Mooring chain corrosion

[SAWDOC]

I laid a new mooring in 2005. Starting from the seabed, the mooring consists of a cast concrete block with ships chain cast in situ connected by shackle to 4 metres of heavy chain, connected to 10 metres of nylon warp which ends in a soft eye, which holds the boat.
Over the winter with our boat on the hard, I thought I would inspect the lower down bits you seldom get to see.The results were very interesting.
The shackle connecting warp thimble to chain was on its last legs.

The corrosion has eaten away the pin to the extent that you can clearly see a gap in the threaded part of the pin.

The action of the chain pull pull pulling on the shackle has caused severe wear on the round portion of the pin.
The chain itself was quite corroded and needed replacement - didn't get a snap of that.
Glad I investigated. I don't think it would have lasted another season.

Another area of concern was where the mooring line went through the stem head anchor chain roller
Because the warp is somewhat narrower than the gap of the anchor-roller the sideways pull on the warp had caused considerable chafe here. At least this is visible from on deck and can be regularly inspected during the season.

I had tried to eliminate this with some soft pipe over the warp - this was pretty successful but a colleague who simply closed the gap with extensive use of hard wearing tape seemed to have avoided any chafe whatsoever.
It is the time of year to be checking this kind of thing.

 

[VicS]

Our moorings are similar. Heavy (1" ??) chain cast into a concrete block (12ft of that IIRC) and the riser, 3/8" or 1/2" depending on mooring category, shackled to that. We don't have a rope section, the riser chain having a loop formed at the end which drops over cleat, bits or what have you on deck.

A pick up buoy is attached with rope long enough to allow the chain to fall to the bottom when the boat is not on it (all winter in effect).

The yard lifts the mooring, as far as the heavy chain, for inspection every year I think.

I reckon if your shackle has been in use for 4 years it has done well. It does not look that heavy but nothing to judge it by in the photo. In my experience shackles from chandlers are c**p, especially the pins.

 

[TradewindSailor]

The trouble with any galvanised chain is that as soon as the zinc has erroded away there is nothing to prevent the chain from rusting ..... and rust is comparatively soft, so the errosion is fast.

Perhaps the mooring chain should be protected with zincs bolted through a link or two ...... any experiences of this?

 

[rob2]

our club moorings are nowadays only reckoned to last two years, often with a change of shackles annually. It say something about the water quality!

I remember sailing books from my youth advocating chain, with a lifting strop to haul it up from the seabed. That eliminates chafe and minimises the rapid erosion normal at the surface.

Rob.

 

[SAWDOC]

Vic
Sorry - should have inserted an object for scale purposes. The warp we use is 25mm diameter. The pin of the shackle in the pic was 20mm originally. It has now corroded down to 16mm and the deformed severely coroded pin is probably around 12mm. The shackle body started off about 100mm in length, and is now down to 86 mm.
We don't have an overseeing body examining our moorings - they are in effect our own property, our own design and our own responsibility. Usually at our club 3 or 4 boats come adrift every year due to some part of the mooring set-up failing - you just can't be too careful.

 

[Bosun Higgs]

Used to keep a boat at a Bristol channel club where the moorings are in the form of an X with the boat at the middle and anchors at the 4 ends. We used rope because it way outlasts chain in that sort of situation - chain wears between the individual links and if you have had the wear you show on the shackles, then the chain needs checking carefully too.

We also used shackles usually at both ends of the rope. At the bottom where they bury under the mud, the corrosion is limited as a black oxide is formed. At the surface end of the rope, there is significant wear of the sort you describe, though I've seen shackles that were 90% worn through.

I'm surprised you get away insurance wise with 3 or 4 boats breaking free each year.

 

[vyv_cox]

This is an example of corrosive wear, not simple corrosion. Note how it is the areas that have been in sliding contact, the pin on the shackle and the upper part of the ring. The mechanism is that corrosion takes place, rust is formed but is immediately worn away, exposing a bright nascent area for corrosion to commence again. This photo shows one of the best examples I know. The chain was attached to a buoy in the Med, with a short scope that allowed the lower end to touch the bottom, Over a couple of years the part in contact with the bottom wore away, although all of the chain was clearly in the same corrosive environment.

Adding zinc or any other anodes will not prevent this mechanism, although it may have a beneficial effect on corrosion of the chain and connectors generally. The only way to prevent it is to drop the mooring chain on the bottom in winter, or to lift it out altogether.

