Rolling Hitch on Snubber

[Kettlewell]

I can't help you there. That's what the data says. It is possible that the BoatUS insured sample is somehow skewed, but its a pretty big sample. It's also pretty clear what the loss ratio says (about which type of line has the greater loss propensity), and that is true independent of the sample representativeness.

Any chance we can see some data on this? So BoatUS says Dacron mooring pennants are the majority and yet none are available on the market from any major retailer?

[conachair]

Originally Posted by estarzinger

^^ nice paper . . . . very close to where I have ended up here . . . . except I am now going one better and using dyneema rather than polyester for chafe resistance.

Will your join be under water? The heat buildup in the nylon did seem to be a factor.

A similar test with dyneema would be interesting, not sure if it was around when the test was done.

The effectiveness of a riding sail was interesting as well, large reduction in rode tension. I've never used one but maybe should investigate.

[Andrew Troup]

I agree that it's nice to see you finally adopting a more nuanced and less dismissive stance on the polyester question, Dockhead, but I've still got a few problems.

The main one is that you are still focussed on thought experiments and examples which involve nylon's acknowledge unparalleled performance at arresting falls, whether they be climbing falls, or, as in this quote from you:

<<If someone would do an experiment to show how many foot-pounds or whatever of energy a given rope can absorb in a short period of time (some fraction of a second) without breaking, I'm sure you'd see a dramatic difference between nylon and polyester -- I wouldn't be surprised if it's 5x or 6x or more. That would be the pushing the weight off the bridge test, or something similar. And the shorter the period of time, the bigger the difference.>>

I don't think anyone could disagree, but that's not the point.

Nylon is exceptional for the rate at which it can convert mechanical energy into another form. Thermal energy. But it's not great at coping with doing so repeatedly. Climbing ropes have to be discarded after a few falls, even relatively minor, even though they might look perfectly OK. That's why you wouldn't buy second hand climbing rope from someone you wouldn't leave alone with your spouse, kids or boat.

Snubbing an anchor line is NOTHING like arresting a climbing fall or two.

Unless you make a habit of anchoring for only the briefest instants....


I think there's a similar misconception, or lack of nuance, at the heart of your contention that a rolling hitch is always strong, and a sheet bend is always weak.

I don't think it's safe to assume that the one-time-only ultimate breaking strength referred to in knot bibles is necessarily a guide to relative survival of cordage under cylical loading over days, weeks or months,

nor do I think one can confidently assert that a knot which is strong in one context (rolling hitch to a rope) will be equally strong in another (rolling hitch to a chain)

or that a knot which is weak in one context (sheet bend to a single rope end) will be weak in an entirely different one (sheet bend to both parts of a sling)

On this latter point, to avoid further red herrings on this very important and fantastically productive thread*, I've started a thread http://www.cruisersforum.com/forums/...ml#post1245448

*(partly due to challenges like yours, which I'm trying to refute rather than discourage ... and I would be happy and enriched if my refutation were to be plausibly disproved.

The one thing - and I'm not addressing this to you - the one thing I neither derive benefit from, nor enjoy, is to have my input dismissed with bald assertions ... which is a regrettably common occurrence on www forums)

[Andrew Troup]

It occurs to me that one of the few materials I'm aware of which rival nylon and yet are suited to cyclic loading is the stuff kangaroos use for leg tendons.

The rate and quantum of energy storage is phenomenal, and clearly they don't suffer from thermal runaway.

I believe that their locomotion is the most efficient, over long distances on level ground of any terrestrial animal, and I think that's by a considerable margin.

The energy recovery from each landing, recycled into the next takeoff, is so close to 100% it's just not funny.

What's not obvious watching them in action is that their centre of mass exhibits very little change in "height above chart datum".

Presumably the albatross manages even higher efficiency ... but it cheats.

[Andrew Troup]

One other thing about the polyester vs nylon comparisons:

I think, Dockhead, that it's disingenuous to dismiss polyester on the basis of a comparison between a resilient construction in nylon and a low-stretch construction in polyester, on the basis that 'they're the most common forms'

A quick Google on "three-strand polyester" got tens of thousands of hits, including most of the major players, and "octoplait polyester" got thousands, so stretchy constructions are hardly unobtainable.

