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Old 03-01-2014, 07:47   #136
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Re: Mantus Chain Hook

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Originally Posted by estarzinger View Post
You don't gain any strength by just running the soft shackle up or down thru multiple chain links. But if you mean to loop it multiple times back and forth between the snubber and chain links (like a lashing) you do gain strength (actually quite a bit, more than linear, because the diamond knot is the weak point and it is only on one loop)
In my testing--and I have not tested this many times--a knot has the effect of weakening every loop in the lashing; they all carry the same load and the weakest one breaks. In testing of climbing sling we see the same thing; the knot breaks at it's rated strength, even if the loop is doubled several times.

This explains why it is best practice to use either a spliced eye or a high-strength knot to start the lashing, and finish with a long chain of half hitches; the half hitches cinch down on the bundle but do not weaken the line. Mostly we just add a a few more turns to allow for this weakening and UV.

Or at least that is how I understand it. Something to test! Parrachute cord lashings with several different knots (a spectra lashing might break the rig!).
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Old 03-01-2014, 09:09   #137
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Re: Mantus Chain Hook

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How about a prussic knot? I dock friend swears by them? Anyone used those?
I use long shackles and prusik knots. Works great no chafe.
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Old 03-01-2014, 10:45   #138
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Re: Mantus Chain Hook

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In testing of climbing sling we see the same thing; the knot breaks at it's rated strength, even if the loop is doubled several times.
That is completely counter intuitive to me . . . (if I understand you)

Don't we know this just from a simple sling - the sling breaks (at the knot) at twice the load which will break that same knot and line in a single rope/straight line pull?

I would have thought that the load on a knot on one strand of say a doubled sling (which then has 4 legs between the load points) would only be (about) 1/4 the entire load.
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Old 03-01-2014, 11:54   #139
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Re: Mantus Chain Hook

OK,

single loop: breaking 1020lbs
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double loop breaking 2240lbs
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Same line and same double sheet bend on both.

It seems clear to me that using a soft shackle as a double loop will cut the loads on the diamond in half and double the system strength.

Ignore the extra safety line I have on the double loop picture to catch the shackle during post break recoil.
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Old 03-01-2014, 12:29   #140
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Re: Mantus Chain Hook

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Originally Posted by estarzinger View Post

It seems clear to me that using a soft shackle as a double loop will cut the loads on the diamond in half and double the system strength.

Ignore the extra safety line I have on the double loop picture to catch the shackle during post break recoil.
This is what I was thinking as well.

My concern with doubling up on chain, is the possibility of pinching or squeezing the loop that passes through the chain link that is closest to the anchor.

I hate to ask you to ruin more of your stuff but I would love to see a test of this concept on chain.

Steve
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Old 03-01-2014, 12:49   #141
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Re: Mantus Chain Hook

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Originally Posted by estarzinger View Post
That is completely counter intuitive to me . . . (if I understand you)

Don't we know this just from a simple sling - the sling breaks (at the knot) at twice the load which will break that same knot and line in a single rope/straight line pull?

I would have thought the load on a knot on one strand of say a doubled sling (which then has 4 legs between the load points) would only be (about) 1/4 the entire load.
I'm sure I did not explain well. Just as likely, I didn't understand you.

Certainly, more passes = less load per pass. Simple proportion.

However, if there are 4 passes (doubled sling), the materials tests 2500#, but the stitching weakens one leg to 2000 pounds, the double sling should fail at 4x2000=8000#. Weakest link.

