colingr
Well-Known Member
Well I chuck out roughly 6x depth with a mix of 7m chain and warp.
Works for me, I don't know precisely why, nor do I care, but it does.
Works for me, I don't know precisely why, nor do I care, but it does.
Anchor scope - why do we teach beginners such rubbish?
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Scotty_Tradewind
Well-Known Member
Well I chuck out roughly 6x depth with a mix of 7m chain and warp.
Works for me, I don't know precisely why, nor do I care, but it does.
Ah! but what weight of boat, what wind, what windage,what type of bottom, what tide, what anchor etc...?
Poignard
Well-Known Member
It's people like you who are responsible for the hundreds of yachts wrecked around our coasts each year due to their anchors dragging.
saltwater_gypsy
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Don't you rubbish me mate!!!!!!
If you sail with your textbook on your lap you'll never make a sailor!!!!
All I said was that RULES are a starting point.
PS a catenery calculation from scratch is not exactly schoolboy physics and maths . It took a long time to realise that this type of curve was not a parabola.(check out http://en.wikipedia.org/wiki/Catenary#History)
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Bosun Higgs
N/A
No - you said "There are no RULES in this matter" which is nonsense. The problem is the reverse - there are too many rules which conflict and have little scientific basis. Like the 3x scope one that everyone knows.
I applaud the original posters attempt to bring a bit of intelligent thought to the decision on what scope to use even if the results are contentious. But after labouring through all the posts and the OP's mathematics ( which was schoolboy maths when I went to school but is mostly beyond me now) I'm not really any the wiser. Craig Smith made a good case for the Rocna withstanding a 1 tonne upward pull until I remembered by last attempt at anchoring in Ilfracombe using my Rocna of that size and with the same 5 times scope when I would have been grateful for 100kg pull let alone ten times that figure.
Maybe the critical angle bit is the most useful result of the debate insofar as it indicates much reduced damping from the catenary. Lob out the anchor, give it a 5 times scope, drag it in with full throttle and then keep a watch on the angle, when the wind rises, as a decision point for putting out more chain. Add some stretchy nylon as a way of absorbing the shock loads you get from passing mobo wakes etc.
colingr
Well-Known Member
Ah! but what weight of boat, what wind, what windage,what type of bottom, what tide, what anchor etc...?
Weight - 2 tons or thereabouts, with crew
Wind - norra lot (mobo see)
Windage - sports cuddy, variable depending on canopy up or down
Bottom - Sand or mud usually by the state of the anchor when it comes up
Tide - various up to 3 knots or so
Anchor - Am I allowed to say??? Oh sod it it's a Bruce - 15Kg
Sorry don't know make of warp.
Talbot
Well-Known Member
What arrogant nonsense.
What CONTRIBUTES to this problem is a lack of understanding about the difference between rules and guidelines. The x3 etc is a guideline, which means that if in the opinion and experience of the skipper, the conditions are a bit extreme, then you may consider letting more scope out, or running the engines to relieve the strain, or setting an anchor watch etc.
Another contributing factor IMHO is that the "guidelines" provided by the anchor manufacturers for use relates really to use as a lunch hook. Overnight and longer term anchoring requires at least one or two sizes larger.
Poignard
Well-Known Member
colingr
Well-Known Member
Quandary
Well-Known Member
Is it over now?
Certainly was a good one and I thought Craig is gradually maturing a bit in the way he presents his opinions, only managed to annoy a few.
Is it time yet to start talking about what to put on the end of the chain or should we give it a week?
Certainly was a good one and I thought Craig is gradually maturing a bit in the way he presents his opinions, only managed to annoy a few.
Is it time yet to start talking about what to put on the end of the chain or should we give it a week?
lw395
Well-Known Member
I'd like a large X-yacht to put on the end of the chain please!
colingr
Well-Known Member
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Scotty_Tradewind
Well-Known Member
Yes I've got a Rocna on one end of my chain and if I changed from my much loved Twister, some of the boats I would consider on the other end would be.... L'Giles38', Rustler36', Nicholson 35,Tradewind 35/39, e.g. something that would look after me in most conditions to get me around the World...
p.s. Craig... Your anchor is wonderful but advise you lighten up old chap and stop upsetting potential punters
craigsmith
Well-Known Member
Hold on I thought the consensus already was that you only need chain?
