Anchor scope - why do we teach beginners such rubbish?

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Andrew_Fanner said:
I recall asking years back something along the lines of "if scope is so important why not let out loads more than advised if space allows". The only answer I ever got was related to variations on "because you have to wind it all back in again".

Anyone able to give a better answer?
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There is no answer, other than a point of diminishing returns which is found somewhere around 8:1.
At 8:1, the maximum angle of pull on the anchor is already capped at just over 7°, and of course the scope must be doubled in order to halve the angle. So even massive increases in rode length give only a few degrees' benefit.
http://www.rocna.com/kb/Scope_vs_catenary
 
I read all the comments regarding anchoring with interest, as I am still coming to terms with trying to get a good nights sleep!

Are there any thoughts on this idea, which I have read about:

http://www.anchorbuddy.co.uk/

As the link says it is to do with a product called "Anchor Buddy", I am interested in the principle not so much the product. A dive belt or lead in a bucket would in fact do the same job.

Your comments are welcome.......................
 
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craigsmith said:
Kellets are essentially a waste of time with regard to performance of the anchor.
Click to expand...

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.
 
Anchoring rules

In nine years of cruising, invariably anchoring when not on passage I have worked to a simple rule:

I found I need a sufficient length of chain let out to achieve 2 boat lengths of chain (approx 10 meters in my case) lying on the bottom plus max depth of water. (This then takes account on needing a greater multiple of depth in shallow and a smaller multiple in deep.) In practice the amount of chain used is very nearly the same in shallow or deep - with the length I have of 45 meters.

I admit I have dragged 3 times - each time in shallow water with a muddy bottom.

Further thoughts :
a)it is how you lay out the anchor and chain that counts. With a CQR (which I happen to have) the inital tug you put on the chain needs to be adapted to the bottom conditions ie hard, soft, weed or whatever
b) If in doubt let it all out - it does not help you if its in the chain locker!
c) dive on the anchor to check if circumstances allow
 
Stu Jackson said:
Two things:

get a good anchor, like one of the new generation like Craig represents

like docking: practice, practice, practice - it doesn't happen by itself - read up on good techniques
Click to expand...

Hi Stu,

How do I know that this is the best type of anchor???
The people selling the Anchor Buddy say their method of anchoring is the best!

As I am still learning my trade so to speak, it is difficult to know which one is actually the best method of staying stuck to the seabed without either trialling or buying both!!!
l
Again you thoughts are welcome.

Mark
 
Oh, dear, is this a troll?
Or do you have a couple of weeks to spend reading all the debates on anchoring in the archive?
Too many answers, too little time!
 
Hi Craig,

craigsmith said:
... (my original deleted for brevity)...

All of these points considering upward pull are incorrect. A set anchor, even with an extended shank, can tolerate much more than its own weight in terms of vertical pull on the rode, indefinitely. The anchor orients to the direction of pull and if at least partially set will (or won't) resist, it is not 'aware' of whether that force vector is above horizontal or not. The weight of the anchor is only marginally important during the setting process. An adequately designed anchor in soft substrate will in fact continue to bury, usually until it finds a harder substrate that stops it, in response to continued high loading.
...
But you want to have a better grip on what the rode is tied to down there on the seabed :)
Click to expand...

What ALL of them? Oh dear, must try harder.

Actually it would be really good to have an anchor designer collaborate on the maths here. I think we may agree that:
- the pull at the anchor doesn't have to be quite horizontal
- that there is an extremely complicated relationship between the max angle at which the anchor starts to lose grip, the anchor design and the type of bottom
- modelling inhomogeneus materials like mud, sand, shells and stones is really, really, difficult, if not actually impossible!

So I used a fiddle factor, lets call it gamma just to show off, as follows:
Allowable vertical force = gamma x anchor weight

I think this is probably an ok representation for a fisherman's anchor, a mooring or a mud weight, but a modern design very possibly does rather better (ie allows more upward force). Indeed, we might even agree that a figure-of-merit could be derived for different anchor designs based on this property.

So could you propose a value of gamma, or a different relationship, and preferably one with an explicit figure of merit? I'd gladly put it into the program and we can see what difference it makes, or if you can write JavaScript, you could try it yourself. I'm afraid I did rather slag off the '8 degrees, irrespective of force and bottom composition' approach since I havn't seen any data supporting that number. I feel that it has to be a function of bottom type at least, and probably, albeit weakly maybe, of force as well.

The saving graces of what I did however are: that it's only a second order effect and so makes little difference in practice, that the formulae/graphs were supposed to be anchor type agnostic to a degree - not everyone has your excellent one yet! - and were supposed to be for a worst case bottom composition. Thus what I did may be a bit pessimistic, but that's probably to err on the right side.

