Rolling Hitch on Snubber

[nigel1]

Originally Posted by s/v Jedi

D12 is the ticket for Marlow 12-strand Dyneema. It is the same as the ones discussed here, may be even stronger (SK78 IIRC). The soft shackle that I put pictures of on-line, only needs 4' of 3/16" Dyneema so the price is not too bad.

You should check out commercial fishing suppliers as they also use this rope instead of wire.

Once again Nick, many thanks for the suggestion to check out the fishing suppliers. Now got myself some 6mm and 9mm 12 strand dyneema at one third of the price being quoted by the yachtie shops

[downunder]

Originally Posted by Cotemar

Not many are using climbing rope as a snubber. Climbing rope is designed to be used by 300lb climbers not a 10,000 catamaran.

If someone gives you climbing rope because it’s not usable for a 300lb climber, why would you ever think it would be a good idea to use it on a 10,000 catamaran snubber.

3 strand nylon line, 8-Strand Plaited & Double Braid Nylon Line have proven themselves in the marine environment for years to secure 10,000 lbs. catamarans to 100,000,000 lbs. ships.

You’re charting your own course using climbing rope as a snubber and best wishes to you in your journey. Hopefully you will not have many followers as that Kool aid may get sour.

Doubt anyone consider he was recommending climbing rope for snubbers just commenting on its use.

Please remember the be nice rule with postings. Kool aid seems more of aUSA thing.

[Dockhead]

Originally Posted by downunder

Doubt anyone consider he was recommending climbing rope for snubbers just commenting on its use.

Please remember the be nice rule with postings. Kool aid seems more of aUSA thing.

Several people report using dynamic climbing ropes with success. If the breaking strength is enough, I don't see why there should be any problem. It's just nylon.

The breaking strength of 11mm climbing rope is not going to be dramatically different from similar size nylon octo. The advantage could be that climbing rope is especially designed to absorb shocks, so the structure may be better suited for this than regular nylon octo.

Climbing ropes are rate for a certain number of falls only, but if you were using regular nylon octo you would do it the same way. An 80 kilo climber will have developed nearly 12,000 Joules of kinetic energy (11,760 to be exact) after falling 10 meters. Not much less than the kinetic energy of a 25 ton sailboat moving at 2 knots (12,500 Joules).

[Andrew Troup]

Originally Posted by svmariane

And apologies for talking about chain hooks when the OP wanted help with a Rolling Hitch problem.

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

On the point you raise, I think a backup snubber is a particularly good idea in bad conditions when the load (in the event the main snubber parted) would otherwise come onto a chain stopper, if it's of the type which cannot necessarily be released under heavy load.

(as opposed to, say, a chain compressor)

I'm personally very uneasy about the idea of having to motor ahead simply to able to release the chain from bondage.

I would prefer to protect the windlass using a chain hook or devil's claw (the latter for a small, HT chain) on a strong strop, but one which could be cut if need be, as others have described.

However this is predicated on my preference for heavy chain (ultra heavy by modern standards) and heavy hardware on the bow. A boat with 'ordinary' chain and especially one with lightweight hardware and foredeck scantlings would, I think, be well advised to rig a fully fledged backup snubber.

For one thing, in those conditions, rigging a new one will probably require paying out more scope, and there may not be searoom for that.

Chain compressors are incidentally a very user friendly option if you DO ever find yourself having to intentionally pay out more scope in a situation where the snatch loads involved would be too much for the windlass, even during the brief period it took to rig a new snubber.

It strikes me that making this period as brief as possible is another point in favour of Evans' soft shackle -- if it proves up to the job in such conditions, which I earnestly hope it will (and it's great to know that we'll hear about it, either way).

In the unlikely event of finding myself in the situation described#, if I had to go to a longer scope with my webbing sling system, I would have to prefit another sling after first bringing the desired amount of extra chain on deck from the locker, by running the windlass in reverse.

And I guess others could do this and go one step further and pre-rig the snubber before starting to run out the scope, provided the connection could be relied on to pass through the bow roller under severe load without compromising anything.

