Choice of Material for Snubbers

[Dockhead]

Originally Posted by conachair

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.

First of all, as to units -- we are all using kilos and tons as shorthand for kg/f or tons/force. I don't think it's very confusing, but yes, probably better to write kg/f. In the specific thing you quoted from me, I did write "tons force", so I think that was precise.

As to the calculations, you are exactly right, that you have to calculate the rate of deceleration and apply that to the mass of the thing you are slowing down. It's pretty simple as far as physics goes.

In my first post in this thread, I used the Marine Safety formula, which is an approximation:

A general formula to approximate the dynamic load on a line (in pounds of force) is

Load =T

∗100 ∗ Vsquared/d

where T is tons displacement of the ship, V is the initial velocity in knots and d is the distance traveled in feet while stopping. You can find it here: http://www.marinesafety.com/research/documents/dynamicload.pdf.

I wouldn't use it now, but anyway, I took 25* 100 * 4/.11 feet (3.4cm) = 90,909 pounds = 41,33 metric tons.

A really precise calculation can be done using this calculator:

Online calculator: Kinematics. The Equations for Uniform Acceleration

You need meters per second for velocity -- 2 knots will be 1.029 m/s

You get 15.57 m/s/s which is 1.588 g. Multiply 25 * 1.588 = 39.7 tons force (to be very precise with terminology: 39700kg/f). That's the precise answer; but the appoximation from Marine Safety was within 4%.

It means that if all you've got to stop the boat from 2 knots is the 3.4cm stretch of the chain at its breaking point, you are screwed -- the chain will explode, or the anchor will be pulled out of the bottom, or your bow roller will be ripped off. Of course there will be other sources of elasticity in the system, so the pratical results will be less dramatic, but this gives you a feeling for the magnitude of the forces involved.

I trust everyone will find these calculations extremely straightforward?

[Dockhead]

Originally Posted by conachair

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.

Ah hah! I think I can help you here.

The polyester won't stop the boat from 2 knots, because the energy which must be transferred to get the boat from 2 knots to 0 will result in a greater amount of force than the polyester can take. That is because of the limited stretch of the polyester -- the boat must be slowed down over a shorter distance resulting in a higher force.

The nylon will stop the boat, because its greater stretch gives more time to "transfer" (I'm using your term) the energy, resulting in lower forces.

Here's the key: force and energy are related by acceleration rate. So a given amount of strength (our nylon and polyester snubbers are the same breaking strength) can mean different amounts of energy "transfer" capacity, depending on the amount of stretch and hence amount of time for deceleration.

It means that stretchier materials have a greater capacity to absorb (store, "transfer", whatever) energy per unit of strength, than less stretchy materials. Chain has almost no ability to "transfer" energy, if you leave out catenary, so it will break if you try to feed much energy through it.

Here's the way -- much more articulate than mine -- the Yale Cordage people put it:

"A vessel at anchor is subjected to varying forces, including both wind and wave action. These forces impart measurable loads (energy) on the hull and superstructure of a vessel. This energy must be absorbed either by movement of the boat through the water, or by the anchor system’s ability to absorb this energy. The anchor rode, to the extent of its physical properties, acts as a shock absorber, preventing the anchor from being pulled out of the bottom, or stopping the fittings from being pulled free of the vessel.

Strength vs. Elongation Chart

Average Breaking Strength
(lbs.)
Horizontal Energy Absorption
(ft. lbs./100')

5/16" BBB Chain 7,600 lbs. 0
1/2" Three-strand Nylon 5,750 lbs. 67,665

1/2" Nylon Brait 6,300 lbs. 114,452

In the case of chain, most of the energy absorption comes from the ability of the chain’s catenary to resist lifting the chain off the bottom. But, during a storm, that capacity is reached quickly and the chain fast approaches its inherent failure point. As the failure point is reached, the
chain becomes bar-taut and the pull-out loads on the anchor build quickly, as do destructive loads on the deck fittings."

http://www.yalecordage.com/pdf/anchoring_tech.pdf

Note that energy absorption of nylon "brait" (that's octo) is much greater than that of Nylon three-strand, although strength is similar. This is on account of the greater stretch of the octo. The difference is much greater for polyester.

