Loading capability of anchor versus anchor chain

[MathiasW]

In another thread of mine, which got a litte hijacked, the discussion got a bit heated when it came to the question whether the vessel is held in place by the anchor chain only (and not the anchor), or the system of anchor chain plus anchor.

It was claimed that the maritime academics and professionals would always view the vessel being moored to the chain and not the anchor, whilst others opposed this as entirely non-sense. I am part of the latter camp.

Now, I have been made aware of a recent paper written by some folks in the Korean Naval School of Architecture, which can shed some light on where maritime academics actually stand in this matter. I guess we all agree with the Korean commercial fleet being as big as it is, they can be taken to be dead serious in the matter.

Now, when you look at equation 4, you will find that they characterise the load share between anchor and chain in a very simple way. The first term is the anchor load, the second is the load the chain takes. This 2nd term is essentially friction of the part of the chain that is lying on the seabed. In essence, they say that the chain lying on the seabed contributes with 3/4 of its own weight towards the load. Finally, when you use their table, you will see that this 2nd contribution is normally much smaller than the load the anchor takes. And obviously it vanishes when the anchor chain is completely off the seabed.

Also to note is that the factor Ka given here is for the relatively poor anchors used for commercial vessels. A Spade or Rocna or Mantis has a much higher value (not 3-4 but 12 or even higher) and so for our leisure vessels, the load sharing between chain and anchor is even more tipping towards the anchor.

But it is always the system of both together, not the chain alone. Maritime academics see it entirely in the same way as most of us on this forum do.

The full paper can be found online when you search for the title.

I guess this may help to explain the different views a little.

Cheers, Mathias

 

[Boathook]

When I anchored in strong winds a few years back, the angle that my chain (8mm) seemed to be taking, meant that none of it seemed to be on the seabed. So (hopefully) the anchor was taking all the load. I have also anchored in calm conditions with no tide and we seemed to be lying to just the chain. It wouldn't be something that I would want to try.

 

[Poignard]

When I anchored in strong winds a few years back, the angle that my chain (8mm) seemed to be taking, meant that none of it seemed to be on the seabed. So (hopefully) the anchor was taking all the load. I have also anchored in calm conditions with no tide and we seemed to be lying to just the chain. It wouldn't be something that I would want to try.

If there is little wind there must be a period at slack water when you are effectively lying to the chain only.

And I suppose if you anchored in a strong wind somewhere where the tide is weak, and then the wind died away, the tidal current might never become strong enough to fully stretch out your chain after the tide turned, so you would be lying to a bight of chain only.

Under those circumstances someone less idle than me might even start the engine and stretch out the chain. I think I would just keep an eye on the wind and if it increased I would just make sure we didn't drag.

 

[Neeves]

Well researched Mathias

This the experiment I ran. I simply stretched 30m of 8mm chain from a high point to a low point and tensioned the chain until all the chain was off the ground.




This is the display when the lowest link was off the ground. The tension would be lower in water.




In this case the scope is 6:1. I did not try this with other chain sizes, I don't have the length of chain required. You can achieve a similar result using a catenary calculator. 30m of 8mm chain weighs roughly 43kg in air, so about half the tension is weight of chain and half is the requirement to stretch the chain out. As Vyv pointed out, in the other thread, a chain without an anchor will not even hold the skin on a rice pudding, I can lay the same 30m, as above, and lay it on the seabed and pull it along the seabed, it actually takes more effort to start it moving - but once moving you can almost gallop along - being a 80kg wimp I doubt the tension needed is 80 kg and that largely 'friction' or resistance imposed by the seabed on effectively a 30m length of chain.

At this point, a freely hanging 30m catenary, there is no friction. You could of course deploy more chain, if you have both the chain and the room. The tension in water equates to roughly 17 knots of wind for our catamaran (roughly similar to a 45' AWB in terms of windage).

The idea that 30m of 8mm chain, without an anchor, is going to hold a yacht, any yacht, in 30 knots of wind is ...... ludicrous. The idea that commercial ships rely on chain deployed (not the anchor) to hold the vessel in, say, 25 knots, is also ludicrous (as evidenced by the Korean technical paper) or the Pasha Bulker which ended up on a beach with both chain and anchor still attached (even the combination was insufficient to save her).


