Stresses on Mooring Lines

[alan_d]

If you're not a climber, I respectfully suggest that talking about using 'climbing rope' - like there's only one type - is the mantra of an ignoramus.

I have no quarrel with what you say about climbing rope(s) but would dissent from your belief that you can suggest that someone is an ignoramus respectfully.

 

[NormanS]

I suppose that it was sadly predictable that an interesting discussion about the stresses on mooring lines, would become embroiled in the completely different situation of anchoring. Fred Drift at work.

 

[lustyd]

dog bone thing (or spring

This category of product is called a compensator whether EPDM or spring based.

They actually work through compression/twisting rather than stretching, hence the wraps of rope.

 

[Neeves]

This category of product is called a compensator whether EPDM or spring based.

They actually work through compression/twisting rather than stretching, hence the wraps of rope.

Thanks Lustyd - but I may forget (please be generous ) - confusing that the spring and a 'rubber dog bone thing' are both 'compensators'.

Do I now have to type 'compensator (rubber dog bone type)'.

Education never ends

Jonathan

 

[Refueler]

You tie those spring snubbers with one continuous line knotted at two points, one to each end of spring, with a short length of line (enough to allow expansion of spring) between knots.

I agree - but many times you will see spring compensators between TWO ropes to create the single line.

 

[Refueler]

Nylon ... Polyprop ... etc. - they really are geographical biased ............

Also when people talk about example 'Nylon' - they are usually talking about a composite format containing nylon .....

Pure Nylon rope - I have been given by a pal in the past and luckily I knew NOT to use it for serious work ... within short period - it had disintegrated into a powder like short bits littering the bottom of my locker.

 

[AntarcticPilot]

The major stress on my lines is on the dock spring which I use to ensure she stops in the right place, and to motor against to keep her there while the other lines are attached. The stress on that line can be calculated quite easily. Mass of boat is about 5000kg; stops in about 0.3m from about 0.5 m/s in about a second. That's roughly 2000 N, or about 200 kgf. A normal 10mm rope has a minimum breaking strain of around 2500 kgf; most of us use larger ropes than that (I use 12mm minimum; 14 more likely). If you can break a rope in good condition, then you are exerting forces of the same order as the weight of the boat. I suspect that the instances of dock lines breaking in bad conditions all arise from mechanical damage to the rope (perhaps arising from the bad conditions) not because of the forces exerted.

 

[Neeves]

The major stress on my lines is on the dock spring which I use to ensure she stops in the right place, and to motor against to keep her there while the other lines are attached. The stress on that line can be calculated quite easily. Mass of boat is about 5000kg; stops in about 0.3m from about 0.5 m/s in about a second. That's roughly 2000 N, or about 200 kgf. A normal 10mm rope has a minimum breaking strain of around 2500 kgf; most of us use larger ropes than that (I use 12mm minimum; 14 more likely). If you can break a rope in good condition, then you are exerting forces of the same order as the weight of the boat. I suspect that the instances of dock lines breaking in bad conditions all arise from mechanical damage to the rope (perhaps arising from the bad conditions) not because of the forces exerted.

I would agree - if your yacht is well secured with a decent web of lines then it simply should not be moving very far or fast. Even 0.5m/s after moving 0.3m is fast - which is why I suspect mooring line failure does occur but is a non event because there are sufficient other lines as back up. The other reason line failure is a non event is because people do use a web of lines - they see the obvious result of abrasion - and they change that line (and hopefully manage the abrasion)


The late Prof Knox and I conferred on a number of 'related' topics and he used 1m/s as a speed at anchor - which I though fast. But I frankly don't know how you measure it - its a best guess.

Jonathan

 

[AntarcticPilot]

I would agree - if your yacht is well secured with a decent web of lines then it simply should not be moving very far or fast. Even 0.5m/s after moving 0.3m is fast - which is why I suspect mooring line failure does occur but is a non event because there are sufficient other lines as back up. The other reason line failure is a non event is because people do use a web of lines - they see the obvious result of abrasion - and they change that line (and hopefully manage the abrasion)


The late Prof Knox and I conferred on a number of 'related' topics and he used 1m/s as a speed at anchor - which I though fast. But I frankly don't know how you measure it - its a best guess.

Jonathan

The figure I gave is derived from actual observations of the stresses when I'm berthing, where I use a spring to stop the boat. I used 1 knot (=0.5 m/s) as the speed because that's about the minimum speed for steerage. The 0.3 is an estimate and may be an overestimate, as is the 1 second - but both are in the right ball-park. I would, as you say, expect the stresses arising from motion while moored to be far less, and, of course, are shared by at least 4 lines. As a back-up to the figures, when berthing without fixed dock lines, it is easy to control the forward motion by snubbing the spring on a cleat; the forces exerted are in the hundreds of kgf, not thousands. I also quoted the lowest figure I could find for the breaking stress of braid-on-braid rope.

