Since I have recommended sailcloth made from high-modulus (low-stretch) fibres for cruising boats, I’m thinking I should expand on that to avoid confusion. Let’s do that:
Back around the end of the time I was sailmaking, a new sailcloth burst on the scene that promised to pretty much banish shape-robbing stretch.
It was made from Kevlar® and it was literally bullet proof1, incredibly strong, and enabled sails to be built that weighed far less than the Dacron equivalents.
So what’s not to like?
I certainly agree that para-aramid (Kevlar, Twaron) and carbon fibre are not ideal for cruising sails. Their slight advantage in a racing situation is vastly outweighed by their high cost and short life.
I would not agree that UHMWPE’s creep has anything to do with this. Creep occurs *very* slowly when the material is under sustained high tension. It is mildly problematic in standing rigging, but in a sailcloth (unless you’re routinely pushing your sails right to the absolute limit of their strength) it would be mostly compensated for without ever noticing it when you adjust the outhaul and halyard tension. Rather, UHMWPE is superior for this because its structure, at the molecular scale, makes it extremely resistant to wear and to chemical attack.
The molecules in para-aramid fibres are (relatively) short. They are attached to adjacent molecules by hydrogen bonds in-plane and by aromatic stacking between planes. If two such fibres slide over each other, as happens when the fabric flexes, they interact strongly enough for some of these molecule-to-molecule bonds within one fibre to be broken. Thus, the fibres wear each other down every time they move. Few things can cut Kevlar more quickly than another piece of Kevlar rubbing hard at right angles.
The molecules in UHMWPE are *much* longer and heavier; 100,000 or more monomers per chain. They interact with each other only weakly, via van der Waals forces. The strength of these molecule-to-molecule bonds is almost negligible! It is only when we stack a hundred thousand of these weak bonds between two perfectly parallel molecules that we get an appreciable total strength. Any one fibre sliding over another fibre simply doesn’t interact with it enough to affect the bonds between the fibre’s molecules. Thus, Spectra, Dyneema, etc. can be bent, folded, and chafed repeatedly with minimal deterioration. And, being aliphatic with no functional groups hanging off the sides of the molecular chains, they’re inherently immune to most aromatic, alcoholic, and water-based solvents.
Hi Matt,
Thanks for the fill and correction. I will take that out, particularly since it does not matter to the point.
That’s a really helpful explanation. Gotta love that sailing straddles the extremely traditional and extremely modern and technical. Thanks for sharing.
John, unrelated but wasn’t sure where to put this: Looks like the link to comments at the top of each article is not longer working. That’s been really helpful to me since the site redesign, especially for checking new comments on posts I’m interested in. Just wanted to give you a heads up! Thanks
Hi Charles,
Thanks for the heads up, but that’s intended behaviour. I’m working on some changes in that area, particularly as it relates to Tips. In the mean time you can always check new comments here: https://www.morganscloud.com/recent-comments/
Hi Charles,
Also don’t forget that you can subscribe to the comments on any article and then get an email any time a comment is added.
DSM presents UV weatherometer data for several fiber types in a graph here.
(There is no information on any coatings or additives that were on or in the fibers tested.)
https://assets.ctfassets.net/q6qgec8ud5tq/2F2XxEQuIxhQnlD9435Ddz/7413e91d3f25447a8f000f94f06aea7d/CIS_YA102_Ultraviolet_exposure_of_UHMWPE_fiber_from_DSM_Dyneema.pdf
Hi William,
Interesting link, thanks for posting it.
After the second seam failure (both during a multi-day cruise) of a 2021 Sobstad laminated jib with aramid structural fibers and a clew radial panel layout (one of Sobstad’s trademarks), we are replacing this jib with a tri-radial design built from Hydra Net. This sail was on our Dragonfy 32 tri when we bought her 2 years ago and I now have no confidence in it for extended hours cruising in any breeze over 12 knots especially at AWA’s of 45 degrees or less.
Hi Bob,
Wow, sorry to hear that, having to replace a 2021 sail is pretty disappointing! It does not bode well for my favourite jib, in the photo above, that came with our J/109.
John,
Don’t worry. I think it has more to do with the clew radial panel layout Sobstad uses rather than a tri-radial layout. I’ve had two sailmakers confirm that to me.
Cheers,
Bob