Choice of Material for Snubbers

[sailorboy1]

Originally Posted by Dockhead

Not me -- although the weather is gorgeous, I am presently 2000 miles from my boat Have to wait until next week

That explains a lot and now I understand the thread.

[Kettlewell]

I posted this also in the Rolling Hitch on Snubber thread. Yale Ropes has a pretty good white paper on anchoring. It is pushing their nylon Brait rope, but it has lots of interesting information on how much energy absorption there is using nylon in the anchor system. In particular, take a look at page 14 and on from there.

[Dennis.G]

Originally Posted by Dockhead

... I don't think the paragraph about the effect of chain catenary at the end is correct...

Let's put a few values to the chain tension as the catenary is straightened out:

(assumptions: 1/2" G40 chain at 2.69 lb/ft (near 12 mm), 150 foot of chain, chain horizontal in air at sea level, end tension of 1.7 ton (3400 lbs) and starting tension of 1/6 of that or 567 lbs)

At start: Boat moving back at 2 knots, 567 lb tension on chain, 13.5' sag on chain, horizontal displacement = 0.

At half developed end tension: Boat moving back at ? knot, 1984 lb tension on chain, 3.8' sag on chain, horizontal displacement = 35 inches.

At end tension: Boat moving back at ? (maybe 0) knots, 3400 lb tension on chain, 2.2' sag on chain, horizontal displacement = 37 inches.


The above spot figures reflect the exponential increase in tension with displacement. Figuring the deacceleration curve of the boat as a function of this increasing tension I am afraid is beyond me, but the fact is:

If 1.7 tons is tension to stop movement of backwards moving boat from 2 knots, this force has been developed over a distance of 37 inches due to chain catenary. I am not sure how to figure the peak force as the deacceleration is variable over this distance, but I think we can agree it would be very much less than your figure (40,749 kg) of stopping the boat over a distance of only 1.3 inches (3.4 cm).

[Jim Cate]

I think that Dennis has a good outlook on this.

Dockheads assumption of the boat accelerating to 2 knots on a wave face does not seem realistic because the tension of the chain/snubber is significant before the accelerating force is applied, and then rises as the boat moves backward and the catenary is reduced. By the time the chain approaches full extension a very considerable deceleration will have been applied and the velocity reduced. Thus the snubber, no matter what its material, should never see the sort of loads Dockhead reckons. In a sustained wind the chain will be near full extension and little displacement of the boat will happen in further gusts, and hence little KE will be built up.

Seems to me this is why there are so few failures of the gear despite exposure to real world experiences.

Cheers,

Jim

[conachair]

Originally Posted by Dennis.G

Let's put a few values to the chain tension as the catenary is straightened out:

I think your figures may be out by treating the catenary as a "whole" catenary instead of "half", which it will be until the chain lifts off the seabed at the attachment point to the anchor, and after that the horizontal component will also have a vertical component as the 2 attachment points aren't at the same height. Not that it matters that much, the result is the same, very small increase in straight line distance between the boat and the anchor leads to a very large increase in tension in the rode.
I've been fiddling with graphs here..

https://www.desmos.com/calculator/ysamntgf91
S=distance from anchor in metres, d=depth in metres, w=weight in kilos per metre of chain, h= HORIZONTAL force in Kg force.

Next is trying to figure out graphing boat position & acc against changing tension, hope google's having a good night, lots help needed

[Kenomac]

Nobody has taken into consideration the damping force provided by the anchor chain weight on the whole equation. Watching my anchor snubber stretch and contract even during low wave action, tells me it's acting basically as an extension of the anchor chain and providing an additional dampening force along with the rise and fall of the anchor chain. The entire anchor rode and snubber act as a huge 700 pound shock absorber right up until the rode it pulled taught during an extreme circumstance.

[DumnMad]

Good calcs Dennis G. So no such extreme load on the ship when using a long chain rode.

Would you give us the figures for 75ft, 150ft and 300ft of chain rode.
I think the effect of having 300ft of chain might suggest a chain or Dyneema snubber to protect the winch would be OK and it would be unnecessary to use a snubber for energy absorption.

But, if we're talking about short rodes and short snubbers and comparing their effectiveness nylon wins hands down for energy absorption. Not much use though if it overheats under cyclic loading from the effects of prolonged storm-wave action.

[conachair]

Maybe of interest to a few..

https://www.desmos.com/calculator/afabyqgjmt

This is a mass decelerating with a constant force being applied:
M is mass in Kg, I is initial speed in Kts, F is force in KgF,
x axis is time in sec, y axis is boat speed in Kts for the blue line, distance traveled in metres for the brown line.

