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Old 27-05-2013, 00:42   #181
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Re: Rolling Hitch on Snubber

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Originally Posted by conachair View Post
Maybe not as big a difference as you might think if you compare similar tensile strengths factoring in a 10 or 15% reduction in strength for nylon when wet.

Exsil | Koordenfabriek Van Houte

It's not quite as black and white as all that.
Good info, conachair, on the relative behaviours of actual nylon and polyester cordage under load.

I have some comparison load/extension graphs like those filed, but I couldn't lay my hands on them just now.

I did recall that the behaviour was non-linear, and that the case being put here bore a few of the hallmarks of "a little knowledge being a dangerous thing".
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Old 27-05-2013, 00:59   #182
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Re: Rolling Hitch on Snubber

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Originally Posted by estarzinger View Post

So, my conclusion is that while the rolling hitch does have the advantage of being easy to tie - we all know how to do it, but it is probably not the absolutely ideal knot struicture for this cyclic loading application, and if you wanted to learn one of the other knots (like the icicle) it would be 'better'. But obviously it's not a huge problem as some of you have used the rolling hitch a lot.

But the soft shackle seems to me to beat either the rolling or icicle hitches.
When you say a knot is "better", you should be specific about which quality you're talking about. An icicle hitch is much better than a rolling hitch for attaching a line to a smooth, slippery object, but this quality is not needed for attaching a line to a chain, which is not at all slippery. I have no idea how strong an icicle hitch is, or how secure it is -- I have seen no data. I have certainly not heard anywhere that an icicle hitch is stronger than a rolling hitch, just that it is capable of generating more friction.

Likewise, I have never seen any information to say that the sheetbend is inherently stronger, on the contrary, I was always taught (which doesn't necessary prove anything) that the sheet bend is a quite weak and quite insecure, if very convenient knot. A casual trawl on Google brings up a lot of stuff consistent with what I was taught:

"The sheetbend is a well known knot. It is quickly tied, very easy to untie and does not jam, even when wet. The bad news are, that it is weak and may slip in some situations. Take care to tie the knot so, that both short ends are on the same side of the knot. Nevertheless, sheet bend is probably the best general purpose bend."

BENDS

"The Sheet Bend is a general utility bend valued for its simplicity yet avoided for important or severe work due to its lack of security."

The Sheet Bend & Double Sheet Bend

According to our own Gord May, the sheet bend leaves 45% to 60% of the strength of the rope, compared to 65% to 75% for a bowline:
http://www.cruisersforum.com/forums/...nots-1885.html

A rolling hitch is essentially a clove hitch with an extra turn, so the strength is supposed to be the same -- 60% to 70% according to Gord and other sources, so quite a bit stronger than a sheet bend. But a rolling hitch can fail to grip, especially with slippery ropes on a slipper object, and is not particularly secure, so it's by no means a perfect knot. But lack of strength is not among the several drawbacks of the rolling hitch.


The soft shackle and hard eye splice does look like an absolutely perfect means of attachment of a snubber . Should be extremely easy to attach and release, and perfectly strong, stronger than any knot. For anyone who has any kind of problem attaching a snubber, I guess this is the killer app The Rocna guys (their Knowledge Base is in my opinion the best single resource on anchoring) say that you shouldn't put all the snubber load on one chain link -- they insist on a rolling hitch and shun anchor hooks for this very reason. That criticism would apply to the soft shackle approach, but I personally don't buy it for one second, even though I agree with virtually everything else in the Rocna Knowledge Base. Why, every single link of the chain bears the entire load, not so? So what's the problem with attaching your snubber to a single link? I just don't see it.
Snubbers (Rocna Knowledge Base)


Concerning failure of Evans' snubber and choice of rope, sorry for being a broken record about it, but here's what a rescue training organization has to say about it:

Types of Rope:

There are a number of different ropes available. They can be made
of:

*Nylon - strong, flexible, resists abrasion, reasonable price -
the top choice for rescue operations.

*Kevlar-very strong, stiff, subject to heat and sharp bends-
used in some rescue applications.

