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Old 30-05-2013, 20:29   #331
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Re: Rolling Hitch on Snubber

Jonjo - My post was really to get people to think about the nylon snubber as , as you say a consumable. The term that's used with nylon is elongation. With the increase in moisture adsorption you loose strength and modulus, but gain in elongation. So with added moisture you get better shock adsorption. If you've ever taken off a nylon plumbing fitting that has been saturated in water and seen it simply give, "smooch" that's the effect of the water degradation. Not really good for rigid parts, but great in rode. As you add fillers and treatments you almost always gain in one characteristic and loose in another.

Nylon reacts to water (fresh or salt) the same way, except that there are technical papers that suggest the salt may not flush out of the rode at the same rate as the water. Also remember that when nylon reaches the ambient humidity level it stops degrading and reaches it's elongation limit.

My point was maybe not as clear as it should have been. Simply put, nylon is an excellent material for anchor rode, snubbers, etc. However, unless you see it chaffing you will never know when it's ready with little effort to break, so like reefing, when you're first concerned about it's strength it's time to replace it.

Andrew Troup - Dupont refers to the effect of chlorine as Fair. On a 4 part scale it the next to lowest value. Either way, it's not a good idea to soak it in chlorine, even diluted in water. I have seen several round the world sailors soak their rode in chlorox laced fresh water. They swear by it, it makes their rode softer and look great, but at what unseen cost.

Nylon is one of those materials that often never tells you you have a problem, till you actually see it manifested.
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Old 30-05-2013, 20:30   #332
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Re: Rolling Hitch on Snubber

Delfin and SP,

Delfin, I would not presume to correct anyone's maths!

The bit that's 'wrong', from my posts is that we do not know the velocity and kinetic energy depends on 0.5 of mass times velocity squared.

Consequently having a handle on velocity is pretty critical, in that my 5t yacht at 1 knot generating 1250 joules suddenly becomes around 10,000 joules at 3 knots. The assumption is that the yacht is galloping across an anchorage being struck by bullets from a variety of directions, another reality is being hit by continuous 4' waves when the rode is bar tight (and if the galloping yacht speed is difficult to measure I'm not sure what to do about 'hobby horsing' at anchor in waves).

But I do like the idea of 'staggered' snubbers, a thinner one for 'normal' usage which is then supplemented, not replaced, by a stronger one (or ones) for more arduous conditions. thus enjoying the elasticity of the thinner one but backing it up with something else (also elastic but less so) for tougher times.

Jonathan
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Old 30-05-2013, 20:39   #333
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Originally Posted by Andrew Troup View Post

I personally think that the slack should not be based on how much the snubber can stretch, but on how much you NEED it to stretch.

And the latter will preferably be a smallish proportion of the former.

Unless conditions were likely to get rapidly worse and/or the boat would be unattended, I would personally err on the side of less slack, and pay out a bit more every time there was a severe snub.
Well, no. You need it to stretch up to 30% in extreme situations and then hold without breaking. You never want the force transferred to the chain and windlass. Erring on less slack means windlass gears ripped up or worse. Assuming there is no chain stopper of course.
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Old 30-05-2013, 20:42   #334
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Re: Rolling Hitch on Snubber

Watercolor,

I was not intending to be critical, if that's what was perceived, sorry.

I tend to forget that cats are different to monos and we have a slight advantage in that we have 2 snubbers in use simultaneously. If one fails, which it has done, with a bang, then we swing side 'on-ish' but we do have another. And the bang cannot be missed. This might be a justification to set up bridles for yachts, if/when one side goes the other would still be there.

You are right, when our snubber broke, we had no idea that is was in any way suspect.

But we carry 2 complete snubber spares (we seem to carry 2 spares of everything). And though we have 2 of everything we also find when one 'thing' goes its mate goes soon after. So if one bridle were to fail, we would replace both.

Currently I do not know how long climbing rope will last. But I have noted that when we tested some, fairly harshly, we do seem to have 'singed' the outer braid (its got crispy bits).

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Old 30-05-2013, 21:06   #335
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Re: Rolling Hitch on Snubber

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Originally Posted by s/v Jedi View Post
Well, no. You need it to stretch up to 30% in extreme situations and then hold without breaking. You never want the force transferred to the chain and windlass. Erring on less slack means windlass gears ripped up or worse. Assuming there is no chain stopper of course.
Given that nylon is potentially going to fail at or before 30% extension you might need to deploy that 30% worth of chain but before, long before, you reach that 30% stretch you need to be thinking of adding a stronger snubber. If you rely on one snubber and it fails (at its maximum extension which might be degraded by use) then you will be sitting on the foredeck in arduous conditions trying to install a new one (and I would not fancy attaching a new snubber to that dyneema soft shackle on the chain under those circumstances.

