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Old 01-07-2013, 06:51   #31
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Re: Anchors and chain

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Originally Posted by Delfin View Post
It's from a Marine Safety white paper on dynamic loads on a rode. The formula is D (in tons)*100 *V2/d. It is shown as "a general formula", but in essence demonstrates that if the snatch load of a rode is dissipated over a distance, that distance is the critical factor in determining the peak load. If a snub line dissipates the force over 5 feet, the instantaneous peak force is 1/5th what it would be if the distance is 1 foot.
.

Got it. Basically F=MA. Bit picky but the " approximately 1/5 what it would be without the snub line." confused me.
This is all assuming the stress/strain of the line is linear, which it seems to be mostly for a lower percentage of load to breaking strain in the likes of nylon & polyester.
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Old 01-07-2013, 06:51   #32
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Re: Anchors and chain

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Originally Posted by gunnado View Post

in my reading of failed tackle after cyclones they almost all failed by chafe at the point of contact on the vessel. if your anchor is good enough to hold the bottom with good scope. you cannot do math on the bottom quality or the heat and chafe caused by large motion. But i would be interested if someone does have a formula?
The advantage of high anchor holding power is solely a function of the force it is expected to counter, with 3rd generation hooks like your Manson having much higher potential holding power because the designs digs in better than older designs and resists plowing once set. The hoop can get in the way of continued burying, but before that happens in most bottoms the anchor is going to be very well set. The CQR, not so much. By reducing shock loading with an elastic rode or a snub line you significantly reduce the force that needs to be countered by the anchor. Nylon 3 strand in larger diameters does heat up when cyclically loaded (out of the water), but you can deal with that by attaching a length of very strong and chafe resistant line to the nylon, keeping the nylon in the water. Probably the ideal snubber is chain securing the snub to the boat, and octoplait or 3 strand in the water attached to the chain to provide 10 feet or so of ultimate stretch. I believe Mr. Estarzinger has elected to use Amsteel for the chafe resistant on board portion of his snub, as have I.
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Old 01-07-2013, 07:03   #33
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Re: Anchors and chain

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Got it. Basically F=MA. Bit picky but the " approximately 1/5 what it would be without the snub line." confused me.
This is all assuming the stress/strain of the line is linear, which it seems to be mostly for a lower percentage of load to breaking strain in the likes of nylon & polyester.
Yes, it is just that, F=ma, but with deceleration of 'a' reducing the F as a function of the distance over which it is dissipated. What's interesting about this formula is that it shows how small the actual forces are that the anchor has to counter when you get the snub line correct. For Delfin, even though she is a tubby b*tch at 65 tons, with a snub line that will stretch 10' and acceleration of 2 knots (who knows, but probably a lot more than real) the snatch load on the anchor is only 2600#. Without a snub line, the force is more like 26,000#, which would require rather heavier ground tackle than I have.

This also illustrates why a light snub line is preferable to a thicker one - you want the stretch and don't want to have to generate anchor dragging forces on the snub line to introduce a decent distance over which the force can be dissipated. The key variable is the velocity, and I have zero idea how to calculate that, but in observing pitching boats I have a hard time imagining they are bobbing or veering faster than 2 knots.
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Old 01-07-2013, 09:20   #34
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Re: Anchors and chain

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Originally Posted by Delfin View Post
Yes, it is just that, F=ma, but with deceleration of 'a' reducing the F as a function of the distance over which it is dissipated.
Think I didn't take a good route with the time based F=MA, possibly an easier way to visualize might be to think of the energy being transferred as equal to the area under a graph of force against distance. Which seems to be fairly linear for a nylon rode at a lowish strain.
A triangle of force increasing to 1 as distance increases to 1 has the same area as a force going up to 1/2 over a distance of 2.

With energy also being equal to 1/2 X mass X velocity squared it shouldn't be too difficult to work out how much a rode would stretch for a given boat speed/weight & rode length/diameter.

In a perfect world with no friction of course

Anyone care to give it a go?



