Anchor Cable Scope

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robbieg

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Is there an online resource that quickly shows the amount of cable to be let out when anchoring at particular depths? My own cable is a mixture of chain & warp & I could work it out on the RYA's 4:1 for chain & 6:1 for warp formula but this is a bit simplistic since I understand more warp than the above formula would indicate may be require if anchoring at very shallow depths & less if very deep. Anyone aware of a resource that takes this kind of issue into account?
 
If you search for 'anchoring' on the web you will see a long list of books, some of which can be ordered from your local library. Possibly the biggest reason some people get problems when anchoring in shallow water(to formula) is that the relevant depth is from the stemhead and not from surface level. In shallow water the height of the stemhead above water level is a considerable fraction of the whole and throws the formulae out.

Geordie
 
I second your shallow water comment. Also shallow water usually leaves chain virtually straight subjecting the anchor to snatch loads. Better in my view to put some spring into the set-up in these circumstances (or use a non-plough anchor).

The opposite I am sure is true - deeper water require less unless the boat is subject to larger loads.
 
This is a topic that's been giving me some cause for thought recently.
I think it must have been revised at some time but I can remember a RYA recommendation of 3x depth for chain (hopelessly inadequate) and 5x for warp (please, not in the same anchorage as me).
Anyway I have got as far as some calculations. I assume that the intention is to ensure, assuming a flat sea bed, that the pull on the anchor does not go above horizontal, ie the links nearest the anchor are resting on the sea bed; and that the chain can be represented as a weight acting at its mid-point (any mathematicians who want to do the calculations assuming a catenary are invited to do so). I also assume that even the best designed anchors do not give their best performance when the pull comes above horizontal to yank them out of the sea bed.
Now assume that the depth sounder correctly reads 4 metres and the skipper remembers that it's set to depth below keel (1 m below sea level) and there is 2 m rise of tide to come. So that's 7 metres, and 20 m of chain should be enough? (He's forgotten that the anchor roller is 1 m above sea level.)
The chain weighs about 2.5 kg/metre in air so that's about 2 kg/metre in water.
Now according to my calculations the chain links near the anchor will start to lift when the horizontal component of the force acting on the boat is 30 kg (about 300 newton, if you must). (Might not look quite so grim if you do the calculations assuming a catenary.)
But if you add 10 metres of rope warp this goes up to 90 kg; if 20 m of warp, 150 kg.
If you have 30 m of chain, then 110 kg; with an added 10 or 20 m of warp, 170 or 250 kg.
And with 40 m of chain, with 0/10/20 m of warp, 180/290/390 kg (assuming the anchor stays put!).
I must admit these figures gave me a bit of a shock. It certainly shows the importance of scope. Also, it doesn't matter much whether the last few metres near the boat are rope or chain.
Only problem now is how to fit into the typical Solent anchorage when the boat is swinging in a circle of (40m chain + 20 m warp +10 m boat) radius.
 
I suggest you adopt the simplest calculation which approximates to the realities of the science. I found that to be a scope of twice the depth expected at HW, plus 20 metres. It is simple to remember and use and comes close enough to theoretical formulae. Using a rode of 27m chain and 60m warp, it has served me well for over a thousand anchor deployments in all depths from 4m to 30m.
 
2 very interesting ideas there.

simonfraser97's is a new one I've not heard before. Is that wind force as in Force 5 or, say, 20kts of force?. Working on it being the Beaufort Scale 'force' I think you may have something. Doing a quick run through of some options I do like it. Nice.

Blackbeard is a thinker, good to see, but your numbers are a bit light regarding loads so if anything you're on the safer side I'd think. That swing room is an issue to watch though.

The basic rule of thumb is generally regarded as 3 times depth (seabed to bow roller) on all chain and 5 times on rope/chain. This is in nice to average weather. As the weather (wind and/or waves) picks up let more out. If in doubt let more out. More is always better than less, swing room allowing. This is a sound but basic 'rule of thumb'.

Of course the rode will have a baring on this. Many are on light gear which means you need more out than you would heavy gear.

Just remember if in doubt let more out, just check behind you first.
 
On all chain I use 5 x depth From bow roller or 30m, whichever is more. I adjust my technique depending on the weather and other circumstances such as wind direction, proximity to danger and other boats, type of bottom, swinging room. There is some anchoring information on my website under Cruising Resources which might be of interest.
 
A wise sailor once said "Put it all out - it's no use in the locker". One may need to temper this advice in Newtown on a bank holiday, but the general idea is good. Having said that, I suspect that more people get problems by dropping it all in a heap rather than through not having enough scope.
 
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I found that to be a scope of twice the depth expected at HW, plus 20 metres. It is simple to remember and use and comes close enough to theoretical formulae.

