Mooring chain corrosion

I used to belong to a club where the members laid and maintained the moorings and these were in an exposed part of the harbour, some in shallow some in deep water and some drying ones in trots of several moorings and others in deep water singles. We had a reputation that very few of our boats ever broke adrift and on the exceptionally rare occasion it happened it was not the club part that failed it was the topmost part supplied and fitted bit the owner. Insurance would certainly have been refused with a loss rate of 3 or 4 per year!

Our moorings were heavy duty, read HEAVY duty. The single moorings starting with two 120lb Danforth style anchors (club made them) then ground chain with 1" or more thick links between the anchors, that chain coming from sources like the local chain ferry after their regular insurance replacement. 5/8" chain risers were shackled into the middle of this rising to 3/4" swivels just below the water to which the owner would shackle his own chain or nylon strop(s). The lower end of the risers were double shackled to the ground chain with a main shackle plus a short doubler section backup left slack across to an adjacent link. All shackles were double wired with heavy steel wire like fencing wire. Galvanised shackles and wire are not needed but good heavy iron shackles are. Swivels should be one size larger than the chain. If a support buoy is needed as in deeper water then the strops were fitted below it not to any through bar that cannot be inspected.

These moorings were lifted and inspected annually and at the very least the shackles replaced if needed and always rewired. Swivels were usually removed if the boats were ashore in winter and usually replaced regardless every two years.

Iron chain sunk in the mud is long lasting as are the anchors. Chain in the water and swivels are subject to wear from silt in the water and chafe across the seabed. Shackles come undone very easily although you wouldn't think so if you try to undo one deliberately, they MUST be wired. Swivels wear on the moving part and sand/silt in the water accelerates the wear, hence use a bigger one and replace regularly.

I find it very difficult to understand that a mooring laid in 2005 is only just being serviced now because relative to the values of the boat the mooring tackle is very inexpensive.
 
Wear on mooring

I must be very lucky where I am at the top of Stanpit Creek in Christchurch Harbour. The mooring was laid in 2000 with 10mm ground chain well buried in gloopy mud and 12mm riser. The only visible wear is on the shackle that attaches the riser to the buoy which is down from 12mm to 9mm on the wearing surface.

I have 8mm chain from the buoy over the bow roller to the cleat. The chain isn't visibly worn but the cheeks either side of the roller are getting nicely smoothed off by the movement of the chain. I would have thought that rope would be better going through a fairlead than over a bow roller with sharp cheeks.
 
When the Menai Marina had swinging moorings in the Menai Straits, large hoops were cast into the concrete blocks. Chain risers were found to corrode badly so an Admiralty method was followed using a large polyproplylene rope. This had a large soft eye at the bottom which was passed through the hoop and then the free end through the eye to secure it. Some sort of plastic was smeared over the inside to reduce wear. As the rope floats, there is no wear on the rope from touching the beach. The swivel was shackled to a rope eye at the top, with the mooring buoy above the swivel. The shackles only lasted about two years whereas the rope risers lasted for a long time.
 
Robin said:
I used to belong to a club where the members laid and maintained the moorings and these were in an exposed part of the harbour, some in shallow some in deep water and some drying ones in trots of several moorings and others in deep water singles. We had a reputation that very few of our boats ever broke adrift and on the exceptionally rare occasion it happened it was not the club part that failed it was the topmost part supplied and fitted bit the owner. Insurance would certainly have been refused with a loss rate of 3 or 4 per year!

Our moorings were heavy duty, read HEAVY duty. The single moorings starting with two 120lb Danforth style anchors (club made them) then ground chain with 1" or more thick links between the anchors, that chain coming from sources like the local chain ferry after their regular insurance replacement. 5/8" chain risers were shackled into the middle of this rising to 3/4" swivels just below the water to which the owner would shackle his own chain or nylon strop(s). The lower end of the risers were double shackled to the ground chain with a main shackle plus a short doubler section backup left slack across to an adjacent link. All shackles were double wired with heavy steel wire like fencing wire. Galvanised shackles and wire are not needed but good heavy iron shackles are. Swivels should be one size larger than the chain. If a support buoy is needed as in deeper water then the strops were fitted below it not to any through bar that cannot be inspected.

