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  #11  
Old 04-28-2013
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Re: Forces on a dockline while docking....

If you are concerned about rope strength, 3/4 inch would be fine. We uses 7/8th polly rope in the Panama canal and two lines held about 100 000 lbs of boats being pushed my a 3 or 4 knot current.
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  #12  
Old 04-28-2013
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Re: Forces on a dockline while docking....

it seems to me the manuever is not only about absorbing motion (energy) into the rope (creating friction and heat) but about changing it's direction. By using the rope as a pivot, it not only absorbs a lot of the boats momentium, but because it is redirecting it, increases the friction on the hull from the water itself.

It's all very simple and elegant really.

you could probably do the same with a rope pretied off to bow and stern cleats and a waiting (high strength) hook attached to the piling in some fashion. As you sail by, clip in to the hook and let it do the work
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  #13  
Old 04-28-2013
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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.
Re: Full Throttle....My inclination in having to make such a landing would be full throttle in reverse on approach to try to check/minimize the forward speed. I suspect the neighbor was doing that and only shifted to forward after the line was secured to hold the boat to the pier.

Re: 40 kt. tailwind....what would have happened if someone fumbled the line....how many seconds to get it right on first try, then how many, if any, to recover the line and try again in 40 kts.? After the fumble, what happens and how expensive? And with the high wind, any wave action to bang the boat against the pier? Was the landing absolutely necessary or were there other, less risky alternatives?
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Old 04-28-2013
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Re: Forces on a dockline while docking....

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Originally Posted by kellysails View Post
We have used this product and love it! It creates a huge easy to use loop.

dockingstick.com - Home (watch the video on cleat use)
Ditto on the positive impression of the dockingstick. The most difficult aspect of springline employment is being able to snag a cleat as you enter the slip. the docking stick makes it damn near foolproof.
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Old 04-28-2013
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Re: Forces on a dockline while docking....

Quote:
Originally Posted by NCC320 View Post
Re: Full Throttle....My inclination in having to make such a landing would be full throttle in reverse on approach to try to check/minimize the forward speed. I suspect the neighbor was doing that and only shifted to forward after the line was secured to hold the boat to the pier.

Re: 40 kt. tailwind....what would have happened if someone fumbled the line....how many seconds to get it right on first try, then how many, if any, to recover the line and try again in 40 kts.? After the fumble, what happens and how expensive? And with the high wind, any wave action to bang the boat against the pier? Was the landing absolutely necessary or were there other, less risky alternatives?

No he was definitely in forward full throttle. The reason was that the wind wasn't exactly on his stern, it was slightly quartered. He was at full throttle so he could have maximum steerage.

What would have happened if I wasn't there? The wife, gets off and secures the cleat, that's how they've done it before in other hairy spots during their circumnavigation. They trusted me as a line handler, but I don't trust strangers to handle lines.

What happens if they fumble the cleat? THOUSANDS OF DOLLARS OF DAMAGE and possibly injuries. The reason I was impressed was that they were 100% committed. Really stupid/lucky or really good... still not sure but since they've been around the world and they did pull it off like it was no big deal, I'm leaning towards very good.

Me, I've done this kind of thing on a side tie, where I could afford to miss. I wouldn't have tried the stunt they pulled. I'd rather anchor on the nasty side of the breakwater and enjoy a crappy night's sleep.

The version of this trick that I do is much less desperate. Usually dead slow or up to 2-3 knots max. Still, I'd like to know the forces because I'm a geek like that.

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Old 04-28-2013
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Re: Forces on a dockline while docking....

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

I would be sure to use as stretchy a line as possible to absorb the impact. I would think either plait or three strand. I don't think this is a good use of double braided. You might want to put in some sort of rubber snubber on it to help make it a bit more comfortable.
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Old 04-29-2013
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Re: Forces on a dockline while docking....

Quote:
Originally Posted by miatapaul View Post
I would be sure to use as stretchy a line as possible to absorb the impact. I would think either plait or three strand. I don't think this is a good use of double braided. You might want to put in some sort of rubber snubber on it to help make it a bit more comfortable.
Yup, the piling might be the limiting factor!

Since the snuber is going to add ~ 4-6 inches of stretch and the line is going to stetch 4-6 feet on this catch, the rubber doesn't affect the math. Rather like folks that put a bungie cord in a handline trolling set-up; the bungie stretches 18 inches, while 150 feet of fishing line stretches 15-30 feet!

If the line were smaller, I might even suggest climbing rope. BTW, climbing ropes are thin braid cover with 10-14 layed inner strands carrying the load; they are not double braid. One more bit of trivia; the inner strands are typically 50% RH and 50% LH to prevent rotation.

I would be comfortable with any nylon line of the correct size; the differences are not that great and the faster handling of braid might be more important.
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Last edited by pdqaltair; 04-29-2013 at 05:56 PM.
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Old 04-29-2013
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Re: Forces on a dockline while docking....

Quote:
Originally Posted by miatapaul View Post
I would be sure to use as stretchy a line as possible to absorb the impact. I would think either plait or three strand. I don't think this is a good use of double braided. You might want to put in some sort of rubber snubber on it to help make it a bit more comfortable.
A stretchy line will also store energy. If that cleat breaks loose from the dock, the more energy stored in the line the more will be transferred to the cleat, making it a several pound metal projectile. (google: Disneyland-boat-accident-death)

This whole maneuver is asking for trouble. If you miss the cleat, you crunch your boat. If you get the loop over the horns of the cleat, but don't get you fingers out of the way, you could easily lose a finger (or more). If the cleat parts ways with the dock, you have a projectile and a still moving boat.

Docking rule #1: never approach the dock faster than you're willing to hit it.
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Re: Forces on a dockline while docking....

The moving boat has "momentum." Momentum is simply mass times velocity, or kg times meters per second. It could be expressed as pounds per miles per hour also. Momentum of a 12000 kg boat moving at 2 meters per second (4mph) is roughly 24000kg or 52800 pounds of momentum. Any 1" nylon line would have broken in an attempt to stop this vessel, assuming the piling didn't break off first.
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