Anchors, cables and catenarys.... or not?

[Bajansailor]

There is an excellent thread on here started by Jimmy Le Constructeur about moving his boat to France, and (as usual) there was a bit of fred drift re anchor chains and catenarys (catenaries?) - JFM suggested starting another thread for those who wanted to continue with this debate, so as no one else has, I will......

I am sitting on the fence here, and will get one of those proverbial packets of pop corn that seem to crop up occasionally whenever a good scrap is expected.....

But just thinking it through, if you have some light chain on your dinghy anchor rope, it really does not do very much in terms of a catenary as it is so light - it is easy to straighten it out, and its main purpose is probably just to avoid the rope near the anchor from getting chafed.

Going to the opposite extreme now, if we consider a large ship (or any ship in fact), it is obvious that the weight of their cables do form an important part of their anchoring effectiveness. Take this right up to super tankers or the 12,000 TEU Maersk container ships, and the anchors are tiny in comparison to ship - they wouldn't hold much if you tried to anchor a ship on one with just a rope.
I have read about how ships have been at anchor in storms with lots of scope out, and when divers inspected the anchors afterwards, found them just resting on the sea bed, with slack in the cable, ie it was just the weight of the cables (and the friction on the sea bed) that was holding them.

So, I think the effect of chain catenary and weight is probably on a sliding scale, from negligible for a very light chain on a dinghy anchor, to very significant on a super tanker. You just have to decide where you are in between..........

 

[AndieMac]

Going to the opposite extreme now, if we consider a large ship (or any ship in fact), it is obvious that the weight of their cables do form an important part of their anchoring effectiveness. Take this right up to super tankers or the 12,000 TEU Maersk container ships, and the anchors are tiny in comparison to ship - they wouldn't hold much if you tried to anchor a ship on one with just a rope.
I have read about how ships have been at anchor in storms with lots of scope out, and when divers inspected the anchors afterwards, found them just resting on the sea bed, with slack in the cable, ie it was just the weight of the cables (and the friction on the sea bed) that was holding them.

I've heard it mentioned numerous times amongst the commercial fishing guys who operate boats between 40 and 70 feet around our coastline, how the principle of significant chain size is a priority over anchor size in their operations.
Their environment is anchoring in shallow sandy bays, 10 to 12 ft of tidal variation and allowing for anything between gale and storm force winds.

 

[kid's inheritance]

I would have thought that a natural for a boating magazine would be
Remove anchor
wait for F5 or more
Find sandy bay
Drop a shed load of chain
Set Anchor watch
Sell shares in whoever owns Rocna these days
But that would mean that Scuttlebutts would have nothing to talk about

 

[Hurricane]

I'm sure that both the anchor and chain have their separate contributions to a "good hold".

Personally and before I see the views of others, I would have thought that the chain is the biggest contribution.

But I'd like to hear what jfm thinks about the subject. It's his comments that started the discussion on the other thread.

 

[Deleted User YDKXO]

Sell shares in whoever owns Rocna these days
:

Aaargh! You've mentioned the R word. You know what trouble that caused on the Scutyerbutt forum
Any moboers got one? I very nearly bought one myself instead of a Delta but didn't in the end because the shank wouldn't fit my bow roller. Glad I didn't now that I know they're made of Chinese self bending metal

 

[jfm]

I'm sure that both the anchor and chain have their separate contributions to a "good hold".

Personally and before I see the views of others, I would have thought that the chain is the biggest contribution.

I agree you first sentence, and I think you have it the worng way round in your second sentence

Sure, in light winds the chain or even a brick on a string can hold the boat. But that's not the issue (I think). We're surely talking about situations where there is a lot of load on the anchor/chain (aren't we?)