I tried making a very long soft shackle and luggage-tagging it around the chain; this way I could get the strength. It was stiff and awkward, to me. Instead, I often use a standard 60cm climbing sling (5000 pounds--enough for my 34' catamaran) girth hitched around the chain (takes about 1 second), connected to the apex of a nylon bride with a 3/16" soft shackle or big SS biner. I like the biner because it only takes 1 hand on or off. If I was forced to cut away the sling to move, who cares. They seem to last through ~ 100 days of anchoring before they look poor. If I wanted to cut costs, the nylon ones are $5, though I just use something retired from climbing. Dyneema is available for $9. Bigger boats would need to splice something; 5' of 1/4" Amsteel would make a nice rabbit runner.
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Old 03-01-2014, 13:35   #142
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Re: Mantus Chain Hook

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Originally Posted by thinwater View Post
However, if there are 4 passes (doubled sling), the materials tests 2500#, but the stitching weakens one leg to 2000 pounds, the double sling should fail at 4x2000=8000#. Weakest link.
.
agreed . . . "number of passes x the weakest link".

and not "(n-1) x full strength + 1 weakest"
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Old 03-01-2014, 13:41   #143
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Re: Mantus Chain Hook

I just did three quick nylon 3 strand pulls, of the three most common rope to chain splices, and they were all "full strength".

The three methods were (a) the conventional 3 strand loop splice thru the last link, ( b ) the 2 strands one way thru the last link and the 3rd strand the other way loop splice", and ( c ) the "up the chain weave'.

All tested to full rope strength, but not so surprising, because while the bend radius inside the first link is less than 1:1 when the rope is fully round with all three strands, it is more than 1:1 to each of the separate 3 strands, and they do separate and mash out when in all these splices (least perhaps in (a) )

But the answer here seems to be "it does not matter so long as you do any of teh splices well".
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Old 03-01-2014, 17:10   #144
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Re: Mantus Chain Hook

Quote:
Originally Posted by estarzinger View Post
I just did three quick nylon 3 strand pulls, of the three most common rope to chain splices, and they were all "full strength".

The three methods were (a) the conventional 3 strand loop splice thru the last link, ( b ) the 2 strands one way thru the last link and the 3rd strand the other way loop splice", and ( c ) the "up the chain weave'.

All tested to full rope strength, but not so surprising, because while the bend radius inside the first link is less than 1:1 when the rope is fully round with all three strands, it is more than 1:1 to each of the separate 3 strands, and they do separate and mash out when in all these splices (least perhaps in (a) )

But the answer here seems to be "it does not matter so long as you do any of teh splices well".
I have read of a 2-strand link splice that is said to pass a windlass more easily and is still 100% strength. It was like a long splice, but I forget the details. It sounds good, so long as redone a bit more often.
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Old 04-01-2014, 05:55   #145
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Re: Mantus Chain Hook

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Originally Posted by Cotemar View Post
Auspicious,

Unfortunately, Life testing a new Bridle Snubber design takes years and I am past year one right now.

<snip>

If you want to do the proper analysis of this Bridle Snubber, you will need a 3D CAD model of the rubber snubber and 3 strand 5/8” dia. nylon line. When you’re done with the 3D CAD model, then you can export out a .stp file and bring it over to your Ansys software for doing your simulation and analysis.

I will be out sailing when you’re doing simulation and analysis.
Certainly don't need a 3D CAD model. The math is simple to model a system like a rode and snubber.

You say the snubber doubles shock absorption. I'm just asking for substantiation.

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Rubber Snubbers in the Bridle Snubber just remove the shock load.
The snubber elements themselves possess a nonlinear stress-strain curve.
The Snubber reduces the expected peak shock load loads 50%
The rubber snubbers you have cited are linear (F = kx) except as loads approach yield. Yield means you are working in a failure regime. That just doesn't make sense to me. You'll have to explain and justify your statement for me to understand.

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This has been discussed before. You need more than just the snubber to take load off the gypsy! The snubber is designed for shock absorption, not UTS! The chain should be belayed in some manner (chain lock, belaying strop) first, then back down. Only then, attach the snubber for shock absorption.
Why? Ultimate strength of the structural member (in this case rode and snubber, with or without rubber snubber) has nothing to do with the loads backing down. You just won't be putting enough power into the water to come anywhere close to ultimate tensile strength while backing down unless your ground tackle is grossly undersized.

My anchoring protocol is to get the anchor down, let the anchor settle, rig the snubber, and run a bunch of chain out to unload the gypsy. There is zero load (aside from the weight of the chain itself hanging in a loop between the anchor roller and the chain hook) when I back down at cruising rpm.