 

[thinwater]

I'm always taking a contrarian view, but fwiw I hate the idea of using climbing rope aboard, especially for mooring lines.

1. Dynamic climbing rope doesn't stretch in the elastic sense, it permanently distorts and gets longer if it experiences force above a defined limit. Once the force has gone it doesn't return to it's original length but stays permanently stretched. So it's actually useless in this application because once stretched it loses its elasticity. Think about it: you really, really, don't want energy stored in the rope if you fall - that way you will hurt yourself by bouncing up again to smack the rock face. The rope has to absorb the energy so you just slow down and come to a blissful stop - hopefully before hitting the ground. That destroys the rope after a couple of falls. The exact opposite to what one needs or expects of a mooring.
2. There are two main types of climbing rope: dynamic and static. Static is what climbing walls use. That's because on climbing walls you have a top-rope and a person (or machine) belaying you so you won't fall far. It doesn't stretch and doesn't therefore distort, but it's just ordinary dyneema or polypropylene. It gets thrown out by climbing walls once it has got old so to buy it for elastic properties is dumb: it never had any!
3. I hate braided ropes anyway for mooring lines. You have absolutely no idea if it's still strong or about to part. You can't see the inner core. If spliced, it's even more of a pig in a poke ( I don't splice braided ropes but that's another debate).
4. That most people get away with this only goes to show how small the forces actually are on mooring lines, so why on earth go to all that trouble for something more expensive but actually inferior to simple 3-strand?

Just choose 3-strand polyester. It's safe, and cheap and works extremely well. You can see if it's getting abraded and past its best when you can simply bin it. Use 8-plait by all means if you want to swank (it's no better, but hey-ho, it looks like you spare no expense).

PS: I'm a climber, I use dynamic climbing rope for climbing and I throw out the rope if I have a real fall because it's now used up its stretch. If you're not a climber, I respectfully suggest that talking about using 'climbing rope' - like there's only one type - is the mantra of an ignoramus.

Really, this post contains little useful information. I am sorry to insult, but misinformation not based on facts require correction.

1. Incorrect. It stretches and rebounds, and has a better fatugie life that other nylon rope. Pure falicy. You have no engineering understanding of dynamic climbing ropes. The fall life of used ropes declines, but that is an artifact of the type of pass/fail testing that is used. Non-climbing ropes fail from the start, and all polyester ropes fail. In fact, I've subbjested climbing ropes to as many as 1000 20-30% pulls without failure. It is only VERY high factor dynamic falls over and edge that cause failures, for reasons unrelated to dock line use. There is some perminant elongation, but only about 6% of the 40-50% they can stretch. And this is less than the permanent elongation found in some other rope types. You are mistaking UIAA drop tests with fatigue testing. Not really related.
2. Climbing walls that use human belayers always use dydnamic ropes, at least in the US. Industry standard. Dyneema would be... stupidly dangerous and never happens except for automated systems with zero slack. Abroists often use polyester for rope accent. Yes, you can climb on a varriety of ropes, but I think it is obvious here that "climbing rope" is an abriviation for UIAA dynamic single rope. You know this.
3. It's not hard to inspect climbing ropes for core failure. But the cover goes first, because it is exposed to UV, because of chafe, because it is less stretchy than the core, and because, unlike yachting rope it carries less than half of the strength. I have worn out many climbing ropes over a 40-year (still climbing) career. A non-concern. Core failures are the result of a fall that results in a pinch and massive compression, and that is not a factor here.
4. Do I use the old ones for dock lines? No, I find better uses, including snubbers and traveler controls. I'd rather use something I can splice. I't may have too much stretch, for boats over 25 feet or so. Too much movement. But using it for dock lines isn't weird. Smae for anchor rode. I used a climbing rope for rode on a 27' cat for 14 years, no problems, smooth ride, easy handling. And it still stretched and it didn't chafe. In fact, after that I pulled a section and pull tested it; the stretch and strength were still 92% and 80% respectively of new, and i was still better than most nylon rope new (pull testing is something I do for test programs).

I think the word ignoramus is strong and doesn't mean what you think it does.

 

[thinwater]

Another thought that might help.

My last boat fit the slip reasonably well. This time I am in a massively oversized slip (nearly double size), on the end, and in certain winds she could really dance. Not a floating dock.