Bajan says above "I have read about how ships have been at anchor in storms with lots of scope out, and when divers inspected the anchors afterwards, found them just resting on the sea bed, with slack in the cable, ie it was just the weight of the cables (and the friction on the sea bed) that was holding them." and have two comments. First, the "slack in the cable" must have got lost in trnaslation if we're talking ships in storms, so I just don't accept that as valid data. Second, this observation about chain lying on seabed is exactly the mistake people make. In that observed circumstance the anchor is still doing most of the job. Remove it and the chain will slide on the seabed. Even a heavy chain has relatively little frictional hold on the seabed compared with an anchor

I mean, your boat Hurricane in 25kts of wind on sandy bottom @10m depth (just to make up a circumstance...) would not hold if you laid out 100m of 16mm chain but if you laid out a 50kg Delta or Ultra anchor on 5m of 8mm chain and the rest as rope it would hold fine

Function of the chain is, in no particular order, (a) anti chafe (b) works wel with windlass (c) catenary effect acts as a shock absorber but irriatingly it is less effective when anchor load is strong so is the "wrong way round" (d) catenary effect makes the pull on the anchor (which is the bit holding the boat) more horizontal. Of course (d) is the significant one

Anyway, I cant remember how this started but I think I'm just saying that the claims "the chain holds the boat not the anchor" are bollox (though might be true in 2 kts of wind, whatever) and the observation that "the chain was flat on the sea bed" does NOT mean the anchor is doing nothing

Other than that I'm just trying to help bajan enjoy his popcorn

 

[kid's inheritance]

JFM,
you say that even a heavy chain has low friction compared to an anchor.
I would think that an anchor, even well bedded ,has a square foot of surface interfacing with sand but 180 foot of chain with 2" links in contact has 30 foot interfacing . I know I am comparing apples etc but ,although I was being tongue in cheek , it would be an easy experiment to do and would solve the question.
Where is this virtual popcorn anyway?
Regards
Rob

 

[jfm]

JFM,
you say that even a heavy chain has low friction compared to an anchor.
I would think that an anchor, even well bedded ,has a square foot of surface interfacing with sand but 180 foot of chain with 2" links in contact has 30 foot interfacing . I know I am comparing apples etc but ,although I was being tongue in cheek , it would be an easy experiment to do and would solve the question.
Where is this virtual popcorn anyway?
Regards
Rob

I'm talking about leisure boat 8-10-12-14mm chain, or even 16-18mm, and I'm saying that size chain doesn't offer huge seabed resistance when you have say 20m on the seabed.

I think you are comparing apples etc with your maths. The anchor goes under the sand, and the deeper it goes the bigger the clod of sand that must be lifted to remove it. Whereas the chain sits on top of the sand. An easy experiment would be in warm med waters, drop the anchor in sand in say 6m of water, and lay out say 30m of chain. 15kts wind on the boat. Then disconnect anchor from chain (by cutting a prepared rope link) and er watch the boat glide back.
There's a gif file for popcorn - MapisM, help!

 

[Deleted User YDKXO]

I think you are comparing apples etc. The anchor goes under the sand, and the deeper it goes the bigger the clod of sand that must be lifted to remove it. Whereas the chain sits on top of the sand. An easy experiment would be in warm med waters, drop the anchor in sand in say 6m of water, and lay out say 30m of chain. 15kts wind on the boat. Then disconnect anchor from chain (by cutting a prepared rope link) and er watch the boat glide back.
There's a gif file for popcorn - MapisM, help!

In the interest of science, I'll try to carry out an experiment this summer in an attempt to throw some light on this. I'm going to get the SWMBO to remove my 40kg Delta (or maybe I'll just drop it into my dinghy if she complains) and lay out 3x depth of just chain in an anchorage in an afternoon seabreeze and see what happens

 

[jfm]

In the interest of science, I'll try to carry out an experiment this summer in an attempt to throw some light on this. I'm going to get the SWMBO to remove my 40kg Delta (or maybe I'll just drop it into my dinghy if she complains) and lay out 3x depth of just chain in an anchorage in an afternoon seabreeze and see what happens

Good plan. Highly scientific. I'll do likewise

 

[kid's inheritance]

