Choice of Material for Snubbers

[Delfin]

Originally Posted by Dockhead

Well, I can tell you this much -- if an even 1 ton boat is moving at even 0.01 knot, and is stopped instantly, I mean literally in 0.000000 nanoseconds, then the dynamic force is infinite -- would explode the whole universe. That's mere mental masturbation, because nothing with any mass can be accelerated or decelerated in 0 time.

The principle is that one G of acceleration will produce one gravity of force -- so a 25 ton boat accelerating or decelerating at 9.8 meters per second per second will be subject to a force of 25 tons. Two knots are 1.029 meters per second. So it's a simple calculation to show that to go from two knots to zero knots in one-tenth of a second will produce a force of 1.05 G, or 26.25 tons. Here's a handy calculator: Online Acceleration Calculator (flexible units)

Distance travelled during this tenth of a second (I am guessing that the average velocity during the deceleration will be (v0 + v1)/2, assuming a constant rate of deceleration, so 0.5145 meters per second; someone please correct me if I'm wrong), is 5.1cm.

So you'll need total stretch of 5.1cm (including the whole system) and strength of more than 26.25 tons, to slow the boat down from 2 knots to 0 in one-tenth of a second.

The more stretch you have, the more time you have, the less deceleration, and the forces go down dramatically. That is the basic principle of a snubber.

Your calculations are correct if your vessel is falling off the edge of the world and gravity is the force you are dealing with. But that's not what is going on with a vessel floating in the water, which is why everyone's practical experience is vastly different from what would be presumed if your method were correct. True, F=ma, but what force is required to move a floating vessel? If you wanted to lift the vessel, it would be the weight of the vessel, but you can move it if floating with your hand. As a result, in their approximation formula, the effect of mass is reduced by a factor of 20, which is why tons times 100 is used for the mass, not pounds. So rather than using F=ma, they use F= mass (in tons) times 100 times the square of the velocity divided by the distance over which the force is countered. But again, the critical difference between your calculations and reality is that a floating vessel is not a vessel dropping off a cliff, so in your example, the force required to "slow the boat from 2 knots to 0 in 1/10th of a second" is 1.32 tons, not 26.25 tons. Big difference.

Physics aside, the reason why a stretchy snubber dramatically reduces the Force that has to be countered by the ground tackle is because it introduces distance into the formula above. With 5 feet of stretch, the force to be dealt with drops from 1.32 tons to 528 pounds. Big deal.

[Kettlewell]

Maybe someone could try a section of whatever Hazelett Marine elastic mooring systems are made out of. They seem to work and be reliable enough to be used on thousands of moorings. Here is a bit of what they say:

Point Load Reductions
Securing vessels with Hazelett Elastic Rodes will reduce point loads at connections points (from docks to anchors and from deck cleats to anchors) by approximately fifty percent.

Wind Load Reductions
Boats secured with Hazelett Elastic Rodes stay pointed directly into the wind with a more constant, gentle pull.

[Dockhead]

Originally Posted by Delfin

Your calculations are correct if your vessel is falling off the edge of the world and gravity is the force you are dealing with. But that's not what is going on with a vessel floating in the water, which is why everyone's practical experience is vastly different from what would be presumed if your method were correct. True, F=ma, but what force is required to move a floating vessel? If you wanted to lift the vessel, it would be the weight of the vessel, but you can move it if floating with your hand. As a result, in their approximation formula, the effect of mass is reduced by a factor of 20, which is why tons times 100 is used for the mass, not pounds. So rather than using F=ma, they use F= mass (in tons) times 100 times the square of the velocity divided by the distance over which the force is countered. But again, the critical difference between your calculations and reality is that a floating vessel is not a vessel dropping off a cliff, so in your example, the force required to "slow the boat from 2 knots to 0 in 1/10th of a second" is 1.32 tons, not 26.25 tons. Big difference.

Physics aside, the reason why a stretchy snubber dramatically reduces the Force that has to be countered by the ground tackle is because it introduces distance into the formula above. With 5 feet of stretch, the force to be dealt with drops from 1.32 tons to 528 pounds. Big deal.