I found that an elastic compensator ONLY on the lee side was the cure. It pulls the boat back into the slip, keeping the slack out of the windward and wake-exposed lines. No slack means no surging or impact. Much better in terms of stress and chafe.

The compensator could be any materil, including a long thin nylon line, but in my case I use this (I forget which model). They look like bungees, but will last >5 years in a tough environment. I've used them in industry to reduce snubbing of industrial wastewater agitators, jerking and covered in nasty crude 24/7.
DAVIS INSTRUMENTS Shockles® LineSnubber™ | West Marine

 

[jdc]

Really, this post contains little useful information. I am sorry to insult, but misinformation not based on facts require correction.

1. Incorrect. It stretches and rebounds, and has a better fatugie life that other nylon rope. Pure falicy. You have no engineering understanding of dynamic climbing ropes. The fall life of used ropes declines, but that is an artifact of the type of pass/fail testing that is used. Non-climbing ropes fail from the start, and all polyester ropes fail. In fact, I've subbjested climbing ropes to as many as 1000 20-30% pulls without failure. It is only VERY high factor dynamic falls over and edge that cause failures, for reasons unrelated to dock line use. There is some perminant elongation, but only about 6% of the 40-50% they can stretch. And this is less than the permanent elongation found in some other rope types. You are mistaking UIAA drop tests with fatigue testing. Not really related.
2. Climbing walls that use human belayers always use dydnamic ropes, at least in the US. Industry standard. Dyneema would be... stupidly dangerous and never happens except for automated systems with zero slack. Abroists often use polyester for rope accent. Yes, you can climb on a varriety of ropes, but I think it is obvious here that "climbing rope" is an abriviation for UIAA dynamic single rope. You know this.
3. It's not hard to inspect climbing ropes for core failure. But the cover goes first, because it is exposed to UV, because of chafe, because it is less stretchy than the core, and because, unlike yachting rope it carries less than half of the strength. I have worn out many climbing ropes over a 40-year (still climbing) career. A non-concern. Core failures are the result of a fall that results in a pinch and massive compression, and that is not a factor here.
4. Do I use the old ones for dock lines? No, I find better uses, including snubbers and traveler controls. I'd rather use something I can splice. I't may have too much stretch, for boats over 25 feet or so. Too much movement. But using it for dock lines isn't weird. Smae for anchor rode. I used a climbing rope for rode on a 27' cat for 14 years, no problems, smooth ride, easy handling. And it still stretched and it didn't chafe. In fact, after that I pulled a section and pull tested it; the stretch and strength were still 92% and 80% respectively of new, and i was still better than most nylon rope new (pull testing is something I do for test programs).

I think the word ignoramus is strong and doesn't mean what you think it does.

I certainly take back ignoramus as unnecessarily rude and not helpful to the debate. And for the avoidance of doubt I wasn't suggesting that non-climbing ropes be used for climbing.

And (genuinely!) thank you for your counter arguments to my diatribe. But practice may not be the same everywhere as climbing ropes used here for the top belay are nowhere as elastic as dynamic ropes; both ('static' and 'dynamic') are sold alongside each other and feel quite different. And for abseiling one generally doesn't want a very dynamic rope as it's too bouncy and so imprecise when stopping reasonably quickly. So I didn't actually appreciate that you meant 'climbing' automatically to mean the very dynamic rope. I guess you must know what you and others mean, but what my local climbing wall gets rid of is what they use, which is pretty much a static rope - say pretty much as it's nowhere as stretchy as one would use for climbing without a top belay where the fall distance may be much further. So beware when buying 2nd hand rope, it may not be what others are referring to.

Do I think dynamic rope is good stuff for moorings? No I still don't I'm afraid. I just don't see the point! Your experience, which I don't doubt, is that it's ok for anchor snubbers, but this tread is about dock mooring lines. Is dynamic climbing rope useful for that purpose? Not in my opinion (as I think you agree - apologies if I've misunderstood).

What I will try following your answer is some old dynamic rope as an actor snubber, because if you're right that it only uses a little of it's elastic stretch then it could indeed be good for that purpose, but I think you'd have to test the brand to be sure, and the rate of snatching will be a complicating factor since the hysteresis implies heat generation.

 

[thinwater]

I certainly take back ignoramus as unnecessarily rude and not helpful to the debate. And for the avoidance of doubt I wasn't suggesting that non-climbing ropes be used for climbing.

