Rolling Hitch on Snubber

[Cotemar]

Using climbing rope must be a local thing as no one around here is using it or giving it away for free.
For me I just do not get the math as 5/8 three strand nylon rope is just $1.39 usd per foot.

It’s high-grade, long-wearing, flexible and easy-to-handle marine nylon rope.

The newer 3-Strand Nylonline is treated with lubricating finish for improved abrasion resistance and increased wet strength.

Very little degradation from sunlight / UV

It’s Supple and easily spliced

Average Tensile Strength: 11,650 lbs (5,285 kg)

Its light Weight per 100 ft: 9.6 lbs

You can buy it everywhere on the planet to any length and color you like

It will visually show you it’s damaged or chafed and needs to be replaced.

What’s not to like about this stuff

I truly believe in using the KISS (Keep it Simple Stupid) principles on any boat.

[Kettlewell]

The problem with the super heavy chain providing catenary theory is that it provides the opposite amount of cushioning you need: it is greatest at low wind speeds and it is least at high wind speeds. I can tell you from experience that it can be nasty when you straighten the chain out and the boat stops moving with a tremendous jerk, and no matter how heavy your chain there is a certain level of wind and waves where you will be pulling out all of the catenary. Someone I knew had his bow roller collapsed when this happened, and then the chain started to saw into the boat so he tried to secure it and lost several fingers in the process. Myself, I like plenty of spring in the system rather than relying on catenary, which is great until suddenly it's not.

[JonJo]

Hi Cotemar,

What's the stretch of 10m of 5/8th 3 strand nylon at 500kg load?

With an ultimate stretch at breaking of, say 30% - that would be 3m at 5t, at 500kg that means your snubber stretches 30cm - that to me is not a snubber. There is nothing wrong with the 5/8th nylon, its just too big for a Mahe, especially if your nylon is your bridle which might be 4m each side (?). If this is the case then your 4m bridles would stretch 12cm (that's on the basis of a straight line pull (which is not the case for a cat bridle)? The other way to look at it is that 10m of 5/8 nylon at 600kg (sorry I did the calculation for 600kg) will absorb 2,000 joules but a 10mm nylon 3.500 joules (and 10mm climbing rope a bit above 4,500 joules). 5/8 nylon will absorp around 4,500 joules under a 1,000kg load.

Check the other posts, its all about absorbing energy and thinner snubbers have a greater ability to absorb energy - but being weaker they will have a shorter lifespan.

Its not about cost, that's a side issue - but being sacrificial, low cost seems something to add the to positive side of the equation. (New climbing rope is expensive).

Climbing rope has a greater elasticity, it has a greater ability to absorb energy than octaplait or 3 strand - its specifically built to achieve that (otherwise climbers would not use it). It might not have the longevity of the same size octaplait, or 3 strand (do not know) but because its can be 25% better at absorbing energy it has some advantages.

So

Your 5/8 nylon is too big for a 35' 6t cat, on the basis a 500kg snatch load is the sort of ball park figure to consider. The elasticity of 'normal' nylon is around 30%, can vary - climbing rope is around 40%+. You could use smaller nylon (than 5/8) but to enjoy the energy absorption of 10mm dynamic climbing rope you will need to use something slightly smaller than 3/8 and then you will need to consider compromising on strength, to something less than 2t, maybe down to as low as 1.5t.

Jonathan

[JonJo]

Originally Posted by Kettlewell

The problem with the super heavy chain providing catenary theory is that it provides the opposite amount of cushioning you need: it is greatest at low wind speeds and it is least at high wind speeds. I can tell you from experience that it can be nasty when you straighten the chain out and the boat stops moving with a tremendous jerk, and no matter how heavy your chain there is a certain level of wind and waves where you will be pulling out all of the catenary.

+1

Most people enjoying strong winds have found that their chain (appears) bar tight (it might not be) but it really does not matter - reality is that the yacht suddenly jerks (not that I have done it but it appears to be like reversing at some speed into a concrete wall). I am quite happy to accept that the theory says there is still catenary - the reality is the concrete wall.

In theory, practice and theory are the same, in practice they are often different.

In the absence of my ability to enjoy the practical benefits of the theory, snubbers seem a good option.



Jonathan

[s/v Jedi]

If 5/8" 3-strand nylon is too big for you, you can always buy 1/2" or 3/8" or 5/16" or 1/4". I don't understand why these things need fighting over... the negative I see wth climbing rope is the difficulty splicing it and I hate to see knots in it.

On heavy chains: nonsense! It has been demonstrated over and over again that whenwe take weight with us, it is best put in the anchor and not in the chain. Why take a ton when a 30' nylon line can do the same job? The weight carried around the bow should be limited as much as possible.

[estarzinger]

Originally Posted by Andrew Troup

I think we've reached a consensus, or at least a truce while any wounds are tended, on the rolling hitch question...

Oh . . . I missed that . . . .what's the answer? I am still very puzzled by it . . . despite having moved on to the soft shackle solution.

By the way, this rope has a rated breaking strength of 5,600lbs. The conditions were let's say F6 'less than gale'.

So what in fact is the strength reduction of the rolling hitch? Does anyone have a good breaking tests of it vs. other knots?