It strikes me as the sort of tactic which is more useful for winning arguments than for throwing light on best ways to proceed.

[Kettlewell]

Here's an interesting article, though it concerns mooring oil tankers. Basically, they prefer polyester for its durability but say you need to double the length in comparison to using a nylon snubber.

[Andrew Troup]

Originally Posted by Dockhead

.... Some things which surprised me:

* Nylon line is much more abrasion-resistant than polyester. I always thought that it was the other way around, so this is something completely new. Nylon octoplait failed after 13,173 cycles of the Coast Guard abrasion test; polyester double-braid failed in 8,134. Same diameter -- 1/2" Table 20.
....

It strikes me that (apart from changing two variables at once - diff matl, diff construction- sigh!):

One reason Nylon has a bad rep for chafe is that in many cases it's the 'fault' of the extra elasticity, causing rubbing and hence chafe.
The CG tests presumably take THIS variable out of the mix.

Interesting to know if the ropes were wet: refer Evans' post above about the supposed better performance of polyester against nylon in chafe, when wet.

I think it bears pointing out that using polyester where it passes the chock is a good way to reduce rubbing at the chock, PROVIDED it's polyester all the way to the sampson post, bollard or cleat. Ideally the cleat should be as close as possible to the fairlead.

If the snubber reverts to nylon and is run all the way aft to a towing cleat at the quarter (which some people prefer, especially on multis) , the polyester will be rubbing as much as a nylon warp would, and all bets may be off.

[Delfin]

Originally Posted by noelex 77

Old polyester fire hose is reasonably good.
Like the climbing rope it is often discarded when its still in good condition, or even unused ( fire hoses in buildings need periodic replacement) so it is available at no cost if you ask around.
Its not as abrasion resistant as Dyneema, but its easy to slip a new length on when necessary. It stores flat so a long hose can be kept and cut up when needed without taking up much space.
For small patches leather is great, cut up old handbags.

Recycling at its best.

Hose pipe is also often used, but it stiffens the line and makes it awkward to stow. To have a large enough internal diameter there is always the risk that it was toilet outlet hose and I am not recycling that.

A stunning amount of knowledge and information can be derived from this thread, so thank you, all contributors.

My humble contribution is in the area of chafe protection. Having tried everything you can think of, I found that leather usually described as "Harley" leather that can be purchased from Tandy Leather in Seattle, and shipped anywhere in the world is the ultimate. It is around 1/8 - 3/16 thick and cured black, like saddle bags. The color of the leather is fast - I've never seen any bleeding. I bought about half a cow's worth and have found a great many uses for it, not the least of which is line chafe gear. I purchased a leather punch from a hobby shop and use that to pre-drill stitching holes 1/4" apart. I use line whipping for the stitching.

After 3 years usage, the chafing protection I sewed onto my mooring lines are essentially unaffected by usage. I finally used a leather cream on them as they were starting to lose color and they basically look like new. After three years!

The technique is to cut the leather about 1/4" under the diameter of the line. I use a razor knife. Then, when stitching, the leather should be so tight that the lay of the strands should be visible. this ensures the leather won't creep out of place with usage. I'm using 3 strand nylon for dock lines.

I have also used the same material to sew onto the rails of the Boston Whaler tender so they don't ding the paint when tied alongside Delfin, as well as to wrap the steel drum of the Pullmaster winches so the cable won't chafe the painted drum, as well as various areas where one thing bumps up against another.

Really, this kind of thick leather, properly applied is to any other method of providing chafing protection what Boudeaux is to Kool-Aid. I'll try to post some pictures if anyone is interested.

Thanks again for all the knowledge transmission here - much appreciated!

[JonJo]

Evan,

If you find a way to splice climbing rope it might be useful, otherwise its bulky knots, which stretch under load and then stay stretched when the load is released (because they are (or can become) so tight. Climbing rope is difficult to knot in the first place. For chafe I'd thought of hollow tube Dyneema, I'm investigating, or Dyneema cordage without an outer braided sheath and running the snubber inside (but not sure it would work). But we have used leather, old fashioned luggage (charity shops) and we too have a leather punch but sew with Dyneema fishing line.

One of the best threads I have read, quite civilised (for the ether).

