HOT INTERFACE ZONE: PART 4 by Danilo Fabbroni
We should mention that the introduction of exotic double braids had their negative counterparts not only seen in higher prices.
An exotic core put together with a polyester cover often ends up being an unbalanced line under stretch. Typically first lines made with spectra core almost all suffered a negative effect of building a significant amount of slack into the cover. A loose cover is highly prone to be snagged by every sharp point, with the result of damaging the line, and/or even stopping the sailing manoeuvre. This was due to polyester and spectra having different amounts of stretch at the same load. The former elongates much more than the latter. If the peak load at which the line is put on strain is within the hysteresis (elastic stretch recoverable over time), once the load is off, both the cover and the core will go back to their original length. If instead the given load will exceed the threshold of hysteresis, the result will be a polyester cover longer than its spectra core counterpart. This over-length is seen as a slack of the cover.
Also with spectra and dyneema lines it must be taken into account that the core, being made with a very slippery material like high modulus polyethylene, is very difficult to bind to the polyester cover, and hence the cover tends to slip towards the direction of the load because the cover is not able to transfer the entire load on it. To try to solve this problem rope manufactures tried to add a third braid in between core and cover, but this complicated the splicing method and above all reduced the amount of spectra core at a given line diameter. Yale and Southern Oceans Ropes1 were the first to minimize the problem by hard coating the spectra core with a chemical dye in order to achieve a better cohesion between core and cover. Also, making the cover on the core when this one is put under load and heat treated does help the former to transfer the load on the latter. Worse than this, and on top of this, there is the creep or a cold flow which is typical of spectra and dyneema. These materials if loaded under a constant static load, like a halyard during an offshore race, suffer consistent elongation due to molecular slippage. Again a very different type of stretching from that seen in polyester.
With spectra and dyneema the point of no return in creep is relatively easy to reach because the material has a very low melting point (critical is 150°F while melting happens at 297°F). In fact this unsatisfactory behaviour is minimized by the latest release of spectra, the DSK 78 version, which is the best so far.
That said, we should not forget the other part of the market featuring OEM boats where locks are not still in use and cordage specifications are very basic. This market deserves a thorough study about how to cope with rope and gear mashing without using high technology and hence high priced materials and fittings.
Surely, jammers should be combined with budget minded lines but nonetheless with covers which can stand the stress.
One condition that should be respected in choosing lines suitable for jammers is firmness: select ropes that are firm and round and hold their shape during use. Soft or mushy ropes will snag easily and abrade quickly causing accelerated loss of strength. Abrasion and over heating are two rope killers. Deck hardware must take this into thorough account if the pleasure of going sailing is to be retained in all of its awesome magnitude.
That's the final post in this series - please let me know if you have any questions!
Danilo
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1 Well known new zealander cordage maker.
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