And (genuinely!) thank you for your counter arguments to my diatribe. But practice may not be the same everywhere as climbing ropes used here for the top belay are nowhere as elastic as dynamic ropes; both ('static' and 'dynamic') are sold alongside each other and feel quite different. And for abseiling one generally doesn't want a very dynamic rope as it's too bouncy and so imprecise when stopping reasonably quickly. So I didn't actually appreciate that you meant 'climbing' automatically to mean the very dynamic rope. I guess you must know what you and others mean, but what my local climbing wall gets rid of is what they use, ie static rope, not UIAA dynamic. So beware when buying 2nd hand rope, it may not be what others are referring to.

Do I think dynamic rope is good stuff for moorings? No I still don't I'm afraid. I just don't see the point! Your experience, which I don't doubt, is that it's ok for anchor snubbers, but this tread is about dock mooring lines. Is dynamic climbing rope useful for that purpose? Not in my opinion (as I think you agree - apologies if I've misunderstood).

What I will try following your answer is some old dynamic rope as an actor snubber, because if you're right that it only uses a little of it's elastic stretch then it could indeed be good for that purpose, but I think you'd have to test the brand to be sure, and the rate of snatching will be a complicating factor since the hysteresis implies heat generation.

Interesting difference in practice between countries, I presume. That is one reason I like this forum. No climbing walls in the US use static ropes, not counting resort and fair walls (which don't count IMO). I have been involved with several on a comercial basis, and it is against industry standards, because human belayers can introduce slack inadvertently. I have used static ropes top roping at the crag, however. No problem for simple striaght up climbs, not OK for on complex climbs with overhangs and such.

Yes, there are reasons I would not use it for dock lines, splicing being one. Even used, there are better uses being another. Chafe does not seem to be a problem; the cover is tighter than any yacht rope, making them quite durable (I have tested on several test rigs). UV may be, since treatments seem to vary; some climbing rope seems quite vulnerable. As you pointed out, dynamic ropes are typically retired from climbing long before UV becomes important.

You can do the math on hysterisis; a climbing rope snubber will disipate heat before it warms at the cycle rates a rode sees. Not enough heat per unit time. This has also been tested by the US Navy (you can find it with Google). Very large (>1" dia) docklines can heat, particularly if covered with chafe gear and subject to very short period, high force loads in a huricane. This is something the US, Florida specifically, has experiencing with. But only larger lines. Small lines cool off too quickly.

Note: ocationally someone will post a pic showing melted ends. This is a misinterpretation. If you pull a new rope one time, very slowly, the rope ends will fuse upon rupture due to the sudden energy release. It 's not multicyle hysteresis heating, but rather the nature of nylon.

---

I've done a lot of rope testing. In my experience, it is more about how the slack is managed and haw the ropes are rigged than the weave (quality being equal). 3-strand is probably best, but it is also a pain to live with on some cleats.

 

[lustyd]

Thanks Lustyd - but I may forget (please be generous ) - confusing that the spring and a 'rubber dog bone thing' are both 'compensators'.

Do I now have to type 'compensator (rubber dog bone type)'.

Education never ends

Jonathan

please type what you prefer, I was hoping to save you having to type the whole thing each time ?

 

[thinwater]

Hysteresis Loops

Springs are, well, springs, while ropes do something different. There will also be some energy dissipated to the water. Not sure about the dog bones--I've never seen a hysteresis plot. (This plot is to about 50% BS, well past normal loading, resulting in faster bedding and greater permanent elongation.) At very low rates and low % WLL nylon is much like a spring, but as rates increase and load increases, there is a lot of damping. A rock climber who falls does not bounce up and down, he rebounds half way back, and that's it.

 

[Neeves]

So beware when buying 2nd hand rope, it may not be what others are referring to.

In Australia we use dynamic rope in gyms.

Rope is retired from gyms if it is suject to a major fall or it has been used for specific length of time. Retired rope - cannot be sold. Selling poses a responsibility on the seller. I have offered to pad out the Xmas party or tea fund - it is always refused.

Retired rope from a gym cannot be sold and must be disposed of ensuring that it cannot be used for height safety. If you source retired rope in Australia from a gym - you will know how it has been used, you will know that there has been no grit or sand in contact with the rope and damage to the cover will be visible and obvious. If it is not recycled it goes to land fill. I believe in the UK retired rope from gyms is simply chopped up (and goes to land fill) - but I only have anecdotal mention of its fate. I actually fail to see how a rope from a gym can suffer from a major fall - unless the belayer goes to sleep - consequently damage to rope is to the outer cover not to the core. In terms of loss of integrity of the outer cover, 30m rope, the section worn can be cut out - leaving a length suitable for a snubber or mooring line. Climbing rope is made with an abrasion resistant cover, resistant to a certain degree - nothing is perfect - which makes the cover ideal to minimise the effects of abrasion in a mooring or snubber application.