It is by accident but because we run our snubbers from the transom up the side deck we can mark a fixed point on one of the snubbers, at a stanchion, and we know exactly how much the snubber has stretched. This is going to be more difficult to achieve, measure, if you run a snubber off a samson post and it disappears into the dark toward its attachment point on the chain.

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Old 30-05-2013, 21:09   #336
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Quote:
Originally Posted by JonJo View Post

Given that nylon is potentially going to fail at or before 30% extension you might need to deploy that 30% worth of chain but before, long before, you reach that 30% stretch you need to be thinking of adding a stronger snubber. If you rely on one snubber and it fails (at its maximum extension which might be degraded by use) then you will be sitting on the foredeck in arduous conditions trying to install a new one (and I would not fancy attaching a new snubber to that dyneema soft shackle on the chain under those circumstances.

It is by accident but because we run our snubbers from the transom up the side deck we can mark a fixed point on one of the snubbers, at a stanchion, and we know exactly how much the snubber has stretched. This is going to be more difficult to achieve, measure, if you run a snubber off a samson post and it disappears into the dark toward its attachment point on the chain.

Jonathan
When the snubber breaks, you have determined you need a stronger snubber, not that you need to put out less slack in the chain.
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Old 30-05-2013, 22:58   #337
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Re: Rolling Hitch on Snubber

Jedi, if it breaks at 40knots I agree, if it breaks at 60knots then I'd be thinking its good enough for 99% of the time and a back up would be good for the 1%. Catering for 60 knots means you might not be catering for 40knots? Maybe this being picky.

There again if it breaks at 40 knots, or 60knots maybe its been fine but its just cycled too many times (its old) and an identical snubber will last perfectly well for another 12 months.

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Old 31-05-2013, 00:18   #338
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Re: Rolling Hitch on Snubber

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Originally Posted by s/v Jedi View Post
Well, no. You need it to stretch up to 30% in extreme situations and then hold without breaking. You never want the force transferred to the chain and windlass. Erring on less slack means windlass gears ripped up or worse. Assuming there is no chain stopper of course.
(my emphasis added)

I don't personally think an elastic snubber should be the last thing between the windlass gearwheel and disaster. There should always, I reckon, be a relieving tackle, strop, chain compressor, or stopper of some sort.

It seems to me the possibility of an elastic snubber breaking is inseperably bound to having it doing its job properly.

And my point was that is should be extended the minimum compatible with doing its job, and that will vary with conditions.

It seems to me the chances of breaking it can only be increased by always worked it to the max elongation, whether or not that is needed.

So I don't see how someone can look at a photo snubber setup and say "that's not enough (or too much) chain slack", without knowing how well matched the snubber is to the boat, and how severe the conditions are.
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Old 31-05-2013, 01:06   #339
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Re: Rolling Hitch on Snubber

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Originally Posted by wsmurdoch View Post


Suggestion to extend life of rubber snubber...

Use Sunbrella to make a loose UV sleeve that's longer than the snubber can stretch.
Sew a short bit of line into each end of the sleeve for closing - like a sail bag.
Tighten sleeve ends a few centimeters away from each end of the snubber.

Just a thought...
James
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Old 31-05-2013, 06:05   #340
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Re: Rolling Hitch on Snubber

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For the spectra loop crowd . . . here is a solution that seems to meet all my criteria:

I made up a very long soft shackle. Wrap it around the chain in any of the possible hitches (this is the klemheist) and then join the soft shackle thru a thimble in the snubber line. The thimble is there to increase the bend radius and make it 'smoother' - not sure if its actually needed.

Attachment 61344

Here is the soft shackle 'undone' . . . this will never jam under load and always be pretty quick to undo (unlike the sheet bend or other knots).

Attachment 61345

Then it struck me there might be an even 'simpler' way to use the soft shackle - thru the chain link, which will obviously never slip and with the spectra I don't think it would chafe. You could take the loop twice thru the chain if you wanted to minimize chafe potential further, but I am guessing it is not necessary and adds a step.

