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Old 19 December 2008, 20:50   #1
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Mooring chain

It isn't for a RIB, its for my dad's Drascombe Drifter, but my RIB might use it one day so it is just about relevant to RIBnet

What sort of gauge of chain would you recommend for a mooring for a 6 metre (ish) boat weighing perhaps a ton and a half? It needs to be fairly robust - it can be windy here though the fetch in the harbour out at the farm is not much, at most about 1/2 mile at high tide so you don't get mega waves. High tide water depth is about 25 feet.

He intends to use something like an old engine block for the weight and then some heavy chain and a riser chain to a buoy, the boat won't live there all the time it will only be for a few days now and then, but as it can get quite windy here with little warning, it would need to be something capable of standing up to a 35-40 knot wind without problems.

He sent off to a chain supplier who came back suggesting "12x42mm" chain for the riser, does that mean 12mm link thickness? if so that seems like a hell of a chain, the anchor chain on my RIB is probably only about 6mm link thickness though I haven't measured it. I have had a look on various websites but those selling chain just seem to give the chain spec as the link thickness and nothing else, what is the second measurement the link length?

The mooring won't dry out as it is in a deep river channel though I don't suppose it makes any difference.

The other thing I wondered is how do you choose the right size of buoy based on chain size/weight, to make sure the buoy doesn't sink!?

Many thanks
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Old 19 December 2008, 23:22   #2
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I would never consider anything less than 12 mm. link diameter for a mooring.
The link length only effects the weight per foot of the chain not the strength.
At our local marina moorings they use 18" dia steel mooring balls.
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Old 20 December 2008, 03:45   #3
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Mooring Setup

Engine block may not really be heavy enough. I would suggest a concrete block of at least 1 ton with reinforcing steel and 5 metres of 1 inch chain laid into mould for block before pouring concrete. This forms your 'deadman' or sinker. The advantage of the heavy short length of chain is that it generally will settle in the seabed and only in extreme circumstances will be lifted and no direct pull is put on the chain to block.
From this you will need at least 25ft for the depth of water plus about 10ft or more depending on how much swing you can allow for with other moorings etc. I would make the riser at least 12mm as an absolute minimum. You will find that as it is probably steel and not galvanised it will be down to 10mm after 1 season due to the constant movement on the links.

This will lead to a Stainless swivel of at least 16mm stainless. Will last for years so worth the investment. Then to the main buoy supporting the weight. Polymarine A5 may suffice but would go with A6 and you will be fine. We use A6 on all our moorings in 40ft of water with chain between 12 and 16mm.

Check out http://www.compassmarine.co.uk/hints_tips.asp

"Buoy Sizing
To work out what size buoy you require you need to determine the weight of the chain it has to support. All chain has a stated weight per foot, calculate the total and add another 25% of the total for safety. Always use chafing gear where mooring lines run through bow chocks or are likely to contact deck hardware, and a thimble on the shackled end will prevent chafe at the buoy.

Note: When picking up a mooring, always use the top ring to pick the buoy with a boat hook. Then shackle the mooring pennant to the mooring chain shackle, rather than to the buoy itself. This puts all the strain on the anchor chain, rather than the buoy."

From the top half of the swivel I would run a 2m length of chain onto your mooring ropes as required. This chain again helps to take the snatch out when the boat is pulling with waves.

Other tips,

Remove swivel, buoy and ropes over winter when the mooring is not in use and drop to seabed with a 30ft rope and small marker. The chain resting in the seabed will not wear.

Turn riser chain end for end each year and inspect welded joints and diameter of chain where links connect. The chain nearest the surface wears more quickly.

Use quality stainless shackles and wire shackles with stainless wire, also add plastic cable ties to shackles. This stops them working undone.

A good way to check Stainless shackles etc is by pinging in with a screwdriver or similar, if you get a dull sound the Stainless may be rotten and no longer useable, if you get a ping it should be fine.


If you got any more quesions, just post them here.
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Old 20 December 2008, 05:30   #4
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Thanks guys. So if I understand correctly the reason for overkill in the chain size is to allow for wear - what is the useful life of a 12mm chain before it would become too worn to be safe? It is in a harbour in the middle of nowhere, no possibility of diving on the mooring to check it or anything so it is pretty much a "plant it and forget it" operation.

Great idea about dropping the chain to the seabed in the off season, Dad is away for half the year and the boat is out of the water so that will definitely be worth doing to reduce the wear rate.

