(the Davis Industries sort--this may not apply to the rubber wrap sort. They are probably also too light duty for boats over a certain size, though I suppose you could double them up. This could also be accomplished with a second thin nylon line or other elastic mechanism.)
I always through these were a gimmick or something for pontoon boats on a lake. But now that I have tried them and looked at some of the successful uses in my marina, I'm changing my view, at least for boats <35 feet.
But I don't think they are for absorbing impact. In a real storm that is too much to ask. The steady load can exceed the elastic capacity. I think they are for controlling slack, particularly in tidal
marinas without floating docks.
I did a bunch of testing for PS on
dock line forces a while ago, and the
single clear lesson was that controlling motion with spring lines dramatically reduced forces, even when the load was not shared by the springs. A few days ago I visited my marina in the midst of 40-knots, to see how the
new boat was riding. In fact, she was dancing pretty wildly, because the slip is a poor fit (too long) and because I can't fit springs easily (trimaran with a bad fit in the slip). Instead of trying to add springs, I simply added a
Dock Shockle to the leeward side, eliminating rebound, and she quieted right down. The
wind force was not that great (40
knot gusts--probably only 200-300 pounds without impact), but rebound was the problem, causing chafe.
At first I tried a pair of Mini-Shockles on a light wind day. However, at 40 knots they were not enough. I switched to a Dock Shockle, and that worked. I would also definitely use the Dyneema sling attachment method rather than clove hitches; the line is not weakened and they are easier to adjust. I only put them on one line, on the protected side of the slip.
Given the length of the lines, I can now set dock lines with Shockles at zero slack, and the stretch will easily manage a 4-foot tide swing. This is also not the same as just pulling lines tight; the
boat can still shift easily enough. There is never much load on the Shockle, and the loop of
rope takes any overload. If I cut the Shockle, my slack would simply revert to normal; I did not change the line length adjustments.
I talked to some other folks in the marina that use them, and they all use them the same way and for the same reason. They hold up for many years, the boats move less, and the lines chafe less. They are a little expensive, but not if the dock lines last longer.
Thoughts?
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