Why not Spectra ?

[Joe Henderson]

Dear Brian,

Why not spectra indeed!
For a while I have been thinking that the majority of classic boats could benifit from fibre rigging. Brion Toss raised this question a while ago in one of his Fairleads columns I think.

About ten years ago I was lucky enough to work from the begining on an IMS Maxi with a very open-minded project manager who was happy for me to spend some time and money on ways to use new fibres in the rig. (This was before Vectran, PBO or high grade Spectra was widely available here in Sydney) One of the things that was bothering us was the creep we would experience in the proposed Spectra core runners and checkstays. The standing backstay was not a worry.
I got over the majority of the creep by pre-stressing the runners to 5000kg and serving and coating them with the load still on. When they were fitted to the boat we had about half a percent elongation from fully loaded to just taut.

We tried Vectran, when we could get some, in the same configuration and the elongation was considerably less, about a quarter of a percent, but the stuff failed under load where it had taken a sharp bend around the leech of the main. Very exciting!

We never found a definitive reason for the failure but I reckon that a bundle of fibres, closely bound together and bent backwards and forwards is a prime candidate for compression and fatigue failure of some of the fibres. In fact now that I think about it, if you wanted to break some Vectran in the laboratory, that is how you could go about it! The uncertaity over the life and toughness of the stuff was the only thing stopping us from using it as diagonals. I did not like the idea of a jib clew thrashing the stays.

Having said that, I can see out of my loft window a pair of my runners that I made out of Kevlar core on the mizzen of a 60 year old classic ketch and they are in good condition after 10 years. There are also miles of fibre rigging around the world on things like radio masts etc. So who knows how long this stuff lasts?

I did a bit of experimenting (limited by time and money and the need to race the boat) with Salvagee strops during the Diagonals trials and I am glad to see Future Fibres have succesfully solved most of the problems I came up against.

After all the above, I like the idea of Vectran, PBO or high grade Spectra in a nice stable braid to accept the torsion put in by the seving, pre loaded and served for standing rigging and spans for a classic yacht.

I think you would have to be careful using drop-over eyes with anything other than Spectra due to the above failure mode, but maybe a looser serving will allow the fibres to slide over each other and take a fair stress during the initial shake down.

My mad South African mate in England has just put together a hard chine Mini transat type hull hull with Carbon spars and gaff rig using Kevlar core because no one wanted it and it was cheap (very good stretch characteristics though) and drop-over eyes and dead eyes and lanyards. Even with the added material for serving and coating it is still cheap and light and with correct re-enforcing at the hounds, stresses the carbon spars kindly without tangs or metal fasteners. Just as you do for timber spars.

Regards,
Joe Henderson