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Old 27 August 2011, 13:45   #1
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Anchor for 7.5 m RIB

Can anyone assist and advise on the following for a 7.5 M RIB

1) Anchor Size (4 /6 / 8 Kg)
2) Chain size (6 /8 / 10 mm)
3) Length of chain

all comments gratefully received by a new RIB owner

many thanks in advance
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Old 27 August 2011, 14:16   #2
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6kg & 8m of 8mm chain should do it
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Old 27 August 2011, 16:37   #3
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I think 8mm chain is bit on the heavy side, its what I use on a 5 tonne yacht.

However, worth having a look at the various manufacturers websites.

Fortress Marine Anchors

Fortress anchors aren't cheap but they are light weight and don't need a full chain rode cutting down on weight again, key to performance in a rib particularly if it's all kept in the bows.

Pete
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Old 29 August 2011, 04:36   #4
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From experience don't underestimate what you may need, consider where you might want to anchor and also an anchor is a good bit of safety kit. Better too much line and chain than not enough. I'm sure you've seen boats dragging their anchors through not enough chain.

I was watching a lovely scorpion rib of about 9m at the air show trying to anchor without success as there was about half a metre swell, they only had about 6foot of chain. I had no problems.

I have a 7.5m rib with I think a 5kg anchor will check, it has about 6m of 8mm chain and 25m of rope. I usually only use about third of that rope in most situations. The combination works fine and I have plenty of rope in case I need to anchor in deeper waters in an emergency. Never drags.

It's stored in the bow sitting in a plastic gardening type of basket like you can get from b and q. I drilled a hole in each side of the plastic basket and use an elasticated cord through the holes and over the anchor to keep it in place as you don't want it banging around your bow when on the move.
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Old 29 August 2011, 04:57   #5
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Here's a question for anchoring experts. For a given weight of chain, which is best - a longer length of lighter chain (say 6mm) or a shorter length of heavier chain (say 8mm)? Assuming the same anchor, and the rest of the line being nylon rope.

Seawolf is a bit nose heavy (largely due to the fat b*(*% in the front seat) and the anchor and chain is one thing I might try and save some weight on at some point in the future - I might get a Fortress and some lighter chain but wondered how much difference it would make if say I got lighter chain but an extra metre or so. I think I've got a 7kg anchor and 6m of 8mm at the moment, overkill being a good thing and all that, it's mostly for if the fan stops because Africa is a fair distance.
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Old 29 August 2011, 05:27   #6
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Longer length of lighter chain for me. The important bit is the ratio of anchor rode length to depth. 4:1 is okay for the picnic ashore in good calm weather when you can see the boat. 5:1 getting better for an over night stop and greater if you can.

Don't forget the 1m from sea level to the top of your tubes if you must have the anchor warp going over the tube but not good practice, use a bow strong point instead.

This is quite a good article:

Fortress Anchors - Safe Anchoring Guide

Another excellent article on catenary:

Catenary & Scope In Anchor Rode: Anchor Systems For Small Boats

What ever anchor you choose, I would suggest avoiding pattern copies. The patent for the likes of the Delta has now expired so Chinese copies are appearing in chandlers at half the price, but the quality of the galvanising and welding is dreadful. CQR and Bruce are another two which are copied.

Some of the better makes, Delta, Fortress, Spade & Manson Supreme. Rocna was good but the quality of the steel is now suspect following the move of production to China and a cost cutting exercise.

Modern high holding anchors are excellent but expensive, however so would be a rib if the anchor dragged and she ended up on the rocks.

If weight and size are a problem, then the Fortress which strips right down and can be assembled in a minute has to be a good choice for a rib. Made from Aluminium and recommended for a chain / rope rode rather than all chain helping to keep the weight down.

Pete
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Old 29 August 2011, 07:27   #7
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You could go with the lighter chain in the locker but carry something aft that could be clipped to the chain as an "angel". Best of both worlds!
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Old 29 August 2011, 07:53   #8
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Quote:
Originally Posted by willk View Post
You could go with the lighter chain in the locker but carry something aft that could be clipped to the chain as an "angel". Best of both worlds!
I've got a bit of an odd setup now-a Danforth that's by norrmal standards slightly undersized for the boat (but fits in my small anchor locker), a 5m length of 6mm chain plus another 2m of 6mm chain and a 2kg grapnel that makes quite a handy kedge or angel-all with stainless shackles to hold it together plus about 80m of line in 2 lengths.

It becomes a bit 'mix and match', but I've yet to find a situation it won't cope with.
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Old 29 August 2011, 13:58   #9
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On our 7.5 metre rib we use an Alluminium "Spade" A60 anchor which is the same physical size as their 10kg Galvanised steel anchor, but is made in alluminium so only weighs 4.5kgs. Its rigged with 7-8 metres of 8mm galvanised chain and 100 metres of 12mm Anchorplait rope. Overkill at 100 metres of rope, but would like to catch the bottom if i break down at sea! (Hope i never need it)

I was advised to use the length of the boat in chain aprox.
Hope this helps
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