Choice of Material for Snubbers

[Dockhead]

In another thread (http://www.cruisersforum.com/forums/...er-104325.html), we had a vigorous argument about whether or not the elastic properties of nylon are essential for snubbers, or not. This followed from my theory that Evans’ polyester double-braid snubber broke not because of the knot he was using, but because the relatively inelastic polyester material failed to dampen the shock loads on his ground tackle.

I am as always grateful to everyone who argued with me, forcing me to think things through which I just took for granted beforehand. In this case the question for me was – what kind of shock loads are generated by a boat at anchor? What properties are essential for effective snubbing of those loads?

I have zero education or experience in engineering or physics, so these concepts are hard for me. A real engineer will surely come up with a much better explanation than I was able to. Some digging and struggling did seem to yield some insight, however, even to this non-engineer.

So the problem a snubber is designed to deal with is dynamic loads. A boat at anchor, pushed around by wind and seas, will snatch up against its anchor chain and will have to be stopped. It’s exactly the same thing as a falling rock climber, although of course the speeds and numbers are totally different. Up to a certain point, these dynamic loads are dealt with by the chain catenary. As the boat is pushed out of its equilibrium position by wind or waves, its motion is counteracted by the weight of the loop of chain hanging down. For the boat to move out of position, the loop has to be lifted and straightened out. This is great, and a great property of all-chain rodes. The only problem is that this effect works only up to a certain force. Once the force of wind and waves exceeds the horizontal component of the force exerted by the chain, catenary no longer does its job. The boat slams against the straightened chain, and the resulting shock load, as we all know, can break stuff – bow rollers, shackles -- it can even break the chain itself.

I found here: http://www.marinesafety.com/research/documents/dynamicload.pdf a formula for calculating the dynamic loads on mooring ropes. Unfortunately it is all in Imperial – I’m sure one of our more engineering-savvy members can find a better formula. But I was able to use it after converting the units.

I don’t know how to calculate how fast an anchored boat can be going when being battered by waves – it will be a very complicated calculation involving lots of factors. But I suppose that it could easily be some fraction of the speed of the waves – as, in the worst case scenario, your anchored boat gets caught up in a wave and is thrown back (I have, unfortunately, experienced this myself). So let’s look at, what? Two knots? Surely not an unrealistic momentary speed in a rough anchorage. What kind of forces can be exerted?

In order to know this, we need to know the “stopping distance”. If we take a 6 meter piece of 16mm nylon octoplait – we know from the Coast Guard study posted in the other thread that this type of material will stretch 25% at 30% of its breaking strength. This material has a breaking strength of 5600 kg, so that means we get 25% stretch at 1,7 tons of force.

A piece of polyester double braid, like Evans’ snubber, of equal strength and also 6 meters long, will stretch 24cm at the same load of 1,7 tons.

I have no idea how to calculate the stretch of the chain, but I guess a rough idea could be gotten from analyzing a 12mm piece of steel bar of the same type as my chain is made out of. Applying the Young’s Modulus of this material of 210 gigapascals, you can see that a 50 meter long bar (I’m assuming I have 50 meters of chain out) will stretch 3,4 cm under a force of 1,7 tons.

OK – so now we have a baseline.

If we apply the “stopping distance” formula, what kind of forces would we get with the various materials? I can’t calculate it, because I don’t have anything like a “Young’s modulus” for the ropes. But I can test to see whether a given force can be accommodated within the available stopping distance, or not.

A 25 ton yacht (like mine) moving at 2 knots will generate the following dynamic loads at various “stopping distances”:

Nylon octo, 1.5 meters – 923 kg
Polyester braid, 24cm – 5773 kg
Chain, 3,4cm – 40,749 kg


I think this is extremely interesting. I think it means that, for a 25 ton yacht moving at 2 knots, the following:

• Nylon octo will easily stop the yacht without nearly reaching its 30% stretch point. In fact, nylon octo could stop the yacht from 4 knots, which would generate 3.7 tons of force over 1,5 meters (in reality, the force would be less than that, because the nylon will stretch more than 25% since the force generated would be – we don’t know, but we know it’s more than 30% of breaking strength).
• Poly braid cannot stop the yacht within its 30% stretch point, in fact the force is beyond the breaking strength of the poly. The braid will stretch further, so the force won't be as large as that. Probably within the poly's breaking force. But the dynamic force will be several times that generated by the nylon, and will probably be approaching the poly's breaking point.
• All hell will break loose on all chain – the chain itself will be shattered if nothing softer (anchor roller? stopper? shackle?) is broken first.

