Mooring Chain Corrosion

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 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 corroded pin is probably around 12mm. The shackle body started off about 100mm in length, and is now down to 86 mm.

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. 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.

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.

I highlighted this issue on a thread on PBO forum and got some very interesting comments:

Catalac08 wrote:

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.

 

William H wrote:

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. 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.

You can read the entire thread by clicking here.

Lessons learnt:

Use heavy shackles – preferably not galvanised – zinc will corrode quicker than non zinc surfaces leaving vulnerable inners exposed.

Shackles will most likely need replacing at more frequent intervals than the chain due to abrasive wear on the pin.

Inspect at least once annually and aim for overall replacement of rising chain & warp every 3 years.

Some suggestion that all chain riser would be better than chain/warp – warp has the benefit of elasticity to dampen shock loads (particularly relevant with big seas in our anchorage)  so I’m sticking with it for the time being. All chain does have the advantage of eliminating chafe at the bow roller when the warp comes in contact with the metal sides.

Barna

Barna Pier

There is a small drying harbour at Barna, on the western outskirts of Galway City. The pier provides shelter from the prevalent westerlies, and the harbour is a nice day sail from GBSC, some 9 NM distance. Donnelly’s Pub, a short walk from the pier is a good lunch stop. There are several perches marking hazards on the approach to the pier, which is straightforward enough if one holds a course bearing 010M  on the eastern end end of the pier, keeping clear of the offlying rocks  to starboard.

Approaching Barna Pier

Approaching Barna, keep the end of the Pier and the large two story house shown here to the right on a transit for a safe approach.

Golam Head

Golam Tower viewed on the approach from Aran , with the magnificant Twelve Bens in the background. The tower was built as a semaphore station in the early 1820’s.

Golam Tower is a wonderful landmark signifying the fact that we have escaped the confines of the inner bay and we are now westbound for open water, through the Inner passage towards Slyne Head and the wild atlantic or northwards to the beautiful setting or Roundstone. It can be seen for miles in all directions – as an “aid to navigation” it is unmistakable, distinct and highly visible. Immediately north of Golam Head is the very narrow entrance to Golam harbour. It can be entered in favourable conditions with a fair wind or under power, but entry is not advisable in a high swell. Careful navigation and chart work is required to avoid the rocks in the entrance. Anchor anywhere inside where it is possible to land by dinghy and go right up to the tower. The shelter is good under all conditions.

Golam Harbour from the NE

A little north of Golam is the broad entrance to Kilkieran Bay with its many islands which extends 14 miles inland. It can be entered in any conditions and gives excellent shelter. The tide runs maximum 2 kn. in the narrows and 1 ½ kn. near the entrance. The ingoing stream commences – 0520 Galway and the outgoing +0105 Galway.
Going southwards around Golam it is best to hold one’s course for at least a mile before turning east to avoid any hazards such as English rock close to the north shore.