Anchor rode length calcs

vyv_cox

vyv_cox

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Now retired, anchor swallowed.
coxeng.co.uk
Can someone please point me to a calculation method for chain rode length/ depth of water. I'm not looking for 3:1, 4:1, 5:1 etc, but one that I have seen on this forum a few times that I think depends on the square (square root?) of the depth, rather than depth itself, to increase proportionately for shallower water.

Links to various spreadsheet and graphical methods not required, thanks, I reckon I have them all.

Thanks for all replies.
 
I work on three times the depth of water, but I have no means of measuring, so I look for a 45% angle off the bow. Then watch the plotter to see if I am moving. If you only have 100 feet in the locker then do not try anchoring in 100 feet. IT WILL NOT WORK.
 
omega2 said:
I work on three times the depth of water, but I have no means of measuring, so I look for a 45% angle off the bow. Then watch the plotter to see if I am moving. If you only have 100 feet in the locker then do not try anchoring in 100 feet. IT WILL NOT WORK.
Click to expand...

Small plastic cable ties are good at 10 m intervals.
 
We never use less than 15m of chain however shallow the water, however light the wind and however little current. I would rather do that than get out a calculator and then allow for the bit between the roller and the water and remember if the cable tie was meant to be in the roller or on the surface.

We have no marks at all in the chain until 15m. If it is that tight with other boats or a steep side I would be nervous about anchoring there at all.
 
vyv_cox said:
Can someone please point me to a calculation method for chain rode length/ depth of water. I'm not looking for 3:1, 4:1, 5:1 etc, but one that I have seen on this forum a few times that I think depends on the square (square root?) of the depth, rather than depth itself, to increase proportionately for shallower water.

Links to various spreadsheet and graphical methods not required, thanks, I reckon I have them all.

Thanks for all replies.
Click to expand...
IIRC it's 12x the square root of the depth for all chain rode, and 20x for a chain/rope combination.

This gives more scope than a linear formula in shallow water, and correspondingly less in deep water.
 
vyv_cox said:
Can someone please point me to a calculation method for chain rode length/ depth of water. I'm not looking for 3:1, 4:1, 5:1 etc, but one that I have seen on this forum a few times that I think depends on the square (square root?) of the depth, rather than depth itself, to increase proportionately for shallower water.

Links to various spreadsheet and graphical methods not required, thanks, I reckon I have them all.

Thanks for all replies.
Click to expand...


some rationale about where the square root of depth comes from can be found here, together with one version of the formula

http://www.anchorwatch.co.uk/index-page04.htm




fwiw, I go depth+freeboard times beaufort strength :smile:
 
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Quote: fwiw, I go depth+freeboard times beaufort strength :smile:
What a brilliantly simple idea. I almost always put out more, but that is neat! Thanks!

Thanks to the last 2 posters for their formulae. I had been 'lurking' on this thread with interest. That helps.

Do the formulae change with either a.) a bower, genuine 25lb cqr that does work, plus 30m chain and miles of anchorplait, c60m, - versus b.)a kedge that is a genuine bruce c.15lb, plus miles of anchorplait, c40m. I try to anchor no deeper than 10m usually less, and I have a high windage Heavenly Twin.?

Thanks. Mike
 
r_h said:
IIRC it's 12x the square root of the depth for all chain rode, and 20x for a chain/rope combination.

This gives more scope than a linear formula in shallow water, and correspondingly less in deep water.
Click to expand...

I avoid these formulae because they are not scale-independent. Anchored in ten metres? That's 38 metres of chain. Anchored in five fathoms? That's 27 fathoms of chain - 40% more.

My personal rule is simple: three times depth for chain, unless it's blowing up in which case every last foot of chain goes out.
 
OK but . . .

My personal rule is simple: three times depth for chain, unless it's blowing up in which case every last foot of chain goes out.
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3 times depth AT HIGH WATER taking into account any echo sounder offset plus the height of the bow roller from the water. If it isn't going to blow.

Fair enough, but why keep any chain in the locker if you have swinging room?

- W
 
How much chain to use

Very simple - always aim to have 2 X length of boat lying on the bottom (regardless of depth) plus (obviously) length equal to max depth at high tide. I have 11m boat and therefore typically use 35 metres in almost all conditions. Remembering that of course chain left in the anchor locker does no good at all!

Footnote: the 3x or 6x formula is basically flawed as it does not take account of shallow or deep water. For an anchor to work it needs a good long horizontal pull and a decent catenary - difficult in shallower water unless you use some kind of 'angel' (or disproportionately more chain)
 
vyv_cox said:
Can someone please point me to a calculation method for chain rode length/ depth of water. I'm not looking for 3:1, 4:1, 5:1 etc, but one that I have seen on this forum a few times that I think depends on the square (square root?) of the depth, rather than depth itself, to increase proportionately for shallower water.