I simply don't understand your point. UTS is not relevant in this scenario.
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Old 04-01-2014, 10:08   #146
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Re: Mantus Chain Hook

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Originally Posted by Auspicious View Post
Certainly don't need a 3D CAD model. The math is simple to model a system like a rode and snubber.

You say the snubber doubles shock absorption. I'm just asking for substantiation.



The rubber snubbers you have cited are linear (F = kx) except as loads approach yield. Yield means you are working in a failure regime. That just doesn't make sense to me. You'll have to explain and justify your statement for me to understand.
I presume the charts published by the line or rubber snubber manufacturers showing stretch as a function of load are accurate so I figure I can leave my Cray 1000 turned off to design a snub line. As you say, elongation is quite linear relative to load and not hard to calculate.

I'm experimenting with an Ultra rubber snubber that allegedly won't fall apart under load. I'm open to such snubbers being pointless, but I added one for a couple of reasons, none of which has to do with "reducing peak shock loads by 50%", which doesn't make sense to me either. My reasons were that, at least for my vessel, to anchor in 60 knots winds would require the stretchy bit to be 3/4" if I wanted to stay within a safe working load on the line. However that is too robust for other conditions, so my thought was to provide energy absorption in 3 stages. First, the rubber snubber stretches along with a 5/8" plait line 30' long. Once the rubber has reached its maximum energy absorption the plait bears the load. This will work fine up to around 35 knots. If conditions are expected to be windier, then I overlay a 3/4" 3 strand line that would begin to take the load after 5 feet of stretch had been induced. The bight of chain would then take the load when the 3/4" line had stretched another 6'. Since all chain hooks I have looked at are no stronger than a Dux soft shackle, I prefer to use those to attach the snubs lines to the chain.

The other reason for using the rubber snubber is to give me a visual indication of load values. I can rig the snubber inboard so it is easy to see from the pilot house and get an idea of what shock loads are actually being absorbed.

Beats me if this is a good approach or not, but time will tell.
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Old 04-01-2014, 10:13   #147
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Re: Mantus Chain Hook

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Originally Posted by Delfin View Post
I presume the charts published by the line or rubber snubber manufacturers showing stretch as a function of load are accurate so I figure I can leave my Cray 1000 turned off to design a snub line. As you say, elongation is quite linear relative to load and not hard to calculate.
Exactly. It's all about F=kx - all linear.

Your other bits make sense also, although I do much the same with just line.
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Old 04-01-2014, 10:30   #148
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Re: Mantus Chain Hook

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The other reason for using the rubber snubber is to give me a visual indication of load values. I can rig the snubber inboard so it is easy to see from the pilot house and get an idea of what shock loads are actually being absorbed.

Beats me if this is a good approach or not, but time will tell.
The Ultra Snubber Bridle looks like a very nice piece of kit. A bit pricy though as one for my cat would cost $307 usd.

My Bridle Snubber cost $120 usd

Purchase ULTRA Snubbers | Quickline USA
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Old 04-01-2014, 10:47   #149
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Re: Mantus Chain Hook

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Originally Posted by Cotemar View Post
The Ultra Snubber Bridle looks like a very nice piece of kit. A bit pricy though as one for my cat would cost $307 usd.

My Bridle Snubber cost $120 usd

Purchase ULTRA Snubbers | Quickline USA
Agreed. I also don't see the point of the chain hook, and prefer to have a bit of Dyneema leading from the cleat to the stretchy line. I put the rubber on the Dyneema, and I think I paid $50 or something for that component. Still pricey, but it does seem very robust.
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Old 04-01-2014, 12:11   #150
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Re: Mantus Chain Hook

Crossing posting with my load testing thread . . . a test of load vs stretch in three nylon constructions:

As expected, nylon double braid is the least stretchy.* But the difference between good old 3 strand and Brait was not as big as expected, and at least with these particular lines (Yale 1/2" brait and Samson pro-set 1/2") and loads the 3 strand was marginally stretchier.

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