Anchor scope - why do we teach beginners such rubbish?

[Andrew_Fanner]

So, in essence, it does come back to "let out as much as space allows but at least x"

I think I need to fix the up switch in the windlass, SWMBO refuses to use the lever

 

[PCUK]

3 times the depth at high water regardless of current tidal height and 5 times in a blow has never let me down. Better to give beginners an easy to remember rule of thumb to start them off than a lot of incomprehensible numbers to confuse them.

 

[cliffordpope]

All these calculations ignore the possibility that it is the chain lying on the bottom as much as the anchor itself that provides the grip.

Anyone done any calculations or practical measurements on a chain without an anchor?

 

[vyv_cox]

Better to give beginners an easy to remember rule of thumb to start them off than a lot of incomprehensible numbers to confuse them.

Some of those beginners might be mathematics professors, so probably not confused

We came across one such this year. He knew every rule, theory and expression for every yachting topic you could think of. The classic academic. But absolutely no idea how to manoeuvre a boat, either astern or forward. We watched him several times finishing up in a berth two or three down from where he was aiming. A friend took him out for a sailing lesson, didn't have any sense of how to do it, so decided he would just motor. They anchored, under the tuition of my friend. All seemed to be well. He then went out for his first solo trip (with crew) to try anchoring. Came back to the marina in an hour or so, never got the anchor in.

 

[craigsmith]

All these calculations ignore the possibility that it is the chain lying on the bottom as much as the anchor itself that provides the grip.

They rightly ignored it. You must have missed the point repeated 10x above that any half decent anchor will allow the rode to straighten and lift well off the seabed.

 

[lw395]

All these calculations ignore the possibility that it is the chain lying on the bottom as much as the anchor itself that provides the grip.

Anyone done any calculations or practical measurements on a chain without an anchor?

Well we had a shackle come undone leading to the loss of a small anchor on an inflatable racing mark last year.
In two knots of tide, I think it was doing about 1.8 knots. it had about 10m of chain which would have been on the seabed, plus a 56lb weight off the seabed to keep the buoy upright.

So I don't think the drag of chain amounts to very much! It would probably have more drag in a tangled lump than laid out in a line?

I guess when the anchor buries, the drag of the chain must get significant, othe wise the anchor would just bury deeper and deeper?

 

[Bilgediver]

They rightly ignored it. You must have missed the point repeated 10x above that any half decent anchor will allow the rode to straighten and lift well off the seabed.

I suggest you chat to folk who make real anchors for ships and oil rigs


Yes in shallow water maybe but in deeper water one should always aim for the catenary to absorb most of the loading.

 

[cliffordpope]

"In anchor tests made by the U.S. Navy and reported by R.J. Taylor in conventional test results at San Diego and Indian Island (Washington DC Technical Note No. CEL N-1581 July 1980) it was found that the chain rode could produce up to two-thirds of the total holding power of the Anchor System"


http://www.dulhunty.com/dmp3.htm

 

[jdc]

civil engineering!

The modelling of "inhomogeneous" shallow sediments is actually carried out frequently in the design of stove pipes and conductors of oil wells (on shore and offshore). The resistance of the sediment to shear is measured from cores and there are representative figures for designers to use. I imagine that Civil Engineers also have access to such data for foundation calculations.

...

I was aware of the measurement of cores (an aside, I was in the oil industry as a geophysicist many moons ago; offshore Angola, so it's quite likely that one on this forum (Skipper_Stu?) flew me in his helicopter). There's bound to be a practicing CE on the forum, who can correct me, but I _think_ the reasons that one can use such measurements, and therefore why civil engineering calcs work, are that (i) an historically validated safety factor is applied, and (ii) the sizes of civil engineering or offshore structures are typically very large compared to the inhomogeneities, so at a macro level it is effectively homogenous.

However our yachts' anchors are rather small, of a similar size to some of the inhomogeneities. Thus typical values of shear strength for the type of bottom, or even from core measurements, can not, I think, be used with any confidence. One has instead to consider the weakest shear resistance one could expect in the vertical direction, due for instance to dislocations and boundaries (stones, bands of mussel shells etc). This is unknowable, and so must be assumed to be quite low.

Thus I have taken the view that an anchor will give of its best - especially important if it's rather an old-fashioned one - if it's allowed to exert only low vertical shearing force, the majority of the force being horizontal. This should ensure that if it does creep, the geometry is arranged so that the anchor would immediately reset rather than skip and we'd never even notice.