Edit
The graph looks like force=1/distance. Ignoring units.
https://www.desmos.com/calculator/hufuophq4m
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Old 01-07-2013, 10:32   #35
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Re: Anchors and chain

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Originally Posted by conachair View Post
Think I didn't take a good route with the time based F=MA, possibly an easier way to visualize might be to think of the energy being transferred as equal to the area under a graph of force against distance. Which seems to be fairly linear for a nylon rode at a lowish strain.
A triangle of force increasing to 1 as distance increases to 1 has the same area as a force going up to 1/2 over a distance of 2.

With energy also being equal to 1/2 X mass X velocity squared it shouldn't be too difficult to work out how much a rode would stretch for a given boat speed/weight & rode length/diameter.

In a perfect world with no friction of course

Anyone care to give it a go?



Edit
The graph looks like force=1/distance. Ignoring units.
https://www.desmos.com/calculator/hufuophq4m
I'm not that smart.

I believe 3 strand reaches 30% maximum stretch at around 1/3 breaking strength. Using my fingers and toes, if I want to peak at 3,000 pounds force that I get to by dissipating shock loads over 10 feet so I can handle 30,000# of initial force I need 9/16" 3 strand, 30' long. I went up to 5/8" because of deterioration over time of the snub line. This would be backed with a 1" octoplait 38' long that would start to take any loads in excess of the 3,000#, and would have an ultimate breaking strength equal to the chain.
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Old 01-07-2013, 11:01   #36
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Re: Anchors and chain

Not sure at all about this one, but might be getting there..

https://www.desmos.com/calculator/gcffowg4zv

Y scale is 100's Kg ( to get the scale readable)
x is amount of stretch in the rode to stop the boat in metres
M is weight of boat in Kg
V is boat speed in m/s

So if the force in the rode increases linearly as it stretches then the stopping a 10,000Kg boat over 1m would need a peak load of 1,000kg.
No idea if that's anywhere near right, anyone have anything to compare it with?
No friction either.
But fun
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Old 01-07-2013, 11:07   #37
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Re: Anchors and chain

G'Day all

You know, I have some difficulty in visualizing and understanding these theoretical scenarios.

IF an anchored boat does not have the chain stretched tight (zero catenary), then when a puff or surge hits, it accelerates backwards, lifting the chain until it does tighten. The act of lifting the chain against gravity and through a viscous medium absorbs some energy, even before the snubber is called into play, thus limiting the velocity that it can achieve and hence the energy that needs to be absorbed by the snubber.

IF the conditions are so severe that the vessel remains at the zero catenary position, then how can it accelerate backwards without the anchor dragging? If it can't accelerate backwards, then there isn't any velocity and hence no shock load from deceleration as the chain comes tight.

I've watched our snubber under fairly severe conditions recently: 45 sustained, gusts around 70, bullets from wide angles causing a lot of sailing about. There was around a cable or so of fetch, more from some directions and we were getting wind waves of around a metre at times. Snubber is 16 mm double braid nylon, about twenty feet deployed on 150 feet of 10 mm chain. Depth around 25 feet. Boat displaces around 12 tonnes in cruising trim, and has a fair amount of windage forward.

I could see the snubber stretch, and by watching the loop of loose chain between boat and chain hook, I could see that it stretched only perhaps a foot, maybe as much as a foot and a half. We experienced no perceptible shock loading.

This real world experience seems at odds with most of the theoretical proposals, so I am a bit confused. What do you think?

Cheers,

Jim
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Old 01-07-2013, 11:22   #38
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Re: Anchors and chain

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Originally Posted by Jim Cate View Post
G'Day all

You know, I have some difficulty in visualizing and understanding these theoretical scenarios.

IF an anchored boat does not have the chain stretched tight (zero catenary), then when a puff or surge hits, it accelerates backwards, lifting the chain until it does tighten. The act of lifting the chain against gravity and through a viscous medium absorbs some energy, even before the snubber is called into play, thus limiting the velocity that it can achieve and hence the energy that needs to be absorbed by the snubber.

