[MHml] Synthetic rigging

[Dave Culp]

Personal opinion, but I wouldn't recommend spectra for such an
application. Kevlar does not creep--not a bit--and has a much higher
temperature/friction damage threshold as well.

You do not need Spectra's one remaining strength--it's light weight.
An additional few ounces of kevlar belowdecks will not effect the
boat's performance. I would consider using a double braid product,
though, to protect the kevlar from both UV light and friction.

Contrary to popular belief, one cannot "pull out" the creep of spectra
rope. It will continue to creep as long as it is loaded. Dan's
recommendation to pre-stretch the line is a good one--for any line
where fixed length is important, but what you are doing is pulling the
line's weave into conformance; all new line is imperfectly woven, and
will lengthen a bit under first loading--this is not the same thing as
creep. Creep is the actual elongation of the spectra fibers--this will
continue as long as there is load. (Before I start an argument, I
should say that many rope manufacturers pre-stretch their line before
it's sold, making this step unneeded.) However, I would not take any
line to within 95% of its yield strength--ever. Not only will fibers
be destroyed in the process, virtually any knot you might tie into the
line to set up such a pre-stretch will cause the line to fail
catastrophically--with potential for grave injury. I expect Dan meant
to stretch it to 95% of rated load, not breaking strain; ie: 20-25% of
rated breaking strain.

Just out of curiosity, why have you eliminated steel wire from contention?

Dave

On 5/4/07, Dan Frenette <Dan.Frenette at sun.com> wrote:
> If you prestretch these you will get better results. That is about 95%
> rated load over night to get it to creep out. Let's see 24' would be
> about 6" of creep if you regard that as a problem.
>
> Plan B maybe to pull carbon yarn through a line cover or at least shrink
> tube. You can wrap it yourself so you have 1 continuous yarn. No stretch
> and pretty cost effective. Kevlar line is also available to do the same.
> The kevlar doesn't like sun but with a line cover you can reduce this
> problem a lot.
>
> Dan Frenette
>
> Lincoln Baxter wrote:
> >
> > On the subject of creep, I am thinking of using synthetic line for my
> > steering system that I am putting together for my 30 foot cat. I have
> > a Lewmar pedestal with a drag link that I plan to connect directly to
> > a quadrant under the bridgedeck, where the lines for each rudder would
> > attach. There would be a total of about 24 feet of ¼" line for each
> > rudder. Is the constant load or dynamic load stretch likely to be a
> > problem?
> >
> > Lincoln Baxter
> >
> > Zbiggy II
> >
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