When and why sweating a dockline DOESNT work - Boat Science!
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- เผยแพร่เมื่อ 10 ก.พ. 2025
- A few questions from a previous video got me wondering- how much force do we generate when sweating a dockline? Most people know it as a 'force multiplier', but how much force does it create? Is there a limit to when it works? What wind speed does it work up to, before it suddenly doesn't? I decided to do some digging- grab the notepad, calculator, loadcell and dockline. We're going outside to play!
Sweating a dockline is a technique used to help our weak bodies (relatively speaking) handle lines under load and create more tension. Commonly used by sailors to adjust lines at the dockside or pull a boat closer in if its blown off in stronger winds. A great technique, but where are its limits?
It may be winter but there is fun to be had!
Hopefully this just opens a fun conversation to join in on, how can we make this testing more accurate? are we on to the right tracks with the stretchy line? is it a red herring? How can we get more force out of the lines? We want to know what you guys think? What do we need to include next time?
I hope you enjoy watching as we had fun making this one! The maths, equations and such contained within are our best guess. We can't guarantee they are accurate, so please don't use it for anything serious where you might get hurt.
Anyway, thanks for watching.
In the next week or so I will add a write up on my old blogsite-
www.bensgame.com
If you have watched and enjoyed and are feeling generous, our buymeacoffee link is here below, we appreciate anything no matter how small-
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Great topic. Best video yet. We just had high winds in our marina. My wind gauge was measuring 20 to 25 knots. Yet at the bow tie, in the most direct winds I was able to easily undo and fuss around with the bowline. When there are gusts there are also lulls. I was surprised that it wasn’t too difficult. And I also sweated the lines.
Sweating lines is how the old schooner sailors shortened the lines they had cleated to the belaying pins. You can see it in the old film clips.
Your force generated numbers are good. Lower than I would’ve thought.
a 70kg person should be able to put at least 0.6 kN ( 60kg x 9.8m/s^2) so your .85kN is impressive. You don’t look like you weigh 90 kg.
The reason sweating works is if you calculate the tension on a wire holding up a painting. If the angle of the line to the nail in the wall is 30° than the tension on each side is equal to the weight of the painting (so 2x total. So whatever your weight is in a hammock, that’s what you’re pulling on each side.) If you tighten the line on the back of the painting until it is a straight line, than the tension would be infinite. This is why sweating works.
Brilliant observation at 22:20! That’s exactly why 30 degrees is easy and 0 degrees is infinity. The flip side here being that you don’t get much out of a slack line. Lesson: Don’t bother sweating a slack line, only a tight one. And your stretch component I simply never considered. This is why so often when I sweat a dock line I get nothing out of it.
Parbuckling I’ve never considered. I will now!
The next step would be to tension a line between two cleats or two of those pilings. Put as much tension on the line as possible, say 5 kN. Now pull on the middle of that line and see how the tension changes. My guess is (if stretch isn’t a factor) initially you’ll get a really big number. (This is the picture wire approaching infinity.)
And your one finger? Any object at rest has a balance of forces, applying any force changes that and if friction is low, you should get a change. (Which is why it works on my 13 ton boat but not on my 0.125 ton refrigerator.) When I taught physics I always thought that in additon to the 6 classical machines (lever, wheel, etc...) buoyancy should be included. (boats, and locks). Sails as well. Thanks for this.
Thanks for that well thought out response to the vid. I didn't want to go higher than 3/3.5kn as even though it is low, i didn't want to be causing damage to anything. Though my presumption is the infrastructure should easily take it. The pilings is a good idea, I have a few large slings. The kevlar rope I have has significantly less stretch under load so I'm hoping a can get far higher tension, just for kicks. I really enjoyed making this vid. Keep the inspiration coming in the comments. Good to know I have a physics teacher keeping me on track. :)
All Hail Ye, a lot of effort you've put into the video, thanks. Will make for great consideration when under pressure (quickly) I need to get my boat into safer position, thanks immeasurably. Some things to practice and train my crew on.