IF the conditions are so severe that the vessel remains at the zero catenary position, then how can it accelerate backwards without the anchor dragging? If it can't accelerate backwards, then there isn't any velocity and hence no shock load from deceleration as the chain comes tight.

I've watched our snubber under fairly severe conditions recently: 45 sustained, gusts around 70, bullets from wide angles causing a lot of sailing about. There was around a cable or so of fetch, more from some directions and we were getting wind waves of around a metre at times. Snubber is 16 mm double braid nylon, about twenty feet deployed on 150 feet of 10 mm chain. Depth around 25 feet. Boat displaces around 12 tonnes in cruising trim, and has a fair amount of windage forward.

I could see the snubber stretch, and by watching the loop of loose chain between boat and chain hook, I could see that it stretched only perhaps a foot, maybe as much as a foot and a half. We experienced no perceptible shock loading.

This real world experience seems at odds with most of the theoretical proposals, so I am a bit confused. What do you think?

Cheers,

Jim
The worst loads I've come across have been when the wind has been gusting up to 60Kn coming from all directions, so the boat gained a lot of speed before the snubber exploded water as it stopped the boat.
Just the wind doesn't seem so bad, even with biggish waves. It's when the boat gets some speed up that the forces seem to be at their worst.
That's without a load cell, just watching the rode go bar tight.
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Old 01-07-2013, 11:44   #39
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Re: Anchors and chain

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Originally Posted by Jim Cate View Post
G'Day all

You know, I have some difficulty in visualizing and understanding these theoretical scenarios.

IF an anchored boat does not have the chain stretched tight (zero catenary), then when a puff or surge hits, it accelerates backwards, lifting the chain until it does tighten. The act of lifting the chain against gravity and through a viscous medium absorbs some energy, even before the snubber is called into play, thus limiting the velocity that it can achieve and hence the energy that needs to be absorbed by the snubber.

IF the conditions are so severe that the vessel remains at the zero catenary position, then how can it accelerate backwards without the anchor dragging? If it can't accelerate backwards, then there isn't any velocity and hence no shock load from deceleration as the chain comes tight.

I've watched our snubber under fairly severe conditions recently: 45 sustained, gusts around 70, bullets from wide angles causing a lot of sailing about. There was around a cable or so of fetch, more from some directions and we were getting wind waves of around a metre at times. Snubber is 16 mm double braid nylon, about twenty feet deployed on 150 feet of 10 mm chain. Depth around 25 feet. Boat displaces around 12 tonnes in cruising trim, and has a fair amount of windage forward.

I could see the snubber stretch, and by watching the loop of loose chain between boat and chain hook, I could see that it stretched only perhaps a foot, maybe as much as a foot and a half. We experienced no perceptible shock loading.

This real world experience seems at odds with most of the theoretical proposals, so I am a bit confused. What do you think?

Cheers,

Jim
Jim, I think what you're observing would be what you would expect from the data, even if you had never been on a boat at anchor. The snub line in the conditions you describe would start out straight in the lulls, if only because the chain is weighting it down. A gust occurs with a force that accelerates the boat and if this force were transferred to the ground tackle instantaneously it would break the chain. That never happens in the real world even with all chain rode because although it might look 'bar tight', it isn't. With a snub line used, it then stretches and the distance it stretches reduces the force delivered to the anchor and gear AFTER some of that force is used up removing most all of the catenary. The more the snub stretches, the more the force is then dissipated. You observe minimal motion, but it is probably a bit greater than your eye tells you.

If you knew what the peak velocity of the boat was when in moved in those gusts, you could calculate the maximum shock load. Assuming 2 knots (high) and 1 foot of residual catenary your boat would generate #10,000 of force. With the 16 mm snub of that length, you might get 5 feet of stretch, reducing the force on the anchor to under #2,000, well within the holding of a decent anchor and the WLL of the chain. Not surprising you don't feel any shock loads since all of this happens dynamically with the boat's velocity dropping off as maximum stretch given the initial force is reached. Does that make sense?