We were sitting in Trial Bay, Australia's east coast sheltering from a southerly wind. A 200' commercial vessel came into the bay (they were on their way to Brisbane for re-furbishment) and anchored about 800m from us. In the morning they left, they were heading north we south. Not long after leaving the large vessel came on the VHF asking is there was a commercial diver available - the local rescue station put them on to a dive school. It turned out they had lost their anchor and had not left but were drifting out of the bay. The winds were maybe 25 knots - or maybe more for them as they were a 'higher' vessel. Their chain did not save them. Their retrieval of their anchor over the rest of the day kept us entertained.

I was on a 80,000t commercial vessel crossing The Bight, from east to west (against the prevailing winds and seas), when 11m seas were forecast for Cape Leuwin (bottom left hand corner of Australia). The Bight is devoid of shelter but there is a vestige of succour at Esperance. The master decided to anchor off Esperance until the seas abated (he had a delicate cargo). He deployed an anchor attached to chain , but it dragged under the conditions. He set up a box, told the on watch crew to keep the ship in 'the box'. They spent the night motoring to the windward edge of the box, dragging back to the leeward end, retrieving the anchor, motoring up and dragging back.... They keep the engines running, obviously, full time in slow ahead as the master was unsure that the chain lock for the windlass would be sufficient to take the strain - so the ground tackle was always being supported in part by the engines (we still dragged)

This is the anchor used in Esperance, state of the art AC 14 10t commercial anchor.



The combination of engine(s), chain and anchor was insufficient to hold the ship.

Jonathan

 

[MathiasW]

Thanks for sharing your experiment, Jonathan, and also for the story on the commercial vessel!

 

[thinwater]

In another thread of mine, which got a litte hijacked, the discussion got a bit heated when it came to the question whether the vessel is held in place by the anchor chain only (and not the anchor), or the system of anchor chain plus anchor.

It was claimed that the maritime academics and professionals would always view the vessel being moored to the chain and not the anchor, whilst others opposed this as entirely non-sense. I am part of the latter camp.

Now, I have been made aware of a recent paper written by some folks in the Korean Naval School of Architecture, which can shed some light on where maritime academics actually stand in this matter. I guess we all agree with the Korean commercial fleet being as big as it is, they can be taken to be dead serious in the matter.

Now, when you look at equation 4, you will find that they characterise the load share between anchor and chain in a very simple way. The first term is the anchor load, the second is the load the chain takes. This 2nd term is essentially friction of the part of the chain that is lying on the seabed. In essence, they say that the chain lying on the seabed contributes with 3/4 of its own weight towards the load. Finally, when you use their table, you will see that this 2nd contribution is normally much smaller than the load the anchor takes. And obviously it vanishes when the anchor chain is completely off the seabed.

Also to note is that the factor Ka given here is for the relatively poor anchors used for commercial vessels. A Spade or Rocna or Mantis has a much higher value (not 3-4 but 12 or even higher) and so for our leisure vessels, the load sharing between chain and anchor is even more tipping towards the anchor.

But it is always the system of both together, not the chain alone. Maritime academics see it entirely in the same way as most of us on this forum do.

The full paper can be found online when you search for the title.

I guess this may help to explain the different views a little.

Cheers, Mathias


View attachment 128935View attachment 128936

Thanks.

A formal way of stating... the obvious!

 

[Neeves]

When I anchored in strong winds a few years back, the angle that my chain (8mm) seemed to be taking, meant that none of it seemed to be on the seabed. So (hopefully) the anchor was taking all the load. I have also anchored in calm conditions with no tide and we seemed to be lying to just the chain. It wouldn't be something that I would want to try.

In 17 knots in a 45' AWB with 30m of 8mm chain deployed at 6:1 scope - none of the chain is on the seabed. By the time you get to 30 knots the chain looks straight, in fact the bit you can see is nearly straight - obviously not straight - but few of us carry a billiard cue to check what amount of 'curve' still exists . Relying on friction between chain and seabed, above 17 knots, demands an amazing act of faith - personally I lack the faith - and rely on the science. Maybe those with the faith are mostly no longer with us.......and have other diversions to occupy their time.

