anchor scope

[Nickel]

Re: Disagree. Completely.

I'm not going to join in the anchoring debate, I am an interested bystander as far as that goes really - but will add that Ships_Cat under his previous pseudonym Mainly_Steam, has already proved himself to me to be a man of poor and rash judgement, happy to throw opinions about even when founded on an absolute and critical lack of knowledge.

Agree with your humility comment entirley.

 

[Santana379]

Re: Downright bad seamanship and irresponsibility!

[ QUOTE ]


Mirelle said "The thing which starts an anchor dragging is the snatching load transmitted down a taut chain or warp. "

--------------------------------------------------------------------------------

Time to (try to) introduce a slightly different topic, which seems like a good idea in view of the fact that those with strongly held views have made their points in this thread, but have not recognised when its time to agree to differ. I can see why duels used to be used to settle arguments!

Snatching load - presumably a reduction in snatching load might help one to reduce scope to 3:1, which all parties agree either is or may be necessary in a very crowded anchorage. On Francis Fletcher we have a stainless steel anchor chain hook, spliced onto a few metres of fairly stretchy rope. If its blowy or bouncy we wrap the rope around one or two bungy mooring compensators. Not only does this help the anchor stay put, but it reduces the shock on the stem, makes it (a bit) comfier aboard, and eliminates all the nasty noise from the chain on the bow roller, whatever scope is out.

Probable downside - no audible clues to the anchor dragging - but I wouldn't know because it hasn't happened when the the rope and hook have been used.

It occurs to me that there might have been more consensus had the thread started with "how long should a piece of string be"!

Keep /forums/images/graemlins/smile.gif ing,

 

[Mirelle]

Er, yes...

Mirelle also has such a device. Actually we have two, as I've been experimenting, and I'm not sure which is better. One is a length of nylon rope with a stainless hook, as described. I find this works particularly well if it is led from one side so as to induce a sheer. However, the hook can wriggle off the chain.

The other, which can only work for boats with bowsprits, is a similar lenth of stretchy nylon which is shackled to the (very substantial!) stem eye for the bobstay. When not in use this is belayed on the bowsprit traveller to keep it out of the way; when in use it is snapped to the chain with a small Wichard hook (breaking load less than that of the chain!)

Edited to add - I agree that the drawback is that one has no audible warning of a dragging anchor - but I have not yet dragged it when using either of these gadgets.

 

[Ships_Cat]

Re: Er, yes...

While we find that we don't need a nylon strop ourselves (I think we don't suffer snatching because of hullshape/weight and we normally have to anchor in deep water) I would be interested in what length strop you use.

John

 

[BlueSkyNick]

Re: Disagree. Completely.

[ QUOTE ]
I'm not going to join in the anchoring debate, I am an interested bystander as far as that goes really - but will add that Ships_Cat under his previous pseudonym Mainly_Steam, has already proved himself to me to be a man of poor and rash judgement, happy to throw opinions about even when founded on an absolute and critical lack of knowledge.

Agree with your humility comment entirley.

[/ QUOTE ]

Rather Harsh - he has also made many interesting and useful posts under both names. Time to draw this thread to an end, IMVHO.

 

[webcraft]

Re: Mainly_Steam

If what you say is true Nickel then I must admit that my respect for this self-acclaimed 'professional' has just taken a dip . . . from a level that was none too high anyway.

I note that he has no details of his sailing experience in his bio - so why is he asking us to believe in him implicitly???

Is he also saying that the various experts I quoted - most of whom wrote for or still work for IPC yachting titles as yachting professionals - are all speaking rubbish?