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The RYA has suggested several different formulae over the years. I reckon that the best one, making allowance for the curve of the catenary, was twelve times the square root of the depth (in metres). The problem was that most people can't quickly do square roots in their heads. A very close approximation, though, is given by three times the depth plus 10 metres. Comparing the scopes, we get:

Depth (D) ........ 2 ...... 4 ....... 6 ....... 8 ...... 10 ..... 15 ..... 20 ..... 25 .... 30

12xSQRT(D)..... 17 ..... 24 ..... 29 ..... 34 ..... 38 ..... 47 ..... 54 ..... 60 ..... 66
3xD + 10 ......... 16 ..... 22 ..... 28 ..... 34 ..... 40 ..... 55 ..... 70 ..... 85 .... 100
2xD + 20 ......... 24 ..... 28 ..... 32 ..... 36 ..... 40 ..... 50 ..... 60 ..... 70 ..... 80

As you can see, 3D+10 is a pretty good representation for depths up to about 10m, but lays out too much above 10. 2D+20 lays out too much in shallow water, but is reasonable from about 6m to 15m or so. I'm an East Coast man, and use 3D+10. If I were on the South Coast or in Scotland, I might remember 2D+20. Either way, the increase in scope/depth ratio for shallow water anchoring is significant.

Sorry about the dots; seems to be the easiest way to get an approximation to a table.

Incidentally, the parabola given by the square root law is a very good approximation to a shallow catenary such as might be found in a strong tide/wind.
 
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The RYA has suggested several different formulae over the years. I reckon that the best one, making allowance for the curve of the catenary, was twelve times the square root of the depth (in metres).

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You just know what I'm going to say here ...
Depth (to anchor roller, at high tide) 169 m (OK it's a very deep water anchorage).
Square root of 169 is 13.
12 x 13 is 156.
Anchor is thus 13 m off the bottom ...
Seems to me that 3x depth plus 10 metres is a better bet (although that might have the opposite effect in very deep water of specifying more scope than you need), remember that it doesn't matter if the last few metres is rope, and let some more out if it really blows ...
as another poster observed, it does no good while in the locker.
and, of course, don't do as I do - do as I say ...
and, finally (well, maybe finally) ... as a result of this thread, I'll be giving a little more scope than I used to. May even put some more chain in the locker (8mm which some would say is OTT on a 25' boat but I don't think so).
 
You're right, of course, in saying that the RYA formula doesn't work at large depths. I reckon that it's OK up to about 30m, but I wouldn't use it past that.

If you are anchoring in really deep water then the bit of cable near the bow is virtually vertical. If it gets deeper, then you only need to lay out extra cable just a little more than the increase in depth.

If you are using a mix of rope and chain, then you need to be aware of chafe of rope on the bottom. If you limit the rope part to just a little less than the depth of the water, then you can't get any chafe. Only works in deep water, though.
 
Hi, many moons ago when I did my day skipper the RYA recommendation was 12 x sq root of the depth at high water for chain and 20 times for rope (in metres). This does address the problem of shallow and deep water anchoring rather better than the simple ratio method. As you will know how much chain you have you can note the maximum depth that the chain will allow you to anchor and calculate the amount of rope required for any extra depth. It works for me.
 
A small contribution, but on at least two occasions, an important and winning one....

[ QUOTE ]
Halfway to the big tidal-gate headland of Portland Bill, we sailed into a persistent ‘windhole’ and, as the tide turned foul, down went our kedge anchor - in 225 feet of water. That’s got to be a record! This old racing tactic of deep-water kedging was used successfully in the ‘30s by RORC legend Adlard Coles in ‘Cohoe’ and has largely been forgotten - but not by crafty and well-prepared crews! Before long, ‘Molly’ and some others re-appeared, with bare steerage way, sailing past us again – backwards - into the gathering murk. This was where she lost her race.....

An expected light breeze slowly filled in from the north-west and, after hauling back on board nearly 500 feet of line, chain and anchor, we were away again, under spi and full main.......

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The 'line', carried deliberately for this specific purpose, was 5mm Kevlar with a polyprop sheath, as used by a telecomms company for cable-pulling - and recycled. A race-winning ploy by both Crafty Old Adlard and the author of the above!

/forums/images/graemlins/wink.gif /forums/images/graemlins/wink.gif
 
[ QUOTE ]
The 'line', carried deliberately for this specific purpose, was 5mm Kevlar with a polyprop sheath, as used by a telecomms company for cable-pulling - and recycled.

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I've got some of that line, as well. I think the sheathing is polythene, though, not polyprop, so that the friction in a duct is lower. Problem was to find a knot which (a) held with the slippery sheath, and (b) didn't weaken it too much. In the end we found that a hard thimble in a hangman's noose gave the least strength reduction.
 
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