The company quoted for "self colour" chain not galvanised, what are the pros and cons of each? I would have assumed galvanised was the way to go so I guess there is a reason for not using it, is it a waste of time as the galvanising just wears off with the chain movement?

One problem there won't be is any other moorings or boats

Location is here:

http://maps.google.co.uk/maps?near=f...38624&t=h&z=15

the mooring is going about 150 metres to the south east of the old jetty you can see, where there is a deep channel. There are also practical issues like getting the anchor weight out to the site as the only boat around is the one that is going to be tied to the mooring, I'm not sure setting a 1 ton concrete block would be practically possible unless some sort of raft can be rigged up to float it out (they did something similar many years ago - in the 1960s I think - to place a mooring for the old farm motorboat which was made out of an old tractor engine block)
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Old 20 December 2008, 07:53   #5
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Quote:
Originally Posted by BogMonster View Post
it is pretty much a "plant it and forget it" operation.
Rather you than me. There will definitely be wear on your gear - if you don't check it, you won't know when to replace.

Given the difficulties, is a mooring definitely the best option for a boat that will only be there for a few days every now and then?
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Old 20 December 2008, 08:30   #6
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its suprising how mooring chain can wear and corrode in a matter of only a couple of years,and it depends on the amount of current and tide as well as the type of water its in ,our local clubs mooring chain was like new in some lengths but within a few meters it was nearly rusted and worn through, although we are situated near a large chemical plant ,also using a concrete block or an old engine block doesent mean its going to stay in the same place as they will loose about 15 or 20 percent of there weight once in sea water .
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Old 20 December 2008, 19:52   #7
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Rather you than me. There will definitely be wear on your gear - if you don't check it, you won't know when to replace.

Given the difficulties, is a mooring definitely the best option for a boat that will only be there for a few days every now and then?
You've got a point and I have been thinking about what Karl said above. How about this for an alternative given it is not going to be used all the time - have a mooring point and chain as per a normal mooring, but leave the full length of chain on the sea bed most of the time, and have a length of nylon rope and lightweight buoy to pick it up and attach the end of the chain to the gear on the boat.

Removes the movement/wear from the equation as the chain will be sitting still for about 350 days a year. Rope can be changed annually (or whatever) without having to dive on it which just isn't possible due to the location and it should greatly extend the life of the chain.

I don't think it would be too much of a pain to use, and if it was being used continuously for a few days the chain could be attached to a bigger buoy for the duration and then dropped back afterwards.

Would galvanised chain be better for this type of arrangement (wear would not be a problem but corrosion resistance might?) and can anybody see any other drawbacks apart from that the initial mooring up would take a bit longer?
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Old 21 December 2008, 03:03   #8
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stephen is there a reason for not just using an oversized anchor and laying it when required, and recovering when not ?
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Old 21 December 2008, 03:07   #9
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You would need at least a tone of concrete or half a tone of steel to make a good mooring (remember that concrete loses half its weight once in the water) but it's the heavy ground chain that you need to get right as it is this that will take the “snatch” out of the boat pulling on the riser chain.

We use Train wheel (700kg) 1.5 inch stud link ground chain (4m) to 12mm riser chain (15m). And that is for a 32 foot yacht. We lift the mooring and renew the riser chain every 2-3 years and do just as you suggest drop it to the bottom and have a rope to a buoy. This dose make the whole rig last a lot longer.

Now the biggest problem you are going to face is getting a tone of gear out and into the right spot before you drop it. We use a 8x3m raft built for the job and can lift 10 -14 moorings in a tide. If you don’t have a means to raise your mooring back up to check and or renew chain as required you will need to lay a new mooring every few years. Or find a diver.

DO NOT
1: drag mooring down beach at low tide.
2: tie on oil drums
3: tow out to the mooring location at high tide
4: cut rope
This is where you discover point
5: the oil drums jump up at you really fast.
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Old 21 December 2008, 05:33   #10
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mdt,

Yep, Dad has been there and done that

He was telling me the other day that the mooring that they laid in the same place for the old farm motorboat back in the 1960s nearly had him down there with it, they floated the anchor weight out on a raft made of oildrums and cut it loose and the chain caught round his watch as it went and nearly took him into the oggin with it. I guess the watch is still down there somewhere but he has that T shirt already

Polwart might be onto something, a big Foff danforth anchor and a buoy might actually be a better bet... I'll suggest it! As said the main thing is the practical problem of getting something out there, laying it and maintaining it afterwards.

Ta for all the suggestions
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