Let me just reiterate my previous quotes:

J.D. Sleightholme:
"The scope of line paid out is popularly quoted as being a requirement of depth of water x 3 for a chain cable and depth x 5 for rope, but this is an absolute minimum for normal weather only - unless a much heavier anchor is used. . . .There must be sufficient length of cable aboard to provide a scope of up to x 10 in really bad conditions . . .
(from 'This Is Sailboat Cruising')
------------------------------------------------------------------------
Nigel Calder:
"I insist on a minimum scope of 4:1, and prefer to lie to 6:1"
(Article, Yachting Monthly)
Nigel Calder:
------------------------------------------------------------------------
Simon Jinks
"Over the last 15 years the recommended chain scope has risen from three times the depth to five times the depth . . . this is . . . related to the increased windage of beamy, high-freeboard modern boats . . ."
(Yachting Monthly, ' Playing Hooky', August 2000)
------------------------------------------------------------------------
Ralph Bagnall-Wild
"The three times the depth with chain five times with rope guide is shown to have no basis in mechanics and to lead to dangerously short scopes in shallow water. Three times the depth pus ten metres is generally a better guide . . .
(Article, PBO, March 1998)
------------------------------------------------------------------------
Prof. John H. Knox
"As I've argued before, a scope of five should be regarded as the minimum for safe anchoring"
('Will My Anchor Hold?" - PBO August 2002)
------------------------------------------------------------------------

Now - never mind that I am insecure, inexperienced and unprofessional so am reduced to pathetically dredging up quotes to support my immature and uninformed opinion . . . is Ships_Cat calling these people idiots, unprofessional, ill-informed, just plain wrong or what??

If so, just what exactly are his divine qualifications that allow him to overrule all these opinions? He appears to be unable to present a coherent argument.

I am all in favour of bold statements that contradict established wisdom, but they must be backed up with sensible evidence and based on more than 'It works for me therefore you must all be idiots'

(Mirelle on the other hand has made it fairly clear that 3-1 works for him most of the time on his boat with his gear - which is a different thing altogether)

I agree with bignick that this thread has nowhere else to go now . . . it seems that anchoring threads are destined to join the no-go area inhabited by Colregs and Flags . . .

- Nick

 

[Santana379]

Re: Sheer, and we\'re not talking Nylons

Please don't use the word sheer in the context of anchoring - it brings back embarressing memories.

After a great morning - 03.45hrs alarm / straight out of the Deben / 3 off the crew saw an eclipse (4th arose of Clacton Pier) / fabulous cruising chute run up the Blackwater (doesn't really have the same ring as "spinnaker run" however) / blazing sunshine / running aground going up to Maldon / quick pint at Maldon - we dropped the hook for lunch in a very crowded anchorage off Osea Island.

We got out the cockpit table, rigged an overboom cockpit awning to keep the sun off, which came down to the gunwhales (the awning, not the sun) so virtually the view out. The tiller was in the way of the magnificent spread of food and drink on the table, so I shoved it over to starboard, securing it with a peg in FF's pin rail. There's a strong tide runs past Osea and the ebb gradually increased in strength towards mid tide. We were on cheese and port when I saw boats passing the back of the cockpit awning at quite high speed for a busy anchorage. Trouble was they were at anchor, and I had induced such a sheer that I think we must have been going round our anchor in circles - I don't know because we got the tent down and anchor up pretty smartish, spilling the port in the process. One of those days when you wish the boat's name wasn't writ large on dodgers.

/forums/images/graemlins/blush.gif /forums/images/graemlins/blush.gif /forums/images/graemlins/blush.gif

I have to say lessons learnt from one's own mistakes are better remembered than lessons taught or read.

 

[Santana379]

Re: Er, yes...

You asked Mirelle, but I'll answer too - our strop extends on only a couple of metres beyond the bow roller, so the chain hook is above the water level. Without the bungy shock absorbers it might have to be a lot longer.

I would've thought pretty well all boats would experience some kind of snatching in bouncy conditions if the shelter is restricted. Obviously we all try to maximise the shelter when anchoring, but on occassions I've certainly found myself anchored in quite uncomfortable conditions.

 

[Ships_Cat]

Re: Er, yes...

Thanks. Yes I think it is commonly found to be an advantage on most small (ie the ones most of us have) boats whether for shock and/or noise.

Is not common to do so on larger vessels, certainly none of the ones I have been associated with, and probably the reasons are obvious. But on one occasion was in an anchorage with a large yacht in very strong gusty conditions and they rigged a quite short line to the chain through one of the bow side fairleads - I think what they were purely aiming for, and seemed to be the result, was that as the boat was set back harder against the chain in the bigger gusts, instead of the boat continuing back to perhaps jerk against the chain it would, because of the angular pull, start swinging around the anchor whilst decelerating and so loosing speed that way.

