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How ultra-high-molecular weight thermoplastic polyethylene improved sailing

2018 July 1


I witnessed an enormous spinnaker acting like a parachute-type sea anchor built for a battleship being pulled through the water on a kite string by a sailboat driven by strong wind and big waves.

The only exaggeration in that sentence is that it wasn’t really a kite string. It just looked like one.

The string was a spinnaker sheet made of high-tech rope so skinny you would have expected it to break the instant the shrimping spinnaker filled with water. I not only expected it to break, I wanted it to break. Desperately.

The spinnaker, victim of a take-down disaster in a near gale, was doomed. Half of it was in the boat when the other half filled with water and hundreds of square feet of red nylon flew out of the companionway, nearly taking three crewmembers with it. 

Recovery was not an option. Getting free of this monster drogue wasn’t working out either, thanks to the aforementioned sheet, which had fouled in a turning block. The decision to cut it wasn’t hard; executing it wasn’t easy. It took a lot of sawing with a sharp stainless steel knife.

The subject here is the marvelous boon to sailors that the invention of polyethylene fiber rope has been, but first let me digress to a tip on cutting this stronger-than-steel material. 

Ordinary sailing knives won’t cut it, at least not quickly or easily. Blades with sharp serrations get tangled in the fibers. Obsidian and ceramic knives, the sharpest in the world, will cut it, but the glass blades are fragile, not to mention dangerous to human digits. For a practical solution, check out the one-hand operated folding knives on the market that are designed to cut safety harness tethers in a man-overboard emergency. They’re not meant for fine work, but several models I’ve tried can cut the toughest line in a hurry.

No matter how you do it, cutting Dyneema rope is easier than breaking it. The smallest available diameter 12-strand Dyneema—one-eighth of an inch—could theoretically lift a medium-size car without breaking. The claimed breaking strength is 3,400 pounds. The breaking strength of a Dyneema line in the diameter of the sheet that towed that ill-fated spinnaker is listed at 13 tons.

Chemistry wasn’t my strongest subject, so I can’t offer much insight into how laboratory geniuses created rope from ultra-high-molecular-weight polyethylene (a subset of thermoplastic polyethylene also known as high-modulus polyethylene) with extremely long chains that strengthen intermolecular interactions, but I can tell you this stuff is a gift to sailors.

It has given us halyards, sheets, guys and tack lines that are stronger, lighter, safer and longer lasting than any that sailors have ever had before. It replaces metal in shackles, lifelines, even, in some cases, in standing rigging. 

In almost every application it is stronger than we need. Quarter-inch Dyneema would be strong enough to serve as a spinnaker sheet on almost any boat, but a larger diameter is used just so it can be easily trimmed by hand or on a winch.

I was reminded of how far we’ve come ropewise when I visited my man cave slash sailing gear storage building recently and bumped into the heavy coil of my boat’s original mainsheet. It’s a five-eighths-inch thick hawser, a polyester artifact of the pre-Dyneema era.

I mentioned stronger than steel. Some of my running rigging artifacts are actual steel—genoa and spinnaker halyards made of wire rope with polyester rope tails. Galvanized cable was used for these beasts because it’s more malleable than stainless steel. Even so, individual wires in the twisted mass were prone to break, creating what foredeck crewmembers affectionately called meat hooks—barbs that left handlers’ hands bloody. Unpleasant to use, extremely heavy and not all that strong, metal halyards were a passing fad for good reasons.

High-tech rope can also be credited with eliminating the need for what many sailors considered to be the most dangerous implement on a sailboat—the halyard reel winch. Sailors of a certain age will recall that all-wire main halyards were once standard because the rope of the time was too stretchy, but using them required a vertically mounted winch operated by a handle locked in place. If the operator let go of the handle it could spin out of control. Broken limbs could result, or worse, as in the case of a friend who suffered grievous facial injuries and a broken jaw.

Chemically conceived rope is truly a sailor’s panacea, but is there anything not to like about it? Well, yes. Call it nitpicking if you want to, but it’s no fun to splice. For those of us who believe that a complete sailor needs at least a passing acquaintance with marlinespike seamanship, this matters.

I know, professional riggers splice this stuff all the time with beautiful results, but for amateurs like me it’s a tedious process, impossible to accomplish without a review of instructions or a diagram, befuddling with its counter-intuitive interaction of the cover and core. 

In delightful contrast, splicing old-fashioned three-strand twisted rope is a rewarding non-cerebral exercise, no measuring, tools or instructions needed, easily leading to the satisfying result of a perfect eye splice.

Fortunately, even in the age of Dyneema, rope like that is still relevant and needed. Three-strand twisted nylon was the best rope for anchor rodes when it first replaced manila and it still is, even though—and because—it is the functional opposite of high-tech rope. Unlike unstretchable Dyneema, nylon is elastic, deriving its strength from its amazing ability to stretch without breaking.

Disciples of the lightweight anchoring school have preached that a small Danforth-type anchor well set in sand and a three-eighths-inch nylon rode can hold just about any vessel lighter than an oil tanker.

That’s an exaggeration, but I have witnessed a skinny nylon rode holding a stout sailboat in a blow. It did the job, but it was scary. The line was so stretched it looked like a kite string.