[conachair]

Originally Posted by Dockhead

As to the calculations, you are exactly right, that you have to calculate the rate of deceleration and apply that to the mass of the thing you are slowing down. It's pretty simple as far as physics goes.

In my first post in this thread, I used the Marine Safety formula, which is an approximation:

A general formula to approximate the dynamic load on a line (in pounds of force) is

Load =T

∗100 ∗ Vsquared/d

where T is tons displacement of the ship, V is the initial velocity in knots and d is the distance traveled in feet while stopping. You can find it here: http://www.marinesafety.com/research/documents/dynamicload.pdf.

I wouldn't use it now, but anyway, I took 25* 100 * 4/.11 feet (3.4cm) = 90,909 pounds = 41,33 metric tons.

A really precise calculation can be done using this calculator:

Online calculator: Kinematics. The Equations for Uniform Acceleration

You need meters per second for velocity -- 2 knots will be 1.029 m/s

You get 15.57 m/s/s which is 1.588 g. Multiply 25 * 1.588 = 39.7 tons force (to be very precise with terminology: 39700kg/f). That's the precise answer; but the appoximation from Marine Safety was within 4%.

It means that if all you've got to stop the boat from 2 knots is the 3.4cm stretch of the chain at its breaking point, you are screwed -- the chain will explode, or the anchor will be pulled out of the bottom, or your bow roller will be ripped off. Of course there will be other sources of elasticity in the system, so the pratical results will be less dramatic, but this gives you a feeling for the magnitude of the forces involved.

I trust everyone will find these calculations extremely straightforward?

Sorry, missed the first post with formula in it, makes more sense now.

Though this seems a slightly odd way to go, but answer should be the same
"You get 15.57 m/s/s which is 1.588 g. Multiply 25 * 1.588 = 39.7 tons force"

from Newton - F=MA, Newtons=Kg * M/s/s,

=25000Kg * 15.57=38925KgF

Online calculator: Kinematics. The Equations for Uniform Acceleration
Good link.
My graph comes out right
https://www.desmos.com/calculator/62w7fe60kj

[s/v Jedi]

Again, I disagree. A chain that gets lifted up from the seabed has in fact absorbed energy and stored it in the form of kinetic energy. It later transfers/releases it by lowering to the seabed again. The fact that this is a different method from storing energy than tensioning a rope, does not mean it is irrelevant or does not work. It is silly to compare stretch of chain with stretch of nylon just like it's silly to say nylon is cr@p because it does not weigh enough to absorb shocks. You must compare the stretch effect of nylon with the "lifting the chain" effect of chain. Better yet: look at how the two complement each other and start to understand why chain rode is always better than rope.

Same for polyester: it will stretch just fine and the same length, provided you make the snubber longer to compensate for the difference in stretch percentage. It all works but it all needs a slightly different approach.

[estarzinger]

^^ this is all interesting basic physics . . . . But in the real world . . .

#1 measured loads on rodes and snubbers are in fact not very high (JoJo has pointed this out with his own excellent measurements and some links to other people's). I had a very high precision (cm level) gps compass on the boat for a while, and I can tell you that at no time did I get anywhere near to 2kts of boat speed while anchored. That is at least an order of magnitude too high.

and related . . . .

#2 rodes and snubbers don't exploded except very very very rarely (they do chafe, and also very occasionally melt). This includes quite a number of boats using all chain and no snubber at all (we know cruisers who do this, but also there is extensive experience from bare boat fleets that operate like this). This includes people using polypropylene as rodes and snubbers (many commercial operators, but also quite a few French cruisers). This includes people using Dacron (Steve Dashew among others) and the Dacron mooring buoy pennants (Which act almost exactly like rode snubbers). So, you are entirely focused on a relatively uncommon failure mode (the exploding shock load).

To bring this discussion back to being at least sort of grounded in the real world, you have to acknowledge that the loads are in fact generally modest and that chafe and heat build up are the failure modes to be concerned with and not shock loaded exploding snubbers.