Hey appsdev299! 😄 glad you enjoyed the video and found it useful! We had fun making it. Fair winds and a happy weekend to you!
Me me me , that's all I needed to hear
Good luck guys
Could you clarify? Its a video about the tension force sweating a dockline creates?
Great to see you working with a strong woman ... oh & good subject matter too!
Keep up the good work.
Awesome video guys! Interesting how the stretch really compromised how much force you can put through the line by sweating it.
Glad you enjoyed. @spongeman further down the comments highlighted the bow of the boat pulling down of the bow also contributes to the limited return. We have done some more testing and isolated the varying factors. Another video coming in a few days. :)
Forget about the elasticity of the lines. It doesn't rob you of force applied. It simply provides for progressive transmission of force. Hang a kilogram from a kevlar line vs a rubber band. It will weigh a kilogram both times. 🙂
Hi SailingSnowGum. The force applied by you doesn't change. But the tension created at different vectors does. for example. The same 400n force applied on the line being sweat by the person can result in 11461n (over 1 1tonne) of tension at 1degree, 2294n of tension at 4degree, 1151n at 10 degree, 585n at 20 degrees and so on, eventually becoming negative. If the line starts stretching at 500n tension,the longer line would go straight past the smallest angles, it can never reach the higher tension forces.
@@theincompetentcrew I see what you're saying. Makes sense! 🙂
@@SailingSnowGum I'm still trying to get my head around it. And I could be wrong. This video presumes a line that has been tensioned by hand initially (fixed to the boat then started sweating it). If the line is under load already, say 50-100kg of wind blowing on the hull. The results are different. Doing some fun testing today. Hope you watch the next vid. :)
You should test between static points. The amount of force you can exert on the boat is limited because as you increase the force you just end up sinking the boat a little and that releases tension in the rope.
Hi Spongman, thats a fair point, the next test shall have static to static to see what can be achieved in more prefect/ more lab like conditions. We are also going to use ropes with less stretch to try and get closer to that perfect point. We figured testing on the boat with the spring line loaded would give a realistic condition slightly mimicking the wind, as the same effect would happen when sweating a boat in from a few meters/yards away. Thanks for adding that.
Still not sure why i would need to sweat a dock line. All i got from this is that Karli always puts out maximum effort while Ben needs some competition in order to improve his effort. So in a nutshell, ill let Karli tie up my boat. No hard feelings Ben. Also, I wonder if something like a truckers hitch, which has a loop in the line so you're pulling at 2:1, would be better then sweating?
Interesting but, as a newbie to sailing, I don't even know what "sweating a dock line" means.
You didn't explain it unless I missed it in all the mathematical equations somewhere.
It means how much sweat you perspire while pulling on dock lines. In the old days you would hold a glass under your chin and then measure the drops of sweat. So from what I got from this video is 1kn of force is equal to 12 drops of sweat. When my wife pulls our stern in when docking if there is a 5 knot breeze she exerts 4 drops of sweat, 10 knots is 11 drops and 18 knots its 25 drops. These techniques would help her a lot. One day I’ll show her this video and I’m sure it will change her life for the better.
@@jameshuntsman6046 Please bear in mind, the more expensive the boat, and the closer the boat to other expensive boats the first boat gets- the more exponential the sweat becomes. :)
Hey Philip, sorry I didn't consider newbies. I will make a quick sort on it as it is a handy technique. But put simply, its wrapping the dockline around a cleat (3/4 of the way around or similar), holding the tail end, and simultaneously pushing/pulling on the side going towards the boat. This pushing perpendicular on the dockline creates tension that pulls the boat towards you. You can then take in the slack and do it again. Its a way to move a boat, which is easier than pulling on the rope directly. The force create is equal to 3-4 people pulling.
@@theincompetentcrew thanks for the explanation. Yes, I'm new to sailing and hope to own my own boat someday. You may want to keep that in mind when creating your videos. Just a thought. I do enjoy your channel and am learning from it. Thanks!
Very cool video! What make/model is that load cell?
