Tuesday, September 27, 2016

Line Selection--Is Low Stretch More Myth Than Function?

Clutching a bit of frayed line in our fist, we trudged to the back of the chandler, where we are met by a wall of multicolored rope spools, ranging from working-class polyester double braid to the most exotic low stretch line with a trademarked name. Some are expensive, and some are breathlessly expensive. I'm not a Luddite. I like Dyneema for soft shackles. But I'm also not convinced that vendors--the folks that are competing for your hard-earned money--are always the best source of information regarding what you actually need.

At the heart of the question is deciding when low stretch is better, and when low stretch is worse. Define the benefit. I've recently been involved in a testing program reviewing jibe force reduction devices. One clear lesson has been that low stretch is not always better.

Jibe Force Reduction Devices. [I'll be reporting on these more later.]
  • Nylon traveler line (8 mm dynamic climbing rope)
  • Nylon rope (sheets and pendants)
  • Gybe Tamer (shock absorber for the mainsheet tackle)
  • Boom brakes (a climber's figure-8 and several other devices)

[From West Marine. Other vendors have similar recommendations--I'm not picking on anyone in particular]

Line Type                            Stretch at 20% Breaking Strength
Nylon dynamic climbing rope                    14 %
Nylon double braid                                   8.5 %
Polyester double braid                               2.5 %
Dyneema and other high modulus              less than 0.6%

Halyards. Reduced weight aloft, reduce chafe, improved sail shape are strong arguments in favor of high modulus halyards. Laminates and other low stretch sail materials demand low stretch halyards. Polyester working sails, on the other hand, are less demanding and can be served by polyester halyards. Spinnakers don't require low stretch halyards for shape maintenance, but spinnaker halyards are notorious for chafe caused by halyard pumping under load. Thus if you have polyester sails you're probably fine with polyester halyards, but laminate sail demand high modulus halyards and high modulus spinnaker halyards will experience less chafe.

Replacing a wire halyard with fiber is a great application for high-modulus line. I replaced the wire main halyard on my Stiletto 27 with a 2:1 Kevlar rope halyard very successfully. Although Kevlar has been largely replaced by other fibers because of its tenancy to fail from bending fatigue, main and jib halyards were always a good applications, because flexing is limited, sheaves are typically large (they were sized for wire, which also requires large sheaves), and it neither stretches nor creeps. Do be careful to de-burr the sheaves; I skipped this step and ended up chafing a spot near the end.

Traveler. Several years ago I found myself with a tattered polyester traveler line and a bit of surplus Spectra line of exactly the right size, a leftover from a testing project. I installed it without giving it much thought, assuming it would be a nice upgrade. Wrong. In fact, no matter how careful I was in my mainsheet management, it was exactly like jibing against a brick. I then read about Evans Starzinger's use of climbing rope for traveler line. He had sailed halfway around the world and was quite satisfied with both sale shape in the reduction in shock loading. I replaced my traveler line with 8 mm nylon dynamic climbing rope and have been extremely satisfied with the result. Because traveler lines are nearly always controlled by a tackle, the line load, even to windward is not very high. Stretch after the strongest gusts is only about an inch. On the other hand, during a rough jibe the traveler remove as much as 3-4 inches to leeward, absorbing damaging impact force.

I wrote about dynamic travelers here.

Practical Sailor reported on climbing rope for travelers, a good practical application.

Climbing rope is optimized for impact loads.

>Vang. Next in importance to the traveler line, in terms of absorbing shock and a rough jibe, is the vang. The mainsheet is typically a multipart tackle and cannot be expected to stretch much. On the other hand, stretch in the vang can allow the boom to rise just a few inches, absorbing critical impact force. Under normal sailing conditions, little stretch in the Vang may allow the boom to lift just a tick in a gust, perhaps a desirable thing. Thus, polyester double braid, again, seems the rational choice. The wrong choice, is to pair high modulus line in both the vang and the traveler; surefire recipe for a mangled gooseneck.

