Scope for anchoring in deep water?

MM5AHO

MM5AHO

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In Loch Fyne for the Scottish Series last weekend got me wondering about anchoring in very deep water.
It's hard enough getting a race mark to stay still there in waters of 100-150m deep.

But what about anchoring a boat? Anyone tried this, and what did you do / use / try ?
Rode of chain (be a big weight to carry)? mixed, all rope? scope you used? Retrieval system?
 
I only do this in one location [ Roseau Dominica ] where all the shallow water has been taken up by moorings. I am forced to anchor in 200 ft.

I use 210 ft of 3/8th chain and then about 150 ft of rope rode. This is only an overnight stop and if I were to drag it is out into open waters. I am glad I have a good electric windlass when I have to recover the anchor.
 
I remember having to pull through some spare halyards on a large raceboat to extend the anchor rode so we could stay put in very deep water when the tide turned against us and the wind disappeared.
I think we had 50 feet of chain and 600 feet of mixed rode to hold us in position in 250 feet of water. It worked and a race crew of 15 didn't seem to have a problem recovering it.

But I remember one day when it was blowing hard at the Scottish Series, the OOD radioed and asked the chartered fishing boat laying the marks, if they could lift and move the windward mark over 20 yards or so. I don't speak fluent Glaswegian, but it was fairly obvious what they thought the race committee could do before they moved that mark, even with a huge mechanical winch and three man crew.
 
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THEORETICALLY, you need less scope in relation to the depth in deep water than you do in shallow. This is because the catenary curve has almost all of its curve near the seabed, where you need it to ensure an almost horizontal pull on the anchor. The curve of an anchor chain is ALWAYS a catenary, no matter how close to a straight line it looks. Just imagining it, I'd be willing to try anchoring in 100+ me of water with about 150 m of rode; that should be enough to ensure the pull at the seabed is close to horizontal.
 
I assume that you only want to anchor to avoid being carried backwards by the tide, and so only when the wind is very light. In that case, although it's quite true that the formula for scope as a function of depth isn't linear, you don't really have to worry over much about scope, and besides you will mostly be on rope.

We anchored very sucessfully in 75m of water half way between the Isle of Man and the Mull of Galoway, and again in the sound of Jura during a 3-peaks race (probaby the best 6 hours for us in terms of our position in the fleet - all the J-boats drifted backwards past us). We uses an aluminium kedge (a Fortress), 10m of 10mm chain and 130m of nylon warp. It was easy enough to recover using the windlass (we have both a chain and a rope part to it), but actually the weight was so little, <30kg, that one person could easily lift it.
 
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Yep done it mid channel, short length of chain and mostly rope not sure how deep but it needed sheets,guys and spare rope tied together. We were racing and wind had dropped so it held us against the tide until the wind filled in.
 
I cannot compete with the racing sailors but I have anchored in 20 metres with 50 metres of chain in gale force winds for a couple of nights. We held well even though when we recovered the Delta anchor it had a polythene bag impaled upon the point.
 
We carry 220m of lorry rope for kedging. We have used it in pretty deep water and several knots of tide.
It would be a scary swinging radius?
 
My chances of being presented with this problem on the East Coast are rather slim, but it has got me thinking about how much warp I could muster if needed. I have 40m of 3/8 chain on my 45lb CQR (don't say it). To that I could add 2x50m of warp kept for when necessary, + about 80m of mooring lines, making, if I am not to far gone, about 220m, so allowing me to anchor in up to 150m of water, say. I think that I'll just motor round in circles.
 
If the cut and paste maths is right then this graph should be scope on the y axes against water depth on the x axis for a 250Kg force to just lift the last link of 10mm chain off the sea bed.

https://www.desmos.com/calculator/fj6alxpxyn

xjmdhBQ.png
 
TQA said:
...Roseau...forced to anchor in 200 ft.

I use 210 ft of 3/8th chain and then about 150 ft of rope rode...glad I have a good electric windlass when I have to recover...
Click to expand...

