Babylon
Well-Known Member
I have 30m of chain (8mm) plus 50m of rode (16mm if I recall correctly), a total cable of 80m. Will this be sufficient scope for a month's cruise on the N Brittany coast?
How much anchor cable for North Brittany?
- Thread starter Babylon
- Start date
Ex-SolentBoy
Well-Known Member
John Barry
Well-Known Member
Assuming you're about 10m long (otherwise 8mm chain is a bit light), you'll be OK in 25Kn of wind up to about 10m of water, or 40Kn of wind in 5m of water.
If you're prepared to accept angulation of course the numbers go up a bit.
If you're prepared to accept angulation of course the numbers go up a bit.
sailorman
Well-Known Member
this summer i took
15Kg Bruce on 60m of 8m/m chain with 50 m of anchorplait spliced to it.
the weather was carp i used it once, just above Shotley on the Orwell in 5m of water,after the return trip back from Boulogne.
now all removed & back to my East Coast rig
10Kg Bruce with 20 x 8m/m chain + 30m anchorplait
doug748
Well-Known Member
Should be no problem.
I have got by with 45mts of chain on a similar boat. I generally scuttle to somewhere more secure/comfortable if the weather is rotten.
I have got by with 45mts of chain on a similar boat. I generally scuttle to somewhere more secure/comfortable if the weather is rotten.
Roberto
Well-Known Member
In the worst places you have maybe 11-12m of tide at springs, add 3m of minimum depth that is around 15m total depth at HW
Your scope will be around 5 which would be ok in most cases, supposing springs and that you anchor in a charted depth of say 3m.
It might be a problem at slack low water, 80m in 3m is more than 25 scope
and the boat risk to go all over the place trying to make company with other boats. FWIW my boat had 80m and cruised North Brittany without too many problems
Babylon
Well-Known Member
In the worst places you have maybe 11-12m of tide at springs, add 3m of minimum depth that is around 15m total depth at HW
Your scope will be around 5 which would be ok in most cases, supposing springs and that you anchor in a charted depth of say 3m.
It might be a problem at slack low water, 80m in 3m is more than 25 scope
FWIW my boat had 80m and cruised North Brittany without too many problems
Thanks for that Rob.
HMS Vancouver 27
heavy old bird. 25lb CQR - only ever dragged once, and that was on rocks![70521]
Well-Known Member
How deep are you planning to anchor in?
Quite sure we use the same gear both sides of la Manche.
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John Barry
Well-Known Member
HMS Vancouver 27
With an 8m boat you have quite a tough rode, which will give 0° of angulation with:
Depth Wind
20m 25Kn
15m 30Kn
10m 35Kn
5m 50Kn
Mind you, 50Kn will give about 500kgf of horizontal force (and similar tension)which, if nothing else, will be hard to pull back in.
With a CQR you should ensure that there's no angulation.
Do you not have a table that gives rode length for depth/wind strength for your anchoring equipment?
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Babylon
Well-Known Member
I have no table.
By 'angulation' I assume you mean horizontal angle as the boat veers around? How can one ensure 'no angulation' whatever anchor one is using?
Why do CQR anchors differ to others?
[Dons a Noddy suit and disappears into a deep underground chamber...]John Barry
Well-Known Member
I have no table.
By 'angulation' I assume you mean horizontal angle as the boat veers around? How can one ensure 'no angulation' whatever anchor one is using?
Why do CQR anchors differ to others?
Angulation is the angle between the rode and the sea bed at the anchor. With a CQR it should be 0° because CQRs start to trip (and before that the holding power reduces considerably) at low anglulations. Modern 'new generation' anchors can accept more angulation, 10° or more is not uncommon.
Angulation is a function of the weight and length of the rode and the horizontal force applied.
The horizontal force is proportional to the boats cross section and the windspeed.
The boat's cross section is normally proportional to the length, depending on boat type.
PM if you want a table.
Poignard
Well-Known Member
From memory, I think St Malo has a tidal range of up to 12 metres. Five times that plus allowance for the distance from water line to the chain locker comes to 65 metres, which is the amount of chain that I carry. So your 80 metres should ensure you have peace of mind.