Completely ignored is the real world with friction through the water, wind etc.

Increasing force with time next, hopefully. Any mathematicians know how to calc that?

[Dockhead]

Originally Posted by Dennis.G

Let's put a few values to the chain tension as the catenary is straightened out:

(assumptions: 1/2" G40 chain at 2.69 lb/ft (near 12 mm), 150 foot of chain, chain horizontal in air at sea level, end tension of 1.7 ton (3400 lbs) and starting tension of 1/6 of that or 567 lbs)

At start: Boat moving back at 2 knots, 567 lb tension on chain, 13.5' sag on chain, horizontal displacement = 0.

At half developed end tension: Boat moving back at ? knot, 1984 lb tension on chain, 3.8' sag on chain, horizontal displacement = 35 inches.

At end tension: Boat moving back at ? (maybe 0) knots, 3400 lb tension on chain, 2.2' sag on chain, horizontal displacement = 37 inches.


The above spot figures reflect the exponential increase in tension with displacement. Figuring the deacceleration curve of the boat as a function of this increasing tension I am afraid is beyond me, but the fact is:

If 1.7 tons is tension to stop movement of backwards moving boat from 2 knots, this force has been developed over a distance of 37 inches due to chain catenary. I am not sure how to figure the peak force as the deacceleration is variable over this distance, but I think we can agree it would be very much less than your figure (40,749 kg) of stopping the boat over a distance of only 1.3 inches (3.4 cm).

Well, I think there's a bit of apples and oranges going on here.

Of course -- we can all agree -- that chain catenary is very powerful. We even made some calculations. If the energy of the moving boat is within the energy absorption ability of the chain catenary, then the snubber will never have to do any work.

The problem is that the energy absorbing ability of the chain catenary is not endless, and vanishes with increasing speed as the chain is tensioned. It is not true at all that the chain catenary is absorbing energy more and more at the end -- on the contrary, it is absorbing energy less and less. Don't confuse resistance with energy absorption. As the loop comes out of the chain, the resistance doesn't go down, but the energy absorption does, as more and more of the load is transferred to the anchor. At the end, there is no more energy absorbing going on at all; the load is all on the anchor, which means that it is all on the limited stretch of the chain itself, and on the anchor's ability to move in the seabed, etc.

What that means is that if you are faced with a load beyond the energy absorbing capacity of the catenary, the boat will not be stopped before the catenary comes out. It is that amount of energy which the snubber has to deal with. If there is no snubber, then it is like running into a brick wall. My figure of 41 tons is correct -- IF the boat is still moving at 2 knots when there's no catenary, and if there's no other flex in the system. I never pretended to analyze the whole system -- just chain ignoring catenary, nylon, and polyester. I say again, and I'm sorry it was so confusing -- my calculations were comparing the energy absorbing abilities of three materials without considering any other parts of the system or other factors such as catenary, anchor pulling out, boat pitching down, anchor roller bending or breaking, boat yawing, etc., etc.

In anchoring situations, you won't get a 41 ton load out of 2 knots of speed, because there are several other factors which will increase the "stopping distance". But if you were to securely fasten the chain in my hypothetical to a concrete pier, and securely fasten it to a hard point on your boat, and motor off at 2 knots, you will get more or less exactly 41 tons of force which will shatter either the hard point, or the chain. It might even rip your boat apart -- 41 tons is a hell of a lot of force. I hope there's not any doubt in anyone's mind about that. Although it's not direclty applicable, because of other factors at play in anchoring situations, it's still highly relevant, when we try to imagine how these materials behave.

My point was that there is a dramatic difference in the energy aborbing abilities of 50 meters of chain, on the one hand, and 6 meters of polyester and 6 meters of nylon, and in the loads resulting from stopping the boat from, say, 2 knots, with these three materials.

[Kettlewell]

Please re-read Modern Seamanship page 172 or go direct to Van Dorn and read what is said about anchor rodes. In a nutshell, all chain in really deep water can work, but in the depths most of us anchor in you need a lot of nylon out too, whether in the form of a snubber or a mixed rode.

[Dockhead]

Originally Posted by Dennis.G

Let's put a few values to the chain tension as the catenary is straightened out:

(assumptions: 1/2" G40 chain at 2.69 lb/ft (near 12 mm), 150 foot of chain, chain horizontal in air at sea level, end tension of 1.7 ton (3400 lbs) and starting tension of 1/6 of that or 567 lbs)

At start: Boat moving back at 2 knots, 567 lb tension on chain, 13.5' sag on chain, horizontal displacement = 0.

At half developed end tension: Boat moving back at ? knot, 1984 lb tension on chain, 3.8' sag on chain, horizontal displacement = 35 inches.

