Can I use 8mm chain on a 40ft boat?

dom said:
20x Wow! You must have big anchorages where you sail ;)
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Agree - this is not right. There is a quite splendid set of graphs in Nigel Calder's Cruising Handbook that shows the scope required for various combinations of wind speed and depth of water, so that when you are in 25 metres you don't put out 4x depth ... unless you are expecting some veeeery serious wind. BTW for peace of mind and from what I have seen of 40+ft boats that use 8mm chain ... I will continue to stick to and advocate 10mm.
 
I wouldn't be happy using 8mm chain on a 40ft, 7 ton boat, anchoring on the West Coast of Scotland. The OP has 35 metres of 10mm chain. Surely the simplest thing is to extend that, when needed, with stretchy nylon warp. That way you get the best of both worlds, with little extra expense, and very little added weight.
 
Kelly's Eye

Many people sail lightweight 35' yachts, or smaller, on heroic passages and some round the world. I met a couple once sailing a 24' yawl built in the early 1900's round Australia. They had endured 10m tides and the Great Southern Ocean and to suggest they, and people with 35' yachts on, say, the ARC are like weekend sailors is arrogance and demeaning. These people simply cannot carry 180m of chain. 180m of 8mm chain would be like having 2 crew extra crew members - both standing permanently on the bow. This is unrealistic, especially as you would emphasise an even larger link size. To base advice and comment on 180m of chain is consequently invalid and very misleading if you do not mention the basis of your experience in detail when you post.

Jonathan
 
sarabande said:
some 8mm chain is proof tested to above the BS of 'ordinary' 10mm.

You would need to look at the specs and proof certificate; it's a techy decision.

But if you believe that chain weight on the sea bed is what stops anchors dragging, then staying with 10mm is 'better'.
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In any sort of blow there's no chain on the sea bed.
 
>Sorry if I seemed to suggest you were stark raving mad - my apologies

No need to apologise I was stark raving mad at the time ;-)

>Does the same apply to your 20:1 scope advice? Somewhere around 6:1 might be a little more credible.

Yes, in shallow water most of the chain is lying on the bottom so you only begin to get a catenary as the wind picks up. The weight of the catenary is also initially less than in deep water where you have big heavy curve in the chain hanging off the bow, in shallow water obviously it's a shallow curve but with enough chain out you will reach a similar weight as the chain lifts.
 
We have some very conflicting advice in this thread.

On one hand we have advocates suggesting in shallow anchorages with a normal conditions that we should be using 20:1 Others are promoting a maximum of 5:1 even in extreme conditions.

The truth lies somewhere in between.

Out of interest here is a practical example. This was the forecast for our anchorage last night. Nothing unusual, but this wind strength is perhaps on the cusp of when larger scopes are used:

image.jpg1_zpsrx2crpp0.jpg



There are no other boats in the anchorage. There is unlimited swinging room for the predicted wind direction.

In other directions the swinging room is more limited. With a westerly wind more than 4:1 would see conflict with obstructions. In other directions 7-10:1 would provide adequate clearance.

The depth is 7.4m with 1.4m to the bow roller. The boats rode is 100m all chain (a snubber is always used) The holding is good in mud. The anchor is an oversized new generation anchor that has good performance in the substrate.

What scope would you use?
 
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Before answering.
If there is unlimited space to the northwest, but restricted space to the east, I would move over a bit before anchoring.
Is the wind strength in knots or mph or what?
 
sarabande said:
some 8mm chain is proof tested to above the BS of 'ordinary' 10mm.

You would need to look at the specs and proof certificate; it's a techy decision.

end quote

Breaking Strength of metric chain is 4 x that of the WLL and Proof testing is twice that of WLL. In terms of short link galvanised, there are three qualities, G30, G40 and G70. The closest quality to your statement would be G70 but even it would only fit your statement for the same sized 8mm chain. However as Vyv points out actual G30 is near G40 quality (and can exceed it) whereas actual G70 just meets G70 specs.

There are lots of chains, 8mm, that are very much stronger than 10mm ordinary chain - but I think you will find it very difficult to galvanise G100 and G120 chain (and retain strength). As far as I am aware your statement is erroneous :) in terms of short link galvanised anchor chain. Maybe you can elaborate, I for one would like to source the the of product you describe.

Jonathan

edit enthusiasm for G70 is very, very muted. There appear to be lots of issues discouraging sales, its expensive, no shackles have a compatible strength, the galvanising cannot be 're-done', the motivation to move to G70 is to downsize chain size and gypsies, for what they are, are extorionate. Abrasion of the raw chain, once you lose the gal, is an issue if you keep it for long - especially if you are a live aboard. There are question marks over the safety factors used.

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