Seeing as KI is San Carles bound in a few weeks time , I will assist in this venture too.However I retain the IP for the magazine article

 

[vas]

JFM,
you say that even a heavy chain has low friction compared to an anchor.
I would think that an anchor, even well bedded ,has a square foot of surface interfacing with sand but 180 foot of chain with 2" links in contact has 30 foot interfacing . I know I am comparing apples etc but ,although I was being tongue in cheek , it would be an easy experiment to do and would solve the question.
Where is this virtual popcorn anyway?
Regards
Rob

from my relatively limited knowledge of physics/structures the above argument doesn't hold. You have:
friction,
weight,
bedding,
shearing
as forces work out, plus a whole load of variables to consider

iow, the 2inch links have to be calculated as mini anchors + weight + friction before you go the next one and max the holding force that these can withstand.
Not to mention that depth has a paramount importance in all that palava.

OTOH, I should mention that a simple survey on 60+ crafts (40%mobo-60%sail) on the hard where mine is shows that 90% of them have anchors of this type (that's mine btw) on around 60-80m chain. whassat, a Bruce clone right?

V.

 

[MapisM]

(d) catenary effect makes the pull on the anchor (which is the bit holding the boat) more horizontal. Of course (d) is the significant one

The anchor goes under the sand, and the deeper it goes the bigger the clod of sand that must be lifted to remove it. Whereas the chain sits on top of the sand.

I think the above are the key points.
Anyone who ever used a shovel can easily understand that.
Just think of the normal way of using a shovel: you push it deep in the soil, and then use some leverage effect to pull/lift it diagonally.
And even then, it does take some effort. Depending on the soil consistency, it might well be a significant effort.
Now, think of pushing the shovel in the soil again, but this time, instead of using the whole length of its handle as a lever, grab it at the bottom, just above the soil level, and pull it horizontally.
Again, it will depend on the soil, but the effort required to move it will be definitely a multiple. And in anything more consistent than soft sand, you could simply forget to move it at all!

Bottom line, I fully agree that the anchor can hold much more than the chain.
But, to hold properly, the anchor does need some chain, because its holding capacity mainly depends (on top of weight, shape, etc.) on how horizontally it can work.
Oh, and by popular demand...

 

[MapisM]

OTOH, I should mention that a simple survey on 60+ crafts (40%mobo-60%sail) on the hard where mine is shows that 90% of them have anchors of this type

LOL, sorry V, but imho the only very simple reason why it's so popular is that it's the cheapest bit of boating metal you can find...

 

[Deleted User YDKXO]

LOL, sorry V, but imho the only very simple reason why it's so popular is that it's the cheapest bit of boating metal you can find...

Agreed. I wouldn't trust my precious boat to a Chinese built Bruce knock off. At least buy the proper article with good solid British metal in it.

 

[Hurricane]

Seeing as KI is San Carles bound in a few weeks time , I will assist in this venture too.However I retain the IP for the magazine article

We can try this experiment in the lagoon but....

The lagoon is absolutely the best place to get a good hold.
The bottom is sticky mud and the anchor will hold without much chain out.
It;s really a good place for "first timers" to sleep on the hook because you have to do something very wrong for it to slip.

On the other hand, the sticky bottom might give a different result for the chain only.

Anyway, the lagoon would be a very safe place to try it out.

The other alternative is the botton just outside the lagoon is sand so anchoring just off the town beach will do a more conventional test. You may want to go a cpuply of miles south down the coast because the fishing fleet arrive at about 17:00 every day and their wash might influence the test!!
Actually, I say "wash" but it isnt really a problem. Nice fresh fish in town though.

Anyway, back to the point.
JFM
Are we thinking of the best hold in all conditions?
If so, you could take the argument to the extreme and consider hurricane force conditions where the chain doesn’t touch the bottom at all.
We certainly wouldn’t have planned on being there.
But my thoughts are to the more usual conditions and IMO, the chain “snakes” its way towards the anchor.
From my point of view, if we get to the situation where it is “straightening out” along the bottom, we should be considering a move anyway.