By simply 'running' the previous snubber (casting it off or cutting it free) with the clutch already slightly loosened on the windlass, and carefully flaking out the new snubber* so it ran clean, the clutch would need never be fully snugged home until the new snubber was already taking the strain.

In such conditions, it's doubtful whether the clutch should EVER be fully snugged home, unless you find yourself in the unenviable situation of trying to get your anchor, in which case, if you don't have enough engine thrust to put a permanent sag in the chain (and someone with considerable chops operating engine and helm, and a great comms protocol), I guess it's likely to end unhappily.


# I should also point out that while my webbing strop might seem like a lot of trouble to go to, I don't expect to need a snubber unless anchoring in an open roadstead.

It's amusing to be repeatedly told that an ultra-heavy chain is not enough, by those who have never tried it: in sheltered waters situations where the problem is wind, rather than waves of roadstead proportions, I'm happy to report that snubbing is not an issue except in very shallow water.

And in roadsteads where it is an issue, it's still always going to be less of an issue with a very heavy chain, (especially as roadsteads are almost always deep water, where heavy chain really shines) unless the weight is so extreme it's holding the bow down.

*(which would probably require breakable 'stops' to be fitted, a bit like old school spinnaker hoist prep, to stop the new snubber being blown or washed into places from which it would not run out freely)

[Dockhead]

Originally Posted by Andrew Troup

It's amusing to be repeatedly told that an ultra-heavy chain is not enough, by those who have never tried it: in sheltered waters situations where the problem is wind, rather than waves of roadstead proportions, I'm happy to report that snubbing is not an issue except in very shallow water.

I think everyone would agree that if you have enough weight and length of chain out, that the force produced by its catenary will snub the rode. I think that it's also been shown by JoJo by actual experimentation that the forces produced on anchor rodes by wind alone (no wave action; no yawing in storms) are modest. So certainly I believe you. I think most of our snubbers are just stopping chain rumble, 96% of the time anyway, even those of us with lighter chains. If you have a super-heavy chain, then all the better.

I think, however, that most cruisers have experienced, at one time or another, what happens when the catenary does get pulled out by strong wave action or violent yawing in a storm. It doesn't happen very often, but this is the situation where the snubber is called on to perform a different kind of work, so different design values apply -- mainly energy absorption, although chafe can also be a critical question. This was the OP's situation, I think, and he ended up with a broken snubber, which was what started the whole discussion. And ultimately led to the birth of an entirely superior snubber design, I think, even if we had a lot of misunderstandings on the path to getting there.

[Andrew Troup]

Cotemar

I, too, think the 'kool aid' comment was rather, shall we say, 'immoderate'.

I don't have a problem with lightly suggesting a group of people might have imbibed 'the kool aid' of an established trend, but I think you need to shoulder a different burden of proof before suggesting that a particular forum member is metaphorically dispensing it, and risking starting a misleading and potentially dangerous trend.

You present instead a rather simplistic assertion -- one which is flawed, to my mind, because it seems to be based on a supposition that technology items will necessarily fail if used in circumstances which they were not designed for.

Climbers would still be using natural fibre ropes if their predecessors had never walked into a boat chandlery and walked out with ('unsuitable', by your reasoning) nylon ropes.

Evans is just returning the favour .... and, as the early climbers probably had the intelligence and integrity to do, he tested it at length, in some rather hard places, before he ever brought it to anyone's attention.

[conachair]

Originally Posted by Andrew Troup

l
It's amusing to be repeatedly told that an ultra-heavy chain is not enough, by those who have never tried it: in sheltered waters situations where the problem is wind, rather than waves of roadstead proportions, I'm happy to report that snubbing is not an issue except in very shallow water

All the "repeatedly told" I've seen was to do with even heavy chain having little effect on the catinary in extreme conditions, it won't keep the chain on the seabed where it attaches to the anchor. Not seen anything about snubbing, though hard to argue against rope being better.

[Andrew Troup]

Dockhead

Thanks for that. Nice to know I'm not completely on a limb on this one.

I think people overlook that, in order to get a 'run up' to snub with, the boat has to be free to move aft or sideways, and/or the bow has to be free to pitch up.

