Questions about anchor chain?

skyflyer

skyflyer

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am I missing something?

AFAIK the primary 'point' of using chain rather than rope on an anchor is not for strength but for weight so there is a good catenary curve and the anchor pulls flat across the seabed and digs in. It would seem that 8mm chain has a SWL approx 4 times the SWL of 20mm braided polyester line if I have read the figures correct. Given the depletion of strength caused by knots and splices the effective load for rope is even less.

So the question is - why are we worried by a small amount of corrosion on the anchor chain once the galvanised zinc coating starts to wear away. The suggestion is that a loss of 10% of the chain diameter mens scrapping the chain but it would seem that it would still be WAY above the breaking strain of any equivalent rope.

Second question.
If you don't have a windlass with gypsy, then a welded or C-link riveted join is presumably not necessary. In that case what is the best method (strongest and safest) to join two lengths of chain, that wont involve shredding someone's hands when lifting or lowering the chain through their hands?
 
Sir, are you aware that for questions about anchors and chains, the original poster is now billed at £5 per reply?

Not really, but we could wish it were so...now, prepare for six or seven pages of responses...:rolleyes:
 
>The suggestion is that a loss of 10% of the chain diameter mens scrapping the chain but it would seem that it would still be WAY above the breaking strain of any equivalent rope.

I'm not sure where rope comes into it, if there is a 10% loss of diameter then the strength of the chain has been compromised and should be scrapped. Personally I would never use rope for anchoring as the boat swings around much more than a boat with chain because there is no weight in the catenary. If I ever saw a boat with rope rode I would anchor well away from it.
 
KellysEye said:
>The suggestion is that a loss of 10% of the chain diameter mens scrapping the chain but it would seem that it would still be WAY above the breaking strain of any equivalent rope.

I'm not sure where rope comes into it, if there is a 10% loss of diameter then the strength of the chain has been compromised and should be scrapped. Personally I would never use rope for anchoring as the boat swings around much more than a boat with chain because there is no weight in the catenary. If I ever saw a boat with rope rode I would anchor well away from it.
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Thats exactly my point - the chain is much stronger than needed to take the load because it is over-specified to give a catenary. Thus a 10% (or possibly even 25 or 30%) loss of tensile strength still leaves it stronger than the rope that many sailors (particularly in USA) use.

Since posting I found a page on the Rocna site http://www.petersmith.net.nz/boat-anchors/catenary.php that says that the effect of catenary is hugely over-estimated and the primary purpose of chain is to prevent abrasion and chafing as it drags over seabed. They suggest using a smaller chain (so long as strength is ok, weight is irrelevant) to minimise weight carried on boat, decrease storage space and allow a bigger anchor instead.
 
Scarboroughsloop said:
How odd, suggesting buying a bigger anchor, from a site that sells anchors......
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Yes, in the short-term obviously, but if the chain breaks and the boat ends up on the rocks with the owner blaming them, that doesn't sell many anchors in the longer term. (as with breaking shanks made from inferior spec steel!)

Leaving aside the commercial interest, the science stacks up. You never get rid of some catenary, without an infinite force, but that's not the issue, there only has to be enough tension to lift the shank of the anchor so that it no longer pulls horizontal to seabed.
 
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Surely one of the major disadvantages of rusty chain is rusty streaks all around the boat. I'm getting my chain regalvanised next week, before it gets bad.
 
>Thats exactly my point - the chain is much stronger than needed to take the load because it is over-specified to give a catenary.

I've never known a boat with over-specified chain, chain and anchor size is always determined by the weight of the boat. No one in their right mind would use chain that has lost 10% of it's diameter. It will be stronger than rope but it's not a valid comparison rope doesn't get severe corrosion.

Since posting I found a page on the Rocna site http://www.petersmith.net.nz/boat-anchors/catenary.php that says that the effect of catenary is hugely over-estimated

They are completely wrong, catenary weight is the greatest determinant of a safely anchored boat assuming the anchor is set properly. As said they are selling anchors and most boats can't have an oversized anchor because the bow roller is designed for the weight of the anchor determined as I said above.
 
KellysEye said:
Since posting I found a page on the Rocna site http://www.petersmith.net.nz/boat-anchors/catenary.php that says that the effect of catenary is hugely over-estimated

They are completely wrong, catenary weight is the greatest determinant of a safely anchored boat assuming the anchor is set properly.
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This comes up often and you are mistaken.
See http://alain.fraysse.free.fr/sail/rode/static/static.htm

In that example it takes just 52kg of force on the boat to lift 15m of 11mm chain off the sea floor at a 3 to 1 scope. And the reduction in depth of the catinery does little to damp forces either as it happens so quickly.
Heavy chain might stop you sailing around when the winds are low but in a blow it makes very little difference, it's off the sea bed when the forces are up.
 