Jonathan

 

[MathiasW]

Thanks.

A formal way of stating... the obvious!

If that's what it takes, I am happy to go the extra mile...

 

[thinwater]

If that's what it takes, I am happy to go the extra mile...

?

 

[TNLI]

When you are at anchor, you rely on a combination of anchor and rode. Sometimes it's the anchor holding you as for example it might do if the chain wraps around a rock or coral head etc. Other times with a short scope, or in water with a strong current or wind, the chain might be clear of the bottom, and only the anchor is holding your boat in place.
That situation is not good if your anchor has not been set correctly, or is something daft, like an alloy Danforth, that might have a warranty for life, but are easily bent or broken.

Drifting off topic, here is my short list of anchors for my 27ft alloy lifeboat:

STORM ONLY: 35lb CQR with 80m of 8mm chain
Normal main anchor: 16kg Bruce with 50m of 8mm chain and 50m of 1 inch nylon line.
Secondary main: 7kg Danforth with 50m of 6mm chain.
Fisherman's classic: 7kg with 10m of 6mm chain and 100m of 3/4 Polypropylene
Emergency or deep water only: 2.5kg folding grapnel with 10m of 6mm chain and 100m of 1/2 inch Poly floating line.
Deep water or wreck fishing disposable: 10lb Rebar grapnel with 10m of 6mm chain and 100m of 1/2 inch Poly. Designed to unbend before the Ploy snaps. You will see several of these on small fishing boats that fish in deep water. Grapnels or classic hooks like the fisherman will grab hold with a much shorter scope, often half that of a Bruce.

You can never have too many spare anchors, and genuine ones are easy to sell to a boat with a duff engine that is dragging around, or has lost their only anchor!

 

[jamie N]

I've done quite a few surveys on anchor patterns from offshore equipment, and have never got anywhere near the anchor itself before the chain has become buried, which is where interest is lost as there's no movement on the chain from that point. The area of interest is usually in the region of a 10-15 meter length at the touchdown point, where movement can be seen. If the touchdown point was anywhere near the anchor, it'd be seen as a potential disaster, likewise if the anchor hadn't been present, the chain wouldn't have held the tanker and it'd be a very big problem.

 

[NormanS]

In 17 knots in a 45' AWB with 30m of 8mm chain deployed at 6:1 scope - none of the chain is on the seabed. By the time you get to 30 knots the chain looks straight, in fact the bit you can see is nearly straight - obviously not straight - but few of us carry a billiard cue to check what amount of 'curve' still exists . Relying on friction between chain and seabed, above 17 knots, demands an amazing act of faith - personally I lack the faith - and rely on the science. Maybe those with the faith are mostly no longer with us.......and have other diversions to occupy their time.

Jonathan

That just shows what will happen to you, if you choose to use 8mm chain for anchoring a 45ft AWB. ?

 

[Neeves]

That just shows what will happen to you, if you choose to use 8mm chain for anchoring a 45ft AWB. ?

But as you know perfectly well we use 6mm chain and have replaced catenary with elasticity of 2 snubbers (aka a bridle) and as you have known this for some time you obviously are endorsing our practice.



As you know catenary is all about energy management.

Here is a simple graph comparing energy management nylon, snubber, and chain.

Above about 300kg of tension chain looks pretty average




But you know all this.

Now, of course you could deploy more chain, if you have lots of room - maybe that yacht behind you is working with a decent snubber (and less weight in the bow) - he is not going to be happy with you sitting on his bow.

Jonathan

 

[TNLI]

Well researched Mathias

This the experiment I ran. I simply stretched 30m of 8mm chain from a high point to a low point and tensioned the chain until all the chain was off the ground.


View attachment 128973

This is the display when the lowest link was off the ground. The tension would be lower in water.