By the time a heavy chain is drawn bar-taut, it's fair to say it has the boat's undivided attention. The ship's head will not be deviating off to one side: it will be pointing in line with the chain, and be trimmed down by the bow.

Even if a deep trough drops the bow six feet, the chain will accelerate downwards at "g", (the wind preventing significant catenary from redeveloping) and unless in a roadstead situation, the next wave will be following so close behind that the boat-chain system will not have had time to rearrange to a significantly lower potential energy level.

This implies that the boat will be pulled through the next wave, exposing the chain mainly to the 'frontal area' forces of the water hitting the bow end-on, which while significant, is nothing a decent chain anchor should struggle to cope with.

Those who claim that a chain catenary cannot work if the wind force is enough to straighten the chain are either thinking of roadstead waves or (more often) simply misunderstanding how catenary absorbs energy. By the time it's straight, the chain (if sufficiently heavy for the energy input) had done its job and prevented the snub.

When it's straight, you're in the position of the state trooper pursuing Burt Reynolds trafficking moonshine, on a very dusty shingle road with lots of sharp bends. (Which tells you how long it is since I watched such a cr*ppy movie !)

Unusual for a Hollywood depiction of a law enforcement officer, this guy was smart and capable of lateral thinking.

His tactic was to reduce his following distance to zero, on the ingenious basis that in that specific situation, when it comes to following distance, more is better, but none is best.

Similarly, you can't snub a chain when it's already bar taut. And provide it's heavy enough to have decelerated your boat to zero in the process, it won't snub when it starts slack, either.

Unless the waves are big enough to increase potential energy (by lifting the entire boat) as well as kinetic energy (by carrying it along) at a rate and intensity the chain catenary cannot handle.

[Dockhead]

Originally Posted by Andrew Troup

Dockhead

Thanks for that. Nice to know I'm not completely on a limb on this one.

I think people overlook that, in order to get a 'run up' to snub with, the boat has to be free to move aft or sideways, and/or the bow has to be free to pitch up.

By the time a heavy chain is drawn bar-taut, it's fair to say it has the boat's undivided attention. The ship's head will not be deviating off to one side: it will be pointing in line with the chain, and be trimmed down by the bow.

Even if a deep trough drops the bow six feet, the chain will accelerate downwards at "g", (the wind preventing significant catenary from redeveloping) and unless in a roadstead situation, the next wave will be following so close behind that the boat-chain system will not have had time to rearrange to a significantly lower potential energy level.

This implies that the boat will be pulled through the next wave, exposing the chain mainly to the 'frontal area' forces of the water hitting the bow end-on, which while significant, is nothing a decent chain anchor should struggle to cope with.

Those who claim that a chain catenary cannot work if the wind force is enough to straighten the chain are either thinking of roadstead waves or (more often) simply misunderstanding how catenary absorbs energy. By the time it's straight, the chain (if sufficiently heavy for the energy input) had done its job and prevented the snub.

When it's straight, you're in the position of the state trooper pursuing Burt Reynolds trafficking moonshine, on a very dusty shingle road with lots of sharp bends. (Which tells you how long it is since I watched such a cr*ppy movie !)

Unusual for a Hollywood depiction of a law enforcement officer, this guy was smart and capable of lateral thinking.

His tactic was to reduce his following distance to zero, on the ingenious basis that in that specific situation, when it comes to following distance, more is better, but none is best.

Similarly, you can't snub a chain when it's already bar taut. And provide it's heavy enough to have decelerated your boat to zero in the process, it won't snub when it starts slack, either.

Unless the waves are big enough to increase potential energy (by lifting the entire boat) as well as kinetic energy (by carrying it along) at a rate and intensity the chain catenary cannot handle.

Very, very interesting. I learned something new from this post. All the logic here seems unassailable, and it seems to indicate that catenary is more effective than it might seem in our simple imaginations. I had sort of thought about the idea that the chain will still be trying to pull the boat back, even when it's bar tight. It means it's actually still snubbing -- just in one direction instead of two. So it's all more complicated.

But I think actually everyone is correct here: Ultimately the effect of catenary is limited somewhere, which you say yourself at the end. At that point your snubber is the only thing you've got (besides your breaking bow roller ) to absorb energy -- at least in one direction.