What I disagree with quite strongly is the view that above a certain wind strength the rode gets bar taut, in which case simple trig would be enough. Even at 1 tonne load, which is what you'd get when anchored in a F10 so I think few will have experienced, there will be some catenary. How much obviously depends on the weight and length of chain, but look at fig 2 for a vertical cross section of what's happening.

I dont have the sangfroid to dive down and have a look next time I'm anchored in F10, but I can and did try it on dry land with one end anchored quite a way up a tree I was pulling down, and the other end tied to a tractor - believe me it's bl**$y hard to take the catenary out of a chain, and I applied _lots_ of force!
 
Just to confuse the issue.
YM anchor test (YM Dec 06) used a 5:1 scope with some chain and mostly rope.
Some anchors of about 15 kg refused to budge with a 5000 lb (sorry about mix of units) pull. (This wasn't consistent though, presumably because sea bed is not completely homogenous, or for whatever other reason; emphasising the need for several measurements in each condition if the test results are to have any practical use.)

With a pull like that, the chain/rope will form pretty much a straight line. So the chain clearly doesn't need to be horizonal near to the anchor, desirable though that might be.
 
This all reminds me of the chap doing his yachtmaster practical and the instructor asks him what to do if the wind gets up and the anchor drags

Let out more chain of course

What if the wind increases again and it still drags

Let out more chain

The instructor - searching for the pupil to suggest starting the engine - asks a third time what to do if the wind increases yet again

Let out more chain

The instructor asks where all this chain is coming from

The same place as the wind replied the pupil
 
bammylovechops said:
Hi Stu,

How do I know that this is the best type of anchor???
The people selling the Anchor Buddy say their method of anchoring is the best!

As I am still learning my trade so to speak, it is difficult to know which one is actually the best method of staying stuck to the seabed without either trialling or buying both!!!
l
Again you thoughts are welcome.

Mark
Click to expand...

Mark,

Started anchoring in 1983, been doing it ever since. Started with the traditional everyone-has-one-it-must-be-good Danforth. Learned it doesn't reset very well. Moved to a Bruce, then to a Rocna. When I set my anchor it wants to pull me off the bow. That's the kind of anchor I like and what my boat needs to allow me the proverbial good night's sleep.

Sure, there are lots of debates about anchors. You asked me, that's my opinion.

But why stop there, try reading this: http://c34.org/bbs/index.php/topic,2705.0.html

Click on the links within the topic and read all the pages.

Sizing of the anchor is critical, too. Try this: http://c34.org/bbs/index.php/topic,4990.0.html

There's no one right answer. This is mine, others will direct you to previous threads. As they say: anchors are like religion. Almost like what's the best electrical system, right guys?
 
Ubergeekian said:
It's better than the rule they used when I took and subsequently taught the shore-based Day Skipper course twenty years ago. Some horror involving a square root, it was - readers of a mathematical bent will instantly see that it was not dimensionally invariant, giving different lengths of chain/rope depending on what unit you were measuring it in.
Click to expand...

It was originally quoted as 12 times the square root of the depth. When I saw Bill Anderson I asked what units he was using, and he said it didn't matter. So I asked him what scopes he would use in 4m (24m), 400cm (480cm, 4.8m) and 0.004km (0.759km, 759m). In the 2nd edition of the Day Skipper book the scope is given as 12 times the square root of the depth in metres.

There is some point in the awkward expression, since for depths up to about 50m it closely follows the catenary. But there's really no need to get out the square root tables; for all practical purposes 3 times the depth plus 10m is the same.
 
jdc said:
...... modelling inhomogeneus materials like mud, sand, shells and stones is really, really, difficult, if not actually impossible!

...... one end anchored quite a way up a tree I was pulling down, and the other end tied to a tractor - believe me it's bl**$y hard to take the catenary out of a chain, and I applied _lots_ of force!
Click to expand...

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.

Your point regarding the difficulty to "take the catenary out the chain" is baffling. There are many examples of suspended objects where chain is used as an architectural / structural tie and is taught and straight.

This has been an interesting thread as I just assumed that the anchor was holding only because the horizontal forces were less than a shear limit in the soils. This of course, is still true. However, apart from gravity I never thought that the anchor was actively being driven downwards by the horizontal force. Obvious considering the shapes off High Holding type anchors!

ps I was taught that for rope rode it was 6 x the depth, not 5 x depth.
 
LittleSister said:
Bloody hell! Do they really? I hadn't realised that inflation had been so rampant in recent years.

snip

By the time the First Mate was doing her Yachtmaster theory a couple of years ago it had gone up to 4x for chain.
Click to expand...

I probably recalled incorrectly :) It could very well be 4x. But it definitely went up as it got windier.

Craigs statement that returns diminish with longer rodes is clearly correct, so 7x seems sensible.
 