I think this is a really vivid illustration of the relationship between the stretch of a snubber and the amount of force which it can absorb, which should make it really clear why we don’t use anything but nylon for snubbers.

There are a lot of approximations and fudges in these calculations – I trust a real engineer from among us will improve this work a lot.

[neelie]

Very interesting. Excellent analysis. Clearly, more stretch is better.

However, can there be such a thing as too much stretch? If so, how much is too much?

Or might it be better to work backwards and calculate the stretch required to produce a certain desired maximum dynamic load?

This has all the hallmarks of a good thread, I am looking forward to the experts weighing in.

[JonJo]

Dockhead,

I like the direction.

Something to think about.

Many tests on materials are slowly incremental. A length of material is tested by slowly increasing the load and measuring the effect, in the case of nylon, measuring how it stretches. The situation you quote is when the chain is bar tight as a result of a large gust coming through and the yacht is simultaneously hit by a largish wave. The actual usage of the snubber is totally different to the way cordage is tested - the load is dynamic, instantaneous - or its instantaneous in terms of the data that is usually quoted (which is slowly incremental).

Is there a correlation, or what is the correlation, between dynamic, snatch (or instantaneous) loading and that gentle incremental loading).

Another way of looking at, and I think you mentioned this on the other thread, take a 20m length of 'rope' and snatch load it with a suddenly dropping 1t weight with a free fall, of say, 3m is the resultant extension the same as slowly loading that same rope with a pneumatic ram (or whatever)?

I think climbing rope is tested dynamically, but only at low (man sized) loads. I have not seen anything else tested dynamically (apart from cars, automobiles) - every test is slow, measured and well controlled.

I do not have an answer and confess I test in the slow incremental way as dynamic testing is hard to set up, expensive, difficult to measure and (at times) frightening.

Jonathan

[wsmurdoch]

Another site to play around on is

Tuning an Anchor Rode

[Kettlewell]

My non-scientific observation is that stretch is good in a snubber, and I have always used 3-strand nylon for the purpose and never broken it in bad weather. I use 3/8" nylon on my 38-foot 20,000 lb. boat, and I used the same thing on my 32-foot catamaran. I have never had a problem with "too much stretch," and in the past I often used mixed rodes with a short length of chain and long lengths of nylon (still very common here on the East Coast). In recent close brushes with the remains of Sandy and Irene I had a couple of Fortresses out on long lengths of nylon--really long--and there was no feeling of "too much stretch." I have also hung on a parachute sea anchor offshore in Force 9-10 on 400 feet of 1/2" nylon. The ride is noticeably smoother and easier when on mostly nylon, in comparison to being on chain with a snubber--maybe my snubbers are too short. Plus, read your Van Dorn and your Frayse. A mixed rode (chain and nylon) is best for the lowest loads on the system and therefore the best holding power.

[estarzinger]

Originally Posted by Dockhead

I think it means that, for a 25 ton yacht moving at 2 knots, the following . . . .

• All hell will break loose on all chain – the chain itself will be shattered if nothing softer (anchor roller? stopper? shackle?) is broken first.

.

All I will say is that what you have written above is obviously false. Typically sized anchor chain does not come anywhere even close to 'shattering' when stopping a boat moving at 2kts.

A surprising number of people anchor (on chain) without any sort of snubber (We can all agree that this is not good practice), and don't come to immediate grief - usually they eventually wreck their windless but they sure don't 'shatter' their anchor chain.

And you may have missed the simple fact that I anchored all last summer (in Nova Scotia and Newfoundland) on Dacron double braid and it never broke. SO, your calculations and your conclusions are WAY off base here.