One thing I neglected to mention at first which seems really important -- snubbing doesn't just protect gear from being broken, it dampens forces which can pull the anchor out of the bottom. The anchor being ripped out of the bottom might prevent chains from shattering in many, perhaps most cases, but that will be cold comfort when your boat goes on the rocks.

[Andrew Troup]

Hmm - Dockhead -

I don't mean it's still capable of snubbing when it's straight, I mean it's DONE snubbing. It doesn't need to snub by the time it's straight.

When there's no slack, there can be no snub.

[Andrew Troup]

I quite agree about dampening the forces for the sake of the anchor.

A chain is not heavy enough to get away without the assistance of an elastic snubber if, in a given set of conditions, the boat is still appreciably moving by the time the chain comes tight.

An elastic snubber, and chain catenary, are two ways of preventing peak loads, greatly in excess of any steady-state loads, reaching the anchor.

The more you have of one, the less you need of the other.

[Dockhead]

Originally Posted by Andrew Troup

Hmm - Dockhead -

I don't mean it's still capable of snubbing when it's straight, I mean it's DONE snubbing. It doesn't need to snub by the time it's straight.

When there's no slack, there can be no snub.

Indeed -- speed will be off by definition. The big question, however, is how fast the boat was still going in the instant beforehand -- which is the moment when the damage will be done.

As you said, if the catenary effect was enough to absorb all the force exerted on the boat as it moved back, then no problem.

But if the force was greater than that, such that there was still unabsorbed kinetic energy at the moment when the catenary came out, then you'll have an impact. I guess your point (one of them) was that with a super heavy chain, there are few scenarios when that could realistically happen -- I get that too.

I am guessing that the amount of force which can be absorbed by chain catenary is a dead simple calculation -- the weight in water (not mass) of the chain multiplied by the average lift. My chain weighs 330kg; if I have half of it out, that's 165kg (ignoring the effect of the water). It's 50 meters long, so a long loop of it in deep water will probably be something like a 12.5 meter average lift. That's roughly 12.5 * 165 * 10 = 20,625 Joules of energy. That's a big number -- an illustration of the power of catenary.

Another point on catenary: when the chain is hanging straight down, any force applied to the end of it will go nearly 100% into lifting the loop, with approaching 0% of the force transferred to the anchor. As the loop gets shallower and shallower, a greater and greater proportion of the total force will be transferred to the anchor. So as the force goes up, the beneficial catenary effect fades away.

[Andrew Troup]

OK, seems we're on the same page on that. I think you're right to focus on 'force exerted on the boat as it moves back', because in most scenarios, a heavy chain will be well able to cope with energy which is stored in the form of the boat's speed.

So it's the incremental forces we have to deal with, such as a phenomenal gust or a wave hit. And the good news is that these do not generally have sufficient time to 'store' energy in the system: it's a real time transaction.

A wave hit does not add as much real-time energy as it might seem, if it's bow on. The yacht in the youtube vid punching out through the surf at

Point Panic! Rogue wave nearly takes out sailboat in Waikiki Harbor - YouTube

does not have a massive prop capable of thousands of pounds of thrust, and it's certainly not going fast enough to make it on stored kinetic energy

The exception is waves big enough to lift the entire centre of mass of the boat significantly. This is a game changer, it seems to me, in that it can store massive amounts of energy very fast, enough, in the extreme, to snap the heaviest chain, because the only relief to a taut chain, once the bow has been buried, is to pull the lifting boat forwards, against the run of play, and the acceleration rates imply very high forces, resisted by the boat's inertia

It's something which is not a problem for big ships, partly because of scaling, and partly because of averaging (they're so long that no waves invading any realistic anchorage will be long enough not to be cancelled out by similar troughs over the ship's length)

I think this is a big part of the reason why big chains 'do it' for big ships, providing a sufficient and complete relief from snubbing in almost all circumstances, in a way even I cannot pretend they do for pleasure boats.

