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post #30 of Old 05-01-2013
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Re: Forces on a dockline while docking....

Originally Posted by Seaduction View Post
Good engineering my friend! I didn't account for the 0.95 seconds of deceleration, but then it was cocktail time also.
I redid a calculation using a boat mass of 11250 kilograms and deceleration time of 0.95 seconds and calculated about 28912 Newtons required or 6500 pounds approximately. Half inch nylon line might not stop the "Capt. Ron" docking style.
(I think I'll stick to my old fashioned methods.)

For the sake of the exercise, Sampson ProSet 3 is nearly linearly elastic over the range of 0 to 20% elongation with loading varying between 0 and 10% of breaking strength. If one needs "average" loading over the range of elongation of 5,793 lbs, at zero elongation the loading is zero and at maximum permitted elongation the loading would need be 11,586 lbs to achieve that average. Note, however, that the line is linearly elastic--in other words, once the yacht comes to rest--all of it's knitic energy has been converted to potential energy in the line (think stretched elastic band) and, unless a constant load is applied, or the elongation of the line is relieved, that puppy's gonna be shot backwards about as fast as it came in. The energy in the surge line has to be burned off as heat, meaning the line has to be tightened and then loosened quickly or snubbed by a knowledgeable line handler. On cannot simply drop the end of the line from a piling over a cleat and hope for the best.


"It is not so much for its beauty that the sea makes a claim upon men's hearts, as for that subtle something, that quality of air, that emanation from the waves, that so wonderfully renews a weary spirit."
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