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  #21  
Old 04-29-2013
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Re: Forces on a dockline while docking....

Med - Our ISPA instructors have to be able to do this single-handed.

The owner of Turicum (the Vic-Maui boat) does it slightly differently. He has a dock line with a hook that attaches to a specific spot on the toe rail.

I do not use full throttle.
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Last edited by jackdale; 04-29-2013 at 07:03 PM.
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  #22  
Old 04-29-2013
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Re: Forces on a dockline while docking....

Quote:
Originally Posted by SlowButSteady View Post
Docking rule #1: never approach the dock faster than you're willing to hit it.
I used to believe this, but that thinking got me in a lot of trouble.

If there is no wind or current, you should approach at dead slow or close to it. Anything more is asking for trouble that you don't need. However, if there IS wind and or current at play, you need to have more throttle (judiciously applied) in order to have more flow (or wash) over the rudder and thus more steerage.

If you're trying to come into a dock, even with 8-10knots of side-on wind and apply above rule #1 the wind will have it's way with you and you'll get to see the consequences of the second part of the rule. At least that's the case on my boat where the windage at the end of the bowspirt is 30-35ft forward of my underwater center of lateral resistance.

Now, I'm no Captain Ron either, charging full throttle into slips that I could approach slowly. My neighbor pulled the stunt I mentioned whereas I would have executed plan B or C. In fact, I have come at my old slip with a similar but less ferocious quartering wind and when I didn't like the way things were shaping up I aborted the landing and poached a side tie until the wind abated.

This technique is usually done at <3knots when I do it, and in the future (with kids occupying the wife) it will be the single-handed method of choice when docking. I just want to make sure that the loads are somewhere near reasonable and that the line (and cleats) are up to the job just in case I had to do it at a higher speed in some kind of emergency.

The fastest I've ever done this was probably 4knots and I used a 1/2" 3 strand line that was probably 20ft long. Nothing broke, nobody died, and it brought us to a halt when things were hairy.

MedSailor

PS I like the climbing rope idea. Perfect application for my line that is about to be retired. PDQ, what do you think, would this be a factor 3 fall?
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  #23  
Old 04-29-2013
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Re: Forces on a dockline while docking....

I often have about a 10-15 kt cross wind (or maybe from just aft of 90 degrees) at my dock. I get the nose of the boat into the slip and then jump off and manhandle her into place. At worst, I can let the fenders do their thing (until I get the windward lines under control) as the wind pushes the boat sideways into the leeward finger. But, I would NEVER approach the dock at anything near full throttle. Even my little 6700 lb boat can have enough inertia to do a lot of damage.
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  #24  
Old 04-29-2013
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Re: Forces on a dockline while docking....

Buy some 3/4" 3 strand and call it good.
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  #25  
Old 04-29-2013
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Re: Forces on a dockline while docking....

Can we have a Google Maps link showing your slip and approach?

Or at least a diagram showing your slip and approach?
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  #26  
Old 04-29-2013
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Re: Forces on a dockline while docking....

Quote:
Originally Posted by MedSailor View Post
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.
It is not a particularly difficult calculation. Google the "Work Energy Theorum" and go from there. The yacht has a kinetic energy equal to its mass x velocity. It's kinetic energy must be consumed/countered. The force necessary is calculated by taking the mass of the yacht multiplied by its acceleration. In this case, the yacht must go from its maneuvering speed as it enters the slip and the surge line is applied as a stopping force to zero speed as its bow reaches the inboard end of the slip and the surge line reaches its maximum extension. If the yacht's speed of approach is say, 5 knots, and its speed upon coming to rest is zero knots, the yacht's average speed over the distance of it's stop will have been 2-1/2 knots as it transitions from approach to landing. Assuming that the surge line extends half the length of the slip, from the end piling to amidships on the yacht, say 20 feet, and the line is nylon and hence cannot be stretched more than 20 percent of its length without rupture, or 4 feet (= .20 x 20), the yacht must transition from 8.43 ft per second (at 5 knots) to zero with an average speed of about 4.215 feet per second. With 4 feet of line "stretch" in the surge line, the yacht will traverse that stretch in, roughly, .95 seconds. The Delta V will be 8.43 feet/second with the Delta T being .95 seconds so the acceleration, which in this case has a negative sign (for slowing) is 8.88Ft/second squared. The required force to effectuate this Deceleration is the yacht's mass (= displacement/32.2 ft/sec^2) x -8.88 ft/sec^2. Let us assume that the yacht displaces say 21,000 lbs. It's mass is then 652.17 slugs and the force required to stop the yacht is about 5,793.30 lbs. The work energy is 5793.3 lbs applied over a distance of 4 feet or about 23,173.20 foot lbs. On the line side of the event, the issue becomes somewhat more complex because the amount of force needed to stretch the line 1% is decidedly different than the force needed to stretch the line 20% and it is the cumulative force moving over the length of elongation that must, in total, equal 23,170.20 ft lbs. Every increment of stretch can be described as a distance dx and the necessary energy to effectuate one such stretch or elongation as F*dx. To elongate the line the second increment, dx2, the force must be incrementally greater which may be, but is not necessarily, linearly related to the first incremental stretch unless the line is perfectly elastic, which of course it is not (tho' nylon in extremis is close). So, one is then confronted with a surge line that is applying a stopping force on the yacht that is not a steady 5,793.3 lbs but is increasing from, initially, zero, to some maximum, with the sum of (or the integral of) the forces applied over each incremental stretch of the surge line totaling 23,170.20 ft lbs from 0 feet to 4 feet. It will take a little pondering but it shouldn't be too difficult to come up with the size line needed from there. Note, however, as someone wisely pointed out previously, all of the kinetic energy of the yacht will be converted to potential energy in the line, which will surely take some time to burn off as heat, although it will, eventually.