Links to various spreadsheet and graphical methods not required, thanks, I reckon I have them all.

Thanks for all replies.
Click to expand...
....................................................................................

Think the answer is, there is not one, think you have to be a nerd to ask for one. There is no equation that can answer the question, except more chain is better than less. I never have a problem anchoring, but then I never asked stupid questions. I normally anchor in about 5/6 meters with 30 meters out. I dont try to put out the critical minimum, what ever that might be. If over night, I might well let out all 60 meters. It's like how much hand brake pressure to be applied to a car.
 
Anchor rode length

It is all about the angle of pull on the anchor when the boat is pulling as hard as possible.ie rode straight.
So if you draw a 2 dimensional diagram you will see that at rode twice as long as depth you get a 45 degree angle (very bad). If you have the rode 3 times the depth the angle changes dramatically to something less than 30 degrees and again if you go for 4 or 5 times the depth the angle decreases but at less reduction of angle as you increase rode length. So the real improvement value is in the 3 to 5 times range.
In practice it is the chain weight that also improves the angle at the anchor. If the boat is not tugging so hard then the chain lays on the bottom and the angle becomes zero. The more chain on the bottom the harder the boat can tug without lifting all the chain off the bottom.

In the end it is a question of how hard the boat tugs and how worried you are about drift.

(NB not a single mention of anchor type) olewill
 
I wonder if this is what you are looking for

vyv_cox said:
Can someone please point me to a calculation method for chain rode length/ depth of water. I'm not looking for 3:1, 4:1, 5:1 etc, but one that I have seen on this forum a few times that I think depends on the square (square root?) of the depth, rather than depth itself, to increase proportionately for shallower water.

Links to various spreadsheet and graphical methods not required, thanks, I reckon I have them all.

Thanks for all replies.
Click to expand...

There are several simple square root of depth formulae around that ignore snatching, but this printed in YM and PBO several times might just be what you are looking for:

http://www.anchorwatch.co.uk/index-page04.htm
 
Thanks for all replies, both helpful and arrogant ones. Contrary to the latter, I am neither a nerd nor inexperienced, anchoring for most of six months every year. The purpose of the question was research for a mag article, trying to find a method that takes into account the increased tendency for chain to lift from the bottom in lower wind speeds when using a linear method.
 
vyv_cox said:
Thanks for all replies, both helpful and arrogant ones. Contrary to the latter, I am neither a nerd nor inexperienced, anchoring for most of six months every year. The purpose of the question was research for a mag article, trying to find a method that takes into account the increased tendency for chain to lift from the bottom in lower wind speeds when using a linear method.
Click to expand...

FWIW, I tried applying the "anchorwatch" method I posted above to my boat, while it gives a theoretically satisfying scope decreasing with increasing depth (that's where the square root of depth comes into play), it also says that I should use a scope of only 2 while anchoring in 14m of water with 15kt wind (30m of chain needed for borderline catenary)... which appears very low indeed, maybe too low?

More funnily, the formula states for example that with a 5knot wind I should use 8m of chain with 10m depth...

Though the "anchorwatch" approach seems very sensible to me, I guess there might be some flaws ?
 
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Vyv,

I'm sure you've done your research and found Alain Fraisses (SP?) anchoring sums. Paraphrased very heavily, his point is simple. The biggest rode stresses occur when there's enough wind to cause a vessel to swing from side to side. These tacking stresses on the rode are then so high that a chain will temporarily lie with no significant catenary, and you can assume a straight line from stem to anchor shackle. If that angle is more than a certain critical angle (which varies between anchor types from 10 to 15 degrees) the anchor will work out of the ground.

Those considerations bypass wind strengths, catenary sums and square roots . . . and shift the emphasis to nice long springy nylon snubbers . . . but I bet you knew that already.
 
I subscribe to the view that a standard length should be laid on the sea bed and then add the depth of water. That length will depend on conditions and the nature of the bottom. If there is plenty of swinging room why not use your chain. It's no good in the locker!
 
Have just produced calculations for my boat using a spreadsheet using the Anchorwatch formula for a number of depths and wind speeds. Didn't get Roberto's problem at shallow depth and have double checked both my spreadsheet and the Anchorwatch algebra both of which are fine.

Flaws I can see are:

What about the force of the a tidal currant?
What about a sustained yaw on the wind caused by a tidal current working against the wind and thereby increasing the windage of the boat?
What about the holding?

All represent potential risks which can only be obviated by veering more cable.

The advantages of putting out the minimum required amount of scope are that your boat will swing less and you will have less cable to recover, either chain or rope. I have an angel and will factor it into the calculations once I have weighed it in order to understand its utility.

Whatever, if in doubt veer more cable and take some bearings or note some transits.

Really useful and interesting thread, thank you vyv.
 
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