Sorry, but you are deeply confused by the fact that acceleration is commonly expressed in "g's", or forces of gravity. That's just shorthand for 32 feet per second per second change of velocity; it has nothing to do with falling.

The fact that a boat is floating does not change one iota its mass. An object moving with 25 tons of mass moving at a particular speed has exactly the same kinetic energy whether it is floating, rolling, sliding, flying, or whatever. Varying amounts of friction from floating versus rolling will play a small role, but the total force to stop 25 tons of mass, including friction, will be precisely the same. And by the way, exactly the same forces will be involved in outer space where there is no gravity at all is involved. Mass and weight are not always exactly the same thing, even if they are expressed in the same units.

You might find this enlightening: https://en.wikipedia.org/wiki/Acceleration

It's Newton's Second Law of Motion:

"The acceleration of a body is directly proportional to, and in the same direction as, the net force acting on the body, and inversely proportional to its mass. Thus, F = ma, where F is the net force acting on the object, m is the mass of the object and a is the acceleration of the object."

See: https://en.wikipedia.org/wiki/Newton...27s_second_law


Note well that it is mass, not weight, which determines the force produced by a given amount of acceleration.

[s/v Jedi]

Originally Posted by Dockhead

Note well that it is mass, not weight, which determines the force produced by a given amount of acceleration.

Exactly. And a boat that is accelerating away because of a too stretchy/thin snubber is building up kinetic energy instead of loosing it.

A 10 ton block of concrete weighs 10 tons on the ground and it's mass is also 10 tons. When submerged in salt water, it's mass stays 10 tons but the weight goes down to 4 tons or so.

[Dockhead]

Originally Posted by s/v Jedi

Exactly. And a boat that is accelerating away because of a too stretchy/thin snubber is building up kinetic energy instead of loosing it.

A 10 ton block of concrete weighs 10 tons on the ground and it's mass is also 10 tons. When submerged in salt water, it's mass stays 10 tons but the weight goes down to 4 tons or so.

I got your point a few posts ago -- you are few steps ahead of me and everyone else.

You mean to say that it's not enough to have x meters of stretch in your system, there also needs to be a certain force applied (probably more precisely -- absorbed).

You are of course absolutely right. I don't really know how to deal with it -- maybe you can suggest? You are an engineer, I believe, a real one? Even if an EE? Please help out this poor techno-ignoramus.

This is a dimension I have not considered, other than in the calculations about the stretch at 30% of breaking force, where we do know the force which corresponds to that amount of extension.

Since we don't have anything like a Young's Modulus for ropes, we simply can't calculate this over the whole range of a rope's extension.

One important implication of this is that a floppy bungee cord will not do anyone any good as a snubber. Grasping at straws here -- is it the spring constant we need to consider?

[Delfin]

Originally Posted by Dockhead

Sorry, but you are deeply confused by the fact that acceleration is commonly expressed in "g's", or forces of gravity. That's just shorthand for 32 feet per second per second change of velocity; it has nothing to do with falling.

Perhaps I am confused. Your post indicates that you believe a vessel at anchor to a chain would experience in normal conditions forces that would be snapping every chain on every boat in existence routinely:

Originally Posted by Dockhead

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

Wow, 90,000# working load for chain on a 25 ton boat. I think I'm going to need a bigger windlass.....

I guess we'll have to agree to disagree, but I would point you to your own post from Professional Mariner if you would like to understand why you're simply wrong. Or, just carry on.

[conachair]

A few thoughts..

What are we trying to achieve with snubbers?

Which seems pretty much agreed upon - reduce peak dynamic loads on the anchor and deck gear.

The obvious first port of call seems to be how much a snubber decreases those dynamic loads but is there anything to suggest that a large elastic member might actually increase dynamic effects by increasing how much the boat sails about at anchor in a blow?
Seems to be almost no data on this, most serious studies seem to be about big boat mooring, presumably as there's more money there. None to be made in sailing boat anchoring dynamic forces.

So what to do when it really kicks off?
Lots scope and nylon in there for a snubber. Works for most people. Trouble is when it's that bad no one is messing around changing nylon to polyester taking readings with a load cell etc to see what's actually happening.

