Chain dragging trials

vyv_cox said:
... Catenary provides a little elasticity at low wind speeds but none over around F5-6.
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That's quite true, I'll take vyv's word for it about the F5-6 but I do know that with enough wind & tide loading a chain rode tends towards being in a straight line and provides no shock absorbtion at all. One can observe vessels lying to chain rodes in these conditions using a shore transit and see that they make no fore & aft movement at all when the surges pass - sure proof that the chain is bar-taut.
 
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BlowingOldBoots said:
Bentonite based clays are used as lubricants in the oil well drilling industry, so, intuitively an anchor chain in water, covered in a saturated clay would not have more friction than an anchor chain on dry, compacted gravel. However, who knows for sure!
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As one who regularly hauls up an anchor by hand, I 'know for sure' a chain covered in clay-ish mud has substantially less friction than a bare chain!
 
Just a thought on this subject; I know from floating around watching 40m of our 12mm chain in 10m of water, attached to our 45kg Spade anchor, in medium grade sand, being pulled on by our 14.6m 30 tonne yacht, with 20 knots of wind, the first 20m of chain lifts from the sea bed. The other 20m gently sweeps the sand. Of course with more wind we would want more scope. In winter mode with, potentially, 50 to 60 knots of wind in 6-7m of water we use 60m of chain and a 25 kilo chum attached to the chain just above the sea bed (so as not to foul the chain).

I used to be as trawlerman and when trawling with a maximum pulling load of 2.2 tonnes I could quite happily pull 2 x 10m of 10mm sweep chain attached to 2 x 50kg trawl doors pulling approximately 200kg of net and ground rope at a depth of 30m at four knots. It was not until there was about a tonne of sand, weed and fish in the cod end that we'd slow down to 1 knot, requiring us to haul.

So I would agree, you do need something rather heavy on the end of your chain to hold you.
 
simple test for those who say the work is done by the chain.

remove the shackle, and attach it with 3 cable ties to the chain. This will be strong enough to carry the weight of the anchor. Then go and anchor your boat. If you're wrong it costs you an anchor. Simples.

see you're not so sure now are you :)
 
One wonders how seemingly intelligent people can be as dumb as to fall for the old catenary lark.

A simple vector diagram quickly proves that most of the pull is translated into effort at the anchor, force is not somehow dissipated within the curve of the chain.
 
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I may be wrong, but I always thought that the chain influenced the direction of the pull on the anchor. If you have plenty of chain out it makes the forces pull the anchor parallel to the sea bed and the flukes dig in and the anchor works. If you don't have enough chain out, the forces surely try to pull the anchor slightly upwards away from the seabed.
My only experience of this is anchoring a charter fishing boat several times a day for several years in the Bristol Channel.
 
Anchored in thick mud in 4m of water with ~50m of 8mm chain out to a 25 lb CQR, and 30 chain / 20 nylon out to a modern anchor, set up in a V. In the night the chain to the CQR became slack so I decided to pull it back in until tight, manually (no winch). Absolutely zero resistance until the CQR hit the bow roller.

So:

a) My CQR was useless (I gave it away)

b) Even with a 25lb weight on the end it was a complete doddle to pull in the chain by hand.


Also: I think when some people say that "the anchor does no work" they might be referring to the physics/engineering term "work" meaning work done by a force. In this context it's true that when the anchor remains set and doesn't drag, it does no "work". Ironically then, only dragging anchors actually "work". (work = distance x force)
 
simon barefoot said:
I may be wrong, but I always thought that the chain influenced the direction of the pull on the anchor. If you have plenty of chain out it makes the forces pull the anchor parallel to the sea bed and the flukes dig in and the anchor works. If you don't have enough chain out, the forces surely try to pull the anchor slightly upwards away from the seabed.
My only experience of this is anchoring a charter fishing boat several times a day for several years in the Bristol Channel.
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What you say is, of course, absolutely true and I don't think anyone is disputing it. What myself and others have disputed is the opinion (given in post #7) that the length of chain suspended in a catenary curve between the boat and where the chain first touches the sea bed, has benefits in "holding the boat" and absorbing wind & wave loadings.
The reasoning is that in a gust (say) the boat will be driven away from the anchor straightening the catenary somewhat and so some of the energy of the surge will be harmlessly dissipated in lifting the weight of the chain against the force of gravity. This would be all right except that in testing coditions, and with any with any chain likely to be used by our boats, the catenary would have disapeared and the chain become near-straight from anchor to bow roller, vyv-cox says that this happens in as little as F5-6. In this situation what you say about a long length of rode giving a more hrizontal pull on the anchor is, of course, especially pertinent.
 
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Sorry, re read it a bit better this morning. You are right, we often used to end up with it bar tight, but at a fairly acute angle due to the sheer amount of cable out. From memory we'd use a 40 or 50lb fishermans anchor, 10m or so of 18 or 20mm chain (a substantial fistful in dia.), followed by 20m or so of 12mm chain, then 30m of lead cored rope, then as much rope as seemed necessary, often the best part of 100m. Luckily we used an Alderney buoy to haul, even then it was pretty knackering!
 
>How can a chain hanging vertically hold anything in place? Or if it comes to that, how can a chain hanging in a near horizontal curve hold anything in place?