Thanks

Jonathan

[JonJo]

Originally Posted by estarzinger

So I think I will make up one of these "Dyneema from the cleat to over the roller and then nylon to the soft shackle" snubber lines.

I have some extra 1/2" amsteel onboard - its 7' from the roller to the cleat. Add 1' to go over the roller, plus 5' to go around the cleat and some extra and it sounds like the dynnema part should be 13'.

I have 5/8" nylon on board but will try to dig up some 1/2" nylon. For this design it seems like max elasticity is correct - that would be nylon brait I guess? I think I might go for 15' of that - splices loops on both ends - cow hitched to the dyneema and stainless thimble for the soft shackle (I know Jedi does not think the thimble is necessary but I like them). The nylon part can always be swapped out later.

Before I make it . . . . any comments . . . . does that sounds about right?

Evan,

Is your 'snubbing' portion not a bit short at 15'? Or is this length defined by the physical attributes of the application? Had you also thought of doing it the other way round, Dyneema to the soft shackle on the chain, which will be the wet portion (potentially all the time) but keep the nylon up on deck which might be a bit drier (though not if its raining).

Jonathan

[SailFastTri]

Originally Posted by estarzinger

Well lets address that with some data . . . In the NE of the USA approximately 60% of the mooring pennants are Dacron double braid, according to boat US insurance data. Approximately 5% are nylon 3strand or brait.

It's pretty hard to tell from the outside if a double braid with a Dacron cover has nylon or Dacron inside. The Boatus data suggests more have Dacron than you think. The Dacron double braid is way less expensive than the hybrid lines, and the loss history (%lost/%used) is not much different.

Do you know anyone other than Yale making a hybrid (nylon core, lower chafe cover)? Serious question, as I am interested in doing some testing. I doubt that Yale has 100% market share of mooring pennants.

Interestingly New England Ropes specialty mooring pennant line is 100% dyneema! Eg they have focused almost exclusively on chafe, and have decided on almost zero stretch as the correct engineering!

Samson has 7 ropes listed for the mooring pennant application. Five of them have no nylon in them. Two have nylon, but neither are 3 strand or 8strand brait.

I suspect the BoatUS data is failing to distinguish between Polyester braid over nylon core hybrid, and is just calling those pennants "polyester".

[Dockhead]

Originally Posted by Andrew Troup

It strikes me that (apart from changing two variables at once - diff matl, diff construction- sigh!):

One reason Nylon has a bad rep for chafe is that in many cases it's the 'fault' of the extra elasticity, causing rubbing and hence chafe.
The CG tests presumably take THIS variable out of the mix.

Interesting to know if the ropes were wet: refer Evans' post above about the supposed better performance of polyester against nylon in chafe, when wet.

I think it bears pointing out that using polyester where it passes the chock is a good way to reduce rubbing at the chock, PROVIDED it's polyester all the way to the sampson post, bollard or cleat. Ideally the cleat should be as close as possible to the fairlead.

If the snubber reverts to nylon and is run all the way aft to a towing cleat at the quarter (which some people prefer, especially on multis) , the polyester will be rubbing as much as a nylon warp would, and all bets may be off.

Wow, so many complaints -- I hardly know where to start

I guess, in no particular order:

1. I got over the concept of "winning arguments" when I was about 15 years old. Alas, that was many decades ago . All the more, with people I don't know, and over the Internet. I participate in these discussions to learn something, and the best possible outcome for me is if it turns out I had believed something which was totally wrong, and someone helped me to see the light on this or that. It happens all the time, almost every day Another, rather rarer reason I participate in these discussions is to try to help some respected friend, like Evans, avoid some error, where he doesn't see something obvious, or has embarked on a wild goose chase. Of course I'm not always right, but sometimes I render a useful service or two.

2. One man's "bald assertion" is another man's statement of a thesis. I apologize if any of mine seemed dismissive of anyone else's input, but I think I was pretty obliging, in digging up facts and evidence, when pressed. I admit that my posts were not masterpieces of nuanced argument, but when you're dealing with obvious propositions, nuanced argument does not always fall to hand; I am no Socrates, I'm afraid . Obvious propositions are dangerous, of course -- they make one lazy, and it's good to be called on them from time to time. But amazingly enough -- obvious propositions are not always wrong!