But this is probably enough from me: I fear I've let my hobby (mathematical modelling that is, sailing's what's important in real life ;-) get a bit out of hand - the precision of the model is reaching diminishing returns; any more and we get into a debate on anchors, which even a theologian might fear!

Besides it's Friday and I'm off sailing this afternoon. Many thanks for all the constructive comments.

 

[Scotty_Tradewind]

I just typed a longish response to this and the bl**** forum co**ed up and lost it upon submission so before I go sulk I'll quickly summarize -

.

Happened to me too a couple of times... learnt now to write up long blogs on 'Word' and then copy/paste onto website reply page

 

[cliffordpope]

Just a final thought. You are anchoring in 10 metres of water with a 2kt current. There is no anchor on the end of the chain (!)

You let out:

a) 30 m of chain. Would it hold ? No.
b) a mile of chain. Would it hold? Yes

At some point between the extremes, chain alone would still hold. (100m ?) So there must be a point at which the chain is making a significant, or even major, contribution to the more usual set up of a chain plus anchor.

 

[Gordonmc]

A bit of Fred Drift perhaps, but I once asked the question on this forum, what has boat weight got to do with anchoring or mooring.
The point is that applying Archemedes, boat weight is neutral on the water, otherwise the boat is a submarine.
I got shot down in flames but later I found Bernard Moittessier's formula for working out the required weight of anchor/mooring and scope. It was all based on the windage of the vessel, complete with a method of calculating windforce on sticks, wires, hull etc.

 

[Scotty_Tradewind]

But can be very useful for the performance of the anchoring system as a whole. From the page you cite:

This is not to say that kellets are entirely pointless; they serve other purposes.

• They can contain and minimize swing radii in light conditions
• They can help dampen “sailing” at anchor
• They can keep a boat stationary when using bow and stern anchors
• They can help ensure that rode is kept down and away from the boat’s keel, rudder, and propeller.

Agreed,but only in 'moderate conditions. What happens in severe conditions when the chain goes tight is really the test of a good anchor.
I used to have a CQR and lots of chain and a skellet.
I practiced in different situations and sometimes/poss 50%, got a fairly good holding in moderate winds and tides, with the weight of the chain and skellet presumably doing a fair amount of 'assist'. But I never used it with enough wind or tide to ever get the anchor chain dead tight to really test the anchors holding. According to what I read, when the chain goes tight, the skellet and anything beyond a certain length of chain has no beneficial effect, as the angle achieved with longer chain becomes errelevant.
The catenary in moderate conditions with around 5x the depth of chain let out, seemed to give a pleasant dampening effect with the CQR. In stronger winds I set the skellet down the chain but this seemed to add to my problems as the line to it often became twisted on the chain and if I needed to raise everything in unpleasant conditions then it all made matters worse.

What does seem apparent is that if you have a modern styled anchor, of a good size related to your boat, then the chances of it holding when the chain goes tight are far greater than with many of the older styled anchors.

Last week in Symi (Greek Island), my friend and I dropped his 35lb CQR from his 37' AWB in 25knots of wind gusting to 30+. It held after we also dropped a second Danforth anchor on chain and line some 90degrees from the CQR.
All other boats except one in the anchorage dragged many times before nightfall and when we swam and looked at our anchor in the morning, we could see that the two had dragged together and got up behind some boulders where they stayed. Only one boat, which was alongside us, did not drag.
He had a modern 'R****' anchor.

 

[noelex]

J So there must be a point at which the chain is making a significant, or even major, contribution to the more usual set up of a chain plus anchor.

Yes but only in light winds when the anchor should hold anyway. When the wind picks up the chain is no longer on the seabed.
Obviously things would change with a mile of chain, but as it would weigh about 6,000Kg and my neighbours would complain about my swinging circle, I think I will stick with the Rocna.

 

[Poignard]

Just a final thought. You are anchoring in 10 metres of water with a 2kt current. There is no anchor on the end of the chain (!)

You let out:

a) 30 m of chain. Would it hold ? No.
b) a mile of chain. Would it hold? Yes

At some point between the extremes, chain alone would still hold. (100m ?) So there must be a point at which the chain is making a significant, or even major, contribution to the more usual set up of a chain plus anchor.

Good point - of course the resistance of the chain has an appreciable effect.