I've said this before the weight in the catenary holds the boat in place and if you dive down on the anchor and lift the chain you will feel there is no pull on the remainder of the chain to the anchor, provided you have enough chain out and he right chain weight for the boat. I've never seen a near horizontal curve except when anchoring in shallow water.
 
Whilst it may seem quite easy to heave an anchor chain, it's worth remembering that its also quite easy to heave a boat. Moving several tonnes of afloat boat is not hard. Nor is holding several tonnes of boat by hand against a moderate wind/tide - we all do that with little effort when we spring off or moor up.

Unless acted on by wind/tide, the force required to hold the boat in place is exactly zero.
For every kg of force the wind and tide exert on the boat, the anchor and chain only have to resist by the exact same amount.

Newton's 2nd and 3rd laws, I think.

Dragging a chain across tarmac also means that very little of the chain surface is in contact with the ground. If it is partially or totally buried in silt, 100% of the surface provides some friction. To illustrate - trying to pull even a small weed out of the garden mud, even if the roots don't go that deep, it is a struggle. Once dug out of the ground, dragging the same weed and it's root system across tarmac - is no effort at all.

IMO, holding the boat against the tide and wind perhaps doesn't require as much "holding" as you may think, and every bit helps - whether that's the chain friction, the anchor itself, the weight of the chain damping wave action or the lie of the chain keeping things horizontal.
 
I anchored in a drying sandy bay a couple of years ago and thought the anchor had set pretty well. But when we dried out I found that about 10m of chain had buried itself in the sand as the boat moved back and the anchor was sitting on the seabed with a little heap of slack chain next to it. Ok conditions were pretty calm but in seabeds which are softer than a car park I think you may well get significant friction between the chain and the bed material.
 
There may be another force acting on the chain in seabed that accounts for more friction or resistance. "Differential sticking" might occur when there is a pressure difference between the top of the chain and the bottom of the chain lying on a seabed. The chain lying on the seabed isolates the hydrostatic pressure of water from the soil immediately below it. At first all is equal but as time passes a small amount of water is squeezed out the formations over the contact area of the chain caused by the weight of chain. The cross sectional area of this contact is the contact surface area that the hydrostatic head of sea water acts on. There is still some pressure caused by the hydrostatic head on the formation before the chain was laid remaining, but not as much after the chain has laid down, hence the differential pressure.

It is of course correct to assume that the hydrostatic pressure is equal all round the chain in general terms, but it is not by a very small amount. This phenomena acts on drill pipes in oil wells to the point where, over a long length of pipe, the cumulative effects of the additional friction caused by a small pressure difference is so high that the pipe cannot pulled out the hole.
 
BlowingOldBoots said:
There may be another force acting on the chain in seabed that accounts for more friction or resistance. "Differential sticking" might occur when there is a pressure difference between the top of the chain and the bottom of the chain lying on a seabed. The chain lying on the seabed isolates the hydrostatic pressure of water from the soil immediately below it. At first all is equal but as time passes a small amount of water is squeezed out the formations over the contact area of the chain caused by the weight of chain. The cross sectional area of this contact is the contact surface area that the hydrostatic head of sea water acts on. There is still some pressure caused by the hydrostatic head on the formation before the chain was laid remaining, but not as much after the chain has laid down, hence the differential pressure.

It is of course correct to assume that the hydrostatic pressure is equal all round the chain in general terms, but it is not by a very small amount. This phenomena acts on drill pipes in oil wells to the point where, over a long length of pipe, the cumulative effects of the additional friction caused by a small pressure difference is so high that the pipe cannot pulled out the hole.
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A similar phenomenon occurs if our jack-up barge (basically a flat bottomed, oblong hull that climbs up it's own legs) is on a muddy seabed. Lifting the hull clear of the sea means that each leg is lifting about 500 tonnes, but when we try to leave from a muddy bottom I sometimes need to pull with 1000 tonnes just to unstick the legs. Mind you, once the vacuum breaks things can get quite interesting when the hulls buoyancy re-asserts itself!
 
"While doing so I was reminded of the many threads in which people claim that the anchor does no work"

Who are these cow boys, ill get a gun and a few dogs to round them up and fire wildly until they give up this nonsense view :-)
 
Are you sure about the bentonite clays being used as lubricants and not to "gel" the liquid mud to support the weighting materials, such as barite or calcium carbonate?

Just asking, because I'm offshore at the moment.
 
KellysEye said:
>How can a chain hanging vertically hold anything in place? Or if it comes to that, how can a chain hanging in a near horizontal curve hold anything in place?

I've said this before the weight in the catenary holds the boat in place and if you dive down on the anchor and lift the chain you will feel there is no pull on the remainder of the chain to the anchor, provided you have enough chain out and he right chain weight for the boat. I've never seen a near horizontal curve except when anchoring in shallow water.
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Why bother with an anchor then? It would be much easier to just bung chain out. A lot easier to just throw it in the locker too without having to faff around with however many kilos of steel on the bow roller.
 
It seems to be forgotten that chain isn't used for anchors just because of it's weight. It's used because it is strong and is very easy and compact to stow compared to rope or wire.
 
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