"THIS IS BEST!" But really we don't know and being the real world perhaps it's impossible to anything more than an vague inaccurate take on what happens.

[Dockhead]

Originally Posted by Delfin

Perhaps I am confused. Your post indicates that you believe a vessel at anchor to a chain would experience in normal conditions forces that would be snapping every chain on every boat in existence routinely:



Wow, 90,000# working load for chain on a 25 ton boat. I think I'm going to need a bigger windlass.....

I guess we'll have to agree to disagree, but I would point you to your own post from Professional Mariner if you would like to understand why you're simply wrong. Or, just carry on.

Sorry, I didn't say that.

I'm repeating myself somewhat, but here goes.

I used stopping from 2 knots as a possible case that a snubber has to deal with in a rough anchorage. I never said that these are "normal conditions". In fact, snubbing 2 knots of motion would be pretty rough conditions which most cruisers will not experience much.

I also said that I'm guessing at that velocity. I have no way of calculating it. It seems plausibly possible in rough conditions.

I also never said that these are the real forces which will practically occur. In fact, I explicitly said that they will be less than that, because there are other "soft" things in the system -- bowsprits, anchor rollers, the anchor itself in the pliable seabed -- which will also contribute to the total stretch in the system.

I was merely trying to demonstrate the forces involved and the DIFFERENCES in the ability to absorb those forces between different materials from which we might make snubbers. I never said that you need 90,000 pounds working load (!!) on your ground tackle.

We can, of course, agree to disagree, but physics are physics. We are all entitled to our own opinions, but not to our own Laws of Motion If you don't care, that's perfectly all right -- as someone above said -- go sailing; just put something stretchy between your boat and your anchor. The operative word being STRETCHY! The rest of it is all for those who have nothing better to do The calculations, however, are correct, until someone demonstrates objectively otherwise. That is the simple reality of the forces which could be exerted on your ground tackle, not my reality, but Sir Isaac Newton's

[Dockhead]

Originally Posted by conachair

A few thoughts..

What are we trying to achieve with snubbers?

Which seems pretty much agreed upon - reduce peak dynamic loads on the anchor and deck gear.

The obvious first port of call seems to be how much a snubber decreases those dynamic loads but is there anything to suggest that a large elastic member might actually increase dynamic effects by increasing how much the boat sails about at anchor in a blow?
Seems to be almost no data on this, most serious studies seem to be about big boat mooring, presumably as there's more money there. None to be made in sailing boat anchoring dynamic forces.

So what to do when it really kicks off?
Lots scope and nylon in there for a snubber. Works for most people. Trouble is when it's that bad no one is messing around changing nylon to polyester taking readings with a load cell etc to see what's actually happening.

"THIS IS BEST!" But really we don't know and being the real world perhaps it's impossible to anything more than an vague inaccurate take on what happens.

I agree with all of this, and that was Nick's point.

He means that too floppy a snubber will not help us. Something which I think is obvious and which we can all agree about.

I have been thinking about Nick's point, which is more and more interesting, the more I think about it, and I realize that we have actually two data points -- we have the extension at 30% of breaking force, and besides that -- we have the breaking force itself.

A bungee cord, because of its design, might stop extending at some point short of its ultimate tensile strength, but regular rope is not like that. Just look at the force-extension curves published by the makers. They are not straight lines (no Young's Modulus for ropes), but our ropes are absorbing more and more force right up to the breaking point -- we can count on that.

So it means that we know that we can absorb up to the breaking point of force, with any piece of our kinds of rope.

A different question is at what point does it become so floppy that it is not doing its job. I think this is a question of the spring constant of the rope versus the forces we hope to deal with, but that's already far beyond my skills to deal with. So on that point, I think we have to revert to our "vague inaccurate take", and do it by trial and error. If the snubber lets you bungee cord around all over the anchorage, then it's too floppy. As Nick said, it might actually make matters worse by letting you accelerate as the snubber extends (the resistance to further extension -- spring constant -- is less than the force of wind and waves, = acceleration).