Its an old Straight Point load cell (pre Crosby).
what is this in slug feet per second? (I kid, I can convert metric to human- but it's oddly easier to start from newtons)
I may be an idiot, but, I cannot see any link to kofi or other contribution possibility. I am an older geeza on an iPad so please help me out! Always enjoyable videos otherwise.
I turned the iPad around and was able to scroll and find the link. Hooray. For some reason no scrolling in landscape mode - steam driven pad I guess
Hey Ian! Thanks so much for the coffee 😁 it’s really kind of you and we appreciate the support very much! Let us know if there is ever anything particular you’d enjoy seeing in a vid! Have a great weekend ☺️
Hi, its rather easy to calculate the force of wind by dynamic formula. Mass in kg x speed m/sek squared. So in 10 m/sek is mass x 100 times. In 30 m/sek is mass x 900 times!!!
Hi Cris, thats true! The problem I have tried to factor is a boat tied up at a dock or being blown off may have no speed, only the wind load on tight dock lines. I used several formulas, including the above you suggested, before settling on the most conservative as the wind adds force without much movement. Just like on any static object with wind.
For instance you could have a steel boat 40ft long 1ft above the water, vs a fibreglass catamaran with 10-15 times the wind force on it
@theincompetentcrew Right. The question about mass is not so easy to calculate, mass of air is 1,2 kg/ m3 and it's all due to resistance by rig and hull...😀👍
@@CrisTait Yep. :) I kinda just wanted to make the point for people that I have produced 1 data set that could be wildly out for their own boat. Thanks for adding to the convo.
@theincompetentcrew Okay. My problem was that mooring ropes snapped in 23 m/sek, Lines was dia. 22. mm and should resist a force of over 3 ton. But did not cause of waves lifting boat (4 ton, 30 feet) when floating pier rolling down. All due to biggest rubber fenders and all.
While this video was entertaining I fail to see where outside of the concept it's self where any of the statistics or the methodology by which you arrived at them is useful. If your boat has parted a line of a cleat has pulled off the dock I doubt your pull out a hand held anemometer, somehow compare that number with a bunch of other theoretical numbers you've cooked up and then somehow decide what to to with another decision making matrix you've prepared for just such an occasion. You'll in reality do what people did 100 years ago. You'll step on that line a somehow get yourself aboard as quickly as possible and use your winches and windlass to make the boat do whatever you wish.
Entertaining video though. Keep it up.
Glad you enjoyed. :)
Great video! Force vectors is of course a great way to illustrate what is actually happening and what it takes to control a boat in challenging conditions. Well done! And it gets even more interesting if we add intermittent dynamic forces (waves, gusts) and momentum forces (a heavy boat in already motion) to these "best case" calculations.
@@PeterFagerberg Haha, yes it does. Glad you enjoyed. I'm working on something a little more dynamic but waiting for a storm to blow through in the next days to get some 'real number'.
I always carry a hand held anemometer just in case I find myself in a situation like the one described here. Consider the case of a moored boat breaking loose from the dock in a storm. I jump on deck and find no one on board. The winch handles are locked inside and the windlass power is shut off. Luckily I also always carry a winch handle along with my anemometer. I may also find that this is a power boat and has no winches. In these situations a rope or two may be all the tools you have. Knowing these techniques and understanding the limits of them can help stop a bad situation from getting worse. Knowing how to apply the maximum force with a minimum tool set is valuable in many types of emergencies. Your smugness reveals your stupidity.
1kg force is only 10N, not 10kN (1G is 9.8H/kg)
Hi aystarik, I haven't said 1kg is 10kn
Have a closer look. The table with wind force has been converted to kg force to make it more relatable. The rest is in Kn
Ahhhh, I've just had a look, I apologize. One of the tables (I'm guessing you are referring the spreadsheet) was originally in newton, I converted it to kn, then decided it was better converted to Kg to make it relatable but didn't update the titles fully. I did also make a typo of kgforce=Knforce*0.101971. This should have read kgforce=N force*0.101971 Is that what you are referring to? Neither of the typos change the figures given though.
Competitive much? Rofl