I think it's funny that mainsheet and genoa sheet are typically placed in the same classification. Because sheet tension largely defines genoa draft, any stretch compromises sail shape. Mainsail draft is largely controlled by cut and outhaul tension. Mainsail sheet loads are typically 2-3 times lower, and typically this is a tackle, reducing line tension another 3-8 times. A mainsheet is shock loaded during jibes, but the genoa sheet is not. Very different.

Mainsheet. Because main sail shape is less affected by sheet tension than a genoa (the boom provides outhaul tension) it is more tolerant of stretch. It would also seem that because the mainsheet is typically controlled by a tackle that stretch would matter less, but this is not true. <0 .6="" nbsp="" p="">Stretch is always proportional to length. If you move a clutch farther from the load, the line will stretch more. If you add a tackle the line is also longer. Stretch is proportional to load. The same load carried by a tackle will carry less load per strand and thus stretch less for a given length of line, but about the same amount overall, because the ratio of length/purchase is unchanged. This may be counter intuitive, but it is the reason that nylon is unacceptable even in multi-part purchases. Polyester double braid seems a rational choice choice because of the slight shock absorption it provides in jibes. High modulous line makes sense for racer, for the increased control it provides, but it should always be paired with either nylon (better) or polyester traveler line.

Genoa sheet. My boat has two genoa sheets; one inside the shroud for close hauled work, and one outside the shroud for reaching work. I use Dyneema core line for the inside sheet and polyester double braid for the outside line. While it is critical that genoa sheets not stretch at very high load, the reality is that typically there is only 5-10 feet of line between the clew and winch, allowing little space for stretch. On the other hand, just a few inches of stretch can significantly increase the draft of a genoa during a gust, which is exactly what you don't want. Bottom line: this can go either way, depending on your budget and sailing style.

Avoid Kevlar blends. They are subject to failure due to fatigue. I had a massively over strength genoa sheet rupture in light winds, the result of years of flogging at the clew.

This sheet was cow hitched to the genoa clew. The failure was at the not exit, where it was subject to flexing. Somehow, the sunburn polyester cover held together even after the core had failed completely.

Spinnaker sheets. Stretch is not a problem, at least not the small amount a polyester sheet will give. A little give actually improves sail stability in lumpy conditions. What does matter is weight on the clew. Thus, always down size sheets; if the genoa sheet is 1/2-inch, the spinnaker sheet should be no more than 3/8-inch and perhaps 5/16-inch, depending on how much wind you like to carry the chute in. Lighter line is good. Soft shackles can also save important ounces on asymmetric chutes, also reducing snags, metal flying about, and accidental opening risk (if you are using a squeezer or furler, as most cruisers do, there is no need for quick release).

I'm not a big fan of Sta-Set X. Although it is a good value in terms of stretch versus cost, it's an abomination to coil and is prone to kinking if used as a sheet or even as a Hilliard when a sail is dropped quickly. Some folks like it, I don't.

Likewise, I dislike single braid for running rigging. The cover tends to be loose and I find it tends to snag on rigging and hardware. It doesn't always like cam cleats or line clutches very well. Some folks like it, I don't.


What lasts the longest? You pay your money, you make your choices. Curiously, high-tech line often does NOT last as long as polyester, because the determining factor is most often the polyester cover. Because the cover will slip against the slick core and bear much of the load, covers on high modulus lines generally do not last nearly as long as those on straight polyester line. Time and again, I've had the cover tear loose from the core of Dyneema line. If your racer, maybe you don't care about replacing lines every few years. . As a cruiser, I bet you do. I like to go fast, but I also like to go reliably and economically.

This feels a bit like a rant because of the way I've departed from conventional wisdom. On the other hand, I've sailed a lot of miles and, to the best of my knowledge, this is the truth about running rigging for 30- to 40-foot boats.

1 comment:

  1. Just a minor correction, it's Evans Starzinger, not Evan.