Climbing rope has been used as mooring lines and as snubbers for decades. There have been no reports of issues - other than an inability to splice and difficulty to knot. The answer to knotting is to sew a loop, which is how some climbing rope is supplied (though not desired by many climbers). You can sew your own loops or a sailmaker will complete very quickly and easily (at, I assume, a cost). Easy if you are patient to sew your own loops which can be sized for your horn cleat.

Gyms in Australia use 12mm ropes, as far as I can ascertain, exclusively. This will be a bit big for a sub-35' yacht - it will lack the required elasticity. But for yachts from 35' - roughly 50' 12mm rope is very applicable as both a mooring line or snubber, and for a snubber as long as possible. Most manufacturers of climbing rope, kernmantle, make sizes bigger that 12mm - for military application (I bought some 14mm rope for a 50' catamaran). I have also found 12mm rope a bit big for our cat (7t x 38' - roughly the same windage as a 45' AWB) and we are now using 10mm , dynamic, climbing rope.

If you can source a sensible length of retired rope - and it should be free - then a free length of rope offers you minimum risk for you to try it in a morring or snubber application. If you find it does not meet your needs you have lost...... nothing. If you find it meets your needs, which is what I would expect - you can continue using the retired product - or go off and buy from new. Neither application, mooring or snubber, are 'critical' - none are life threatening and each application does not use one rope in isolation (the fall back for the snubber is the chain, for a mooring - you should not be using a single rope).


I encourage contrarian views, I like ideas 'outside the box' - I abhor contrarian views totally absent of any user experience and based on unsubstantiated arguments. Insults have no place in this forum especially when the individual expressing the insult is a better example of the insult than the possible targets.

I am grateful for Thinwater's detailed reply - he knows his stuff and restricts his comment to his core competencies.

Jonathan.

 

[Refueler]

At the end of the day ... anyone who relies on single line mooring at each end of boat are gambling with their boat. Reduce that risk by using multiple lines - from different points etc if possible - to ensure that whatever happens ie one line parting etc. - your boat is secure.
My mooring line parting in Yarmouth was not risking boat - as it was only one of a multiple lines ...

Doing so - then basically allows as I do - use of recycled cordage.

I always remember another club member sailing on my boat and when going to locker to prepare mooring lines - commented on how many lines and of different lengths I had ... as he said - enough to tie up a battleship !

 

[thinwater]

At the end of the day ... anyone who relies on single line mooring at each end of boat are gambling with their boat. Reduce that risk by using multiple lines - from different points etc if possible - to ensure that whatever happens ie one line parting etc. - your boat is secure.
My mooring line parting in Yarmouth was not risking boat - as it was only one of a multiple lines ...

Doing so - then basically allows as I do - use of recycled cordage.

I always remember another club member sailing on my boat and when going to locker to prepare mooring lines - commented on how many lines and of different lengths I had ... as he said - enough to tie up a battleship !

Consider this. There are many applications, beyond boating.

Which is better?

• A single 10.5 mm climbing rope or a pair of 7.5 mm ropes? Twin ropes provide less risk of total system failure, but if only a single rope is loaded at one time failure is more likely. Twin ropes can provide rigging advantages, but there is more complexity. Total strength is very similar. I've used both systems.
• A double walled tank with a 1/4-inch inner wall and a 3/16-inch outer wall, or a 3/8-inch tank? Depending on the instalation, there will be no way of inspecting the outer wall (may be on the ground) or the inner wall (would need to inspect an ultrasound from both the inside and outside), which may not be practical). Consider if it was a hull with no inspection of the interstitial space. I've inspected both and observed both from installation for as much as 40 years.
• New rope or 2 used ropes of unknown conditions. Age and visual appearance are not a reliable metrics based on testing.
• One 3/4-inch rope or 2 x 1/2-inch ropes. The 1/2-inch lines will chafe on each other somewhere (chocks or coming off the cleat) unless independent points are possible, which is not always possible. Total strength is similar. One of the 1/2-inch lines will likely carry more load than the other. The total strength it similar. I've tied up boats for long periods with two ropes on a cleat, and secured boats for tropical storms and huricanes with a spider's web of ropes.

Add that:

• Cleats may not accommodate a larger line or two lines. One solution is adding a Dyneema loop.
• The spring rate of a larger or old line is higher, meaning greater force. About 40% greater for a rope double strength.
• Which is better, a cored hull (like a double wall tank) or solid hull with the same amount of fiberglass? (Stiffness, flexural strangth, and penetration resistance will give 3 different answers.)

Not simple, and I intensionally did not declare which was better in each case. Often, such as the climbing rope case, it depends on other variables.