3. My position on polyester versus nylon for snubbers has not, actually, become more nuanced; I don't really see much room for nuance. Using polyester double braid for a snubber is like making a shock absorber for your car out of a wooden block. It's theoretically possible, and yes, wood is not "incompressible". Yes, I should have said "for all practical purposes" in front of various attributive adjectives like "inelastic". I don't that this position should be hurting anyone's feelings -- no one uses polyester snubbers, not even Evans, as of a couple of days ago.

4. I compared polyester double braid to nylon octoplait and three strand, because this was the exactly relevant comparison -- the former is what Evans was using, and the latter is what the entire rest of the cruising world uses. Sure, you might soften the wooden block a bit by using polyester octoplait, but what's the point? It's an empty theoretical question, which wouldn't make any principle difference anyway.

5. Climbers falling on their dynamic lines and sailboats snatching up against their anchor chains when the catenary runs out are highly analogous situations, from the point of view of physics, demanding exactly the same materials properties, so this was a very apt and usefully illustrative line of discussion, I think. These are both cases of violent snatch loads which can break gear loose, snap chains, or kill falling people, as the case may be. Dampening a violent snatch load demands of the material that it should absorb energy, so the material needs to have the ability to absorb a lot of energy relative to a given quantum of strength, so that it will resist breaking in the process. I think this is really obvious to most sailors without further explanation or proof, but for those more demanding I did dig up the data demonstrating this. For a given amount of strength, the type of snubber used by 99.9% of cruisers absorbs approximately 6x more energy than the type of snubber which Evans was using and which he has fortunately now abandoned.

6. There is one fundamental and relevant difference between climbers/dynamic lines and sailors/snubbers -- climbers can't afford to carry lines which are much bigger than the minimum required to save their lives once, whereas it makes no difference to us whether we use an 18mm or 22mm nylon line. And we don't need the snubber to save us once; it needs to save us repeatedly. So yes, climbers destroy their dynamic lines in one fall, but this is merely the result of the sizing of their dynamic lines, not because there is any basic difference between the snatch load created by a falling body, and that created by the catenary coming out of an anchor chain. Nylon will happily take a cyclic load almost forever as long as the load doesn't go to zero at each cycle, and as long as the peak force is not more than 60% of the breaking strength (see the Coast Guard study). So that's why our nylon snubbers work perfectly well and last for years, although they are doing fundamentally the same work as climbers' dynamic lines.

7. As far as knots are concerned, it doesn't seem to me that you read my posts at all. I do not press the rolling hitch on anyone; it is a modest knot of modest properties and I never claimed anything else for it. I specifically said that just about any knot will do for a properly designed snubber (right size and length; energy-absorbing material), even a couple of half-hitches. The point of everything I wrote about knots in this thread was merely to say that of all the many drawbacks of the rolling hitch (slipping; coming undone), lack of strength is not one of them. Evans was on a wild goose chase, worrying about his knots, and I was just trying to help him back onto the right track. For me it was as if a friend of mine had just installed solid wooden shock absorbers on his car, and was surprised when the suspension a-arms were ripped out of the frame after a run on a bumpy road, and starts worrying in great detail whether he ought not have better used titanium rivets, instead of aluminum ones, to install his a-arm brackets. "Mate! You've got wooden shock absorbers!! Forget the rivets! We all use aluminum rivets; they are not the problem here!" was kind of what I was trying to tell him, in the discussion about rolling hitches.

8. As to chafe -- I tend to agree. Everything I ever read or heard indicates that nylon is much less chafe resistant than polyester, and I suspect that this is a function of its elasticity, as you wrote, and the Coast Guard study did not persuade me. I was merely reporting as a matter of interest, as something surprising (which I specifically said). In my own cruising life, I have had pretty good luck with various types of nylon ropes, including snubbers, holding up reasonably well to chafe, but I keep this probable weakness of nylon to chafe in mind, preventing chafe as much as possible, and inspecting frequently.


I seem to have irritated you with my posts; this was not intended, and for this I apologize . Peace, brother. It should all be good fun and occasionally educational -- it's only the Internet.