 

[Quandary]

Is it good or bad practice to use galvanized ground chain?
If you use uncoated chain, as always recommended around here, should you then avoid galvanized shackles. Is it good practice to shackle a galvanized riser to an untreated ground chain or should the galvanized chain and shackle be restricted to bits you can pull up to inspect at LWS.
I suppose what I am asking is 'does galvanizing actually accelerate corrosion of the ungalvanized stuff?'

 

[vyv_cox]

Is it good or bad practice to use galvanized ground chain?
If you use uncoated chain, as always recommended around here, should you then avoid galvanized shackles. Is it good practice to shackle a galvanized riser to an untreated ground chain or should the galvanized chain and shackle be restricted to bits you can pull up to inspect at LWS.
I suppose what I am asking is 'does galvanizing actually accelerate corrosion of the ungalvanized stuff?'

I can't think of any reason why galvanising of one section of chain could have an adverse effect on another, although I'm ready to be educated

When I had my mooring in the Menai Strait I used ungalvanised ground chain and riser simply because it was second-hand (ex Manchester Ship Canal) and came like that. However my swivel was galvanised, as was my pick-up chain and the shackle holding it to the buoy. I know of some contractors who cast large zinc anodes onto risers, or the joint between ground chains and riser, to preserve them.

 

[SAWDOC]

I use galvanised chain and galvanised shackles. As Vyv mentions above, every winter it is gathered up and dropped to the bottom into the anaerobic mud of the bay. Thanks for illustrating the corrosion by abrasion issue Vyv - what was the seabed matreial in the area in which that chain was used?

To compare the before and after shackle size, I am posting a pic of the replacement shackle which to the best of my knowledge is very simiar to the corroded shackle above.

I agree with VicS that one has to be extremely wary of shackles of unknown origin from whatever source. The green pin shackle in the next pic come with a CE mark and I assume they are of a good standard. I would be interested if anyone has any comments on their performance.

 

[tabernacleman]

mooring

I gather from your post that you were surprised at the amount of wear, my club advises and if we could we would insist, that all groung tackle is examined annually. How long did you expect the gear to last, in most cases there is a lot of investment swinging from these little bits of quite inexpensive metal bits, I never understand not renewing annually, most harbour authorities round here require certification by a local mooring contractor.

 

[SAWDOC]

I gather from your post that you were surprised at the amount of wear, my club advises and if we could we would insist, that all groung tackle is examined annually. How long did you expect the gear to last, in most cases there is a lot of investment swinging from these little bits of quite inexpensive metal bits, I never understand not renewing annually, most harbour authorities round here require certification by a local mooring contractor.

Well, I started off by saying this was a newly laid mooring. It was also the first mooring I laid so I honestly didn't know. Five years on, I am of a mind to inspect annually with a view to replacing every three years. It is probably something that is user and location specific.

 

[webcraft]

.
Our riser lasted five years and was worn to about 8mm from 19mm when we replaced it. That sounds like its bad, but our anchor chain is 8mm!

- W

 

[Bilgediver]

I use galvanised chain and galvanised shackles. As Vyv mentions above, every winter it is gathered up and dropped to the bottom into the anaerobic mud of the bay. Thanks for illustrating the corrosion by abrasion issue Vyv - what was the seabed matreial in the area in which that chain was used?

To compare the before and after shackle size, I am posting a pic of the replacement shackle which to the best of my knowledge is very simiar to the corroded shackle above.

I agree with VicS that one has to be extremely wary of shackles of unknown origin from whatever source. The green pin shackle in the next pic come with a CE mark and I assume they are of a good standard. I would be interested if anyone has any comments on their performance.

Green pin shackles are a trade mark of one of the better hardware companies and as they are extensively used in the offshore business and shipping they should be OK for you.



http://www.vanbeest.nl/products/product_updates/item/22

 

[SAWDOC]

Thanks for the link Bilgediver - must give their catalogue a look. Are all green pin shackles vanbeest?

 

[impact30]

Hi Tony
On the Shannon estuary moorings are similar but all use riser chain. Rope can get caught on keel during change of tide as there can be strong currents. It is almost unthinkable here to put a boat on a mooring at the start of the season without first inspecting the mooring. Shackles are changed annually and riser chains every other year, sometimes they can last 3.
Tested shackles (green pin) seem to fare better. PTFE tape on the threads can preserve them and aids removal. Dont even think of putting grease on anything down there!
It is usual here to use uncoated ground chain with galvanised shackles, swivel and riser chain. Anodes for chain are usually magnesium/aluminium type but are seen as prohibitively expensive for their effectivness around here.
Sin a Bhfuil
Padraic

 

[William_H]

Mooring hardware wear

The stories and pictures are typical of wear of iron on moorings in Swan River except perhaps faster here in warmer water.