* Polypropylene-low strength&durability, floats-sometimes used for water rescue.

*Polyester-low stretch, takes shock loads poorly-not used for climbing or rescue.

* Manila (hemp)-weak, deteriorates rapidly-not acceptable for climbing or rescue

http://www.morganrescue.com/Docs/Bas...nd%20Knots.pdf

Snubbers are similar in basic purpose to dynamic climbing ropes -- energy absorption.
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Old 27-05-2013, 01:09   #183
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Re: Rolling Hitch on Snubber

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Has anyone any idea if it is possible to buy tubular Dyneema to use as a chafe guard. One could slide the snubber though the hollow and position the Dyneema where necessary. Dyneema has good chafe resistance, seems ideal - though maybe too expensive.

Jonathan
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.
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Old 27-05-2013, 01:11   #184
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Re: Rolling Hitch on Snubber

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Originally Posted by conachair View Post
Maybe not as big a difference as you might think if you compare similar tensile strengths factoring in a 10 or 15% reduction in strength for nylon when wet.

Exsil | Koordenfabriek Van Houte

It's not quite as black and white as all that.
Well, that would be true if you're comparing the same rope structure. Nylon double braid is only 2 1/2 times more elastic than polyester double braid of the same strength, as your tables show. But we're comparing nylon three-strand and octoplait, the common forms of nylon cordage, to polyester double braid, which is the common form of polyester cordage, and there the difference is 4 to 5 times.

What that means is that if a 10 meter nylon octoplait snubber is the right amount of elasticity, then to get the same amount of elasticity you would need a 50 meter double braid polyester snubber of the same strength. Factoring in some loss of strength due to water absorption of nylon, ok, say 45 meters.


I have been taken to task by several of you for framing this in simple terms "inelastic polyester" and so forth. OK, ok, already. Yes, you're right, it's not exactly that simple.


The thing we all rightfully worry about in nylon is failure due to heat buildup, which we all know about from Dashew. It should be kept in mind that this is the result of nylon's doing exactly what it is supposed to do in this application -- absorb energy and dampen shock loads. The lesson is that nylon should not be used near its limit of strength. But no rope should be used near its limit of strength. Because of its poor resistance to shock loads, polyester will be even more subject to failure below its theoretical limit of strength, maybe far below its theoretical limit of strength, unless it is protected from shock loads by an elastic snubber.

Because of Dashew, I chose polyester and not nylon for my secondary rode, which is all rope. But like Nick, I always snub it with nylon
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Old 27-05-2013, 01:44   #185
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Re: Rolling Hitch on Snubber

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Originally Posted by Dockhead View Post
Well, that would be true if you're comparing the same rope structure. Nylon double braid is only 2 1/2 times more elastic than polyester double braid of the same strength, as your tables show. But we're comparing nylon three-strand and octoplait, the common forms of nylon cordage, to polyester double braid, which is the common form of polyester cordage, and there the difference is 4 to 5 times.
Any links for that 4 or 5 times? Data on this seems to be a bit hit and miss on google.

Not sure where you get the 2 1/2 times from, the tables in the link don't spec which construction, for worked rope @ 20% of load I make it about 7.5% elongation for poly & about 12.5% for nylon, which makes polyester having about 60% of the elasticity of nylon. Which, fair enough, still needs a 16.66m poly to match a 10m nylon but not as chalk and cheese as people might think.

And how much difference will that make in the real world? Is it even possible to get a handle on such a dynamic non linear system? Dunno, we all use what seems to have worked so far and think we're right

Even more so on the internet.
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Old 27-05-2013, 01:55   #186
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Re: Rolling Hitch on Snubber

When passing the soft shackle through a chain loop the acute bend radius would concern me. Dyneema etc are sensitive to this.
Does anyone know what break load would be in this application after factoring in the bend radius?
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Old 27-05-2013, 02:02   #187
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Re: Rolling Hitch on Snubber

Google's on form this morning

A couple of good links on chafe..

http://pdos.csail.mit.edu/decouto/my...s/seagrant.pdf

BoatUS: Seaworthy



edit:
And for those with some time on your hands..
http://www.dtic.mil/dtic/tr/fulltext/u2/a087106.pdf
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Old 27-05-2013, 02:46   #188
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Re: Rolling Hitch on Snubber

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Originally Posted by conachair View Post
Any links for that 4 or 5 times? Data on this seems to be a bit hit and miss on google.