All this is correct, but I can say one thing -- if the ultimate tensile strength is enough to deal with the forces you will encounter, then you will have enough spring constant, ipso facto. Because our ropes (three-strand, octo, whatever) are not bungee cords -- it takes ever increasing force to get more extension, right up to the breaking point.

Which does not eliminate the necessity of "tuning" the snubber so it resists extension to the right degree, for a comfortable and stable ride at anchor. It's easy with a simple nylon snubber, as you can simply take in or let out the length.

[Kettlewell]

A few random thoughts:

A lot of production boats have a couple of bow cleats so close to the deck edge forward there is no need for chafing gear and there is a perfect lead from the cleat to the anchor chain. Rig a bridle snubber from those two cleats and it will help to hold your bow into the wind, is unlikely to chafe, and if one leg should for some reason let go, the other leg will still be there taking the strain.

On a cat I owned I used to take a foot or so each side of the middle of a 3/8" snubber line (marked by a piece of tape) and used the loop to tie a rolling hitch on the chain. The boat ends of the snubber line were connected to eyes on the bows down near the waterline, so the length wasn't easily adjustable, but once I arrived at a reasonable length to form a nice V off the bows I used that set up in most normal anchoring situations. Provided a comfortable and secure ride and the bridle made a big difference in how well the boat pointed into the wind.

[s/v Jedi]

Gee, it's nice when I manage to express myself good enough that people start to understand what I mean... doesn't happen so often

I'm just an EE so I don't know enough about all these mechanical things, but I used common engineering sense to come to these conclusions:

- the snubber must stretch a lot, but it would be really nice if it doesn't break.

- the stretch percentage only matters for how long the snubber becomes in the end: a nylon 3-strand one will be shorter than a polyester 3-strand one for the same gain in length of the snubber under tension.

- I want it as simple and cheap as possible within the required quality, because the snubber wears out and is a replaceable item like oil filters.

- chafing has two faces: one where it rubs on hardware like a roller or toe-rail and another where it rubs on it's own fibers, i.e. internal chafing, which is more of a problem with stretchier materials (they rub the fibers more).

With the above, the problem that I see most often is that people don't have a clue what size snubber they need. I see boats half my weight with 3/4" snubbers. That is why I keep stating that your snubber is too thick until you break one in the middle (not due to external chafing) and go up one size. I broke 1/2" so I'm with 5/8" and it never broke until in a full cat4 hurricane. It survives 60-80 knot squalls that last 20 minutes, although the 80-knot squall resulted in hard spots, where the fibers had melted together. This gives an exact data point and IMHO, one that confirms 5/8" 3-strand nylon is the right size/type combination for my boat.

About length: it's easy because after you determine the size, just make a 30 or 40' snubber and start experimenting. I find no extra benefit when the length of the snubber becomes more than 20' (excluding the part on the cleat). At 30% stretch, that is 6 foot, and within those 6', it fully absorbs the kinetic energy. That's not true, the chain helps too. With 3-strand, the snubber torques when it stretches and this wraps the slack end of chain around it which is a great indicator of force that is on the snubber. It shows that the chain (stretching and lifting from seabed) takes the first part, followed by the snubber.

About chafe: every boat is different, but my snubbers have no chafe protection because I consider the snubber itself as the chafe protection. But they never wear out on the stainless steel hardware that they loop through. They wear out at the outboard splice. When they have been overloaded in squalls, they get the molten hard spot halfway, which is also the part where I broke the 1/2" snubber with visible melting damage.
I see no benefit in Dyneema leaders for that reason; they would be just an expensive complication without benefit for me. If your snubber chafes and fails at the chock or roller first then it may make more sense. But even then, when it happens after 2 years of use, I would be happy and just replace before it gets that far; chafe is visible.

[wsmurdoch]

Originally Posted by Kettlewell

Maybe someone could try a section of whatever Hazelett Marine elastic mooring systems are made out of. They seem to work and be reliable enough to be used on thousands of moorings. Here is a bit of what they say:

Also look at Home Page which are (I think) the moorings used in the public mooring fields in Tittusville, FL and at Dinner Key in Miami, FL.

It looks to me like they use a bungee cord like thing as the shock absorbing element.

