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Old 21 December 2008, 04:44   #11
Country: UK - Scotland
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Originally Posted by BogMonster View Post

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
The advantage you have with a mooring is that you have got 4+ meters of thick heavy ground chain. Which when the boat is blown one way and is getting lifted and dropped by the waves is take all the shock loads out and slowing the boat relatively softly.

With out this you are directly tied to the bottom, via a tight line with no give in it. Hence why you need to pay out so much anchor line / chain when you are in the ruff. Also over a few days or a week with the wind and tides changing direction an anchor could work its self out.

Glad your dads t'shirt fits you too, best of luck

“The only difference between men and boys, is the price and size of their toys”
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Old 21 December 2008, 15:37   #12
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I think that the simple truth will be this ...

... the easier it is for you to put down and recover, the less secure it will be.

Yes, a big anchor might work, but will you be able to sleep when the wind blows?

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Old 22 December 2008, 02:29   #13
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Originally Posted by mdt View Post
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.

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.
Why not slowly fill the drums with sand when your in the right place, when neutrally buoyant leave the lids off and let the whole lot go down as they fill with water and use them to increase the anchor weight
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Old 24 December 2008, 05:36   #14
Country: UK - Scotland
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Originally Posted by BogMonster View Post
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

My 38 ft steel ketch weighed about 11 tons. She had a 35lb. Danforth anchor and 30 fathoms (say 60m) 3/8", approx 9mm chain as ground tackle. We sailed beginning of April to end October and lay at anchor many times in force 8 to 11. With the anchor set properly we never dragged, even with the enormous windage of the rig and inertia of a heavy boat. And yes, I did sleep.

If the sea bed is the usual mud or muddy sand of harbours, I can't see that you would need anything more for the summer mooring for a Drascombe. There's little inertia and not much windage. You could probably manage with 20 fathoms/40m if swinging room is a problem - even less if you use heavier chain, but then that gets hard to pull up. In winter you can haul it up - perhaps put a marker buoy down to mark the spot.

It's standard small boat kit and galvanised. I use some second hand 9mm chain and small anchor (20lb) for my RIB mooring. No problems.

Concrete blocks need to be heavy. They work best in soft-ish mud, where they sink a bit and the suction prevents them dragging. Anchors don't need to be anywhere near as heavy to generate the same holding power. Old engine blocks, etc. are not a good bet unless they're very heavy.

Cheers, Tony
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Old 24 December 2008, 08:02   #15
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Originally Posted by alystra View Post
Anchors don't need to be anywhere near as heavy to generate the same holding power.
Concrete blocks need to be heavy. They work best in soft-ish mud, where they sink a bit and the suction prevents them dragging.
I agree. And, stating the obvious, a concrete block isn't a mechanical anchoring device, it's just a weight. Concrete weight varies depending on its mix but for a good mix you're looking at approx 2400kgs for 1cu metre in air. But, water weighs 1000kgs per cubic meter which the concrete block will displace. So, even if you managed to make and drop your 2400kg block of concrete it would become only 1400kgs when in the water. Not nearly so good.
Realisticly, you're only likely to be able to manhandle about a tenth of that and even then it'll likely take 3 of you to dump it over the side and still you'll only end up with a 140kgs weight on the end of your chain. To put it's weight in perspective, per volume, concrete is lighter than aluminium.

Old engine blocks, etc. are not a good bet unless they're very heavy.
Cast iron is almost twice the weight of concrete so a better bet weight wise. But to put that into perspective, this anvil weighs 150kgs and a couple of self tappers can easily take it's weight.

Also, flat things try to fly in the water when tugged so a disc shape as an anchor weight is not such a good idea.

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