At end tension: Boat moving back at ? (maybe 0) knots, 3400 lb tension on chain, 2.2' sag on chain, horizontal displacement = 37 inches.


The above spot figures reflect the exponential increase in tension with displacement. Figuring the deacceleration curve of the boat as a function of this increasing tension I am afraid is beyond me, but the fact is:

If 1.7 tons is tension to stop movement of backwards moving boat from 2 knots, this force has been developed over a distance of 37 inches due to chain catenary. I am not sure how to figure the peak force as the deacceleration is variable over this distance, but I think we can agree it would be very much less than your figure (40,749 kg) of stopping the boat over a distance of only 1.3 inches (3.4 cm).

To address this narrow question, and for the avoidance of any doubt:

Stopping the boat in 37 inches will obviously result in different forces from stopping it in 1.3 inches. That was not what I analyzed. I analyzed purely the forces resulting from stopping the boat with the stretch of the chain.

As to your "exponential increase in resistance" -- that is, at the same time, an exponential increase in force! This shows the weakness of catenary as a shock absorber, as it approaches the end of its energy-absorbing capacity. The force spikes up, and when this exceeds the breaking strength of parts of the system, things start to break. That's why you must have a snubber, if there is any risk that the real forces will approach the energy absorbing capacity of your catenary.

Big ships in deep water with tons and tons of chain out don't need snubbers. But as others have mentioned, the energy-absorbing capacity of your catenary will be greatly reduced in the kind of shallow water many of us anchor in. In that same shallow water, you can get the kind of short, sharp seas which can impart a violent motion to your boat in storm conditions in imperfect shelter. All these are the reasons why snubbers, and their energy-absorbtion ability, are so important to us.


Another thing to keep in mind: boat motion in also not the only force which your catenary and snubber have to deal with -- there's also wind forces. We're using boat motion just because it is easy to calculate. But obviously you have to subtract from the energy absorbing capacity of the system any wind forces being absorbed.

It's hard to generalize about wind forces, because boats are so different. My boat has -- as a rough guess -- 25m2 of windage when reasonably bows-into the wind? 5 meters of beam, 2 meters of freeboard at the bow, then a 75 foot mast, cabin top, spray hood, etc., etc. So I can roughly guess that a 50 knot gust of wind will produce 0.5 * 1.2kg/m3 * 25m/s squared = 375 Pascals (formula here: Re: wind load calculations?) * 25m2 * .7 (rough guess at coefficient of drag) = 6,562.5 Newtons. That's 669kg force, a lot more than the 330kg weight of my chain. Even if I'm off by a factor of 2, I will still have no catenary left at 50 knots of wind, even with all 100 meters of chain out and hanging, which would require very deep water. This accords with my practical experience.

So keep in mind when you're thinking about catenary and snubbers -- your catenary can absorb a lot of energy, at least if you're in deepish water, but the wind might eat up this capacity, leaving your snubber (or whatever of its stretch is left) as the only thing you have to absorb wave motion, inertia from yawing, etc.

[conachair]

A few thoughts here, might be worth zooming out a bit, all IMHO...

• A major role of a snubber in an anchoring system is to keep the horizontal force to a minimum. Horizontal is what stops the boat.

• I think the highest forces come from the boat veering about, though this is just from watching the snubber and could be up for a seperate discussion. I don't know for sure were the highest forces come from to be honest. But lets look at boat movement.

• Energy absorption is not a good term to use IMHO, it seems to suggest that the energy has been sucked up somehow and doesn't exist anymore, not the case, you can't do that. It has to go somewhere. Why not use energy transfer instead.You can't make it go away, just put it somewhere else.

• It might help to think of boat momentum rather than kinetic energy, momentum is Kg times metres per second. Velocity times mass, easier to link in with force which can be thought of as causing the rate of change of momentum.

So when a boat gets slowed down the energy stored in the moving mass needs to go somewhere, but not too quickly. And preferable much less energy would get transferred back into the boat as motion. Nylon seems a bit bad at that, springs the boat back after a gust but in practical real world terms nylon with something chaff resistant to attach to the boat seems to be the front runner so far.

[conachair]

Originally Posted by Dockhead

In anchoring situations, you won't get a 41 ton load out of 2 knots of speed, because there are several other factors which will increase the "stopping distance". But if you were to securely fasten the chain in my hypothetical to a concrete pier, and securely fasten it to a hard point on your boat, and motor off at 2 knots, you will get more or less exactly 41 tons of force which will shatter either the hard point, or the chain. It might even rip your boat apart -- 41 tons is a hell of a lot of force. I hope there's not any doubt in anyone's mind about that. Although it's not direclty applicable, because of other factors at play in anchoring situations, it's still highly relevant, when we try to imagine how these materials behave.