These moorings were lifted and inspected annually and at the very least the shackles replaced if needed and always rewired. Swivels were usually removed if the boats were ashore in winter and usually replaced regardless every two years.

Iron chain sunk in the mud is long lasting as are the anchors. Chain in the water and swivels are subject to wear from silt in the water and chafe across the seabed. Shackles come undone very easily although you wouldn't think so if you try to undo one deliberately, they MUST be wired. Swivels wear on the moving part and sand/silt in the water accelerates the wear, hence use a bigger one and replace regularly.

I find it very difficult to understand that a mooring laid in 2005 is only just being serviced now because relative to the values of the boat the mooring tackle is very inexpensive.
Click to expand...

I don't disagree with any of your well made points. In terms of the service interval, of course this was down to myself based on my assessment of the degree of corrosion from an annual or at best twice annual inspection over the gunwhale of the dinghy at lw springs. As I replied earlier, some experience of local conditions has been gained and I intend "servicing" at 3 year intervals henceforward.
 
ayearsley said:
When the Menai Marina had swinging moorings in the Menai Straits, large hoops were cast into the concrete blocks. Chain risers were found to corrode badly so an Admiralty method was followed using a large polyproplylene rope. This had a large soft eye at the bottom which was passed through the hoop and then the free end through the eye to secure it. Some sort of plastic was smeared over the inside to reduce wear. As the rope floats, there is no wear on the rope from touching the beach. The swivel was shackled to a rope eye at the top, with the mooring buoy above the swivel. The shackles only lasted about two years whereas the rope risers lasted for a long time.
Click to expand...

That's interesting, CHT still use rope risers on the trots in the River Seoint. They do lie on the mud as the tide goes out, but they also seem to last well.
 
William_H said:
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.
Click to expand...

Use the nylon thimbles - they seem to last forever. And if you can, miss out thimbles and shackles altogether and tie the ropes to each other
 
Searush said:
That's interesting, CHT still use rope risers on the trots in the River Seoint. They do lie on the mud as the tide goes out, but they also seem to last well.
Click to expand...

We almost never have to replace the ropes we use on moorings - just on the odd occasion when a big ship crosses one in winter and cuts it, or someone has a go with their shaft rope cutter.
 
SAWDOC said:
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.
Click to expand...

I think you're right Sawdoc. Its a small price to pay for peace of mind, and in any case shackles are cheap. As someone has said, its size and thickness you need more than an upmarket expensive brand. They will all corrode and wear.
 
Wear oinb chain and shackles

I think the wear comes from the movement of the chain with pull. So that chain which is lower down spends more time sitting on the bottom not moving while further up it is constantly moving. The condition of the bottom has little wear effect. it just keeps the chain shiny on the outside.
Hence heavy chain fixed to the weight or between weights will not move much so won't wear compared to the riser. olewill
 
I have a private mooring on the Hambleand have the risers inspected annually and renewed every three years;there is self-certification requirement to my licence each year. This year I was informed that the main ground chain for the whole trot of my moorings were overdue for replacement. The cost is going to be just over £800 including VAT, so came as a nasty surprise to say the least!.
I spoke to the moorings contractor who showed me the chain he is going to lay as a replacement ,some 65ft of about 1 1/4"dia ungalvanised ground chain, 2 large shackles about the same diameter @ £25 each,and two risers of 12mm brand new chain about 10 metres each;he will secure to my own mooring buoys and warps. He said that the ground chain had been down on the bottom for 16 years and that the new would be changed in 10 years time; the risers as and when needed, I have to replace,or get them done.
The ground chain he uses has a web cast into it on each link,and no galvanising.
I imagine they get the larger stuff from broken up tankers' anchor chains,just guessing though.
My boat is 26ft so the new length gives a decent angle fore and aft to each buoy,its almost straight up and down at the moment and the buoys contact the hull at change of tidal current, not very good.
I use proof stamped ungalvanised shackles,none of the Chinese rubbish that some of the chandleries have that rust before they hit the water!