Attachment 61346

This is all independent of the debate about what sort of line to use behind the soft shackle - it could be any type.
Sorry to be a bit out of step, but what size are these 5mm,6mm or 8mm dyneema?
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Old 31-05-2013, 06:38   #341
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Re: Rolling Hitch on Snubber

Earlier in this thread there were some questions about suitability of climbing rope, as it's designed to be stretchy in breaking the one-time fall of a ~200 pound climber.... whereas we're talking about the application in absorbing repeated cyclic shocks for a 10,000+ pound boat.

I also noted the following except from Yale ropes write-up in a whitepaper pitching Yale Brait: (source: http://www.yalecordage.com/pdf/anchoring_tech.pdf) "Three-strand nylon line stretches quite easily when a load is first
placed on it, absorbing little energy. It quickly gets busy as the load
passes 20% of the line’s breaking strength; however, that puts it beyond its
recommended working load. The stretching process within the threestrand
lay generates heat within the fibers, accelerating the rapid fatigue
failure of the line."

Putting this information together in context, I would worry about climbers rope's suitability for use as a snubber in storm conditions.

Please note edits after initial post, in italics
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Old 31-05-2013, 07:44   #342
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Re: Rolling Hitch on Snubber

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Originally Posted by SailFastTri View Post
Earlier in this thread there were some questions about suitability of climbing rope, as it's designed to be stretchy in breaking the one-time fall of a ~200 pound climber.... whereas we're talking about the application in absorbing repeated cyclic shocks for a 10,000+ pound boat.

I also noted the following except from Yale ropes write-up in a whitepaper pitching Yale Brait: (source: http://www.yalecordage.com/pdf/anchoring_tech.pdf) "Three-strand nylon line stretches quite easily when a load is first
placed on it, absorbing little energy. It quickly gets busy as the load
passes 20% of the line’s breaking strength; however, that puts it beyond its
recommended working load. The stretching process within the threestrand
lay generates heat within the fibers, accelerating the rapid fatigue
failure of the line."

Putting this information together in context, I would worry about climbers rope's suitability for use as a snubber in storm conditions.

Please note edits after initial post, in italics
Dynamic climbing rope is not three-strand. Yale are pushing their brait (octo), which is much more suitable than three-strand nylon, but three-strand nylon is very good compared to the alternatives.


Besides that, you can't compare the 200 pound climber's fall to snubbing a boat in a storm unless you know how far the climber is falling, and how much motion the boat has. In fact, we saw that the kinetic energy of an 80 kilo climber falling just 10 meters is similar to the kinetic energy of a 25 ton sailboat at 2 knots. See this post: http://www.cruisersforum.com/forums/...ml#post1247662
We have determined that 2 knots of speed is a lot of motion, perhaps more than most of us every have to snub, enough to produce a 39 ton load if you snatch up against a rigid piece of chain with no other flex in the system.

It follows that a climbing line designed to save an 80 kilo climber from a 20 or 30 meter fall might be very well capable of snubbing a 5 ton yacht year in and year out. It's a question of whether the forces exceed the working load or not. But the job of snubbing a 5 ton yacht might easily be 1/10 or 1/100 of the load resulting from the fall of an 80 kilo climber, putting it well within a load which such a line could handle continuously.
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Old 31-05-2013, 08:19   #343
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Re: Rolling Hitch on Snubber

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It follows that a climbing line designed to save an 80 kilo climber from a 20 or 30 meter fall might be very well capable of snubbing a 5 ton yacht year in and year out.
To be precise, it is not only theoretically possible, but is a simple proven empirical fact. We have used climbing line as our snubber for over a decade on a 15 ton boat - thru a rather wide range of anchoring conditions.

And horror of horrors . . . . that climbing line even had a knot in it (a double sheet bend), and did not explode or self destruct.





Calculations are very nice, and I am all for them . . . . but when a calculation disagrees or is inconsistent with actual real world experience . . . . . its usually the calculation that's wrong
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Old 31-05-2013, 09:24   #344
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Re: Rolling Hitch on Snubber

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Originally Posted by estarzinger View Post
To be precise, it is not only theoretically possible, but is a simple proven empirical fact. We have used climbing line as our snubber for over a decade on a 15 ton boat - thru a rather wide range of anchoring conditions.

And horror of horrors . . . . that climbing line even had a knot in it (a double sheet bend), and did not explode or self destruct.



Calculations are very nice, and I am all for them . . . . but when a calculation disagrees or is inconsistent with actual real world experience . . . . . its usually the calculation that's wrong
Hysterical!