They use a figure of 6000 lb for the storm shock loading of an all chain mooring.

[Kenomac]

I use one of those jumbo rubber mooring shock absorbers wrapped with 6-7 winds of 3/4 inch braided nylon rope attached to a stainless chain hook. It works great, just the right amount of stretch and less complicated.

[Dockhead]

Originally Posted by s/v Jedi

Gee, it's nice when I manage to express myself good enough that people start to understand what I mean... doesn't happen so often

I'm just an EE so I don't know enough about all these mechanical things, but I used common engineering sense to come to these conclusions:

- the snubber must stretch a lot, but it would be really nice if it doesn't break.

- the stretch percentage only matters for how long the snubber becomes in the end: a nylon 3-strand one will be shorter than a polyester 3-strand one for the same gain in length of the snubber under tension.

- I want it as simple and cheap as possible within the required quality, because the snubber wears out and is a replaceable item like oil filters.

- chafing has two faces: one where it rubs on hardware like a roller or toe-rail and another where it rubs on it's own fibers, i.e. internal chafing, which is more of a problem with stretchier materials (they rub the fibers more).

With the above, the problem that I see most often is that people don't have a clue what size snubber they need. I see boats half my weight with 3/4" snubbers. That is why I keep stating that your snubber is too thick until you break one in the middle (not due to external chafing) and go up one size. I broke 1/2" so I'm with 5/8" and it never broke until in a full cat4 hurricane. It survives 60-80 knot squalls that last 20 minutes, although the 80-knot squall resulted in hard spots, where the fibers had melted together. This gives an exact data point and IMHO, one that confirms 5/8" 3-strand nylon is the right size/type combination for my boat.

About length: it's easy because after you determine the size, just make a 30 or 40' snubber and start experimenting. I find no extra benefit when the length of the snubber becomes more than 20' (excluding the part on the cleat). At 30% stretch, that is 6 foot, and within those 6', it fully absorbs the kinetic energy. That's not true, the chain helps too. With 3-strand, the snubber torques when it stretches and this wraps the slack end of chain around it which is a great indicator of force that is on the snubber. It shows that the chain (stretching and lifting from seabed) takes the first part, followed by the snubber.

About chafe: every boat is different, but my snubbers have no chafe protection because I consider the snubber itself as the chafe protection. But they never wear out on the stainless steel hardware that they loop through. They wear out at the outboard splice. When they have been overloaded in squalls, they get the molten hard spot halfway, which is also the part where I broke the 1/2" snubber with visible melting damage.
I see no benefit in Dyneema leaders for that reason; they would be just an expensive complication without benefit for me. If your snubber chafes and fails at the chock or roller first then it may make more sense. But even then, when it happens after 2 years of use, I would be happy and just replace before it gets that far; chafe is visible.

LOL.

Now backing WAY back away from theory, and moving on to practice.

I think very few cruisers actually anchor out in conditions which will take the catenary out of their chains, so this is a totally abstract conversation for most of them. Of course, the catenary is doing all the work in a reasonably calm anchorage, and the snubber is doing nothing but stopping chain rumble.

That was not Evans' case, which is just one reason why it was so interesting.

So all this snubber design is really for fairly rare cases -- when the going gets tough, and the chain catenary is not enough. What's frightening is that the forces grow with a very steep curve from that point.

So I will defend Evans' hybrid snubber design -- it is not made for ordinary conditions, where just about any old snubber will do. It is made for those cases where the snubber is saving your boat. Here I really do worry about chafe -- we may be talking about conditions where it's not safe to even go to the foredeck to check the snubber. I really like it and I want one. My snubbers are great, but I would never have wanted to anchor out in a hurricane in one. I'd like to have at least one super-snubber on board for when the fit hits the shan. In those conditions, the snubber is really the weak point in the whole system keeping your boat off the rocks. I'd like to attach the dyneema part to the nylon part with a cow-hitch. And I think a soft shackle will be good for the chain attachment.

[Nor'Sea 27]

Jedi

When suggesting a snubber of 30' in 5/8 diameter do you refer to a bridle - thus two legs each 30' - or do you mean a single line at 30'?

Charles