ianat182
 
William_H said:
I think the wear comes from the movement of the chain with pull. So that chain which is lower down spends more time sitting on the bottom not moving while further up it is constantly moving. The condition of the bottom has little wear effect. it just keeps the chain shiny on the outside.
Hence heavy chain fixed to the weight or between weights will not move much so won't wear compared to the riser. olewill
Click to expand...

I agree with that. I would add that conditions in the mud at the bottom are more anaerobic than nearer the surface and this should also reduce the rate of chemical corrosion as opposed to abrasive corrosion.
 
I am surprised that in amongst all the well informed comment about moorings that the subject of preventing corrosion of the pin threads was not considered important. I once had a boat go walkabout which puzzled me as the shackles werewell sized, only about 2 years old and were moused. What had happened was that the threads had corroded away and the mousing had worn through allowing the pin to fall out. Thankfully no damage to the boat then and ever since I have always used Sikkaflex to protect the threads on mooring shackles from corrosion.
 
Catalac. -
I take your point about the thread corrosion,obviously because there is no plating on it; but failure more likely to be the non-threaded end where the aperture clearance is often a millimetre or two ,allowing the chain to lever the pin on the non-threaded side. the suggestion of PTFE taping the threads seems counterproductive for a secure pin to the shackle, as are all lubricants. Sikaflex sounds a better option threadwise.

ianat182
 
ianat182 said:
Catalac. -
I take your point about the thread corrosion,obviously because there is no plating on it; but failure more likely to be the non-threaded end where the aperture clearance is often a millimetre or two ,allowing the chain to lever the pin on the non-threaded side. the suggestion of PTFE taping the threads seems counterproductive for a secure pin to the shackle, as are all lubricants. Sikaflex sounds a better option threadwise.

ianat182
Click to expand...

Check the shackle pin thread and the shackle body thread. The threads of most galvanised shackles I have seen have been galvanised too. When the zinc is removed by galvanic action there isn't much thread there to keep the shackle pin in, and by that time it is loose anyway.
 
catalac08 said:
I am surprised that in amongst all the well informed comment about moorings that the subject of preventing corrosion of the pin threads was not considered important. I once had a boat go walkabout which puzzled me as the shackles werewell sized, only about 2 years old and were moused. What had happened was that the threads had corroded away and the mousing had worn through allowing the pin to fall out. Thankfully no damage to the boat then and ever since I have always used Sikkaflex to protect the threads on mooring shackles from corrosion.
Click to expand...

Thanks for making an excellent point. The first pic in my initial post does show the problem of a corroded shackle pin - which I was able to pull rather than twist loose. However your post highlights the problem and proposes a solution which my earlier effort didn't.
 
SAMYL said:
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.
Click to expand...

I am amazed by your comment as I would have thought - perhaps incorrectly- that most users would have opted for the green pin type shackle simply because they are of proven strength. Am I wrong?
 
Sawdoc - I belong to a club where we lift and inspect moorings each year. I would suggest (based on our experience)

Above the water chafe is the big killer. Run your warp(s) through plastic tubing where they come aboard or anywhere else they could chafe.

Below the water; electrolytic corrosion kills shackles. Seize them with monel wire or cable ties or (ideally) weld them. A new shackle each year is a small price to pay.

Swivels are weak points - if you need to use one go oversized if poss and inspect / renew each year
 
You must log in or register to reply here.

Other threads that may be of interest

mogmog2
Maxwell windlass 10 VW looks interesting (to a windlass noob)
2 3
Replies
59
Views
1K
mogmog2
mogmog2
N
Viking AnchorsI
Replies
0
Views
124
Neeves
N

Share this page

Top