Anchor scope - why do we teach beginners such rubbish?

[Ubergeekian]

Current RYA Day Skipper booklet, page26, chain 4 x depth & warp 6 x depth.

Perhaps new generation anchors need more cable out?

 

[jimbaerselman]

I also believe the frequent assertion that the catenary flattens and the rode becomes straight in any sort of blow to be false in general(1): how do you know? How have you determined it? Just by looking at the few meters near the stem-head, from above?

As a mathematician, you can calculate the angle. We know that a typical 15 to 20kg anchor can hold up to 5,000lb force, that it will be connected by an 8mm chain whose weight and displacement are known, giving us a down force over the length of the chain. Let's say 42m of chain out in 6m of water . . . what is the reduction in angular pull from a straight line?

But 8 degrees remains a mantra without anybody providing either empirical evidence or theoretical justification. I at least tried the experiment with a tractor and 60m of heavy chain.

Errm. Was the chain under water? And, of course, much of the point here is that the angle at the anchor differs between anchors - and perhaps bottom structure - to the degree (groan!) that we're working with over 10% uncertainty.

Becoming too hung up with accuracy and differences less than 10% between outcomes is probably fruitless.The bottom may be sloping by 5 degrees!That sends all calculations into the rubbish bin for that situation - although it doesn't mean we shouldn't do the sums.

 

[vyv_cox]

I also believe the frequent assertion that the catenary flattens and the rode becomes straight in any sort of blow to be false in general(1): how do you know? How have you determined it? Just by looking at the few meters near the stem-head, from above?

East coast of Sardinia, four years ago. We sat out a gale at Cala di Volpe, just south of Porto Cervo, anchored on a 15 kg Rocna for four days. We were in very shallow water, not a lot more than 2 metres, so less than 1 metre beneath our keel. Just over 20 metres of 8 mm chain out, short 12 mm nylon snubber. Nowhere near as impressive as Craig's photo but it shows much the same thing.

Wind speed was frequently 40 kts but the water temperature was at swimming pool level. I dived on the anchor several times each day, for fun rather than concern. The rode was clearly bar-tight on many occasions, visibly lifting the anchor stock from the bottom. It certainly appeared to be straight both above and below the water.

 

[jdc]

apology

I'm afraid my last post was unnecessarily abrasive and impolite - many aplogies. And many thanks to those who pointed out their real-life experience of diving on the anchor in a blow (not mad, but certainly brave imho!).

I never did expect that the maths be taught to anyone, least of all a beginner; it was included for interest and for peer review. Afterall what I actually do is let out about 10m + 3 - 4 x depth, but I then look at my graph and adjust a bit if necessary - arriving at a scope not so far fom what everyone else does. But I had thought that it would be a conceptually simple thing to have a graph of how much to let out vs depth, for a variety of wind strengths.

I feel that an understanding of what one is trying to achieve is a good thing. That any approach is approximate and there are always more effects not considered does not make a search for better understanding misguided: I think it always worthwhile attempting to quantify the unquantifiiable, but I'm concious that I may take this to extremes! I believe I did take into account dynamic forces, and that the chain lifts at the anchor when the wind is strong, but following posts from several, including Vyv's above, I'm not not happy with the criterion I used for how much the chain can be allowed to lift at the anchor stock. I'll have to think a bit more about the physics of this.


j(dc)

PS: Did anyone actually get the JavaScript version to work?

 

[craigsmith]

Yes, and it's nicely set-up, but what is the point supposed to be? As labored posts above try to explain, you essentially have a garbage-in-garbage-out formula. The point at which the rode lifts of the seabed is essentially completely unimportant*. I think everybody appreciates the effort, but you need to revise your presuppositions.

*I assume you are familiar with Alain Fraysse's work, if not you should become familiar with his published stuff here:
http://alain.fraysse.free.fr/sail/rode/rode.htm
I dislike referencing this directly because Fraysse's conclusions still do not properly account for dynamacity and trend a little far toward stretchy rope. However this modeling of the rode and angles of force are well done, and rightly ignore the anchor which can only be correctly analyzed separately and then only really by real world testing. You cannot then determine from first principles "how much the chain can be allowed to lift at the anchor stock", as it depends completely on the particular anchor and the particular set in the particular substrate (and therefore on a massive set of experimental data you don't have).

Experienced scientists warn us that when you don't have enough data you tend to twist the facts to suit your theories... when of course you must rather change your theory to suit the data until it is provable and only then actually applicable.

 

[webcraft]

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Are we nearly there yet? Just been out teaching 4x chain, 6x rope, am I a murderer??

- W