They did not drag, but were not, like in the story you relate, eating their lunch /forums/images/graemlins/smile.gif.

John

 

[Santana379]

Re: Er, yes...

I think the technique you describe is the same as Mirelle's deliberately induced (slight) sheer.

I don't think anyone in their right minds would deliberately induce as much sheer as I did on the occasion in question. As I said, lesson learned! /forums/images/graemlins/blush.gif

(I'm not sure we actually dragged, I supect we just struck fear into all the crews around us as we veered about uncontrollably!)

 

[Ships_Cat]

Re: Er, yes...

Thanks for that - I wondered if was part of what he was aiming for. Most refer to the elasticity and I had not thought too much about inducing sheer on purpose. We are off in the boat again for 2 or 3 weeks in a few days (work permitting) and might do a bit of experimenting along similar lines.

Our favourite trick when at anchor and having lunch is, because we don't have fiddles on our saloon table, ending up with it all in our laps but that is another story.

John

 

[Guest]

Re: Disagree. Completely.

Thanks for the moral support /forums/images/graemlins/smile.gif Such a savaging gives a bit of a jolt.

Just to make it absolutely clear, I'd like to point out that I do not doubt what Mirelle says with respect to his boat and heavy anchoring gear used on board, especially now that he has clarified that he will veer more in stronger winds. Anyway, I have even less experience of anchoring boat's like Mirelle than Mirelle has of anchoring AWBs.

Unfortunately I just cannot see that that is good practice in AWB's and such-like without heavily specified gear. I doubt that many AWB's carry anchor and chain of comparable specification to Mirelle's.

 

[Mirelle]

About same as FF

Maybe a tad longer. I would reckon about 9 feet outboard from the fairlead, and the one on the stem eye is about the same, so that it tidies itself away when belayed to the traveller.

12mm 3 strand nylon, but maybe 10mm would have been better.

I had not thought of FF's trick with the snubbers, but will now copy it!

 

[Mirelle]

Re: Er, yes...

Yes, that is just what I am trying to do; I got the idea from an eigteenth century textbook on seamanship, and it does seem to work. Mirelle tends to wander about a bit at anchor due to windage of bowsprit and sheer forwards with moderately cut away forefoot, and the slight sheer settles her down.

We have, by common consent, the worst cabin table afloat, but I designed and built it and it's staying!

 

[peterb]

Re: RYA (long)

[ QUOTE ]
Surprised no-one has mentioned the RYA yet (perhaps we're all too experienced on here!) but the standard scope is 4:1 for chain and 6:1 for warp as recommended on yer Yotmeister course with provisos about swinging room and the way different types of boats lie.

[/ QUOTE ]

Yes, it is now. But it has changed over the years. Back in the 70s I believe that it was 3:1 for chain and 5:1 for rope. Then the RYA introduced a rule of 12 times the square root of the depth for chain (modified in its second version by defining the units as metres), and IIRC 20 times the root of depth for rope. So many people laughed at the idea of carrying square root tables that they changed back to the linear system with the 4:1 and 6:1 ratios.

I once started to do a mathematical study of the scope required. I started with the idea that any anchor would have a maximum possible horizontal pull that it might withstand without dragging. I looked at the results of various tests on anchors and found that most good anchors (in reasonable holding ground such as mud or sand) were able to take loads of about 10 to 20 times the anchor weight before they dragged. Using published advice on sizes of chain to use with different anchor weights, I should then be able to work out the scope required in order that with the maximum acceptable anchor load the pull on the anchor would still be horizontal (conventional wisdom).

I was originally planning to publish the study, but didn't because the answers I came up with were so far from reality. For instance, in 10 metres of water I would need 60 metres of chain. In shallower water it was even worse; in 4 metres I would need 38 metres of chain, a ratio of nearly 10:1. I've checked and re-checked the figures, done the calculations in several ways, but still come up with the same answers. And the theory of the catenary is not ground-breaking stuff, it's been known for centuries.

So what relevance does it have to real-life anchoring? I suspect very little. Thinking back, whenever I have dragged I've almost always found the anchor fouled in some way (often by weed, but once memorably by a baked bean tin!). I suspect that the fouling prevented the anchor setting properly, even though it resisted my test pull; maybe I hadn't pulled hard enough.

