To protect a dock line, as well as its belaying points against damage from surge loads, the line must have three things: 1) Stretch least 8 percent stretch at 15 percent loading.

This essentially limits dock lines to nylon rope-strand, 8-brait, or 12-braid; 2) Strength tensile strength that is at least 8 times the maximum load that will be on the line… that means utilizing “big” diameter lines, plus belaying points to match; and, 3) “Stretch length” ­for example, a rope that has 10 percent stretch, but is only 10 feet long will stretch 1 foot, but that same rope, only 40 feet long, will stretch 4 feet… a marked improvement on that rope’s ability to cushion surge loads. For gale force winds, use lines that are at least 30 feet long, and for storm force winds, at least 40 feet in length."

In exposed slips, especially as the weather becomes more severe, the more important these three” percentage of stretch”, tensile strength, and “stretch length” become. Without these features, lines suffer from accelerated wear, chafe, or melting, plus belaying points are at a higher risk for bending or breaking. As an added advantage, lines sized to meet these requirements also perform admirably well in lower wind speeds.

If chafe is a concern, switch to polyester lines or one of the high-tech ropes, though the tradeoff will be too little or no stretch in the line. Double braid, even if polyester, is not recommended as it chafes too easily. And regardless of which line is used, be sure to employ whichever anti-chafe techniques are suitable for the circumstances.

Attaining adequate stretch in a dock line deserves one more consideration … if the boat’s slip does not provide enough room that allows the use of lines long enough to have sufficient “stretch length”, or if “low-stretch” lines, such as polyester, double braid, small diameter nylon, or one of the high-tech ropes are used, some other mechanism that can allow stretch into the line must be employed. One product that can allow for stretch are those big, thick, black stretchy bands often marketed under the name of a “dock line snubber”; there are also others products that can produce the same effect.