I disagree re quality of shackles (and chain) the only thing you need is iron. So far better a bigger shackle of unknown origin than a high quality fully tested shackle that is smaller. In a mooring the gear needs to be so big that actual strength is not an issue. You need large size so large amount of wear still leaves a lot of metal.
Some people have reverted to rope for mooring riser here with some success. You need a large diameter rope and the steel thimbles will still wear away. As will any shackles used.

Our mooring authority suply and require the use of a large yellow buoy. Too heavy to lift on board. It comes with a stainless steel rod through the middle with a swivel built in. I think the metal about 16mm diameter.
Originally they expected boats to use the swivel however the stainless rod failed on many occasions. (about 2000 in use) so replacements deleted the swivel so you had to connect under the buoy. Not necessarily better but gets them out of responsibility. Though many people still rely on the built in swivel.

The stainless steel however caused accelerated wear/corrosion of the Galvanised Iron shackle. I have now gone for all stainless with home made swivel and chain. The chain a gift from a friend out of some furnace.
Well it has been stainless now for a few years with no signs of wear. The smallest bolts in the swivel are about 10mm. I do have some GI connecting additional weights/ anchors but this is isolated by sections of rope so no GI to stainless connection.

I did get a shock a few months back when the authority rang to say the buoy was found on the beach. the rod up the middle had failed. There was a nut on the threaded rod with a pin through it to stop the nut coming loose. The rod had failed at the hole and the nut unscrewed. Fortunately I had moved the boat connection to the riser so only the buoy escaped. They gave me a new rod for the buoy now 18mm stainless. That should last better than the original.

As for attachment to the boat. I use a rope to a snap shackle on the winch out eye half way up the bow. This eliminates all chance of chafe. I then have a second line onto the standard cleat on the foredeck. I remove the bottom connection on arrival by dinghy and reconnect before leaving the boat. The top line is used to cast off and piuck up.

So yes even with the best heaviest gear a mooring needs to be inspected every year. In shallow water I check mine visually by diving every few weeks. This may help olewill

 

[SAMYL]

I use galvanised chain and galvanised shackles. As Vyv mentions above, every winter it is gathered up and dropped to the bottom into the anaerobic mud of the bay. Thanks for illustrating the corrosion by abrasion issue Vyv - what was the seabed matreial in the area in which that chain was used?

To compare the before and after shackle size, I am posting a pic of the replacement shackle which to the best of my knowledge is very simiar to the corroded shackle above.

I agree with VicS that one has to be extremely wary of shackles of unknown origin from whatever source. The green pin shackle in the next pic come with a CE mark and I assume they are of a good standard. I would be interested if anyone has any comments on their performance.

In Belfast Lough and Strangford Lough it is prudent to change shackles every year as they seem to corrode very quickly as per the photos shown above in some of the postings.
As for the green pin shackles; my son works in a chandlers and was advised by the traveller from a shackle company that the green pin shackles should never be used under water, they are supposed to be 'high tensile' or very tough, so corrode quickly in sea water.
I know one of the sailing clubs here some years ago bought chain from a coal mine in England which was closing but the chain was 'high tensile' or whatever, thinking they had had a great deal but it lasted less than a year when used for moorings!!!

 

[William_H]

A worn shackle

here is a photo of one I keep as a reminder. It was in service for 3 or 4 years then i saw it was wearing and fitted another in parallel. They are a devil to remove when underwater so I left it there. Eventually nature freed it. All in about 5 years. yes it started as a 10mm shackle. olewillhttp://www.ybw.com/forums/attachment.php?attachmentid=3734&stc=1&d=1266627811

 

[vyv_cox]

Thanks for illustrating the corrosion by abrasion issue Vyv - what was the seabed matreial in the area in which that chain was used?

The chain was recovered from the seabed at Pollensa, Mallorca, where the bottom was sand. The chain is only 8 or 10 mm, on a very small mooring close to the shore. From the surface there was no reason to suppose that there was anything wrong with it. We remained in the anchorage for more than a week, during which I dived on lots of other moorings, finding many which were in similar, although not so severe, condition

Moral - be careful when picking up unkniown moorings!