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Go Back   SailNet Community > General Interest > General Discussion (sailing related) > Forces on a dockline while docking....
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Thread: Forces on a dockline while docking.... Reply to Thread
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Topic Review (Newest First)
05-02-2013 01:49 PM
T34C
Re: Forces on a dockline while docking....

Quote:
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..
05-02-2013 12:12 PM
jrd22
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.
05-02-2013 12:09 PM
Tallswede
Re: Forces on a dockline while docking....

Quote:
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.

Kevin
05-02-2013 11:19 AM
svHyLyte
Re: Forces on a dockline while docking....

Quote:
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.
05-02-2013 10:04 AM
T34C
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.
05-02-2013 09:56 AM
svHyLyte
Re: Forces on a dockline while docking....

Quote:
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...
05-01-2013 09:40 PM
pdqaltair
Re: Forces on a dockline while docking....

Quote:
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.
05-01-2013 09:11 PM
MedSailor
Re: Forces on a dockline while docking....

Quote:
Originally Posted by svHyLyte View Post
Yes. You did. Quite well and quite succinctly. Note, however, that when one is addressing an audience relatively unschooled with respect to a particular bit of esoterica, sometimes it is helpful to illuminate the sequence of the logic so that at least some will "get it". IMHO it is not enough to assume that everyone or even most will "take one's word for it" 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. But then, what do I know...
I appreciate both the theoretical and empirical explanations.

I will also report back if I ever actually end up trying this, Captain Ron style, from the point of view of SailNet's first human crash-test dummy.

MedSailor
05-01-2013 08:11 PM
svHyLyte
Re: Forces on a dockline while docking....

Quote:
Originally Posted by pdqaltair View Post
Funny, I covered all of this in post 4, ....
Yes. You did. Quite well and quite succinctly. Note, however, that when one is addressing an audience relatively unschooled with respect to a particular bit of esoterica, sometimes it is helpful to illuminate the sequence of the logic so that at least some will "get it". IMHO it is not enough to assume that everyone or even most will "take one's word for it" 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. But then, what do I know...
05-01-2013 07:45 PM
Seaduction
Re: Forces on a dockline while docking....

Quote:
Originally Posted by MedSailor View Post
What's not to buy about this? While I would never try to get into a dead end slip with a 40kt quartering tail-wind they chose to do so (and pulled it off).

How would you have approached getting into the slip under the same conditions?

Using a spring line to approach the dock with enough speed for steerage is a technique that I recommend everyone at least consider adding to their toolbox before poo-pooing it. You might need it some day, and it's a pretty standard technique.

T34 Changing approach angles is not always possible. A strong wind blowing from the beam is not likely going to allow you to approach the dock at <1kt (the speed you'd like to hit it at). Is there something else to your change of angle that you can describe for me? I'm having a hard time picturing it.

Medsailor
I guess I can't visualize the situation you witnessed, ie. dock layout and approach, etc. The good outcome speaks to the experience of the captain. I use a bow-spring led aft everytime I dock my own boat at home to do a 180 degree turn around a piling, so I can attest to the efficacy of spring lines with single screw full keel sailboats.
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