View attachment 128974


In this case the scope is 6:1. I did not try this with other chain sizes, I don't have the length of chain required. You can achieve a similar result using a catenary calculator. 30m of 8mm chain weighs roughly 43kg in air, so about half the tension is weight of chain and half is the requirement to stretch the chain out. As Vyv pointed out, in the other thread, a chain without an anchor will not even hold the skin on a rice pudding, I can lay the same 30m, as above, and lay it on the seabed and pull it along the seabed, it actually takes more effort to start it moving - but once moving you can almost gallop along - being a 80kg wimp I doubt the tension needed is 80 kg and that largely 'friction' or resistance imposed by the seabed on effectively a 30m length of chain.

At this point, a freely hanging 30m catenary, there is no friction. You could of course deploy more chain, if you have both the chain and the room. The tension in water equates to roughly 17 knots of wind for our catamaran (roughly similar to a 45' AWB in terms of windage).

The idea that 30m of 8mm chain, without an anchor, is going to hold a yacht, any yacht, in 30 knots of wind is ...... ludicrous. The idea that commercial ships rely on chain deployed (not the anchor) to hold the vessel in, say, 25 knots, is also ludicrous (as evidenced by the Korean technical paper) or the Pasha Bulker which ended up on a beach with both chain and anchor still attached (even the combination was insufficient to save her).


We were sitting in Trial Bay, Australia's east coast sheltering from a southerly wind. A 200' commercial vessel came into the bay (they were on their way to Brisbane for re-furbishment) and anchored about 800m from us. In the morning they left, they were heading north we south. Not long after leaving the large vessel came on the VHF asking is there was a commercial diver available - the local rescue station put them on to a dive school. It turned out they had lost their anchor and had not left but were drifting out of the bay. The winds were maybe 25 knots - or maybe more for them as they were a 'higher' vessel. Their chain did not save them. Their retrieval of their anchor over the rest of the day kept us entertained.
Anchor watch on a sunny day:

I was on a 80,000t commercial vessel crossing The Bight, from east to west (against the prevailing winds and seas), when 11m seas were forecast for Cape Leuwin (bottom left hand corner of Australia). The Bight is devoid of shelter but there is a vestige of succour at Esperance. The master decided to anchor off Esperance until the seas abated (he had a delicate cargo). He deployed an anchor attached to chain , but it dragged under the conditions. He set up a box, told the on watch crew to keep the ship in 'the box'. They spent the night motoring to the windward edge of the box, dragging back to the leeward end, retrieving the anchor, motoring up and dragging back.... They keep the engines running, obviously, full time in slow ahead as the master was unsure that the chain lock for the windlass would be sufficient to take the strain - so the ground tackle was always being supported in part by the engines (we still dragged)

This is the anchor used in Esperance, state of the art AC 14 10t commercial anchor.

View attachment 128976

The combination of engine(s), chain and anchor was insufficient to hold the ship.

Jonathan

Interesting post, and a few points worth thinking about:
1/ Test results from out of water chain or anchor tests need to be corrected for the static weight in water. Not very significant for anchors, but more significant for chain, as it weighs less under water.
2/ Always keep the main engine ready in dodgy conditions at anchor.
3/ The classic commercial version of a fisherman's anchor is not too good in very soft mud or sand, a CQR or delta is far better. The other option is to add another inline anchor, or deploy the ships second anchor in a V pattern.
4/ Make 100% sure you know how to rig snubbers or similar methods of attaching an anchor to a chain or rope rode. I use the normal chain hooks and V pattern to 2 steel bollards that would not pull out too easily, but I sometimes just use my Simpson Lawrence Hyspeed manual windlass for my rather light 27ft composite alloy lifeboat, and they are not known for failures under load.
5/ I might not have dragged into anyone, BUT I did run my last steel Van Der Stadt 34 aground in the inner lagoon of St Martin deliberately to avoid the total mayhem in the main anchorage in a minor hurricane, as it was far too crowded to allow a sensible all chain scope to be safe. Very comfortable night stuck in the putty, but had a lot of trouble trying to get out of the putty a few days later.

