Joining mooring ground chains

[jdc]

Two years ago I bought a mooring in NW Scotland (Plockton), in place and 'as is'.

It consists of two x 100kg mooring anchors joined by a heavy stud link chain, with the normal long-link riser, swivels, buoy etc. I am advised that for my boat (42' cutter, ~14 tons) I should really have three such anchors. I'm happy to add a third and since the whole mooring needs comprehensive inspection and the riser chain replacing anyway now is a good time to take care of it.

But I could do with advice: (i) what is the best way to make the join on the sea bed at the junction between mid point of existing ground chain and the new one connecting to the new anchor (thus making a point at which the three anchors are spaced at approx 120 degrees)? and (ii) does that change how one joins the riser to the ground chains, and if so how am I best to do this?

What is puzzling me is that any way I think about it using normal, albeit beefy, shackles I can conceive of cross loads on a shackle, and they aren't designed to withstand these.

Final question on materials. I understand that I should use black, ie not galvanised, chain and shackles on the sea bed. But what about the riser?

At some point in the progression of riser chain - shackle - shackle - swivel - shackle - shackle - chain through buoy - shackle - hard eye - mooring strop I should switch to galvanised fittings.

I'd love to hear where others have found it's best to do this (at the moment it's the shackle just below the swivel).

 

[Bodach na mara]

First point, who gave you the advice regarding three anchors? If it a local, then they may know what they are talking about. Or maybe not. I ask because in my area (Gourock) I do not know of anyone using three anchors even though there may be strong winds from the north as well as east and west.

Secondly, joining three chains is tricky in normal chain and very tricky in stud-link. You require a joining point that is equally strong for each direction of pull and the usual method I have used is to use a ring and shackle each chain to that. A different method can be used to attach a riser chain to the ground chain, a simple shackle is usually used, but even that is hard (or impossible) with stud-link.

Thirdly, have you considered doing away with the swivel? That would remove a potential main weak point (the swivel, which probably will not swivel anyway) along with four others (the four shackles.)

Finally, galvanising is a waste of time for riser chain (our contractors do not use it), but it is possibly worth it for the hard eye thimble. The shackle attaching thimble to riser chain will probably be galvanised as it is easier to get galvanised shackles than un-galvanised ones. So probably all the shackles are galvanised when new. It will not last long.

 

[jdc]

Thanks for good advice

It would be a relief not to have to add a third anchor - the advice came from a local but not I think based on actual knowledge of the boat, more following the reasoning that the mooring was originally laid for a 38 foot boat and mine is a little longer so it might need to be beefed up. I only use it in high summer anyway, keeping my boat in Falmouth most of the year, so it never experiences the strongest winds.

What would be most useful would be some guide relating the boat weight or cross-section to mooring anchor requirements, given some assessment of exposure of the location (which is not too bad at Plockton, it's only open to the NE with a 2 mile fetch across Loch Carron, and my mooring is under the shelter of the island anyway).

The point about difficulty of attaching to stud link hadn't escaped me, hence the original question! I've not seen a ring for sale, but could fabricate one easily. I have a reasonable source of black iron shackles and other bits but fully agree that the galvanised shackle attaching the strop doesn't last long!

 

[Quandary]

Surely your ground chain and any shackle are designed for weight and corrosion resistance and are massively over strength anyway. However having just lifted my three anchor mooring it will be going back down again with two anchors and a heavier ground chain, the complication and effort involved in handling it is out of proportion to a two anchor arrangement.
Another Scottish contributor recommended 'Seasteel' mutiplait risers in preference to chain as used on fish farms and I am giving it serious consideration, massive strength, much less weight and nothing to corrode away as long as you address the issue of chafe effectively; information on the Gaelforce website.

 

[wotayottie]

My old club in the Bristol channel use nothing but rope on their moorings because it lasts way longer than chain. The rope is leaded to ensure it sinks. The rope is tied to the ring on the anchors rather than shackled because shackles corrode and steel thimbles can become razor sharp. Again at the boat end some people fasten rope to rope rather than using shackles. Others use shackles there ( they are easy to inspect) and then maybe nylon thimbles rather than badly galvanised modern third world steel jobbies.

To the best of my knowledge, there have been no problems with this mooring system due to rope failures and I believe it has been used now with variations for more than 50 years.

 

[Bodach na mara]

The ring that I used to use came from a skip lifting bridle. I am sure that a scrappy may be able to source one for you.

 

[William_H]

Mooring hardware

As said the shackles etc need to be hugely oversized (limited by what size you can get through the chain links) They need to be oversized to last as long as possible considering rust/wear with movement. So strength itself is not a consideration.
Adding the additional anchor or better still 2 anchors (90 degrees and no need to move existing anchors) reduces the tendency for the mooring centre to be pulled sideways with cross wind which may tend to dislodge the anchors. 4 anchors means that all pull is only in line with the anchors set. Even much smaller anchors at right angles will improve things enormously.
The additional anchors result in much less movement of the horizontal chains so less wear. And of course you have more back up.
The attachment of chains for the additional anchors can be simply shackles onto the main between anchors chain or on the riser at the bottom. Or you can simply pass the new chain around the existing one and shackle back onto itself. A rope might be needed to locate the sliding joint near the riser. Or wrap it around the riser. You can use rope to join the chains if you like.
You might ask how long the chain needs to be to new anchors. This depends on how high the centre of the mooring will lift under load. So the longer the chain between the original anchors the more it might lift so something comparable to existing chain or even down to half as much but not really short or the lift might dislodge the new anchors.
As said galvanising is pointless as it soon wears where it matters at the interface between metals of chain and shackles. There is much to be said for using rope. I have a friend who has used rope for many years on the riser. I am not sure I have the courage to do that. Currently I have all stainless steel riser which is lasting well but is joined to black iron parts by rope only to minimise galvanic corrosion.
Much will depend on your diver and what he finds and what he can do. I dive on mine myself and
because it is easier to add than remove old and replace it can become a bit of a mess. good luck olewill