Mooring chains risers and shackles

[ianat182]

My mooring on the Hamble is maintained by a subcontractor. I believe the Ground tackle itself is of large dimensions and of 'black iron' metal. The risers are large dimension chain in the lowest section and the upper 4 metres in a lighter one (12mm ) to the Buoy.
I have the mooring inspected and the risers replaced at 3 year intervals,however in between times I am having to replace the shackles which seem to wear through and rust in 6 or 9 months or so,sometimes leaving the boat moored to this chain by only one of the two warps fore, and aft.
My question is regarding the black iron chain.I believe the heavier chain is made of this metal, I'm not sure about the lighter chain of the riser,but neither are galvanised; I use 2 separate galvanised shackles of good quality on each of these risers and it seems that only they get rusty and badly worn eventually failing.

What is the make-up of the 'black iron that it seems to have a longer life than the galvanised material - chain or shackles. Should I have only black iron throughout, including the shackles?

As regards the shackles I know these are sometimes of 'chinese' origin and not worth the money or risk,but should those with stamped weight limits be reliable as I thought?

 

[webcraft]

Black iron

.
Black iron usually just means iron or steel with no finish or painted.

The ground chain will be very thick chain and it will be buried in the mud in an anaerobic environment so it cannot rust.

Chain risers tend to rust particularly badly in one or both of two places. The first is where the riser lifts in and out of the mud at the bottom with the rise and fall of the tide, and the second is near the surface where there is more oxygen available. Normally when you replace a riser only part of it is badly worn, much of the chain is good as new.

Local conditions can affect the rate of wear, notably type of mud - there are places in our bay where risers wear much faster than others. Another factor is are silt or sand suspended in fast tidal streams, which can cause rapid abrasion. In this sort of situation some people prefer to use rope risers.

It is not normal or economic to use galvanised chain as the galvanising gets worn off quickly at the bottom end with the chain rising and falling and dragging around on the seabed. Not much point in galvanising if it quickly comes off at the point that needs protecting most.

Our riser is 19mm long link chain, which is pretty standard for yacht moorings in this neck of the woods. The shackles are 25mm tested - not galvanised. You should definitely use tested shackles. (Again, the shackles tend to be wear points, so galvanising isn't going to last long). Your riser at 12mm seems very light to me. The cost of the chain is minimal compared to the cost of losing your boat, and if you are having to replace 12mm every three years then 19mm will probably last five.

When you say your mooring is maintained by a subcontractor, who does the subcontracting? Is it your moorings association, because if so you should perhaps bring up your concerns with them.

We have gone over to inspecting and maintaining our own mooring as it has been difficult to find a reliable and affordable company to do it here. However, we are fortunate in being able to hire a local skipper and his creel hauler to lift the mooring if we need to work on it - not everyone will find it so easy. Also, if you have full insurance you may not be covered if your mooring breaks and it is not 'professionally' inspected. (We only have 3rd party insurance, so not an issue).

Sorry to have gone on at such length - hope some of it is useful. (No doubt someone else will be along in a minute with contradictory advice, but heyho that's the forum for you )

- W

 

[Billjratt]

Get black iron shackles. That's what we use in Troon and they last for years.
compatibility is the name of the game.

 

[longjohnsilver]

I'm surprised you only inspect and replace the riser every three years. I'd want to look at it at least once a year and probably replace every two years. As has already been said the ground chain probably won't wear much unless it's not buried and moves around a fair bit. And the riser will wear at the bottom where it's effectively being constantly ground away by being in contact with the sea bed, and towards the top where it will rust away over a period of time.

 

[ianat182]

Thank you for your reply; as to the subcontractor they inspect my mooring annually(a requirement of my licence).
I am responsible for the risers replacements and they do this on my behalf.
The lighter chain was put on so that I could maintain the upper risers and shackles,and didn't have to lift all the weight of the heavier tackle at MLS (I may be wrong regarding its size)

I was of the impression that Black Iron contained some added constituent like, say, molybdenum ,but I didn't check giggles,so I'll have look there too.
I have on board 2 very large shackles I bought fully galvanised in a bag; both have never touched water or damp and both have rust on them in patches after 6months(they were too big for the riser chain) Now they are unnecessary weight, and useless unless I have need of a weight like an 'angel' or something!.

 

[vyv_cox]

As has been said, black chain simply means ungalvanised. In the majority of cases its composition will be a mild steel, i.e. carbon content less than 0.2 and no major alloying elements, just traces of odd ones. Corrosion normally only becomes an issue towards the surface as the oxygen content is lower further down. Much of the chain used for moorings in the NW of England/Wales is secondhand stuff that comes from Manchester Ship Canal. It often arrives heavily greased or tarred but is all otherwise unprotected.

When I had a mooring for 10 years or so the links were all 'paper clip' types made from mild steel, welded along the joints. Some contractors cast anodes on to the chain close to the joints between riser and ground chain but mine never did. All my chain up to the buoy was black but the pickup between the buoy and boat was galvanised and lasted two seasons mostly.

One cause of rapid corrosion is one which commonly occurs in pipeline welds. If the pipeline (chain) contains small amounts of other elements and the weld (shackle) is totally free of alloying elements, it will be anodic to the main body and far smaller, so galvanic corrosion will occur rapidly. Shackles are often quite cheaply made and fairly soft, so a low or zero alloy content is quite likely. The answer may be to buy shackles of better quality, if such a thing exists. Try Bradney chain or one of the lifting gear specialists.

 

[ianat182]

Thank you all for the information,looks like the answer is definitely the un-galvanised black iron shackles duly stamped with SWL figures.Google didn't enlarge on the black iron formulae.

ianat182