Anchor rode length

Andrew_Fanner said:
I recall asking years back something along the lines of "if scope is so important why not let out loads more than advised if space allows". The only answer I ever got was related to variations on "because you have to wind it all back in again".

Anyone able to give a better answer?
Click to expand...

Yes more length of anchor rode is better. However it gives a much bigger swing circle which may be a problem. The point is that the more chain/rope you have out the less gain you get by increasing the length.
As I said before this is just simple trig and can be seen in a scaled diagram. So up to 5 times depth you get a real useful improvement in approach angle with extra length let out but beyond 5 times there is not much improvement. Of course it is a continuous diminishing of improvement in a logarythmic manner. Certainly for rope 4X is not enough 6x is better than 5X but not necessary unless you are really worried. Beyond 5X if you want more safety you look for bigger anchor more or heavier chain or extra weight on the chain. olewill
 
jdc said:
What ALL of them? Oh dear, must try harder.

Actually it would be really good to have an anchor designer collaborate on the maths here. I think we may agree that:
- the pull at the anchor doesn't have to be quite horizontal
- that there is an extremely complicated relationship between the max angle at which the anchor starts to lose grip, the anchor design and the type of bottom
- modelling inhomogeneus materials like mud, sand, shells and stones is really, really, difficult, if not actually impossible!

So I used a fiddle factor, lets call it gamma just to show off, as follows:
Allowable vertical force = gamma x anchor weight

I think this is probably an ok representation for a fisherman's anchor, a mooring or a mud weight, but a modern design very possibly does rather better (ie allows more upward force). Indeed, we might even agree that a figure-of-merit could be derived for different anchor designs based on this property.

So could you propose a value of gamma, or a different relationship, and preferably one with an explicit figure of merit? I'd gladly put it into the program and we can see what difference it makes, or if you can write JavaScript, you could try it yourself. I'm afraid I did rather slag off the '8 degrees, irrespective of force and bottom composition' approach since I havn't seen any data supporting that number. I feel that it has to be a function of bottom type at least, and probably, albeit weakly maybe, of force as well.

The saving graces of what I did however are: that it's only a second order effect and so makes little difference in practice, that the formulae/graphs were supposed to be anchor type agnostic to a degree - not everyone has your excellent one yet! - and were supposed to be for a worst case bottom composition. Thus what I did may be a bit pessimistic, but that's probably to err on the right side.

What I disagree with quite strongly is the view that above a certain wind strength the rode gets bar taut, in which case simple trig would be enough. Even at 1 tonne load, which is what you'd get when anchored in a F10 so I think few will have experienced, there will be some catenary. How much obviously depends on the weight and length of chain, but look at fig 2 for a vertical cross section of what's happening.

I dont have the sangfroid to dive down and have a look next time I'm anchored in F10, but I can and did try it on dry land with one end anchored quite a way up a tree I was pulling down, and the other end tied to a tractor - believe me it's bl**$y hard to take the catenary out of a chain, and I applied _lots_ of force!
Click to expand...
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 -

Blackbeard referenced what I would too, WM's independent testing, Rocna 15 consistently exceeded 5,000 lb-force limit at 5:1 scope. That's 1,000 lb-force = 454 kgf vertical lift on the anchor, less any benefit from catenary but the testers were only using 6 m of 8 mm chain with the rest rope (go put that in your model and examine the resulting straight line :) ). That was in hardish sand with the anchor set then immediately loaded, no nice soft clay and time to penetrate deeply.

This should shine a light on why it's a deep flaw to focus on the force level required to lift the chain. It's a useful point to use as a benchmark to investigate patterns and trends but it's not relevant to the anchor.

Your gamma would require a large amount of experimental data with every combination of seabed substrate arrangement, anchor type and size, and quality of set. Even then it would only be a useful predictor if the seabed type to be anchored upon was known in advance.

I don't trust your data w.r.t. force levels. The usual culprit is wind force measurements, boaters typically overrate wind speeds with elevated and very mobile mast heads. Otherwise you have a boat with a low windage and well set-up rode that is dampening shock well; the dynamics from surge and gusts in some situations definitely will give massively higher peak loads. The photo I posted above shows a different reality with chain that's practically straight, as the math will predict, and that's with no surge. The article I linked to above shows the catenary profiles in conditions that we know still won't trouble an adequate anchor. All that matters is scope.
 
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There are no RULES in this matter, only guidance notes which form a starting point after which common sense and experience should prevail.
I do however agree with the comments on shallow water anchoring where a scope of 10x the depth or more is often appropriate.
It may be worth pointing out that many anchors wil only tolerate a pull of 8 degrees from the horizontal before breaking out. This occurs at a scope of 7x depth so 10x depth or more is very necessary if the anchor rode is bar taut in strong conditions.
 
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