Ubergeekian
Well-Known Member
p.s. Craig... Your anchor is wonderful but advise you lighten up old chap and stop upsetting potential punters
I'm afraid the tone young Craig takes here is the reason I would never consider a Rocna anchor. Well, that and the photographs of them turned up like a jester's shoe after hitting a rock. We have rocks round the west of Scotland, you see, and I can't be doing with an anchor which needs hammered flat again every time it hits one.
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lw395
Well-Known Member
At least you've got the rocks to hammer it flat with.
Reminds me of an RYA Young Skippers escapade where we demonstrated the holding power of a Danforth dropped into a gap in the rocks.
5m depth
6m rode
Got the anchor out by putting all the sails up and tacking on the spot.
aquaplane
Well-Known Member
I got to the end of page 2 and thought everything that needed to be said had been. Put out what you like, if it's not enugh you will find out.
jimbaerselman
Well-Known Member
Very interesting debate. But far to little emphasis has been placed on dynamic loads, as some have pointed out.
Those who recommend x3 for chain and x5 for rope have usually not experienced winds above 30kts in their anchorages, and are usually talking from experience in yachts which don't sheer from side to side once the wind is above 25kts . . .
So, the loads on an anchor system depend on wind strength, the lateral behaviour of the boat in stronger winds, and (if it's there) nodding in a seaway.
The constant drag due to wind has been nicely graphed above.
Yawing in wind depends to what degree wind drag acts in front of the point of lateral resistance in the water - the "head falls away in a wind". It has nothing to do with displacement. But once yawing starts, it is useful to watch the behaviour of the rode. About two thirds of the time after a "tack" the rode is slack until the next pull. That pull has to arrest the boat and yaw it, and provide enough windward inertia to survive the next period of slack rode.
So, crudely, some three times the static wind force you expect has to be exerted, since the rode is only pulling one third of the time. Crude, because there is damping effect absorbing some energy.
At this stage displacement becomes critical. Heavy displacement yachts have a high inertia, so a lot of force now goes into just reversing its motion, as well as handling all the wind effect in one third of the time. My ten ton 40 footer would bring the chain up bar taut each tack - no discernible catenary. I might just have well had a rope rode (and indeed, always used a long snubber, some 20 metres).
So I ran a series of simple tests using the three anchor types available (CQR, Britany, Spade) using just a rope rode (well, 5m of chain at the bottom end) to see what the critical scope was for rope.
The test? Anchored, and required the anchor to hold full astern. The 48 HP engine could just push the boat ahead at 2 kts against a 45kt wind in flat water at 3,600rpm (4,000 max). The symmetrical feathering prop just reached the same revs when static astern. So this was the same (steady) force as a F10 with a two kt current. Test was done in a lovely big sandy bay, off Elefonisos, the SE corner of the Peloponese. Hard sand, flat bottom, 6m depth.
With each anchor, I started with 50m scope (7 times depth from stemhead, give or take an inch). All anchors held full astern, though the CQR had to be pulled about 20m before it dug in. Lovely groove of evidence in the sand. No anchors buried deeply - hard sand I guess. I then shortened scope, and pulled again. Shortened, and pulled again. Shortened, and pulled again.
The Britany started to drag at x6. The CQR at x5. The Spade at x4.
So I tried setting them at these figures, without "coaxing" the hook into the ground (which I have noted may people do with their CQRs). The Britany needed all x6 to dig in, and did so reliably. The Spade needed a bit more than x4 to dig in, but did so reliably. The CQR always travelled unpredictably before digging in, irrespective of scope.
Conclusions? Assuming that the dynamics in a big wind cause your boat to sheer a lot, you are going to load your rode sufficiently to bring it bar taut, even if it's chain. So the scope you will need for your anchor to get the best out of it can be determined by a rope only test. And the scope will vary depending on the anchor type, and perhaps the bottom.
So, go out there and try it for yourself. Two certainties. One; there's no need to put out more scope that the rope amount indicates. Two; any less when using chain does not allow for the bar taut case caused by heavy yawing (let alone the shift in pull angle each end of the tack).
And the yawing? Ah, if more than 20kts was likely, always used a forked moor. Then at least the pull was always correctly aligned with the anchor shank, so the chance of being yanked out of the ground by a sideways tweak no longer arose. Yes, that's why so many Caribbean boats use forked moor at anchorages - those steady 25 kt winds sometimes lift a little . . .