I think I understand the chafe issues, but Heat build up is something that I would in fact like to know more about - is 8 plait the same or better than 3 strand? Is the kermantle semi braided core better than highly braided lines? Would a dacron snubber with 3x the length (say perhaps running right back to a stern cleat) avoid the heat problem entirely? etc There is something very useful for us to learn there, if you cared to direct all your obviously vast energy to helping look into it.

I really and honestly think you have fully tapped out the discussion potential of F=MA.

[noelex 77]

Originally Posted by s/v Jedi

Again, I disagree. A chain that gets lifted up from the seabed has in fact absorbed energy and stored it in the form of kinetic potential energy.

I would hate a physicist to have a heart attack reading CF.

[conachair]

Originally Posted by Dockhead

Ah hah! I think I can help you here.

The polyester won't stop the boat from 2 knots, because the energy which must be transferred to get the boat from 2 knots to 0 will result in a greater amount of force than the polyester can take. That is because of the limited stretch of the polyester -- the boat must be slowed down over a shorter distance resulting in a higher force.

Very likely so, but I've been wrong on a daily basis for so long it would be nice to calculate it to be completely sure, which means figuring out calculating time and distance to stopping with the increasing force over time. Long time since I was a school

(our nylon and polyester snubbers are the same breaking strength)

Gotcha, see now.

These forces impart measurable loads (energy) on the hull

Naughty naughty!! Force is not energy! That whole section doesn't sound like it was written by someone who really really knew their stuff.

[Kettlewell]

I think I understand the chafe issues, but Heat build up is something that I would in fact like to know more about - is 8 plait the same or better than 3 strand?

Well, Yale says their 8 plait called Brait has less heat build up than 3-strand.

Yale Nylon Brait capitalizes on the elasticity of nylon fiber, and
incorporates a unique construction lay to yield similar breaking strength as
three-strand, but with far greater energy-absorption capability. More
energy is absorbed at lower loads (note in the graph that the shallow
slope of the curve is longer), minimizing the pull-out load on the anchor
and deck fittings. Plus, the strain on the nylon fiber itself is lower, reducing
heat buildup and lessening fiber fatigue.

[s/v Jedi]

Originally Posted by noelex 77

I would hate a physicist to have a heart attack reading CF.

Yep, still dazed about how I could write that ... getting older I guess

[conachair]

Originally Posted by noelex 77

I would hate a physicist to have a heart attack reading CF.

Wouldn't be a problem, there would be 3 pages of medical advice withing 10 minutes

[noelex 77]

Originally Posted by s/v Jedi

Yep, still dazed about how I could write that ... getting older I guess

I am actively doing my best to forget everything I have learned to make room for some new stuff.
Some of those new knots are tricky.

[Dockhead]

Originally Posted by s/v Jedi

Again, I disagree. A chain that gets lifted up from the seabed has in fact absorbed energy and stored it in the form of kinetic energy. It later transfers/releases it by lowering to the seabed again. The fact that this is a different method from storing energy than tensioning a rope, does not mean it is irrelevant or does not work. It is silly to compare stretch of chain with stretch of nylon just like it's silly to say nylon is cr@p because it does not weigh enough to absorb shocks. You must compare the stretch effect of nylon with the "lifting the chain" effect of chain. Better yet: look at how the two complement each other and start to understand why chain rode is always better than rope..

I'm not sure who you're arguing with -- I'm pretty every single person here is in full agreement about this.

Catenary is our first line of defense, and why just about all of us use all chain. The discussion was all about what happens when the catenary runs out -- what do you do then? As you said, nylon snubbers are the perfect complement to a nice chain rode.

Originally Posted by s/v Jedi

Same for polyester: it will stretch just fine and the same length, provided you make the snubber longer to compensate for the difference in stretch percentage. It all works but it all needs a slightly different approach.

It's theoretically possible to make a polyester snubber with the same strength and energy absorption capacity as a given nylon snubber, but why would you bother? Nylon octo has 6 to 8 times the energy absorption capacity for given length and strength as polyester double braid -- so your poly double braid "snubber" would need to be 36 to 48 meters long, to match the energy absorbing capacity of a nylon octo snubber of the same strength and 6 meters long. You'd just about have to attach such a "snubber" to your anchor. Either that, or live with much less strength than a normal nylon snubber would provide.

You could do better with polyester octoplait or even three-strand, compared to double braid, but again -- why?

