Forces on a dockline while docking.... - SailNet Community

   Search Sailnet:

 forums  store  


Quick Menu
Forums           
Articles          
Galleries        
Boat Reviews  
Classifieds     
Search SailNet 
Boat Search (new)

Shop the
SailNet Store
Anchor Locker
Boatbuilding & Repair
Charts
Clothing
Electrical
Electronics
Engine
Hatches and Portlights
Interior And Galley
Maintenance
Marine Electronics
Navigation
Other Items
Plumbing and Pumps
Rigging
Safety
Sailing Hardware
Trailer & Watersports
Clearance Items

Advertise Here






Go Back   SailNet Community > General Interest > General Discussion (sailing related)
 Not a Member? 


Like Tree17Likes
Reply
 
LinkBack Thread Tools
  #1  
Old 04-28-2013
MedSailor's Avatar
Closet Powerboater
 
Join Date: Mar 2008
Location: Anacortes PNW
Posts: 2,694
Thanks: 109
Thanked 57 Times in 49 Posts
Rep Power: 7
MedSailor is on a distinguished road
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.

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.
__________________

To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.


I have a sauna on my boat, therefore I win.
To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
Reply With Quote Share with Facebook
  #2  
Old 04-28-2013
SloopJonB's Avatar
Senior Moment Member
 
Join Date: Jun 2011
Location: West Vancouver B.C.
Posts: 10,821
Thanks: 58
Thanked 52 Times in 49 Posts
Rep Power: 4
SloopJonB will become famous soon enough
Re: Forces on a dockline while docking....

Quote:
Originally Posted by MedSailor View Post
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 ?
__________________
I, myself, personally intend to continue being outspoken and opinionated, intolerant of all fanatics, fools and ignoramuses, deeply suspicious of all those who have "found the answer" and on my bad days, downright rude.
Reply With Quote Share with Facebook
  #3  
Old 04-28-2013
MedSailor's Avatar
Closet Powerboater
 
Join Date: Mar 2008
Location: Anacortes PNW
Posts: 2,694
Thanks: 109
Thanked 57 Times in 49 Posts
Rep Power: 7
MedSailor is on a distinguished road
Re: Forces on a dockline while docking....

Quote:
Originally Posted by SloopJonB View Post
E=Mc\2 ?
I hope not!!!

Minnesail likes this.
__________________

To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.


I have a sauna on my boat, therefore I win.
To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
Reply With Quote Share with Facebook
  #4  
Old 04-28-2013
pdqaltair's Avatar
Senior Member
 
Join Date: Nov 2008
Location: Deale, MD
Posts: 2,246
Thanks: 1
Thanked 35 Times in 35 Posts
Rep Power: 6
pdqaltair is on a distinguished road
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.
__________________
(when asked how he reached the starting holds on a difficult rock climbing problem that clearly favored taller climbers - he was perhaps 5'5")

"Well, I just climb up to them."

by Joe Brown, English rock climber




To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
Reply With Quote Share with Facebook
  #5  
Old 04-28-2013
Senior Member
 
Join Date: Feb 2007
Location: New Jersey
Posts: 1,993
Thanks: 4
Thanked 50 Times in 49 Posts
Rep Power: 8
Tempest is on a distinguished road
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 ;-)
__________________
Tempest
Sabre 34
Morgan, NJ
Reply With Quote Share with Facebook
  #6  
Old 04-28-2013
kellysails's Avatar
Puget Sound Pounder
 
Join Date: Nov 2008
Location: Bainbridge Island, Port Madison, WA
Posts: 594
Thanks: 16
Thanked 6 Times in 6 Posts
Rep Power: 6
kellysails is on a distinguished road
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.

Last edited by kellysails; 04-28-2013 at 01:38 PM.
Reply With Quote Share with Facebook
  #7  
Old 04-28-2013
Senior Member
 
Join Date: Oct 2011
Posts: 1,325
Thanks: 6
Thanked 24 Times in 24 Posts
Rep Power: 3
Seaduction is on a distinguished road
Re: Forces on a dockline while docking....

If the rope breaks while "full-throttling" in to the dock:
Attached Thumbnails
Forces on a dockline while docking....-long_beach-_ny_-_october_30_a_boat_sits_on_the_dock_20121030172915_640_480.jpg  
Reply With Quote Share with Facebook
  #8  
Old 04-28-2013
SloopJonB's Avatar
Senior Moment Member
 
Join Date: Jun 2011
Location: West Vancouver B.C.
Posts: 10,821
Thanks: 58
Thanked 52 Times in 49 Posts
Rep Power: 4
SloopJonB will become famous soon enough
Re: Forces on a dockline while docking....

Quote:
Originally Posted by pdqaltair View Post
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.
Seaduction likes this.
__________________
I, myself, personally intend to continue being outspoken and opinionated, intolerant of all fanatics, fools and ignoramuses, deeply suspicious of all those who have "found the answer" and on my bad days, downright rude.
Reply With Quote Share with Facebook
  #9  
Old 04-28-2013
pdqaltair's Avatar
Senior Member
 
Join Date: Nov 2008
Location: Deale, MD
Posts: 2,246
Thanks: 1
Thanked 35 Times in 35 Posts
Rep Power: 6
pdqaltair is on a distinguished road
Re: Forces on a dockline while docking....

Quote:
Originally Posted by Tempest View Post
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.
capta likes this.
__________________
(when asked how he reached the starting holds on a difficult rock climbing problem that clearly favored taller climbers - he was perhaps 5'5")

"Well, I just climb up to them."

by Joe Brown, English rock climber




To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
Reply With Quote Share with Facebook
  #10  
Old 04-28-2013
MedSailor's Avatar
Closet Powerboater
 
Join Date: Mar 2008
Location: Anacortes PNW
Posts: 2,694
Thanks: 109
Thanked 57 Times in 49 Posts
Rep Power: 7
MedSailor is on a distinguished road
Re: Forces on a dockline while docking....

Quote:
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)

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
henrysailor likes this.
__________________

To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.


I have a sauna on my boat, therefore I win.
To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
Reply With Quote Share with Facebook
Reply


Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools

 
Posting Rules
You may post new threads
You may post replies
You may post attachments
You may edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Forces acting on mast plate Barquito Gear & Maintenance 25 02-22-2013 04:25 PM
Wow, bad dockline setup.... SHNOOL General Discussion (sailing related) 12 08-09-2011 07:15 PM
transient dockline inventory -single vs. double braid? hazmat17 Gear & Maintenance 3 06-04-2010 09:51 AM
Dockline failure from snubber argofred Gear & Maintenance 24 07-24-2009 05:13 PM
Loads & Forces on a Sailboat FishFinder Gear & Maintenance 6 05-01-2009 04:27 PM


All times are GMT -4. The time now is 10:37 PM.

Add to My Yahoo!         
Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2014, vBulletin Solutions, Inc.
SEO by vBSEO 3.6.1
(c) Marine.com LLC 2000-2012

The SailNet.com store is owned and operated by a company independent of the SailNet.com forum. You are now leaving the SailNet forum. Click OK to continue or Cancel to return to the SailNet forum.