How do you work out what size mooring line you should have

tudorsailor

tudorsailor

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SWMBO and I have been having a discussion about mooring lines - sad but true. Now we both wonder if one can calculate the strength of the line required rather than eye-balling a line and thinking - "that looks strong enough". My argument is the greater the windage of the boat (if that is the right term of exposed surface to the wind) is more important than the weight of the yacht. SWMBO thinks that the weight of the yacht is more important. Of course it is probably a combination of both

So when I next buy a line, how should I work out what breaking strain I should go for! I have these nice to handle dockline but I wonder if they are oversized

TudorSailor
 
tudorsailor said:
I am aware of Jimmy Green's guidance. Question is how do they arrive at their advice and what if I want different lines?

TS
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Rule of thumb and commonsense. Unlikely you will ever get anywhere near the breaking strain of dock lines in normal use. The main forces are weight and windage, but the latter is not constant in either strength of direction. The biggest enemy is chafe which will occur if there is lots of movement on the boat when it is moored and can be reduced by using some form of dampener in the line.

Sailorman's 16mm for normal use and 18mm for absolute security if you are leaving the boat unattended for periods of time makes sense. FWIW i use 14mm on my 37ft for both short and long stay and double up in the winter on the lines that take the greatest load.
 
Again FWIW I have never known a dock line break, and believe windage is the big issue. I think 16 mm is more than adequate for a 36 ft boat and a snubber in the system (rubber or a steel spring) is a big help to reduce snatch loads. Doubling the lines in winter is reassuring.
 
According to The Complete Anchoring Handbook (Alain Poiraud et al):-

The estimated peak wind loads on a moored 50' sailing boat are 4800lb (I think this is taken from ABYC data). This is stated for a permanent mooring and includes the effect of current and wave action. Presumably alongside berthing, using "docklines" might be less if there is a degree of shelter?

The estimated average wind loads on an anchored 50' sailing boat are 1600lb at 30kn, 3200lb at 45kn and 6400lb at 60kn (again taken from ABYC data).
 
37' -b oat normally on a 24-28mm strop and chain backup. But in the marina over winter it's on 14-16mm lines - doubled up. Windage is a consideration - hence the sails have been removed. But still have the tent up and it's all fine (I hope!)
 
Once I have seen dock lines break, one by one; 50 ft boat, 25 mm lines, quite enough normally. It was in some F8 but not windage was problem - swell entered the harbour. Boat was going some 2 meters up and down.
 
Fantasie 19 said:
Blimey... you'd think the cleats would go first on lines that thick...
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Well, this is for question - "how do you work out the size of line" - you go for a bigger, once something breaks :)
Those were proper cleats. Here you can see them. And new lines I bought. https://picasaweb.google.com/lh/photo/KfclItHiPbO5rOVOTf8YENMTjNZETYmyPJy0liipFm0?feat=directlink
For comparison - rail is 5 cm high (2")
Boat is traditional (wood, 15 tons) and built to construction rules, similar to Lloyds, such things used to be calculated in good auld times :) But this reminds me - once somebody did not allow enough slack for tide, in mooring line from bows, so the bow actually hung from dock on the line; so I was told. Cleat held, but part of wooden rail with fairlead was torn from deck...
 
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I've never had a broken mooring warp, but I have seen posts here where other people have. It will almost always be due to snatch loads rather than continuous, and since a snatch is what happens when rope has to resist the momentum of the moving boat, it will be down to weight (momentum is mass times velocity) more than windage.

Pete
 
P'raps it's best to use thick and thin lines in parallel - a slightly shorter, very light line, whose breaking strain only slightly exceeds the greatest it is likely to encounter...so its elasticity will be excercised to the maximum extent, reducing the shock of snatch loads on cleats, etc...

...meanwhile, a slightly longer, much heavier line backs up the thin stretchy stuff, so that if the light one were to break, the yacht wouldn't drift anywhere...although in practice, the heavier might never carry any load at all.
 
Rope is a lot stronger than it appears. YM had Marlow test several to destruction a year or two ago. The problem is the knots, many of which halve the rope strength. Even splices reduce it a lot.

I have had the same anchor snubber line for years. It is 11 mm braid on braid nylon and in use is the only line connecting the boat to the anchor. Boat is 34 feet, 7 tons, the snubber has been used in winds to about 48 knots. But, it is well protected from wear, which is probably the biggest factor in mooring line breakage.
 
Had a 12mm polyester warp snap whilst our 25' approx 2 ton jaguar was moored to a floating pontoon in a F6-7 last year. As it went with a bang while being pounded by lumpy seas I reckon it was mostly the snatching movements that made it go. Had plenty of (and thicker) other lines out at time so no problem, but think I'd always go for as big lines as I could unless in a sheltered marina.
 
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