[JonJo]

Cotemar, Any comments on Kool Aid went right over my head - I have no idea to what you refer, so no offence taken. An advantage of a different culture and a sheltered life

But

A significant number of people use climbing ropes as snubbers (I am an advocate). 'Second' hand they are cheap and not much different to a used octaplait (of the same dimensions) - and people would use octaplait for years (and once its been used on a six month cruise its as second hand as a retired climbing rope). Most people use their snubbers until they actually fail (guilty as charged) or until they see actual chance of failure. Snubbers are (important, critical etc etc) but disposable (and are not the ultimate, we still rely on the chain).

The difference between climbing rope failing (and injuring a climber) and a failed octaplait is that the former is a huge lawsuit and the latter, a sudden reliance on the anchor chain. The first is embarassing? and expensive, the second an inconvenience. Which is why climbing ropes on artificial rock walls are retired. Additionally anyone using retired climbing rope knows its 'retired' and accepts the responsibility.

But climbing rope of 11mm has the same strength within +- 10% of 11mm octaplait, or 3 strand. Its just Nylon. If you can find anything to contradict this, I'm interested. But dynamic climbing rope is specifically designed to be elastic, which is what snubbers are all about. So we have a nylon, specifically designed to be more elastic than 'normal' nylon with a similar strength (and if you use retired climbing rope, cheap).

Climbing rope enjoys more realistic testing, for climbing application (and, by accident, for snubber application) than 3 strand nylon or octaplait. Climbing rope, dynamic - enjoys dynamic testing, not so octaplait nor 3 strand nylon (I have not seen cyclic testing of octa nor 3 strand). What we know of climbing rope is that under the test regime it fails quite quickly, we have no idea about octa nor 3 ply. The realilty is that Octa and 3 ply last quite long periods and I suspect as does climbing rope (our second hand has lasted well, but we have spares on board) and I suspect the loads are much less than the loads in the climbing rope test regime.

The dynamic climbing rope test regime is a number (from memory around 10?) of 1.2t snatches (that's what their tests equate to) but halve that load and failure is 75 snatches (and at 75 there is only a failure of one or 2 internal strands). Consequently I think our, anyone's, snatch loads are less than half of 1.2t - under common conditions. Once you get to beyond, say, 50 knots (once winds get to over 40 knots we, personally are into covering all options rather than get up at 2am) loads might be higher but I might suggest different tactics would be used, more anchors, more snubbers, more chafe resistance (check the forecast etc).

The other factor is that the dynamic climbing rope test is as true instantaneous as you can get, our rodes (with snubbers) also have an'end point' that can (and presumably does) move - the anchor. It is difficult to believe that an anchor 'snatched' at say 500kg does not move (and it probably moves much more than an anchor chain can 'stretch' - so I ignore deformation of chain, as anchor movement is probably greater). But this movement of the anchor 'helps' in the grand scheme of things.

Unfortunately the maximum size of climbing rope is 11mm (maybe 12mm) and for a 60' yacht of 25t this would not be considered adequate and as there is no alternative one would need to use a larger sized cordage, nylon or dacron (and there is not the same opportunity to buy retired cordage). Or you could maybe use 2 climbing ropes, one slightly longer than the other?

So I advocate retired climbing rope because its strong (or stronger than the loads imposed), its cheap (in my case free), its elastic and I accept the shortcomings (though cannot find any major short comings vis a vis octa and 3 strand - except the restriction of size.

And returning to something that someone said earlier - its not about strength (and climbing rope seems more than strong enough) its about energy absorption.

Jonathan

[conachair]

Originally Posted by Dockhead

I am guessing that the amount of force which can be absorbed by chain catenary is a dead simple calculation --

depends on your definition of simple

Describing the shape of a catenary is easy

y=a*cosh(x/a) where a is a scaling factor.

does this link work? Should be a graph with a slider for a.
https://www.desmos.com/calculator/flgasletap

A non linear catenary such as an anchor chain seems much more complex, for my struggling synapses anyway, seems like a sort of cross between a bridle leg and a catenary once the chain lifts off the sea bed at the anchor attachment point with the force required to straighten the catenary increasing exponentially. You can play around with some numbers here..
Cable Sag Error (Catenary Curve Effect) Calculator

Me & Mr Google working on graphing it