I'm not sure where you obtained your rope strength information. Using Liros' data the strength of rope somewhere between 14 and 16 mm is the same as the break force of Grade 30/40 chain. http://www.liros.com/fileadmin/user...pekte/LIROS_Katalog_2012_engl_72dpi_klein.pdf

I prefer not to get into arguments about catenary, it has been well covered in the thread referred to. Suffice to say that in other than light and shifty winds, where it helps to stop the boat from drifting about, it has no benefits.

The 10% wear figure is well recognised. The vast majority of wear occurs where the links cross, which is where the chain fails in overload, see pics on my website. The value can only be a guideline but it draws on the experience of corroded chains that have failed in adverse conditions. Three or four anchor chain failures per year are reported or come up in forums and most seem to be well rusted ones.
 
GHA said:
This comes up often and you are mistaken.
See http://alain.fraysse.free.fr/sail/rode/static/static.htm

In that example it takes just 52kg of force on the boat to lift 15m of 11mm chain off the sea floor at a 3 to 1 scope. And the reduction in depth of the catinery does little to damp forces either as it happens so quickly.
Heavy chain might stop you sailing around when the winds are low but in a blow it makes very little difference, it's off the sea bed when the forces are up.
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Well, the sums baffled me into submission, but the conclusion under section 7 looked very wonky.

I prefer this explanation:

http://www.ybw.com/forums/showthread.php?364388-Anchor-Scope/page3

See post 29
 
>In that example it takes just 52kg of force on the boat to lift 15m of 11mm chain off the sea floor at a 3 to 1 scope.

You don't say what the depth is so what you have said is meaningless. I've always said the shallower the water is the more chain you need and the amount decreases with deeper depths which the table clearly shows. What I use is depth 5 metres 10 x depth, 10 metres 7 x, 15 metres 5 x, 20 metres 3x.

Depth H Minimum scope N Minimum chain length Lc
4.6 m (15 ft) 5.6:1 26 m (84 ft)
9 m (30 ft) 4.0:1 37 m (120 ft)
18 m (60 ft) 2.9:1 54 m (176 ft)
 
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KellysEye said:
>In that example it takes just 52kg of force on the boat to lift 15m of 11mm chain off the sea floor at a 3 to 1 scope.

You don't say what the depth is so what you have said is meaningless.
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Doesn't it follow that the depth must be around five meters?
 
In the example I have quoted so many times, in between 2 and 2.5 metres depth we had 35 metres of 8 mm chain out. In about 30 knots of wind the whole lot was off the bottom and the shank of the anchor was lifting in the gusts. Catenary was contributing zero but my nylon snubber was doubtless taking most of the snatch loading. Cala Volpe, Sardinia. Almost identical situation in Galixhidi, Gulf of Corinth. The water was warm and clear and I dived on the anchor many times to watch what was happening.
 
>Doesn't it follow that the depth must be around five meters?

No. As I said the length of chain diminishes as the water gets deeper. In 5 metres I use 10 times chain and in 20 meters 3 X chain. Perhaps GHA will tell us what the 3 to 1 scope depth was.

>2 and 2.5 metres depth we had 35 metres of 8 mm chain out... In about 30 knots of wind the whole lot was off the bottom and the shank of the anchor was lifting in the gusts. .

Ouch, you need 200 to 250 metres of chain for that which is not possible a second anchor would have been the answer, provided you carry a second length of chain and anchor. We had enough chain for three anchors for storm conditions but we did have a heavy displacement boat so the weight wasn't a problem.
 
Why is it with forum posts that you try and keep it simple then end up having to fill in detail that you left out originally to explain something that has subsequently come up ;)

Background to the question: I too am having my chain re-galvanised. (taking some forumite's chains with me in a batch!).

I laid it out on the drive yesterday and had a good look at it and there are some corroded links. Difficult to say whether there is any significant loss of material, I measured with callipers and all links are well above the nominal 8mm but then the non-corroded links are about 8.5mm anyway!

They are not 'worn' in the way Vyv describes, i.e. where the links bear on each other under load. This corrosion has come about from mechanical wearing away (scuffing) of the zinc coating over time and no doubt the corroded bit was at the bottom of the chain locker sitting in salty water for the longest time!

So the reason I asked the question was that it seemed to me (Vyv says otherwise) that if people (not in the UK much maybe) are happily anchoring with 20mm rope rode, I needn't worry about a bit of surface corrosion on 8mm chain because the load a 20 mm rope can take can be withstood by 6mm chain quite happily, the only reason we were supplied with 8mm originally being for weight (catenary) and not strength.

As always - there seem to be ten people with eleven different opinions on that though :roll eyes:

Vyv - as an afterthought - is the reason for failure not due to the reduction in cross sectional diameter but small irregular pitting causing stress points from which a fatigue crack will propagate? If that is the case then I can see that any significant corrosion presents a risk.
 
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