And we can argue about percentages of use, but people do in fact use Dacron successfully for mooring buoy pennants, and as you see Dacron was in fact RECOMMENDED (over all nylon) by those MIT engineers after hurricane Bob. You are just all wet here, with both an inaccurate calculation and just looking at one factor of a multi-factor engineering problem.

[lancelot9898]

Someone asked about having too much stretch and I can relate to that question. I was anchored via a single anchor during a Cat 1 hurricane(never again) and experienced gusting winds in the 90 knot range. The boat would surge one direction or another and the rode would stretch and then as the gust moderated that rode would slingshot the boat as if sailing. Don't know how that anchor held up under those conditions. No snubber at the time however was using around 80 ft of 3/8 BBB chain and that was spliced unto around 100 ft or more of 5/8 3 strand nylon. And that nylon rode looked like a banjo string under the strain.

One other comment is that the weight of the boat plays an important role in the amount of dynamic loading. Not only will shifting gusting wind impact upon the boat but wave size will bring that bow up and down with large impact loads on the attachment points.

[JonJo]

It has repeatedly be mentioned that you can have too much, or too long, a snubber as too much elasticity will allow your yacht to behave either like a yo-yo or a jumper on the bungy jump, surging back and forth and thus compounding problems of yawing etc.

I would like to say we have not found this to be the case, with our 14m snubbers, but there again I've not sat out with the 14m snubbers and switched to a 6m snubber and checked the difference.

As Kettlewell is suggesting many people use lots of nylon (or dacron) in mixed synthetic/chain rodes, without anyone mentioning that their yachts bounce about.

So is this idea of yo-yoing a myth or is there any evidence it happens?

Jonathan

I'm with Evans - the numbers of people with failed chains are not very great.

Lancelot - I've just read your post (it was not there when I started drafting!), you have provided an answer!

[Fuss]

I once anchored in 45kts of breeze and did a bit of an emergency test.

Im not into all that stuff about putting the anchor down carefully, letting it sink in , gently backing down as the chain rolls out etc etc In an emergency I like to think that I can just dump it and it will do its job.


The way I did it was... let the clutch go in 6 meters of water, then the bow blew off and I let the 10mm chain fly out until the 80 meter mark was reached. I then connected the half meter internal snubber that is designed to only take the weight off the windlass, which only takes a few seconds. The boat weighs 25 tons and by the time the chain started to take up, I reckon we were doing a good 4 knots.

Perfect, bow came into the wind, nothing broke.

Gave it some reverse for a bit, looked good and then attached the external snubber that is designed to take the weight off the bow roller and also do what snubbers do.

My snubber is about 10meters of 10mm old climbers rope I think.

[Cotemar]

Is this really that big of a problem?

Just put something stretchy from boat cleat to anchor chain (snubber) and call it a day.
No rocket science needed here. There are literally hundreds of ways to make a snubber. Pick the one that works best for you boat and go sailing.

It is this simple. Put something stretchy from boat cleat to anchor chain (snubber)

[Fuss]

Originally Posted by Cotemar

Is this really that big of a problem?

Just put something stretchy from boat cleat to anchor chain (snubber) and call it a day.
No rocket science needed here. There are literally hundreds of ways to make a snubber. Pick the one that works best for you boat and go sailing.

It is this simple. Put something stretchy from boat cleat to anchor chain (snubber)

Yep, but in order to know what works best for you, you have to lose one to chafe and maybe lose a chain hook. So there probably is a bit of rocket science in there somewhere.
So we all hope that the people reading these threads get information that helps them make good decisions.

[Cotemar]

Originally Posted by Fuss

Yep, but in order to know what works best for you, you have to lose one to chafe and maybe lose a chain hook. So there probably is a bit of rocket science in there somewhere.
So we all hope that the people reading these threads get information that helps them make good decisions.

I have never lost a snubber to chafe and have never lost a chain hook.

I have tested numerous snubbers and chain hooks and you know what, they all worked well.

Sailing is far more fun than worrying about the zinchoid cravostat molecular line cordage on leap year.