Incidentally, if the chain is nearly bar tight without an elastic snubber, the issue becomes the pitching of the boat, so you would have some significant shock loads experienced as the boat tries to rise to the waves.
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Old 01-07-2013, 12:03   #40
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Re: Anchors and chain

Rebel Heart. Turtle Bay late December. Blowing off of the land. I was anchored near the entrance, so there was plenty of fetch. A very uncomfortable night._____Grant.
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Old 01-07-2013, 16:02   #41
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Re: Anchors and chain

Practical Sailor will have an article on Kinetic and Potential Energy of yachts and snubbers in the next couple of months.

The big unknown is the speed at which the yacht moves. The formula depends on velocity squared thus 1m/s or 2m/s makes a real difference. yachts when they drag can move at a fair old speed, even pulling chain and anchor, but speeds actually at anchor - so far its a guess.

The reality is that the chain and the snubber share the energy, some is stored in the elasticity of the snubber and some in the staightening of the catenary. As the catenary straightens it will need an increasing load to straighten it further and then the snubber will take a larger share. However I cannot quite work out how they share the loads, or energy. The stretch of nylon is not linear, but given the number of other variables might be considered linear, at least upto, say, 50% of total elasticity (say 15%). But removal of catenary is not linear, needs more load as it straightens. As the chain straightens it really does not matter if its 12mm or 8mm - the snubber will be taking most of the energy.

Delfin, my understanding is that nylon stretches 25%-35% (3 strand or braided - 3 strand is better upto about 15% elasticity) and then fails. Climbing rope stretches more than 40% (but sacrifices some ultimate strength to achieve this) A min breaking strength of 10mm is about 2t, 14mm 4t, 16mm 5.3t though actual or typical breaking strengths are 2.7t, 4.7t and 6.6t.

Jim's point is valid - if the yacht is being held relatively steadily as a result of small seas and maybe a bit of tide and a wind in the same direction then theoretically the yacht will not move, or accelerate. So velocity squared starts to become less important (its just the force of the wind on the windage). My limited experience suggests that thought seas and tide might be a constant direction but the wind shears or veers - and its this that causes the yacht to move and thus have velocity. (If the seas are big enough to overcome wind shear I personally, with yacht, would try to be somewhere else.)

The idea of 2 snubbers, side by side, seems sound. A thin one to use upto say 40 knots and a thicker one for beyond. You would need to experiment to work out how much slack the thicker one would need so that it only started to work at, say, 40 knots. They could be attached to the same chain hook (or soft shackle) and the same Amsteel, or chain.

Jonathan
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Old 01-07-2013, 16:14   #42
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Re: Anchors and chain

I've just checked, Muirs do a 6mm gypsy and suggest their 6mm gypsies will fit the imperial equivalent. Their advise is take the gypsy in when you buy your chain and try it. My experience is that chandlers have no idea but a good chain retailer, one that does lifting chains etc, will be able to test the chain against the gypsy (or they can in Sydney).

I'd normally be suspicious of Muirs claim (no reflection on Muirs, just I'm a cynic) that their metric gypsies take imperial chain - except I have one of their older imperial gypsies 5/16th inch and have run 8mm chain for 12 years without a problem.

I do not know if a Muir gypsy would fit a Maxwell, etc windlass.

Jonathan

PS We need to buy new chain in the next 12 months or so (its cheaper in Sydney than trying to regal). We run 8mm and had thought of G7 (we also have G5 in Oz) 6mm, but I'm not sufficiently convinced! I looked at 6mm and it looks incredibly small against 8mm (which looks equally small against 12mm!). 6mm looks the sort of thing for a small dog, not a 6t yacht!
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Old 01-07-2013, 16:19   #43
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Re: Anchors and chain

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Delfin, my understanding is that nylon stretches 25%-35% (3 strand or braided - 3 strand is better upto about 15% elasticity) and then fails. Climbing rope stretches more than 40% (but sacrifices some ultimate strength to achieve this) A min breaking strength of 10mm is about 2t, 14mm 4t, 16mm 5.3t though actual or typical breaking strengths are 2.7t, 4.7t and 6.6t.