 

[Neeves]

Interesting post, and a few points worth thinking about:
1/ Test results from out of water chain or anchor tests need to be corrected for the static weight in water. Not very significant for anchors, but more significant for chain, as it weighs less under water.
2/ Always keep the main engine ready in dodgy conditions at anchor.
3/ The classic commercial version of a fisherman's anchor is not too good in very soft mud or sand, a CQR or delta is far better. The other option is to add another inline anchor, or deploy the ships second anchor in a V pattern.
4/ Make 100% sure you know how to rig snubbers or similar methods of attaching an anchor to a chain or rope rode. I use the normal chain hooks and V pattern to 2 steel bollards that would not pull out too easily, but I sometimes just use my Simpson Lawrence Hyspeed manual windlass for my rather light 27ft composite alloy lifeboat, and they are not known for failures under load.
5/ I might not have dragged into anyone, BUT I did run my last steel Van Der Stadt 34 aground in the inner lagoon of St Martin deliberately to avoid the total mayhem in the main anchorage in a minor hurricane, as it was far too crowded to allow a sensible all chain scope to be safe. Very comfortable night stuck in the putty, but had a lot of trouble trying to get out of the putty a few days later.

The weight of the chain in water is less - but does not alter, in this case, the post - the differences are not great.

In soupy mud our anchor of choice is something daft, like an alloy Danforth (commonly called a Fortress and now also a Lewmar LFX). We have no experience of the LFX but assume it will perform similarly to the Fortress, set at 45 degrees.

For details see:

Chesapeake Bay Anchor Test - The World's Best Anchors!

For your interest I've attached some background on snubbers

A Snubber & Hook for all Occasions - Practical Sailor

YM has an article on snubbers in the July 2021 issue

also

How to: Dealing with Snatch Loads in an Anchorage

Anchor Snubber Tips

We are keen advocates of anchoring in a 'V' primarily to quell wind sheer or veer.

We also take to the ground (or beach, its a comfortable way to keep the hull(s) clean. We have some king tides at the beginning of Feby and once we have cleared Chinese New Year we will be secured with 3 anchors (to stop us floating away over night).

and also

The art of snubbing, in the nicest possible way - Mysailing


I confess there is a bit or repetition - but the articles are published for different geographic markets.



Jonathan

 

[Stemar]

It seems entirely clear that, when it really matters, the only thing that will hold you is the anchor.

I have sat to the chain once that I know about. A calm day on the Medina, opposite Island Harbour. I don't know why I didn't dig in more, but it was a good while ago, so maybe I didn't know any better. When we dried out, I could see the Delta anchor on its side, sitting on the mud, not in it, with the chain in a curve. It was good thick mud, unlike the mud soup you get in parts of Newtown Creek, so chances are, had it needed to do something more, the anchor would have dug in and held but, in those conditions, it certainly wasn't doing any more than a rock of the same weight would have.

 

[thinwater]

...
1/ Test results from out of water chain or anchor tests need to be corrected for the static weight in water. Not very significant for anchors, but more significant for chain, as it weighs less under water. ...

The "K" values were obviously determined in water, therefore the SG correction is included.
....

 

[TNLI]

The weight of the chain in water is less - but does not alter, in this case, the post - the differences are not great.

In soupy mud our anchor of choice is something daft, like an alloy Danforth (commonly called a Fortress and now also a Lewmar LFX). We have no experience of the LFX but assume it will perform similarly to the Fortress, set at 45 degrees.

For details see:

Chesapeake Bay Anchor Test - The World's Best Anchors!

For your interest I've attached some background on snubbers

A Snubber & Hook for all Occasions - Practical Sailor

YM has an article on snubbers in the July 2021 issue

also

How to: Dealing with Snatch Loads in an Anchorage

Anchor Snubber Tips

We are keen advocates of anchoring in a 'V' primarily to quell wind sheer or veer.

We also take to the ground (or beach, its a comfortable way to keep the hull(s) clean. We have some king tides at the beginning of Feby and once we have cleared Chinese New Year we will be secured with 3 anchors (to stop us floating away over night).

and also

The art of snubbing, in the nicest possible way - Mysailing


I confess there is a bit or repetition - but the articles are published for different geographic markets.