[Delfin]

Excellent analysis, Dockhead, although there must be a decimal placement issue going on in your conclusion. I am certainly no engineer, but I think (and this being a forum if what I think is wrong, I'll find out from 38 people), is that if a 24 ton boat were moving at 2 knots and you brought it to a stop instantly, the force would be around 9600 pounds. Since the force goes up exponentially to the acceleration, if you make the speed 3 knots, the effective weight of the vessel in terms of the load on the gear goes to over 21,000# of force.

But that's not what happens with boats at anchor - there is always some distance they travel under increasing load on the ground tackle before they stop. If the distance is 5 feet, then the 2 knot, 24 ton example yields a force of 1,920#; for the 4 knot example, 4,200#. This is why the snubber is so important - after the catenary of the chain would have been lost you have the stretch of the snubber that increases the distance the vessel travels before it comes up hard, and this reduces the force by a factor of the distance traveled.

That's why I wonder if Evans 15' of nylon is long enough (did I get that length right?) since that allows for around 4' of distance to travel, reducing the force to 1/4 of what it would have been, which is great but less reduction than he would get if the stretchy part of his NASA designed snubber were twice as long. Maybe that's why most recommendations are for a minimum of 30'? Beats me, but if my math is correct, then if Delfin is bucking on a wave at 2 knots acceleration (seems reasonable, but I don't know) the force I have to deal with would be 65 tons * 100 * 4 / 7 (stretch of 30' of nylon) = 3,714#. The key to accuracy is clearly the effective displacement of the vessel - (does a 65 ton vessel place a 65 ton load on the ground tackle when bucking at anchor in waves?), and the actual speed of acceleration (2 knots seems reasonable, but is it?)

These relatively manageable forces is the reason why no one much seems to break anchor rodes/chains, and why you can use pretty small diameter snub lines even for bigger vessels.

[Singleprop]

Good subject - can't contribute much myself yet - except to say that it doesn't matter how good ones anchor is if the rode and tackle is not up to the task... and that I have a G-70 chain - not much stretch in that chain - so I need a REALLY good rope snubber........

[Dockhead]

Originally Posted by estarzinger

All I will say is that what you have written above is obviously false. Typically sized anchor chain does not come anywhere even close to 'shattering' when stopping a boat moving at 2kts.

A surprising number of people anchor (on chain) without any sort of snubber (We can all agree that this is not good practice), and don't come to immediate grief - usually they eventually wreck their windless but they sure don't 'shatter' their anchor chain.

And you may have missed the simple fact that I anchored all last summer (in Nova Scotia and Newfoundland) on Dacron double braid and it never broke. SO, your calculations and your conclusions are WAY off base here.

And we can argue about percentages of use, but people do in fact use Dacron successfully for mooring buoy pennants, and as you see Dacron was in fact RECOMMENDED (over all nylon) by those MIT engineers after hurricane Bob. You are just all wet here, with both an inaccurate calculation and just looking at one factor of a multi-factor engineering problem.

Well, first of all, mooring pennants don't necessarily have the same dynamics as snubbers, or serve the same purpose. I am not familiar with those recommendations and can't comment. If someone wants to make some arguments based on the MIT recommendations, I am all ears. Did they make calculations of the dynamic loads and snubbing ability of the pennants, or did they focus exclusively on chafe? I haven't read it.

My calculations were done carefully and are correct, as far as I know. If someone wants to dispute them, they could provide figures and their own better calculations, and not just a statement that they are false.

Now my calculations are THEORETICAL, that much is true -- they assume that the there is no elasticity in the system besides the chain, and this assumption is not quite realistic. In fact, the anchor will move under such huge forces, the bow roller will at least bend, the stopper will give, etc., etc., so the reality will undoubtedly be less dramatic.

However, the point was different, and remains: as the "stopping distance" of the rode system is shortened because of less and less elasticity in the system, the dynamic forces increase dramatically, and quickly reach levels where things break. I think this is plain physics understandable even by a techno-ignoramus like me. If someone can dispute this proposition, I am all ears.