I dragged 65 metres of 8mm chain (no anchor on the end) into a straight line across a tarmac carpark in order to measure it for marking. I seem to remember there being enough resistance to give me a back-ache next day!

I have also used my chain cable (without anchor) to 'drudge' up the Pontrieux river. Depending how much is paid out and the strength of the current (and wind) you can easily keep the boat bow on to the current and steer her using the rudder.

 

[craigsmith]

"In anchor tests made by the U.S. Navy and reported by R.J. Taylor in conventional test results at San Diego and Indian Island (Washington DC Technical Note No. CEL N-1581 July 1980) it was found that the chain rode could produce up to two-thirds of the total holding power of the Anchor System"
http://www.dulhunty.com/dmp3.htm

I suggest you chat to folk who make real anchors for ships and oil rigs

If you were to read some of material referenced above and elsewhere, you would find that this is perfectly predicted as the factors scale. The ideal way to anchor large ships with relatively small symmetrical anchors, or rigs with either drag embedment or installed anchors subject to unidirectional constant forces in set-ups such as taut leg spread moorings, is very different to the ideal way to anchor a small day sailer. Conflating the two is a major cause of the traditionalist mindset and ignorance that pervades this topic.

The 1980 USN test illustrates nothing more than the anchors tested were small and essentially rubbish (but nonetheless practical and adequate for their application, since you cannot put a 20 tonne Rocna pattern on an aircraft carrier). Without referencing exact sizes it's a meaningless statement anyway. Two thirds, 67%, of what? A mile of chain tied to a brick and it'll be 99.99%. A sensible rode make-up with modern high tensile chain and a modern anchor and it'll be something like 1%. Go figure.

 

[duncan]

"In anchor tests made by the U.S. Navy and reported by R.J. Taylor in conventional test results at San Diego and Indian Island (Washington DC Technical Note No. CEL N-1581 July 1980) it was found that the chain rode could produce up to two-thirds of the total holding power of the Anchor System"


http://www.dulhunty.com/dmp3.htm

another very selective 'quote' that really requires to be taken in context (in this case poor anchor designs such as the CQR ...their words)

they also say

"In bottoms of soft mud however, the chain itself tends to lie on the surface, preventing the anchor from penetrating deeper. When constantly anchoring in soft mud, deeper penetration and higher holding power can be achieved with the use of a wire rope, instead of chain."

and

relating to all chain rodes and winds over 30 knots "With tensions of this order, and with the reduction of weight of chain in seawater, the catenary effect almost disappears."

the only real conclusion (which is what many have argued on here for years) is that scope (7:1) and a well set, well designed anchor are your friends...

 

[Leighb]

A bit of Fred Drift perhaps, but I once asked the question on this forum, what has boat weight got to do with anchoring or mooring.
The point is that applying Archemedes, boat weight is neutral on the water, otherwise the boat is a submarine.
I got shot down in flames but later I found Bernard Moittessier's formula for working out the required weight of anchor/mooring and scope. It was all based on the windage of the vessel, complete with a method of calculating windforce on sticks, wires, hull etc.

I would agree that windage and underwater drag (tidal forces) are the main loading forces, however surely the weight of the vessel will have an effect when there are snatching loads?

If a heavy yacht with the same windage as a modern lighter one, not at all unlikely, is anchored with the same equipment the loads on the anchor in rough conditions must surely be higher?

I would expect though that the heavier yacht would be fitted with a larger anchor and possibly chain.

 

[lw395]

A bit of Fred Drift perhaps, but I once asked the question on this forum, what has boat weight got to do with anchoring or mooring.
The point is that applying Archemedes, boat weight is neutral on the water, otherwise the boat is a submarine.
I got shot down in flames but later I found Bernard Moittessier's formula for working out the required weight of anchor/mooring and scope. It was all based on the windage of the vessel, complete with a method of calculating windforce on sticks, wires, hull etc.

I think it's windage in flat water.
Displacement comes into it as soon as there is enough chop to move the bow up and down.
It's the boat's mass rather than its weight.

In one way, higher displacement might help (for constant windage) because the boat will shear around less in gusts?

It all quickly gets hard to analyse, as the conditions are never the same twice.

 

[Bosun Higgs]

Bloody hell! Do they really? I hadn't realised that inflation had been so rampant in recent years.

I was taught 3x depth for chain, 5x for warp, for my Yachtmeister theory,

You should worry - I'm still teaching that. The RYA hasnt notified me of any change