Those who recommend x3 for chain and x5 for rope have usually not experienced winds above 30kts in their anchorages, and are usually talking from experience in yachts which don't sheer from side to side once the wind is above 25kts . . .
So, the loads on an anchor system depend on wind strength, the lateral behaviour of the boat in stronger winds, and (if it's there) nodding in a seaway.
The constant drag due to wind has been nicely graphed above.
Yawing in wind depends to what degree wind drag acts in front of the point of lateral resistance in the water - the "head falls away in a wind". It has nothing to do with displacement. But once yawing starts, it is useful to watch the behaviour of the rode. About two thirds of the time after a "tack" the rode is slack until the next pull. That pull has to arrest the boat and yaw it, and provide enough windward inertia to survive the next period of slack rode.
So, crudely, some three times the static wind force you expect has to be exerted, since the rode is only pulling one third of the time. Crude, because there is damping effect absorbing some energy.
At this stage displacement becomes critical. Heavy displacement yachts have a high inertia, so a lot of force now goes into just reversing its motion, as well as handling all the wind effect in one third of the time. My ten ton 40 footer would bring the chain up bar taut each tack - no discernible catenary. I might just have well had a rope rode (and indeed, always used a long snubber, some 20 metres).
So I ran a series of simple tests using the three anchor types available (CQR, Britany, Spade) using just a rope rode (well, 5m of chain at the bottom end) to see what the critical scope was for rope.
The test? Anchored, and required the anchor to hold full astern. The 48 HP engine could just push the boat ahead at 2 kts against a 45kt wind in flat water at 3,600rpm (4,000 max). The symmetrical feathering prop just reached the same revs when static astern. So this was the same (steady) force as a F10 with a two kt current. Test was done in a lovely big sandy bay, off Elefonisos, the SE corner of the Peloponese. Hard sand, flat bottom, 6m depth.
With each anchor, I started with 50m scope (7 times depth from stemhead, give or take an inch). All anchors held full astern, though the CQR had to be pulled about 20m before it dug in. Lovely groove of evidence in the sand. No anchors buried deeply - hard sand I guess. I then shortened scope, and pulled again. Shortened, and pulled again. Shortened, and pulled again.
The Britany started to drag at x6. The CQR at x5. The Spade at x4.
So I tried setting them at these figures, without "coaxing" the hook into the ground (which I have noted may people do with their CQRs). The Britany needed all x6 to dig in, and did so reliably. The Spade needed a bit more than x4 to dig in, but did so reliably. The CQR always travelled unpredictably before digging in, irrespective of scope.
Conclusions? Assuming that the dynamics in a big wind cause your boat to sheer a lot, you are going to load your rode sufficiently to bring it bar taut, even if it's chain. So the scope you will need for your anchor to get the best out of it can be determined by a rope only test. And the scope will vary depending on the anchor type, and perhaps the bottom.
So, go out there and try it for yourself. Two certainties. One; there's no need to put out more scope that the rope amount indicates. Two; any less when using chain does not allow for the bar taut case caused by heavy yawing (let alone the shift in pull angle each end of the tack).
And the yawing? Ah, if more than 20kts was likely, always used a forked moor. Then at least the pull was always correctly aligned with the anchor shank, so the chance of being yanked out of the ground by a sideways tweak no longer arose. Yes, that's why so many Caribbean boats use forked moor at anchorages - those steady 25 kt winds sometimes lift a little . . .
saltwater_gypsy
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Good comments from Jim regarding dynamic loads and yawing.
Another way to minimise yawing is to put up some riding sail. Say half of the mizzen if you have one or a special riding sail on the backstay (which I have seen but never tried)
Another way to minimise yawing is to put up some riding sail. Say half of the mizzen if you have one or a special riding sail on the backstay (which I have seen but never tried)
vyv_cox
Well-Known Member
My anchor-to-chain connector failed when we were anchored in Ibiza. Conditions at the time were tideless and less than force 3 wind. We had around 25 metres of chain out in about 3 - 4 metres of water. I heard it fail with a bang, typical of stress-corrosion cracking. Immediately we began to drift astern and had to start the engine very quickly as the anchorage was busy. The contribution of the chain may have been to slow us down a little but it made absolutely no contribution to remaining in the same spot.