I would add that sometimes it is also because the real-world situation was misunderstood. In fact, calculations work best when extrapolating from one well-understood expereince to another, from an engineer's perspective.

FYIW, the practical and real world reason climbers do not splice; when you go to pull the rope up the cliff behind you the splice with catch in a crack. Knots are removed before hauling up. Additionally, climbing ropes do not break in the knot, they cut over a rock edge, so knot strength doesn't generally enter in. Third, very short falls can be the worst, since there is little rope out, and it has been learned that the knot cinching up absorbs a material amount of energy in these short nasty falls. Downsides? The knot can snag, but experienced climber learn to tie very compact end knots when that is a factor.
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Old 31-05-2013, 17:00   #345
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Re: Rolling Hitch on Snubber

The test for climbing ropes is free falling a 80kg weight, something a bit over 2m. The climbing rope was run through a carabiner, it now runs over a 5mm surface. This is roughly equivalent to pulling with a 1,200kg load. The braking strain of climbing rope is not measured or quoted but people have tested and its around 2t. The measure of climbing ropes is how many 80kg free falls at 2m+ the rope will stand, its not much - about 7-10. 5 mins is allowed between each fall. Presumably the rope fails as a result of heat generated slowly degrading strength. Other tests were conducted dropping a half weight, 44kg, same distance - the life of the rope increased to allow 75-100 drops and then only a few strands broke. Its not mentioned where the failures occur, in the long strand or at the point where it runs over the 5mm edge.

The forces, sorry if someone does not like the term but cannot think how to convert it (quickly) we experience are maybe of the order (max) of the half load test, of 44kg/2m+ fall (or 600kg) but we have much longer snubbers than 2m+, ours are also not shock loading as quickly as the climbing rope test (my guess is the climbing test is seconds, a second, our snatch loads are over seconds - many. Our snubbers are not loaded over a 5mm edge (we, all of us, try to run our snubbers as a straight line)

So halving the internationally agreed climbing rope test increases life factorially, maybe 8-10 times. Our loads are imposed over a period of time, not instantaneously, (this will increase life of the snubber) our snubbers are much longer than used on the standard test (this will increase life). We do not run our snubnbers over a sharp edge. If loaded sufficiently our end point, the anchor, has the ability to move, 1cm-5cm - whatever and - if its a decent anchor - it will stay set. Additionally though the yacht might move at 1.5 knots at its fastest it will be slowed (by the chain catenary) as it reaches the end of its tether - so 1.5knots might be a maximum, its the velocity as the yacht starts to rely on the elasticity of the snubber that is needed to calculate the energy that needs to be accommodated. (So far we are guessing - but a conservative guess might be a good one!).

Another factor not mentioned is that the catenary is being helped by having an elastic snubber. As the yacht moves back the catenary straightens but less than theory will suggest as some of the kinetic energy needed to straighten the catenary is already being taken up by the snubber (equally the stretch of the snubber is less). This also, possibly explains why yachts do not yo-yo - not all of the energy is in the snubber, some is in the chain.

There have been doubts raised over the use of climbing ropes, because they are designed to catch a falling body of say 80kg (and our yachts weigh 10t-25t). Interestingly they are made from nylon, the same as nylon 3 strand or octaplait. The method of construction is designed to increase elasticity, there seems to be a slight loss of ultimate breaking strength of 10mm climbing rope over 10mm octa or 3 strand - but climbing rope is actually tested as close to how we use a snubber as possible. Not so Octa or 3 strand, we know its ultimate breaking strength but no one has done a cyclic loading test. The reality I suspect is that climbing rope probably has a life, when used as a snubber, slightly shorter than 3ply or octa, maybe 1 year less, but you get an extra 25% elasticity (over 3ply or octa). I do confess if I had a 60' 30t vessel I might not use climbing rope, though I might use 2 together (not thought about it) but for our 6t x 38' and I suspect a 12t 45' I would remain quite comfortable.

The other downside to climbing rope is the inability to splice, though maybe Evans has or is looking at that. Kermantle can be sewn and the sewn eye can be stronger than a knot.

Much has been made of heat degrading nylon and the fact that nylon might not work so well when wet - does being wet reduce the impact of heat? Does rope construction impact the degradation of water or heat.

I note Evans measure of yacht speed, as less than 2 knots, which starts to put a figure on the energy developed but I think Evens was measuring yawing - anyone with ideas of movement through wave action, pitching, hobbyhorsing?

I also note the complaints over the use of terminology, apologies to the purists (who are correct).

Jonathan
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