Forces on a dockline while docking.... - Page 4 - SailNet Community
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post #31 of 47 Old 05-01-2013
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Re: Forces on a dockline while docking....

A spring line is a wonderfull tool but an old Maine salt advised me to never approach a dock at a higher speed than I would be pleased to ram the dock.
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post #32 of 47 Old 05-01-2013 Thread Starter
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Re: Forces on a dockline while docking....

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A spring line is a wonderfull tool but an old Maine salt advised me to never approach a dock at a higher speed than I would be pleased to ram the dock.
That point was brought up a couple pages ago. What about when the wind is high and blowing you off? Sometimes speed (water flowing over the rudder) is the only way to maintain any control.

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post #33 of 47 Old 05-01-2013
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Re: Forces on a dockline while docking....

OK.... I'm just not buying it anymore! Coming into dock at full throttle with a 40 knot tailwind. Good grief, what gives here???
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Re: Forces on a dockline while docking....

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Originally Posted by MedSailor View Post
That point was brought up a couple pages ago. What about when the wind is high and blowing you off? Sometimes speed (water flowing over the rudder) is the only way to maintain any control.


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I would recommend changing you approach angle...

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post #35 of 47 Old 05-01-2013
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Re: Forces on a dockline while docking....

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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.)
Funny, I covered all of this in post 4, based upon actual dynamic rope testing. Yes, I know the ideal calculations (high school physics), but the reality, including sizing the rope so that it can survive this punishment dozens of times, is better satisfied with testing. Good that we all came to the same, rather intuitive, numbers. Slow down, use something bigger than 1/2-inch. In fact, the answer turns out to be a standard length, standard weight spring line. How about that; it's good when established practice and theory match.


It's a shame that rope manufactures have not established a common method for dynamic testing and fatigue ratings that they would like to publish; I suspect they don't want to be judged in public, at least not enough of them to form a consensus standard. The testing would also be a good deal more expensive. Climbing ropes are tested in this way; in fact, they do not carry strength ratings, because it is less relevant (it's about controlling fall energy, not strength).
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post #36 of 47 Old 05-01-2013 Thread Starter
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Re: Forces on a dockline while docking....

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Originally Posted by Seaduction View Post
OK.... I'm just not buying it anymore! Coming into dock at full throttle with a 40 knot tailwind. Good grief, what gives here???
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.

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post #37 of 47 Old 05-01-2013
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Re: Forces on a dockline while docking....

How would you have approached getting into the slip under the same conditions?
About the time my bow got close to the slip I would have been in full throttle... only reverse..... to stop the boat, knowing the 40K tailwind would be pushing me into the dock, then after securing a stern spring, used forward throttle to keep the vessel alongside the dock.
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Re: Forces on a dockline while docking....

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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.

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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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post #39 of 47 Old 05-01-2013
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Re: Forces on a dockline while docking....

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

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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.

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