We clearly need to allow some scope. If the anchor were lowered until it just touched the bottom, then it is certainly not going to set. A scope of 3:1 (all chain) probably allows us to put on a sufficient pull to allow the anchor to set. I suspect that most anchors can withstand more vertical pull than we might think, so that with the short scope the anchor still holds even though the chain isn't horizontal. I've never seen any study of the effect of non-horizontal pulls on anchor loads; maybe there's scope (no pun intended!) for a Yachting Monthly investigation?

Incidentally, if the catenary idea holds, then the RYA's "12 times the root of the depth" rule has some justification. Because of the curve of the catenary, in shallow water the scope ratio needs to be higher than in deep water, and this is what the square root law provides. But there is a much easier method of calculation; 10 metres plus 3 times the depth gives an almost identical answer for depths up to about 10 metres; above that just use a 3:1 scope.

Incidentally, my study started with the intention of looking at the effcts of "Chums" or "Angels"; weights which could be attached to chains to improve holding power. Initially I ignored surge loads, and simply looked at steady pulls. I found that the most effective place to put the weight was as close to the anchor as possible. But then I started to think about snubbing and surge loads. If the effect of the angel was to prevent the anchor from moving, and the angel was close to the anchor, then the angel itself wouldn't move. If, however, the angel were halfway down the cable, then under light loads the angel would be on the bottom and would then lift under surge loads. It would then have to be given kinetic energy, and would suffer drag forces in moving. I could see that this might be beneficial, but without a lot more information (such as the behaviour of anchors under non-horizontal jerk loads) I wasn't going to get anywhere. That's when I stopped work and decided not to write an article about it.

All of which has left me even more confused. Hope it hasn't done the same for you.

 

[Sea Devil]

Re: Disagree. Completely.

The problem with having a ships cat on board is that it thinks it knows everything and is in complete charge of the world.... Bless it! Sweet things but I would never have one on board my boat - but that's the whole point -

The wonderful thing about boats and sailing is that there is no definite answer - no 'right' way or 'wrong' way. It is the way that suits you best - that in your experience is best or having read lots of books and taken a degree or two can be 'definitely prooved ' to be correct!

the Pussy did start off by saying 3-1 chain scope is a good basic starter kit - most books and experienced people will agree that. I think he was right but there are stacks of circumstances when it is not nearly enough and as Sealeopeoard pointed out there are times when it is too much.

If I were seriously to give advice I would say that provided the anchor and chain are appropriate to the boat then 3-1 will be fine in good holding with sufficient swinging room. After that it's a matter of experimenting and having several possible solutions up your hawser hole...

 

[Anonymous]

Re: RYA (long)

[ QUOTE ]
I looked at the results of various tests on anchors and found that most good anchors (in reasonable holding ground such as mud or sand) were able to take loads of about 10 to 20 times the anchor weight before they dragged.

[/ QUOTE ]I think this is out by an order of magnitude and maybe the reason why you got strange figures? I would hope that my 33kg Bruce wouldn't drag at between 1/3 and 1/2 tonne load. Of course on some bottoms it might, but they wouldn't be regarded as very good holding.

 

[peterb]

Re: RYA (long)

Higher loads on the anchor (meaning greater horizontal components of tension in the chain) would make the discrepancy still greater.

 

[ashanta]

Re: Disagree. Completely.

I would agree with that. I have my anchor set up now with 30mtrs chain (10mm with 35lb CQR) but I also have 5 mtrs of 10mm spliced to 30mtrs 3 strand nylon. If circumstances determine it I feel that I have sufficient options on board to cope. (or was that scope?) People who are racing yachts will not want too much weight on board. I do not know but maybe this applies to planing mobo's also? All that I want know as a cruising sailor is do have sufficient equip on board to tackle the various conditions that we can be facing at times. Hence my agreement with your comments about the various options you need to have available.

Regards.

Peter.

 

[Boathook]

Normally anchor 5 to 1 though as the tide comes in this can alter !! The more the better though there is the worry as to what others have let out and some boats; just push the button and let all the chain pile in a heap on the anchor ....