I'm not sure mine'd dead lift the whole chain and anchor too. Our worst/deepest was the little island above Lanzarotte (can't remember the name) but it was black as a coal-miners bum and blowing 30+ knots when we arrived; half the yachts anchored weren't showing lights, so I dropped 70m of chain into 28m of water out the back, dug it in with the engine - not sure how much good that did with that depth/chain-length, then went blissfully off to sleep. No worries, there was no chance that Lesley was going to sleep in those conditions, so I might as well do!
 
GHA said:
If the cut and paste maths is right then this graph should be scope on the y axes against water depth on the x axis for a 250Kg force to just lift the last link of 10mm chain off the sea bed.
Click to expand...


Your graph shows scope required decreasing with increasing depth - can't be right!

The formula is Scope = sqrt(y^2 + 2 * lambda * y) where y is depth and lambda is force / weight of chain per metre so a typical value might be 100 (200kg / 2 kg/m) So in 50m you'd need sqrt(2500 + 10000) = 111m. You most likely won't have this much! So I think that appeal to simple catenary is not useful in very deep water (by which I mean >30m). Instead you have to allow for mixed chain - rope rode, and allow some angle at the anchor - they can take it after all, some more than others.
 
jdc said:
Your graph shows scope required decreasing with increasing depth - can't be right!
Click to expand...
Tis right , say you have 4m chain out in 1m water, the (horizontal) force is much less to lift the chain off the ground than 40m chain in 10m water. Greater the depth, less scope is required for the same force to lift the last chain link off the sea bed.


The formula is Scope = sqrt(y^2 + 2 * lambda * y) where y is depth and lambda is force / weight of chain per metre so a typical value might be 100 (200kg / 2 kg/m) So in 50m you'd need sqrt(2500 + 10000) = 111m.
Click to expand...

How do you get scope as metres? Scope is a ratio.

Ah, understand now - you are looking at length of chain, not scope. Turn your formula into scope and the graphs match. :cool:

pjmmixy.png
 
GHA said:
...Scope is a ratio...
Click to expand...

OK, I see wht you did now: glad we agree (we normally do on this sort of thing).

I'm not sure that scope is a ratio mind, there seem to be two definitions, one ratio and one defining it as length of rode. Personnaly I discourage the concept of ratio as it implies that there should be a linear relationship where in many matters the relationship is a power law (with the power not equal to 1).

Btw, you can use http://www.awelina.co.uk/anchor_rode/rode_length_graph_only.html to plot rode length vs depth for arbitrary mixes of chain and rope, and for a seecified angle above horizontal at the anchor. See (and let me know) what you think.
 
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Yes, I was surprised also. I think it may to some extent exlain the superior performance of latest generation anchors. If one insists on the chain being always being horizontal at the anchor you have to use rather a large rode length, and empirical evidence says that these lengths are neither necessary nor used in practice! I've rather chnged my view on teh subject over the last couple of years since I added this extra factor to my graph.
 
Our kedging setup used to be a Fortress, 10m chain, 150m of rope. Successful in at least 100m depth. Incidentally some chain is pretty essential with the Fortress, just to make sure it finds its way to the bottom rather than fly like a kite in the stronger tide under the boat.
 
jdc said:
I'm not sure that scope is a ratio mind, there seem to be two definitions, one ratio and one defining it as length of rode. Personnaly I discourage the concept of ratio as it implies that there should be a linear relationship where in many matters the relationship is a power law (with the power not equal to 1).
Click to expand...

That's confused me. :confused:

How can the relationship we refer to as "scope" be defined as a length of rode? Surely the length of rode doesn't mean anything at all unless it is expressed as a length of rode relative to the depth of water?

Richard

Having now seen Macd's post I realise that your definition of "scope" as the "length of rode" is a more fundamental re-definition of the word "scope" than I was expecting.
 
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GHA said:
How do you get scope as metres? Scope is a ratio.
Click to expand...

He got scope as metres, as did I, because your 'y' axis was labelled ambiguously. Scope may indeed be considered as a ratio, but it can equally be expressed as a distance. If, prior to dropping your hook you inquire of an anchored boat what scope it has out, I suspect you'll get (and possibly expect) an answer in absolute units.

But mainly, thanks for the graph. Interesting.
 
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