Sorry to be picky but your units are a bit off there, 41 tons isn't a force, or a weight but strictly speaking a mass. "you will get more or less exactly 41 tons of force" - you need to calculate the negative acceleration to work that out-apply a force of 41t on a 41t boat travelling at 2Kts, I make it 10m/s^2, stopping in 0.1s over 50mm. But could be wrong.

https://www.desmos.com/calculator/udf9ulitdc

Maybe a bit picky but for any of the physics to mean anything it's important the the units are clearly defined.

[Dockhead]

Originally Posted by conachair

A few thoughts here, might be worth zooming out a bit, all IMHO...

• A major role of a snubber in an anchoring system is to keep the horizontal force to a minimum. Horizontal is what stops the boat.

• I think the highest forces come from the boat veering about, though this is just from watching the snubber and could be up for a seperate discussion. I don't know for sure were the highest forces come from to be honest. But lets look at boat movement.

• Energy absorption is not a good term to use IMHO, it seems to suggest that the energy has been sucked up somehow and doesn't exist anymore, not the case, you can't do that. It has to go somewhere. Why not use energy transfer instead.You can't make it go away, just put it somewhere else.

• It might help to think of boat momentum rather than kinetic energy, momentum is Kg times metres per second. Velocity times mass, easier to link in with force which can be thought of as causing the rate of change of momentum.

So when a boat gets slowed down the energy stored in the moving mass needs to go somewhere, but not too quickly. And preferable much less energy would get transferred back into the boat as motion. Nylon seems a bit bad at that, springs the boat back after a gust but in practical real world terms nylon with something chaff resistant to attach to the boat seems to be the front runner so far.

Some very valuable observations.

Just a couple of quibbles:

Momentum is kinetic energy, and the easiest way to connect the job with the capacity of the tool is to measure it in Joules, as JoJo and I have been doing, and compare it to the Joules of energy absorption available.

As you say, force and energy are different things. A given amount of energy will create different amounts of force depending on the amount of time over which it is released or transformed or absorbed. That's why stretch and "stopping distance" are crucial to keeping forces under control -- keeping forces below the breaking strength of our anchoring system -- they spread the release of energy over time.

We don't care very much whether the energy of boat motion is absorbed and dissipated or is stored and re-released, in this situation. In any case, none of the tools we have to much dissipation of energy. Mostly the boat will spring back in the opposite direction when the force is released, which is fine. Nylon is probably the most energy-dissipating of any of these things, due to the great amount of stretch and internal friction. This makes is dampen better, but it also means heat builds up, creating a risk of failure. A double-edged sword.


A piece of nylon octo of a given length and breaking strength will absorb 6 to 8 times as much energy as a piece of polyester double-braid of the same length and breaking strength. I hope that no one by now doubts that this means that the nylon octo is dramatically more effective as a snubber. The bottom line is energy absorbtion (or transfer, or storage, or whatever, as you prefer) -- that is the work which a snubber does. When the energy capacity of the snubber is exhausted, it will break, so a piece of polyester double-braid of given length and strength will break at an energy load in Joules (not force, but energy) which is one-sixth to one-eighth what it would take to break a piece of nylon octo of the same strength and length.

[conachair]

Originally Posted by Dockhead

Some very valuable observations.

Just a couple of quibbles:

Momentum is kinetic energy, and the easiest way to connect the job with the capacity of the tool is to measure it in Joules, as JoJo and I have been doing, and compare it to the Joules of energy absorption available.

Yup, I find it easier to visualize momentum as it's force and time, instead of energy which is force and distance.

A piece of nylon octo of a given length and breaking strength will absorb 6 to 8 times as much energy as a piece of polyester double-braid of the same length and breaking strength. I hope that no one by now doubts that this means that the nylon octo is dramatically more effective as a snubber. The bottom line is energy absorbtion (or transfer, or storage, or whatever, as you prefer) -- that is the work which a snubber does. When the energy capacity of the snubber is exhausted, it will break, so a piece of polyester double-braid of given length and strength will break at an energy load in Joules (not force, but energy) which is one-sixth to one-eighth what it would take to break a piece of nylon octo of the same strength and length.

Not sure I buy that exactly, if you have a piece of anything which stops a boat from 2Kts then whatever it is will have a load of energy transferred via a force going somewhere, the 6 to eight times doesn't come into it, the force might be substantially less as the rate the energy has been transferred is lower but if you stop the boat the energy must go somewhere, doesn't matter what the rope is, it will do something with all of the energy.
But yes, nylon is better.