Not sure where you get the 2 1/2 times from, the tables in the link don't spec which construction, for worked rope @ 20% of load I make it about 7.5% elongation for poly & about 12.5% for nylon, which makes polyester having about 60% of the elasticity of nylon. Which, fair enough, still needs a 16.66m poly to match a 10m nylon but not as chalk and cheese as people might think.

And how much difference will that make in the real world? Is it even possible to get a handle on such a dynamic non linear system? Dunno, we all use what seems to have worked so far and think we're right

Even more so on the internet.
"We all use what seems to have worked so far and think we're right "

Yes -- of course That's because practical experience is usually much more effective than theory. But theory can be fun, too, sometimes, and sometimes even enchances practical experience


I just ran across this: http://www.dtic.mil/dtic/tr/fulltext/u2/a087106.pdf

Which just about exhausts the subject; a very cool paper right on point. Not only strength and elasticity of nylon and polyester in various layups, but even dynamic loading charachteristics, and as if that were not enough! All the ropes are analyzed again after five years of exposure to marine conditions. Very cool!

Just a few tidbits:

* The apparent (dynamic) spring constant of nylon octoplait is more than three times less (that is, 3+ times stretchier in dynamic conditions) than polyester double-braid. Table III. Spring constant is the ". . .most important and most easily understood property of the synthetic line. . ." [for mooring purposes]; p. 36.

* "Nylon line of 8-strand plaited and double-braid construction stretches 2-3 times more than polyester line of the same construction.. . It was also observed that the dynamic elasticity of nylon lines is 2-3 times greater than that of polyester lines. 8-strand plaited lines stretch more than double braid lines. For example, at 30% of rated strength, nylon double-braided line and plaited line stretches approximately 15% and 25%, respectively. Polyester line of the same construction stretches 4% and 11%, respectively." p. 29 So -- according to this, I actually understated the difference between nylon octoplait and polyester double braid -- it's actually more than six-fold, not five-fold.


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.

* Nylon rope is highly resistant to cyclical load failure -- provided, however, that the load is not reduced completely to zero at each cycle. Under cyclical loads where the line goes completely slack, nylon rope can break at much less than its rated load. The Coast Guard explains that with some complicated explanation of the structure of nylon fibers. Perhaps this is the real cause of the hurricane failures described by Dashew, not internal heating. This is undoubtedly a drawback of nylon which should be kept in mind.
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Old 27-05-2013, 02:53   #189
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Re: Rolling Hitch on Snubber

Quote:
Originally Posted by noelex 77 View Post
When passing the soft shackle through a chain loop the acute bend radius would concern me. Dyneema etc are sensitive to this.
Does anyone know what break load would be in this application after factoring in the bend radius?
I thought about that, but there should be a huge reserve of strength. Probably worth checking, however.
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Old 27-05-2013, 02:55   #190
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Re: Rolling Hitch on Snubber

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Originally Posted by noelex 77 View Post
When passing the soft shackle through a chain loop the acute bend radius would concern me. Dyneema etc are sensitive to this.
Does anyone know what break load would be in this application after factoring in the bend radius?
I would be worried about the dyneema getting nipped between the slack link of the chain and either chaffed or caught so it's hard to release. I am keen to see how Evans tests go. Personally I would stick with a prusik loop in my soft shackle, simple to tie, very secure and always easy to undo.

I still think the whole soft shackle snubber idea is pure genius.
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Old 27-05-2013, 03:23   #191
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Re: Rolling Hitch on Snubber

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I would be worried about the dyneema getting nipped between the slack link of the chain and either chaffed or caught so it's hard to release. I am keen to see how Evans tests go. Personally I would stick with a prusik loop in my soft shackle, simple to tie, very secure and always easy to undo.