Babylon
Well-Known Member
Thanks - I have a simple mind, so I like simple solutions!prv
Well-Known Member
vyv_cox
Well-Known Member
Having swum to my anchor many times in fresh to strong winds I disagree with these figures. In Cala Volpe on the east side of Sardinia we anchored for several days in about 2 metres total depth with 35 metres of chain out. Wind speed varied during the period from around 25 to 42 knots. On very many occasions the chain was bar tight, no catenary whatsoever and none of it touching the bottom. In Galixidhi in the Gulf of Corinth we anchored in slightly more water, about 2.5 metres, with around 30 metres of chain, with similar observations. With a mixed rode I would expect this to occur at lower wind speeds. Delta on the first occasion, Rocna the second. All chain, total carried 60 metres.
To the OP - those who point out that the range of the tide is the important factor are correct. With your rode composition you need a scope of about 4 or 5:1 for the maximum depth you will experience. In areas with a big range and stronger currents you may need to be concerned about the rope part snagging your prop or even your keel when the tide turns. This is one of the few occasions where you may find an angel to be useful, holding the rope on the bottom.
John Barry
Well-Known Member
Quite happy to justify the figures with details of the various forces involved - vertical, tensional and horizontal. They're based on a standard shaped 8m monohull, 30m of 8mm chain and 50m of warp, with up to 30° of yaw, and do not include snatching forces.
Perhaps yours is a larger boat, or different chain? If you tell me the size of your boat and anchoring gear I'm happy to give you equivalent figures to see if they match your experience.
'no catenary whatsoever' is an interesting comment. Perhaps your chain was 'bar tight', but it would have been in a curve unless the chain was made of a very light metal
A mixed rode, with 30m of chain as here, may make less difference than you think.
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John Barry
Well-Known Member
Just noticed a typo above that I've now corrected.
It originally said
15m at 20Kn.
It should have been
15m at 30Kn
Sorry about that...
It originally said
15m at 20Kn.
It should have been
15m at 30Kn
Sorry about that...
John Barry
Well-Known Member
Based on the data you've given and assuming 60m of 8mm chain on a 10m boat.
In 3m of water (the extra 1m is to the stemhead) 29½kn of wind will just lift 35m of chain.
With another ½m of water, 30m of chain will be just lifted by 23½Kn of wind.
With 42Kn as you stated, you'd get 2½° of angulation in the first case, and 4½° in the second. Both of these may appear 'bar tight' - there's about 500kgf of horizontal force for 42Kn on a 10m monohull.
vyv_cox
Well-Known Member
Quite happy to justify the figures with details of the various forces involved - vertical, tensional and horizontal. They're based on a standard shaped 8m monohull, 30m of 8mm chain and 50m of warp, with up to 30° of yaw, and do not include snatching forces.
Perhaps yours is a larger boat, or different chain? If you tell me the size of your boat and anchoring gear I'm happy to give you equivalent figures to see if they match your experience.
'no catenary whatsoever' is an interesting comment. Perhaps your chain was 'bar tight', but it would have been in a curve unless the chain was made of a very light metal
A mixed rode, with 30m of chain as here, may make less difference than you think.
My boat is 10.6 metres length, 8 mm chain.
You are by no means the first to post theoretical information here, and many of us are familiar with the work of Alain Fraysse, and his website. 'No catenary' is my observation, I can believe that there was a small curve but sighting along it both from the anchor and the bow roller provided little evidence of it. Craig Smith showed photographs of his father's boat anchored in very strong winds in Patagonia, with 100 metres of apparently straight chain. Most people who have researched the topic agree that the contribution of catenary is effectively negligible at more than around Force 6 - 7.
John Barry
Well-Known Member
A 10.6m boat will give results slightly worse than those I quoted above for a 10m boat, and they seem to match your observation reasonable.
42Kn in 3.5m of water and 30m of chain will give 4.7° of angulation (6.6° would be a straight line).
The catenary effect is progressive. In the above case you get 4.7° at 42Kn, 5.5° at 55Kn and 5.9° at 65Kn (1300kgf).
The problem at higher windspeeds is normally with snatching, which is not so much of an issue for the OP as he had 50m of warp on the end of his rode, and possible wasn't expecting to anchor in F7 and above.