will_Lee]

Don't forget Dyneema usually means SK75, which is usually considered too stretchy and creepy for most rigging applications.

Chemistry (not rigourous):

Dyneema/spectra is polythene pulled out so the molecules are more lined up and I think the polymerisation process may be better than ordinary polythene so you get more carbons in each chain. Each unit is a carbon and two hydrogens (CH2).

SK78 is 500,000 of these, which is 15% of a mm long (this is my own calculation, not from a source, so may be wrong)
Presumably SK75 presumably is less, SK90 more.

Antidotes uppers are 3mm SK78. The lowers are 5mm SK78. The D2s are 3mm SK78 but 5mm may be better.

The forestay is 3mm wire (though could be SK78 or probably SK75, and people have made these from vectran too), as are the shrouds. This is really because we lacked the bottle to keep the mast up with rope.

The shrouds are 3mm wire, and 3mm SK78 is far less stiff than this, though 5mm SK78 is about as stiff as 3mm wire. I asked Davro whether people use 3mm wire for its strength or its stiffness, and the historical message seems to be that there were many breakages with thinner wire shrouds. This suggests it is the strength of 3mm wire rather than its stiffness which has made everyone standardise on this in recent years, though thinner shrouds may also have been too stretchy of course. In turn this suggests that, because 3mm dyneema seems to be stronger then 3mm wire (~600kg vs ~1300kg in our test), then 3mm SK78 would be ok for shrouds.

That said, we were not too happy with holding the mast up with rope that thin. We bought a load of 5mm SK78, but is so huge I don't think that is suitable. Not wishing to seem like a fudger, if 4mm SK78 exists then it would seem like a good compromise!



SK78 (CH2

[john_hamilton]

thanks for that insight into the best set up rig i have ever sailed under will, i shall take heed, thoug i also think ill lack the bottle to use rope for main shrouds and forestay

does SK78 have a trade name, like SK75 has dyneema

edit:
is this the stuff you used: http://www.marlowropes.com/index.php?option=com_content&view=article&id=78:excel-d12-max-78&catid=51:dinghy-windsurfing&Itemid=231

also, how/what are antidotes control lines set up/made of

[Will_Lee]

I think they are all types of dyneema.

[Tim Noyce]

Will is right, SK75 and SK78 are just grades of dyneema.

More geeky info can be found here. http://en.wikipedia.org/wiki/Dyneema

Will, do you still have the PBO rigging, or did you replace it with SK78?

I'd personally go wire for mains at least as they are cheap and lets you get the mast up with the right rake, then you'll have a better idea of how long the others need to be.

[Phil Alderson]

I am currently using 5mm dynema almost certainly SK75 for the fore stay on Pocket Rocket. My thinking was use 5mm to get back towards the stiffness of wire, and as it is inside the luff of the jib for most of its length it does not affect windage.

[phil_kirk]

Rope rigging does present the issue of chafe and while there are a number of methods used to overcome this I suspect you would have to replace rope rigging more often that wire.  Due to the creep efect it may take a while longer to get the rig set up perfectly.  Just saying that weight, stiffness and windage are not the only consideration.

This forum is maintained by UKCCOA for more information visit http://www.uk-cherub.org
If you have any comments or queries then please Contact Us.
Powered by SMF 1.1.14 | SMF © 2006-2011, Simple Machines LLC
Template based on the Blueace design by Crip | XHTML | CSS