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Sail & Rigging - 44 backstay
15 October 2015 - 17:16
#1
Join Date: 01 October 2013
Posts: 33

44 backstay
SAFA's original backstay should be replaced and I have been thinking dyneema rope instead 10/19 wire. The breaking strength is enough, it's cheaper and the work can be done without professional rigger's hands. Has someone experiences, opinions etc. using dyneema backstay ... ? By the way, has someone measured 44's long rig backstay length?

Markku Silvonen / Swancountry

16 October 2015 - 20:06
#2
Join Date: 02 January 2008
Posts: 1547

Dear Markku
Comparison of wire and rope based on breaking strength has been discussed in a Forum thread "Dyneema rope for checkstay" dated 14 Aug. 2014.

The rope needs to be spliced each end, and a professional may be needed for this. The splices cause about 10% strength reduction. 12 mm Dyneema appears to have suitable strength here, and the weight is 20% of the wire.

For a backstay it is also necessary to look at the elastic stretch under load, as there is some difference between wire and rope. The backstay is about 19 m long, and 10 mm wire stretches 0.25% or 47 mm under 21 kN of working load.

12 mm Dyneema rope stretches more, the manufacturers give somewhat varying numbers. If 0.38% is assumed, this is 71 mm - nearly the double - and it can be concluded that this can affect the required stroke for the backstay tensioner.

If one choose to use 14 mm diameter the stretch will be reduced to 0.27%, practically the same as for the wire, but the wind resistance will go up. The weight is then 28% of the wire.

If kept under constant load Dyneema will creep, i.e. slowly extend, but there are grades available where creep has been eliminated.
Kind regards
Lars

18 October 2015 - 19:51
#3
Join Date: 01 October 2013
Posts: 33

Thanks for information Pro. The 14 mm dyneema seems to suit best. Chafing is also no problem in backstay. Splicing is one of my skills too (one of the few ...) so let's do it!

Rgds, SAFA_sailors

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