Some people just consider snubber "chafe gear" and regularly toss them, and don't worry about nylon chafing. Others, like Evans, cow-hitch their nylon to dyneema for chafe resistance.

I don't know why you would put up with such dramatically lower energy absorption capacity in a polyester "snubber", even if you do get better chafe resistance. Actually, the greater susceptibility to chafe may itself (I think Andrew pointed this out) be a result of the energy absorbancy of nylon. If you want to get away from that, the answer is to do something like what Evans has done, not toss the baby out with the bath water by using an intentionally less energy absorbant snubber.

[Dockhead]

Originally Posted by estarzinger

^^ this is all interesting basic physics . . . . But in the real world . . .

#1 measured loads on rodes and snubbers are in fact not very high (JoJo has pointed this out with his own excellent measurements and some links to other people's). I had a very high precision (cm level) gps compass on the boat for a while, and I can tell you that at no time did I get anywhere near to 2kts of boat speed while anchored. That is at least an order of magnitude too high.

and related . . . .

#2 rodes and snubbers don't exploded except very very very rarely (they do chafe, and also very occasionally melt). This includes quite a number of boats using all chain and no snubber at all (we know cruisers who do this, but also there is extensive experience from bare boat fleets that operate like this). This includes people using polypropylene as rodes and snubbers (many commercial operators, but also quite a few French cruisers). This includes people using Dacron (Steve Dashew among others) and the Dacron mooring buoy pennants (Which act almost exactly like rode snubbers). So, you are entirely focused on a relatively uncommon failure mode (the exploding shock load).

To bring this discussion back to being at least sort of grounded in the real world, you have to acknowledge that the loads are in fact generally modest and that chafe and heat build up are the failure modes to be concerned with and not shock loaded exploding snubbers.

I think I understand the chafe issues, but Heat build up is something that I would in fact like to know more about - is 8 plait the same or better than 3 strand? Is the kermantle semi braided core better than highly braided lines? Would a dacron snubber with 3x the length (say perhaps running right back to a stern cleat) avoid the heat problem entirely? etc There is something very useful for us to learn there, if you cared to direct all your obviously vast energy to helping look into it.

I really and honestly think you have fully tapped out the discussion potential of F=MA.

I think probably all of us can probably agree with nearly all of this

Just a couple of points:

1. This thread did start with an exploding snubber failure. So I think it was not out of line with the original post to go down that road (rode?) and explore the possible causes.

2. It is certainly true that 99.9% of nights at anchor do not result in broken snubbers or ripped off bow rollers. But there's nothing wrong, IMHO, with spending a good bit of time to understand how to deal with a 0.1% situation. I guess you're right about 2 knots of boat speed, but I'll bet that we might actually see something like that in a really rough anchorage in a storm and imperfect shelter -- probably from pitching. So it may be unusual, but I bet it's not impossible, therefore possibly worth planning for.

3. Snubbers and mooring pennants are very different beasts subject to very different forces. I don't think they are comparable at all. See the post above by someone about how mooring balls provide considerable damping of forces by their flotation.

4. I wouldn't even know where to start with the heat question. I wouldn't know how to measure it or anything. I wouldn't know where to find any data. It seems reasonable that a really long polyester snubber maybe led back to the quarter cleats might be less susceptible. But a thicker nylon snubber led back to the quarter cleats might be even that much better, and stronger to boot. I really don't know. You could also get more length in your snubber with a double-purchase arrangement with a block. There are probably lots of things you could do -- only, the old-fashioned simple nylon snubber with an old-fashioned humble rolling hitch does seem to work ok in just about all conditions, so why bother?

[s/v Jedi]

Originally Posted by Dockhead

I'm not sure who you're arguing with -- I'm pretty every single person here is in full agreement about this..

Well, you were writing about stopping a boat in 3.4cm stretch if chain and comparing that to nylon. There is no logic to that from my POV.

Nylon octo has 6 to 8 times the energy absorption capacity for given length and strength as polyester double braid -- so your poly double braid "snubber" would need to be 36 to 48 meters long, [...]
You could do better with polyester octoplait or even three-strand, compared to double braid, but again -- why?