Meanwhile, the OP has proven himself to be much wiser than all of the rest of us, and has already moved on far, far beyond these subjects. I would expect nothing less from Evans, who -- it is no secret -- is one of the CFers I respect most of all, and read with the most interest. He has reintroduced elasticity into his snubber and is now pushing the envelope of snubber design with novel methods of attachment and spectra leaders to prevent all chafe -- wow! I started out thinking that this is grossly overengineered -- a solution looking for a problem which doesn't exist -- but I am starting to be attracted to this super-snubber, and I'm starting to toy with the idea of making one myself!

[JonJo]

Dockhead

+1, mostly.

I query your para 6 where you suggest nylon is safe if cyclically loaded to less than 60% of its breaking strength. Or I do not query the 60% but -

Climbing rope has a breaking strength of around 2.5 tons, so 60% is 1.5t! its the load I query, not the 60%. Whereas this load might eventuate in extreme circumstances this would be, surely, not considered normal. I know it depends on the boat but I was thinking 40' not 70' (or 30t). Consequently whats wrong with climbing rope for normal situations (and a 40' 10t yacht) and when extreme conditions are forecast - simply (if simply is the right word) use 2 climbing ropes. The idea of using 16mm octaplait for a 40' x 10t yacht seems to defeat the object, as the elasiticty will not be there, unless the load is 1.5t. Actually if the forecast is such that loads of 1.5t are to be anticipated one might have more than one anchor deployed anyway.

Jonathan

[Dockhead]

Originally Posted by JonJo

Dockhead

+1, mostly.

I query your para 6 where you suggest nylon is safe if cyclically loaded to less than 60% of its breaking strength. Or I do not query the 60% but -

Climbing rope has a breaking strength of around 2.5 tons, so 60% is 1.5t! its the load I query, not the 60%. Whereas this load might eventuate in extreme circumstances this would be, surely, not considered normal. I know it depends on the boat but I was thinking 40' not 70' (or 30t). Consequently whats wrong with climbing rope for normal situations (and a 40' 10t yacht) and when extreme conditions are forecast - simply (if simply is the right word) use 2 climbing ropes. The idea of using 16mm octaplait for a 40' x 10t yacht seems to defeat the object, as the elasiticty will not be there, unless the load is 1.5t. Actually if the forecast is such that loads of 1.5t are to be anticipated one might have more than one anchor deployed anyway.

Jonathan

Points all well taken.

The 60% comes from the Coast Guard study.

And I'm not sure whether you can express shock loads in such terms at all. A regular load, like what we rate ropes for, is a sustained force; a shock load is a certain amount of mechanical energy released in a short period of time. This is way beyond my amateurish engineering skills. One thing I was forced to discover in the course of this thread was that the engineering is much more complicated than I imagined, with hysteresis and all kinds of other factors in the mix.

My own approach to "engineering" snubbers -- if you can call it that -- is simple trial and error. A snubber is strong enough if it can withstand a good whack when in rough weather and poor shelter a wave pulls all the catenary out of the chain and slams your boat into the snubber. It's elastic enough if it dampens and softens the impact. It's too elastic if it lets you bungee cord all over the anchorage. For calm weather, you can use a lighter and shorter one.

For my boat, 6 meters of 16mm nylon octoplait works well for the lighter and shorter one. 10 meters of something thicker is what I use for rougher conditions.

Update: Here is something on how to calculate shock loads: How to Calculate a Shock Load | eHow
Indeed, it is a different calculation from x kilograms of load. It can be seen that a 25 ton boat moving at half the speed of a wave will generate a huge amount of force when snatched up against a chain. This is the force that the snubber must dissipate. I guess snubbers could actually be engineered, using these formulae.

[carstenb]

This thread is going to need some serious rereading in order to follow everything. There were some comments earlier about "how much elasticity etc", and here is my (very) humble 2 cents:

It seems to me, that in order to reduce "sailing" we want the snubber to act like the string on a double recurve bow. In essence, be difficult to stretch at the start and get easier along the way, opposite when returning to original position.

If we agree on the above, then the question becomes how to rig this? A bridle is basically the same as a bowstring. Would an answer be to rig a recurve bow - like object across the bows of the boat? This would be difficult - are there any other methods?

I'm thinking a bit outside the box here, so feel free to shoot me down, but why not?