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Old 04-28-2013
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Forces on a dockline while docking....

The coolest bit of sailboat seamanship that I was ever witness to was when my next door slip neighbors in Seattle came into their slip with their Moody (45?) ketch with a solid 40kt tailwind.

They came roaring in at full throttle and one crewmember handed me a dockline with an eye in the end and asked me politely (but with some urgency) to please place the eye on the aftmost dock cleat. I complied.

The line in question was run through a foot block at the quarter and then to the primary winch and back to the skipper at the helm. He hauled in the slack and the line stopped the boat. While still powering in forward (because the line was right at the picot point of the boat) it stayed stationary at the dock perfectly parallel to the finger pier.


I've used this technique quite a few times after witnessing this (though without as much throttle or tailwind!) and really like it. I'm planning on making a dedicated line that is the correct length for my dock, but when I got to sizing the line, I wondered, "how much force would be on this puppy?"

I used to feel competent with basic Newtonian physics, but now I'm too rusty. Seems like such a simple equation.... 30,000lb boat moving at x knots, stopped by line that stretches x inches....

Anyone able to help with the calculations? Otherwise I'll just grossly overbuild everything like I always do.
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