I can only counter with a question: why, why would you compare an 8-plait nylon with a double braid polyester and come up with these numbers? I can tell the only answer that I can think of: because you want to come up with the most exaggerated numbers possible, by choosing a more elastic fiber with the most elastic construction on one side, with the less elastic fiber with the least elastic construction on the other side.
If you want to have a sensible discussion, you should have come up with numbers for nylon vs polyester octo. Or both 3-strand.

The reason to pick an octo-plait polyester over nylon, is to get the same stretch with a more durable fiber. And that without tying or splicing Dyneema to it or other complicating constructions. I find it highly amusing to imagine people going to such extremes for a snubber while they use fire hose for chafe protection instead of a Dyneema sleeve.

I don't know why you would put up with such dramatically lower energy absorption capacity in a polyester "snubber", even if you do get better chafe resistance. Actually, the greater susceptibility to chafe may itself (I think Andrew pointed this out) be a result of the energy absorbancy of nylon. If you want to get away from that, the answer is to do something like what Evans has done, not toss the baby out with the bath water by using an intentionally less energy absorbant snubber.

When you tie nylon to Dyneema, the overall result is something like a snubber made completely from polyester, without cow hitches.

I used just nylon, but I see the advantages and disadvantages of other options and in the end they all work when you use the right approach for each option. When you use the approach that works for another option instead of the right one, it is like trying to row using an outboard instead of a paddle. You can repeat a paddle works better for eternity but I will keep saying that an outboard is pretty neat when used correctly too

[Dockhead]

Originally Posted by s/v Jedi

Well, you were writing about stopping a boat in 3.4cm stretch if chain and comparing that to nylon. There is no logic to that from my POV.

A lot of people misunderstood that, so I guess it was poorly presented. The purpose was to give a baseline -- show what huge forces are generated with very small stretch. For the further comparison to polyester and nylon. In fact I think, poorly presented or not, it was useful. Several people simply couldn't believe that the forces could be so high. One person even thought that the "effect of mass", and thus kinetic energy was reduced 20 times by the fact that the boat was floating in water. I think some learning happened in the end, which is always a good thing

Originally Posted by s/v Jedi

I can only counter with a question: why, why would you compare an 8-plait nylon with a double braid polyester and come up with these numbers? I can tell the only answer that I can think of: because you want to come up with the most exaggerated numbers possible, by choosing a more elastic fiber with the most elastic construction on one side, with the less elastic fiber with the least elastic construction on the other side.
If you want to have a sensible discussion, you should have come up with numbers for nylon vs polyester octo. Or both 3-strand.

The reason to pick an octo-plait polyester over nylon, is to get the same stretch with a more durable fiber.

I will. I realized that the best way to compare all the different choices for snubbers is by energy absorption in Joules at a safe working load for a given ultimate breaking strength.

But nylon octo versus polyester double braid followed from the OP -- Evans was in fact using polyester double braid, and the original point of all these calculations was to try to show him how little energy absorbing capacity that material has compared to normal snubber material.

Comparing the same construction, nylon still has about 3.5 times the energy absorbing capacity as polyester. So no -- you can't get the "same stretch with a more durable fiber" by using polyester octo. Polyester octo still has very little energy absorbing capacity, and still makes a poor snubber, although more, of course, than Evans' double braid.

Originally Posted by s/v Jedi

When you tie nylon to Dyneema, the overall result is something like a snubber made completely from polyester, without cow hitches.

I don't think so -- Evans has 5 meters of nylon in his snubber. You would need 15 meters (about) of polyester octo of the same strength. I really like Evans' super-snubber, and I think I'm going to make a similar one to have ready in case of storms.

Originally Posted by s/v Jedi

I used just nylon, but I see the advantages and disadvantages of other options and in the end they all work when you use the right approach for each option. When you use the approach that works for another option instead of the right one, it is like trying to row using an outboard instead of a paddle. You can repeat a paddle works better for eternity but I will keep saying that an outboard is pretty neat when used correctly too

Well, as a general principle, sure. Except that using polyester for a snubber is a bit like trying to row your dinghy with a 1" wooden dowell. There really isn't a "when used correctly" scenario for that. It's possible, but it doesn't make a whole lot of sense.