I still think the whole soft shackle snubber idea is pure genius.
Just an idea - but maybe think of Dyneema tape, rather than rope (or cordage) it might take the sharp bend more easily and be less prone (having a greater contact area) to chafe. In my younger days that's why we used tape running belays. Its easy to make into a loop, as well - we use it (tape) as a soft shackle for blocks - but using nylon tape (its cheaper and chafe is not an issue).

Thanks for the contacts on hollow Dyneema. If I get anywhere I'll disseminate.

A little knowledge goes a long way - a few weeks ago I was ridiculed on this forum for running my snubbers from the transom, the same person (who was keen to ridicule) is now quoting the practice as common.

Jonathan
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Old 27-05-2013, 05:05   #192
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In a place so windy that you worried about the snubber parting?

I've left the boat as well but only in a decent anchorage.
How windy? I'll repeat that our snubbers have no problem with squalls up tp 80 knots wind which is all you can ask for imho... We were also gone for weeks when hit by a hurricane which vaporized the snubber at 120 knots sustained but the anchor still held, just had to replace the chain.
I only leave the boat is good anchorages of course, but squalls and storms don't travel around the good anchorages...
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Old 27-05-2013, 05:09   #193
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I had not been following this thread, I've been out of touch, but have now read the whole thing.

A comment was passed that a snubber should not be more than 6m long. People have been using mixed rodes for decades, as far as I know always 3 strand nylon but more recently octaplait. There has never been an issue of yachts yo-yoing (or surging) as result of the elasticity of the nylon. Based on this snubbers could be any length, much longer than 6m? There is in fact a whole sub-industry selling chain, say 30m spliced to nylon say 40m - its common and popular (or is in the UK).
No, you can't compare snubbers with rope rodes. A snubber is much thinner and thus stretches much more at equal loads. That plus the chafing factor is why we use a snubber on rope rode too.

A 20' snubber of the right diameter stretches enough to absorb the shock loading on the anchor rode.
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Old 27-05-2013, 05:15   #194
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Originally Posted by noelex 77 View Post
When passing the soft shackle through a chain loop the acute bend radius would concern me. Dyneema etc are sensitive to this.
Does anyone know what break load would be in this application after factoring in the bend radius?
I have 3/16 dyneema lashing between chain hook and snubber that does tight radius and no problem. I also have Amsteel Blue runners that are spliced onto a stainless steel chain link without thimble that don't have a problem either.

As the snubber is rather weak compared to the rode, it should never give any trouble for the much stronger shackle.
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Old 27-05-2013, 05:48   #195
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Re: Rolling Hitch on Snubber

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No, you can't compare snubbers with rope rodes. A snubber is much thinner and thus stretches much more at equal loads. That plus the chafing factor is why we use a snubber on rope rode too.

A 20' snubber of the right diameter stretches enough to absorb the shock loading on the anchor rode.
I've just checked Jimmy Green in the UK and they sell, as standard, 12mm nylon spliced to 8mm chain to use as an anchor rode. The UK tends to use G3, or Grade 30 chain. This for a 10/12m yacht. We use 8mm chain and 14m x 11mm climbing rope as a snubber. To me the difference between 11mm and 12mm is not great, or not to say that 11mm is 'much thinner'. The climbing rope will have more elasticity, for a given length, than either octaplait or 3 ply but as the nylon, Jimmy Green's supply, could be 20m-40m it will have a much greater surging effect, or yo-yoing, than our 14m (and no-one using mixed rodes in the UK ever mentions it (but maybe they think it normal).

Admittedly if we used, say, an 8mm snubber of 6m length it might stretch as much as 14m of 11mm - but it would have a shorter lifespan and I'd rather not wake up at 2am with Noelex's bang of a snapped snubber (it has happened to us as well!). So slightly thicker and slightly longer seems to work. Its a balance.

This not to say mixed rodes will work everywhere, we use chain because the seabed can have a voracious appetite for nylon (of many other synthetics). But assuming the owner can stomach the potentially shorter lifespan of a mixed rode they equally stomach the yo-yoing (if it actually occurs - and we do not find it an issue, at all).

Jonathan
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