[Cotemar]

Originally Posted by JonJo

Your 5/8 nylon is too big for a 35' 6t cat, on the basis a 500kg snatch load is the sort of ball park figure to consider. The elasticity of 'normal' nylon is around 30%, can vary - climbing rope is around 40%+. You could use smaller nylon (than 5/8) but to enjoy the energy absorption of 10mm dynamic climbing rope you will need to use something slightly smaller than 3/8 and then you will need to consider compromising on strength, to something less than 2t, maybe down to as low as 1.5t.

I agree with you that my 5/8 three strand nylon snubber is too big for my boat. Instead of going with smaller line I will try these rubber snubbers this year which will give my 3 feet (1 meter) of stretch per bridal leg. The reason I do not go down a size from 5/8 line is because a broken snubber on my cat would allow the tightened chain to hit my hulls at the worst time in storm conditions.

I know some of you guys do not think these rubber snubbers are any good, but a few cruisers at my marina have been using them for 10 years without a failure, so time will tell if it will work on my boat.

[colemj]

I expect the rubber snubber will be just fine for many years in Mystic. Down in the tropics, they won't give much life.

Mark

[Kettlewell]

Yale Ropes has a pretty good white paper on anchoring. It is pushing their nylon Brait rope, but it has lots of interesting information on how much energy absorption there is using nylon in the anchor system. In particular, take a look at page 14 and on from there.

[conachair]

Originally Posted by Kettlewell

The problem with the super heavy chain providing catenary theory is that it provides the opposite amount of cushioning you need: .

https://www.desmos.com/calculator/hvbvbyylh8

Another graph to play around with. Seems to tally up with the sag distance here so might even be a bit right

S is horizontal distance from anchor in metres
d is depth of water in metres
w is weight of anchor chain in Kg/metre
h is HORIZONTAL force in KgF (kilos force = 10 newtons)

I might have a look at displaying how much the boat moves as the chain straightens per unit force and maybe see if there's anything could be done similar for nylon.

All good fun

[Cotemar]

Originally Posted by colemj

I expect the rubber snubber will be just fine for many years in Mystic. Down in the tropics, they won't give much life.

Mark

It’s just a test at this point to see how well they work. Your right that in the tropics they may not last at all.

Still too young to retire and cruise the Caribbean at this time, so that rubber snubber tropics test will have to wait.

[estarzinger]

Originally Posted by estarzinger

Oh . . . I missed that . . . .what's the answer? I am still very puzzled by it . . . despite having moved on to the soft shackle solution.

By the way, this rope has a rated breaking strength of 5,600lbs. The conditions were let's say F6 'less than gale'.

Given jojo's load testing and our own (less complete) load cell measurements, I would be very surprised if the peak load on the snubber was more than 2000lbs. Does that mean the rolling hitch maintains only 35% of the line breaking strength?

[Dockhead]

Originally Posted by estarzinger

Oh . . . I missed that . . . .what's the answer? I am still very puzzled by it . . . despite having moved on to the soft shackle solution.

By the way, this rope has a rated breaking strength of 5,600lbs. The conditions were let's say F6 'less than gale'.

So what in fact is the strength reduction of the rolling hitch? Does anyone have a good breaking tests of it vs. other knots?

A rolling hitch is just a clove hitch with an extra turn, so the geometry should be the same. So the rolling hitch should be somewhat stronger than a sheet bend and somewhat weaker than a bowline. Google will give you a few references; Gord May posted figures a few years on this forum for example.

It is doubtful that the choice of knot played a significant role in your broken snubber.

Your soft shackles will increase strength probably by 40% (the knot, any knot, will have been the weak point), but what you've done to increase elasticity and thefore capacity to absorb energy in your new snubber will be the decisive change. This will have greatly reduced the forces your gear has to withstand.

[Dockhead]

Originally Posted by estarzinger

Given jojo's load testing and our own (less complete) load cell measurements, I would be very surprised if the peak load on the snubber was more than 2000lbs. Does that mean the rolling hitch maintains only 35% of the line breaking strength?

The peak load will go way up if there was any snatching so it might not be that any of those measurements reflect what was really going on with you that night. It will depend on the waves more than wind, I would think -- was it rough? Were you pitching at all?

[estarzinger]

I would have guessed that the rolling hitch was very roughly around a 50% strength knot.

One other diagnostic piece of information . . . there was no 'gunshot' noise. Usually when a rope breaks with a 5,000lb 'shock load' there is a clear and definitive 'gunshot'. All I heard was more chain noise and the snubber 'squeaking' disappeared. To me that indicated more a 'slow' break, rather than a single shock load break.

JoJo's load testing data was with a 100% dyneema 'snubber', so would have significant shock loading built in. My own load cell testing was with this exact same type of snubber (that broke) (edit: except I always used a chain hook previously). I am really sure the peak shock load would not have been more than 2000lbs.

My own personal conclusion is that this line got 'eaten' eaten either by a rough spot on a chain link or by cutting itself over the last bend of the rolling hitch. I don't really like either of those conclusions but they seem more likely than any others I have heard/considered. I don't like the chain roughness conclusion because this is new (winter before last) never been regalvanized acco chain, which is generally quite high quality. And I don't like the eaten itself conclusion because so many other people have had good experience with rolling hitches in nylon, and I think nylon should in fact be more prone to eating itself than Dacron.

So, as I said, I am still puzzled by this event. But I personally consider the 'eaten' conclusion the most likely given the facts.