FWIW...
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Last edited by svHyLyte; 05-02-2013 at 08:59 AM. Reason: Correct Misspellings
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  #27  
Old 04-29-2013
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Re: Forces on a dockline while docking....

Per Faster,
Quote:
People ferries and water taxis use a similar technique when they make their quick unload/reload stops.. Usually in fwd gear and the rudder turned away from the dock.. Thay basically motor against the spring line holding themselves on the dock.
Have Watched it many times.

My dock neighbor has perfected this technique as he has limited steerage.(he singlehands) I have used at it at the home dock on occasion. (while singlehanding) Once in a while i do come in hot due to weather conditions,BUT, my boat handles extremely well and i can stop her on a dime. I have confidence in my motor and the boat. I go around if i don't think i have it right.
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Old 04-29-2013
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Re: Forces on a dockline while docking....

I based my original post on drop testing and other dynamic tests of rope energy absorption. It will vary from one line to the other. However, This is a case where energy absorption is what matters, not strength, and the test methods are different. Unfortunately, energy absorption capacity drops off with use faster than strength (ropes stiffen), making the calculation more complex and in the end, empirical.

The other important factor that some understand but a few have missed (those reasoning everything will break) is the length of the line. For example, I'm betting I can stop a 175-pound swimmer with a head start with 15-pound fishing tackle; did it once at the local pool just for laughs. It's all about the math.

-----------

Bottom line: a boat-length of standard dockline (3/4-inch for this size boat) will do nicely.
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Old 04-30-2013
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Re: Forces on a dockline while docking....

Quote:
Originally Posted by svHyLyte View Post
It is not a particularly difficult calculation. Google the "Work Energy Theorum" and go from there. The yacht has a knitic energy equal to its mass x velocity. It's kinitic energy must be consumed/countered. The force necessary is calculated by taking the mass of the yacht multiplied by its acceleration. In this case, the yacht must go from its maneuvering speed as it enters the slip and the surge line is applied as a stopping force to zero speed as its bow reaches the inboard end of the slip and the surge line reaches its maximum extension. If the yacht's speed of approach is say, 5 knots, and its speed upon coming to rest is zero knots, the yacht's average speed over the distance of it's stop will have been 2-1/2 knots as it transitions from approach to landing. Assuming that the surge line extends half the length of the slip, from the end piling to amidships on the yacht, say 20 feet, and the line is nylon and hence cannot be stretched more than 20 percent of its length without rupture, or 4 feet (= .20 x 20), the yacht must transition from 8.43 ft per second (at 5 knots) to zero with an average speed of about 4.215 feet per second. With 4 feet of line "stretch" in the surge line, the yacht will traverse that stretch in, roughly, .95 seconds. The Delta V will be 8.43 feet/second with the Delta T being .95 seconds so the acceleration, which in this case has a negative sign (for slowing) is 8.88Ft/second squared. The required force to effectuate this Deceleration is the yacht's mass (= displacement/32.2 ft/sec^2) x -8.88 ft/sec^2. Let us assume that the yacht displaces say 21,000 lbs. It's mass is then 652.17 slugs and the force required to stop the yacht is about 5,793.30 lbs. The work energy is 5793.3 lbs applied over a distance of 4 feet or about 23,173.20 foot lbs. On the line side of the event, the issue becomes somewhat more complex because the amount of force needed to stretch the line 1% is decidedly different than the force needed to stretch the line 20% and it is the cumulative force moving over the length of elongation that must, in total, equal 23,170.20 ft lbs. Every increment of stretch can be described as a distance dx and the necessary energy to effectuate one such stretch or elongation as F*dx. To elongate the line the second increment, dx2, the force must be incrementally greater which may be, but is not necessarily, linearly related to the first incremental stretch unless the line is perfectly elastic, which of course it is not (tho' nylon in extremis is close). So, one is then confronted with a surge line that is applying a stopping force on the yacht that is not a steady 5,793.3 lbs but is increasing from, initially, zero, to some maximum, with the sum of (or the integral of) the forces applied over each incremental stretch of the surge line totaling 23,170.20 ft lbs from 0 feet to 4 feet. It will take a little pondering but it shouldn't be too difficult to come up with the size line needed from there. Note, however, as someone wisely pointed out previously, all of the knitic energy of the yacht will be converted to potential energy in the line, which will surely take some time to burn off as heat, although it will, eventually.

FWIW...
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.)
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Old 05-01-2013
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Re: Forces on a dockline while docking....

Quote:
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.)
Hummm...

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.

FWIW...
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