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-   -   Forces on a dockline while docking.... (http://www.sailnet.com/forums/general-discussion-sailing-related/98938-forces-dockline-while-docking.html)

MedSailor 04-28-2013 11:48 AM

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

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.;)

SloopJonB 04-28-2013 12:03 PM

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

Originally Posted by MedSailor (Post 1022422)
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.;)

E=Mc\2 ? :D

MedSailor 04-28-2013 12:53 PM

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

Originally Posted by SloopJonB (Post 1022423)
E=Mc\2 ? :D

I hope not!!!:eek:

https://encrypted-tbn1.gstatic.com/i...L0Nz7-Me4aOC4g

pdqaltair 04-28-2013 12:55 PM

Re: Forces on a dockline while docking....
 
1. This is more about energy absorption than strength. While you could plan on easing the line and do this with steel cable, I think you want to build a passive system. I'm betting he slipped some line, though.

2. What is the maximum load you want on the cleats and line? I don't know your boat.

3. The length of the rope matters. Ropes will absorb X energy/ft.

4. Speed. Convert to feet/second.

4. The energy of the boat is about 1/2(M/32)V^2. Pounds and ft/s. At 5 ft/s this is about 12,000 ft-pounds. at 5 knots this is more like 35,000 ft-pounds.

5. The below is the maximum for GOOD nylon rope to survive a dozen times (empirical testing). You'll need larger. The risk of too large is that the forces go up fast. Knots and sharp turns reduce strength.

7/16"---300ft-pounds/ft. Peak load about 2000 pounds.
1/2"---400. Peak load about 2700 pounds.
3/4"---900. Peak load about 6000 pounds.

Assuming 30 feet of rope, 1/2" line will do it at 5ft-sec, but 3/4" rope will be needed at 5 knots. A conservative sizing would be 1" inch and peak loads would be over 8000 pounds! A big range of answers. If anything breaks someone gets hurt. I wouldn't dare this above 3 knots, and you shouldn't need to. In that case, ordinary dock lines are about right, if they are in new condition.

----

Unless fusion or fission are involved, that is E=1/2 MV^2.

Tempest 04-28-2013 01:05 PM

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

I think you might be over-thinking this. And I'm too lazy to do any math right now.

1st...I don't know why you need throttle at all when the wind is astern. I'd be in neutral with just enough way on to steer. ( approach a dock at the speed you're willing to hit it). Sounds like you do that already.

As for your neighbor: Suppose you missed his line handoff...or worse got your finger caught against the cleat. what does he do when you're not there?

Once you pick up your line, you can then put it in gear (idle) and it should tuck you into the finger dock and hold you there while you attach your other lines. Your normal docklines should be able to manage this nicely if they are properly sized to begin with. I'm sure they are ;-)

kellysails 04-28-2013 01:31 PM

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

For our 24,000 lbs sailboat the line and loop are composed of six feet of 3/4inch, 8 plait high quality nylon line. I would not want to go in any faster than 1kt though.

Seaduction 04-28-2013 01:45 PM

Re: Forces on a dockline while docking....
 
1 Attachment(s)
If the rope breaks while "full-throttling" in to the dock:

SloopJonB 04-28-2013 02:13 PM

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

Originally Posted by pdqaltair (Post 1022435)
4. The energy of the boat is about 1/2(M/32)V^2. Pounds and ft/s. At 5 ft/s this is about 12,000 ft-pounds. at 5 knots this is more like 35,000 ft-pounds.

Looks like the pilings will be the limiting factor. :)

pdqaltair 04-28-2013 02:18 PM

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

Originally Posted by Tempest (Post 1022439)
As for your neighbor: Suppose you missed his line handoff...or worse got your finger caught against the cleat. what does he do when you're not there?

That is the most frightening bit. For example...

Some marinas send line handlers that can manage any botched approach. I can think of one marina that has a nasty current across the docks that must really train the hands.

Some marinas send line handlers that you are better not using at all. I've had line handlers let lines run that I clearly and specifically asked that they simply secure so that I could power against them. No trouble, but confusing until I realize the problem.

I can't see trusting unknown help. In the PO's example, they may simply not understand that your intention is to apply 2-tons of force in a few seconds and try to hand-hold, believing that is better.

MedSailor 04-28-2013 02:20 PM

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

Originally Posted by kellysails (Post 1022451)
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)

For our 24,000 lbs sailboat the line and loop are composed of six feet of 3/4inch, 8 plait high quality nylon line. I would not want to go in any faster than 1kt though.

Interesting.... Most of the docking gadgets are total crap in my opinion. This one might be simple enough to actually work.... I have a lot more interest in single handed docking now that there's a kiddo aboard.

PDQ, the "magic line" as we call it, is about 15' long as to be long enough to reach the cleat easily but short enough to stop the boat before I reach the end of my slip. I was thinking of attaching it to a SS climbing bolt hanger and those are rated to 10K lbs. Surprisingly high loads you've calculated out there...

MedSailor


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