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  #41  
Old 05-01-2013
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Re: Forces on a dockline while docking....

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Originally Posted by svHyLyte View Post
... or intuitively understand or appreciate that kinetic energy is equal to the integral of the dot product of the velocity of a body and the differential change of the body's momentum....
Yup, that's one way of making it complicated.

I guess what I'm suggesting, in a way, is a simplification; wouldn't it be nice if ropes were rated for both strength and energy absorption per foot over a few thousand cycles. That matters more, for most applications. It also makes the calculations for dock lines and anchor rodes simpler, since the surge energy is what matters. Two ropes may be of the same strength, but what about fatigue life?

Climbing ropes are rates simply; the number of standard falls survived, with a limit on extension and impact force.
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  #42  
Old 05-02-2013
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Re: Forces on a dockline while docking....

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Originally Posted by pdqaltair View Post
...I guess what I'm suggesting, in a way, is a simplification; wouldn't it be nice if ropes were rated for both strength and energy absorption per foot over a few thousand cycles. That matters more, for most applications. It also makes the calculations for dock lines and anchor rodes simpler, since the surge energy is what matters.....
I agree with this sentiment entirely although, frankly, I suspect most non-technical types might not understand the information and thus apply it improperly. Frankly, I would wager many do not understand the significance of the information presented in this thread though it really can be quite important. Knowing how to use a snubbing line/spring to warp a boat around, or stop her, can be the difference between relief and (serious) regret.

FWIW...
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Re: Forces on a dockline while docking....

Knowing the energy absorption of the dockline might be a great idea, but whats the energy absorption of the cleat on the dock? Or the bolts holding the cleat, etc... Seems like using a better docking procedure might be a better idea since you would eliminate many of those variable.
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Re: Forces on a dockline while docking....

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Originally Posted by T34C View Post
Knowing the energy absorption of the dockline might be a great idea, but whats the energy absorption of the cleat on the dock? Or the bolts holding the cleat, etc... Seems like using a better docking procedure might be a better idea since you would eliminate many of those variable.
Unless I really know a marina's equipment, I will only surge/snub from a piling. I have seen more than one bulkhead/dock cleat ripped out by a snubbing/surge line and with the line under much pressure, they are slung like a rock from a sling-shot.
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Re: Forces on a dockline while docking....

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Originally Posted by svHyLyte View Post
Unless I really know a marina's equipment, I will only surge/snub from a piling. I have seen more than one bulkhead/dock cleat ripped out by a snubbing/surge line and with the line under much pressure, they are slung like a rock from a sling-shot.
Ain't that the truth! I've seen more than one cleat pulled out and a couple launched. Visiting backwater marinas while exploring is a good way to learn alternate docking procedures. Most are not very well equiped with cleats so pilings and other protrusions have to suffice. I really like the looks of that docking thingy for use with a boat hook and after reading this thread will go out and practice other methods besides just going slow.

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Old 05-02-2013
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Re: Forces on a dockline while docking....

40+' sailboat, full throttle in forward, 40 knot tailwind, one line coming tight on a cleat = something is going to give. Unless it's a perfectly straight, unobstructed path from the deck to the dock cleat there's going to be a chafe/break spot, and unless both ends are spliced eyes any knot is going to reduce the breaking strength of the line by about 50%. I suspect that the skipper was allowing the line to slip around the winch to avoid the shock load.
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Re: Forces on a dockline while docking....

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Originally Posted by svHyLyte View Post
Unless I really know a marina's equipment, I will only surge/snub from a piling. I have seen more than one bulkhead/dock cleat ripped out by a snubbing/surge line and with the line under much pressure, they are slung like a rock from a sling-shot.
I've also seen old pilings that were previously thought very strong simply break off..
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