Jonathan

The test of the Fortress anchor was designed to prove it was very good in mud or sand. In the real world when your main anchor gets jammed under a rock or something and the wind starts blowing causing a chop significant enough to put some serious force into the anchor, a Fortress can be bent, as can any Donforth look alike. I've never seen a CQR or Bruce bent, which is why I would only use one in good conditions as a main anchor.

One thing I want to mention is that using all chain rodes can be a mistake if the bottom has a lot of coral heads, wrecks or debris like old cars that the chain or heavy line can wrap around when the wind or current changes direction too often. You can easily finish up with the anchor chain or line jammed up enough to be nearly vertical. Not a good situation if it gets bumpy. That is when you either need to set up some type of moor, or use polypropylene or other floating line.

 

[Neeves]

TNLI

There is no one perfect anchor, they are all a compromise. If you are sensible, like your self, you carry a range of anchors to suit different seabed environments. You are correct a Fortress is a liability in seabeds with coral or rocks (but then you should not anchor in coral anyway) but for soupy mud it is unbeatable (and gives a very good account of itself in sand or if you need to deploy an anchor by dinghy). There are other lightweight anchors, Anchor Right's aluminium Excel and Viking Anchor come to mind - giving a choice of 3 highly different designs. Your choice of a CQR or Bruce are admirable and supported by many but anchor design has moved forward and there are better anchors available now that offer the same strength but twice the hold (or if you like you can use a lighter model and still have better hold than offered by a CQR or Bruce).

The absence of any anchor of a modern design in your quiver of anchors suggests you are not a convert to modern designs, and may have no experience of them, which would put you at odds with most of the members here. You will find much support here for the new designs, Spade, Rocna, Supreme, Excel (even Fortress ) and the converts to the new designs would never, ever, return to using a CQR, Bruce or Delta. Most people here were brought up using the old designs and are well versed in their usage. They now prefer the ease of deployment and speed of setting, the high strength imparted by the use of high tensile steels and the outstanding reliability of the new designs in a wider range of seabeds than the more restricted application of the older models.

If you are more than happy with the old designs - go for it. But you might find the new designs a revalation (though a bit pricey compared to buying a second hand Bruce, genuine, or a free CQR (you cannot give them away).

Jonathan

 

[TNLI]

Genuine CQR anchors are worth real money on both Gumtree and Fleabay in particular. I got both my Bruce main anchor and CQR storm anchor from Gumtree contacts. Make 100% certain that you do not buy a copy of a CQR in particular, because very few of them will dig in correctly due to lack of lead in the pointy end. The copies also rust very quickly due to a very thin layer of Zinc. The real McCoy versions say made in Scotland on the CQR, or made in Belgium on the Bruce if you buy one from a European dealer, yard or boat. About 3/4 of the CQR anchors I saw listed were copies, and they are the ones that don't sell too well.

Trying to find good used calibrated BS 766 chain is far more difficult for some unknown reasons, although I did manage to buy 50m of 6mm in almost new condition for only 50 quid in my local area. The Fisherman's, Danforth copy and folding grapnel anchors are cheap new, so I bought those three from Force 4 chandlery online.

My famous 5 selection has changed over the years because of some real hard times anchored in various Islands in the Pacific, close to mud banks inside Australia's barrier reef and in very odd river estuaries in South American rivers.
I made an error in a previous post by saying coral, when I meant coral heads and rocky areas with some coral covering that are designated anchorages in the Western Pacific. The most problematic is definitely a thin layer of sand over a flat rock strata, which seems OK to start with, BUT you then find that the anchor fails to hold when you apply max continuous RPM in astern to check if it really is holding. I gave up in Vanuatu, but still managed to stay anchored after running a line ashore and digging in a Danforth.

I have updated my list as regards the Bruce, as I used to be a CQR devotee, as many old timers still are, BUT they head for the centre of the earth when it gets bouncy are trip lines get mistaken by visitors for mooring buoys, so I have moved with the times to a Bruce main anchor. The Bruce also holds better on a 3:1 scope, which is important in a crowded anchorage.