An example – do you remember last year leaving us and Warlock at anchor in the Iles de Lerins? We had a fabulous evening under the stars but at about 4 o’clock in the morning it blew up. I left it about 30 minutes to see how things “panned out” and then decided to head for Antibes arriving just before 06:00. As always, I had prepared the boat for sea before going to bed so it was very much under control. The point is that I didn’t like the way that our chain was “bar taught” and IMO wasn’t contributing to holding. I suppose that this example makes your point – the anchor was holding. But my confidence wasn’t – I will always want the chain properly on the bottom.

Another example
I was swimming with a mask in Mallorca and actually watched a friends anchor dragging. IMO, it dragged because the chain was straight and the pull was along its length. IMO if you put enough chain out, you can achieve the “snaking effect” and thus the chain is holding not the anchor. Again subject to the conditions of course.

 

[mjf]

Its a combo I feel - like others many times you can see with scuba gear the craft anchored riding to the deployed cable. But we have all seen times when the anchor 'digs in' after a blow and this is a result of the chains purchase drawing the flooks into the ground.

On small leisure stuff and large commercial ships you see times when you are obliged to heave the anchor vertically in order to trip it and release its holding - we have all seen the results with 1/2 ton of the bottom at the end of the cable and no anchor to be seen as its hidden under the crud. Certainly holding ground is an issue as we know.

On many occasions my ample scope (for the depth) has been bar tight and position held in a blow - problem is when going to bed on lee shore always the issue in places like Newtown Ck is will the anchor (not the cable) hold as often the simple solution of lying to more chain (and taking some the wt off) means running the risk of grounding as chain pulls out forcing boat into shallow water.


There is a fab story of a chap up for masters fg cert and the examiner says you are at anchor and wind picks up and anchor suspected of dragging - chaps says he will put extra shackle out , wind picks up more says examiner, ok says candidate more chain out,
more wind says examiner, more chain retorts the candidate.

Finally exhausted the examiner say where Sir are you getting all this anchor chain - candidate says same place as your bloody wind!

 

[MapisM]

The point is that I didn’t like the way that our chain was “bar taught” and IMO wasn’t contributing to holding.

That's very much understandable, and maybe I would have done the same at night, along the lines of better safe than sorry.
But surely you didn't jump in the water with a scuba torch in that occasion, I suppose?
I'm saying this because when I'm in a similar situation in daylight, before deciding whether to stay or not, I always give a look at what happens on the bottom.
And no matter how tight the chain looks out of the water, I never saw it suspended above the bottom, at least in the last few meters.

 

[vas]

LOL, sorry V, but imho the only very simple reason why it's so popular is that it's the cheapest bit of boating metal you can find...

you imply me anchor is no good then? shame on you!

MM, I merely pointed a fact. TBH, I doubt ALL craft in the marina had their anchors nicked, broken, bent, whatever and hence replaced with Bruce...

Agreed. I wouldn't trust my precious boat to a Chinese built Bruce knock off. At least buy the proper article with good solid British metal in it.

just wait, come Easter I'll start a thread for best med anchor and matching bow roller assembly for my Lofrance Tigres windass [just need to wait for things to calm down before I start this thread ]

btw, this bruce was only good for nice clean sand with no weed patches on, anything else and you've had it....

V.

 

[MapisM]

you imply me anchor is no good then? shame on you!

MM, I merely pointed a fact. TBH, I doubt ALL craft in the marina had their anchors nicked, broken, bent, whatever and hence replaced with Bruce...

Oh no, far from implying that - depending on your cruisig habits, it might well be good enough.

But re. the boats in your marina, I wouldn't be surprised if most builders were the first to be careful about costs, to start with...!
You know, for most large production boats, builders are more interested to offer them with an appealing price tag, rather than with the best equipment available.
Btw, anchor aside, actually this is not so true for your own boat. Many original equipments I've seen in your threads were the best money could buy, back in those days.