Jonathan
Quite right, thank you.
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Old 01-07-2013, 17:43   #44
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Re: Anchors and chain

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I'm not that smart.

I believe 3 strand reaches 30% maximum stretch at around 1/3 breaking strength. Using my fingers and toes, if I want to peak at 3,000 pounds force that I get to by dissipating shock loads over 10 feet so I can handle 30,000# of initial force I need 9/16" 3 strand, 30' long. I went up to 5/8" because of deterioration over time of the snub line. This would be backed with a 1" octoplait 38' long that would start to take any loads in excess of the 3,000#, and would have an ultimate breaking strength equal to the chain.

When nylon is put into tension such that it is over 30% of breaking, IT GETS HOT. The heat is internal to the rope and if you pull hard enough on it, it will permanently deform.

I did my own tests with my Ford 2500 Econoline, an old 5/8" three stand nylon anchor rode and some brush in my yard that I tried to rip up by the roots. I got some up, I distorted the rope and I even broke the rope tugging on it with the van.

Hurricane Bob hit some of the harbors here in Massachusetts pretty hard. There were hundreds of boats up on the beaches. I forgot which organization did the study but they found nylon almost melted as it went over the chocks.

Another example which I am not proud of happened two years ago at Martha's Vineyard. I had a length of nylon seat belt material hanging in my dinghy, used it to lift the dinghy into and out of the water with my davit crane. Somehow that length of seat belt material that has/had a SS hook on the end got into the water. All of this was at the stern of my 40' Silverton. The dinghy had a hunk of nylon attached to one of the boat's cleats.

I knew the dinghy was tightly tide so I backed out of my slip and all hell broke loose. Anyway, that seat belt material wrapped around the prop AND MELTED! Yeah, in the water and it melted!! I still have the piece on my boat. If it had not melted major damage would have occurred because the dinghy would have been pulled into my props.

Foggy
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Old 01-07-2013, 18:19   #45
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Re: Anchors and chain

Foggy,

We can confirm what you have done, though I'm not sure what you mean by 30% of breaking?

We tested climbing rope to 50% of its (approx or estimated) breaking load (thats 1t load) (so we stretched about 20% of the available 40% of elasticity). The outer case had little, tiny, beads of melted nylon where the hairs on the casing had melted in some areas. We also tore the outer braid at the knots (and needed a vice to undo them). We did this dry. We were using a 4x4 and the speed of our stretch seemed not dissimilar to the way our cat moves back and forth.

Climbing rope is a braid and I'm not sure if 3 strand might work better or worse (maybe not much in it). But 3 strand is a better option (over normal braided nylon) upto 15% of of the available 30% elasticity (but they have similar ultimate elasticity and strength.

Nylon has a reduced breaking strain when wet, by 10%-20%. So I'm not convinced about keeping the snubber wet (though cannot see how you can keep it dry when its raining?! even if its not in the water)

But based on something similar happening with a snubber - to stretch by 15% (of the 30%) or to stretch to half of the actual breaking load of say a 10mm nylon means a load of well over 1t. I'd be suggesting that a 10mm line is too thin for anticipated 1t loads and that a thicker line should be used (it will still take the same 1t+ but the stretch will be say a max of 10% (not the 15%).

Its a compromise, you must have as much stretch as possible without reducing the life of the snubber to something so short as to be ridiculous. The shorter your snubber the more work it needs to do, so you want maximum snubber length that's convenient. I recall Evan is working to 6m, which I think parsimonious - but if it works for him (we are using 14m - each side).

For a 35'/40' x 10t yacht 10m of 10mm seems fine but I'd back it up with (have not sat and thought about it much) 11m of 14mm to step in when the wind is above 40 knots (the 10mm will still work, further stretch is available, from the 10mm, but the 14mm will manage the stretch (manage the rate of stretch) of the 10mm - if that makes sense?)

Jonathan
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