Anchor Scope

parsifal said:
Examples, please.
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Since September last year Practical Sailor have run a series of articles, roughly every other month, on anchor rodes the series is ongoing and is scheduled well into early next year. As the name suggests, practical articles. The series covers loads on rodes, (at various wind speeds and scopes), snubbers, chain size etc. Professor John Knox has had a series of articles over the last 15 years on anchor and anchor rodes in PBO. 15 years might seem a bit dated - but the technology has not changed that much. Both the Knox and PS articles are as close to controlled testing as you can get. The articles have all been practical, based on real life data - backed up with theory. Rocna have their data base, which I assume is based on practical experience (though I am not aware it is backed up with any controlled experimentation). The Fraysse website is both theory and practical experience (again I do not know if he tested the theory with controlled experimentation). If you check the Morgan's Cloud website you will find a wealth of practical experience and there is enough to draw some decent conclusions yourself, you need be careful as the MC debates do tend to follow a specific lead. If you dig into the anchor section of the Cruisers Forum (or many other forum) you will find a wealth of detail - though forum threads do require patience:). In fact google and you will be overwhelmed, but if you looked at some of the ones I try to highlight it would be a good start.

Sadly, apart from PBO there is little detail (Vyv's articles excluded) on the subject in UK mags - it appears there is not sufficient interest from UK subscribers.

Jonathan

edit I'm not providing links as I'm at sea, well offshore, and only have limited and very expensive internet access and finding and copying the links takes time (thus money), regrets:(
 
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>Pulling anchors along a beach, using a tackle bears little resemblance to real life anchoring.

Agree, the problem with that is that at anchor it's usually angled snatch loads not a straight line pull, which most anchor tests are.
 
KellysEye said:
The catenary is the curve in the chain before the chain lying on the bottom, there is no pull load on the anchor when there is still chain on the bottom. How can chain be irrelevant?
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I thought it was worth one more try.

I have just been for a dive and my anchor has rotated about 40 degrees to align with new wind direction.
The anchor is 55 Kg and is just about completely buried so it needs a fair bit of force to rotate it.
There is still chain on the bottom. The wind is now a bit stronger than it was last night.

Where did this force to rotate the anchor come from?
 
GHA said:
http://www.science20.com/chatter_box/why_anchors_dont_work

A bit of physics :cool:

Kellyseye is right in a clearly defined physics way, a non dragging anchor does no work.
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That's the trouble with using physics definitions of everyday words in an inconsistent way.
Anything that has a function 'works' if it performs that function, but it may not be doing any work in a strict physics sense.
The shelf here has the function of supporting my books.
It does that job, it clearly works, but it isn't doing any work in the force x distance sense.
It performs the job of exerting the force, but does not move any distance.

Blurring that by referring to dissipating a bit of kinetic energy as heat is just unhelpful.
Any real physics trained engineer would realise you could have a theoretical lossless anchoring system that did no work, in the same way that the handbrake on your car does no work when your car is parked.

That's not to say that the damping and energy loss may not often be beneficial in controlling the motion of the boat.
 
noelex said:
I thought it was worth one more try.

I have just been for a dive and my anchor has rotated about 40 degrees to align with new wind direction.
The anchor is 55 Kg and is just about completely buried so it needs a fair bit of force to rotate it.
There is still chain on the bottom. The wind is now a bit stronger than it was last night.

Where did this force to rotate the anchor come from?
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The chain obviously.
The chain puts some force on the bottom due to its weight, some force along the bottom as friction, and some force pulling on the anchor.
Unless it's accelerating, the vectors of the forces on each link of chain must sum to zero. Those forces are gravity, reaction from the seabed, friction, drag and the tension applied to each end.
 
>I have just been for a dive and my anchor has rotated about 40 degrees to align with new wind direction.
The anchor is 55 Kg and is just about completely buried so it needs a fair bit of force to rotate it.
There is still chain on the bottom. The wind is now a bit stronger than it was last night. Where did this force to rotate the anchor come from?

There is nothing unusual in that, the wind changed direction and the chain dragged the anchor round, it doesn't have to come off the bottom to turn the anchor. In the tropics you anchor in crystal clear water and it's easy to see how the chain and affects the anchor. I hope that's the last try given the post two up from yours.
 
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KellysEye said:
There is nothing unusual in that, the wind changed direction and the chain dragged the anchor round, it doesn't have to come off the bottom to turn the anchor. In the tropics you anchor in crystal clear water and it's easy to see how the chain and affects the anchor. I hope that's the last try given the post two up.
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Agreed, nothing unusual at all.
So there is a force, or "pull load" on the anchor even though the chain is still on the bottom. Surely this contradicts the quote

KellysEye said:
The catenary is the curve in the chain before the chain lying on the bottom, there is no pull load on the anchor when there is still chain on the bottom.
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GHA said:
Is it?
Doesn't it point to the very heart of the matter, we want transfer the energy from a moving boat somewhere else. Relatively slowly to keep the force on the anchor down.
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I thought in the simplest case, the idea was to keep the boat fairly still so there isn't much kinetic energy to dissipate.
When things get dynamic, yes damping comes into it.
But a lot of that damping will be the hull moving through the water and also the anchor moving through the mud. I expect to fully analyse the situation you might have to regard the seabed as a heavy viscous fluid.

As I said a few days ago, sometimes the damping of a kellet can be a useful tool to have in your box.
That's the trouble with these debates, people choose their simplifications to suit their preconceptions or what they are trying to sell.
Or the situation they choose to analyse is one that has got them thinking or troubled them, in my case trying to work out why 10kg of scrap lead made such a difference.
(I'm not trying to sell you any scrap lead).
 
lw395 said:
I thought in the simplest case, the idea was to keep the boat fairly still so there isn't much kinetic energy to dissipate.
When things get dynamic, yes damping comes into it.
But a lot of that damping will be the hull moving through the water and also the anchor moving through the mud. I expect to fully analyse the situation you might have to regard the seabed as a heavy viscous fluid.
.
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From standing at the bow with a foot on the snubber I'm fairly sure the largest loads come from the boat sailing around in the gusts, if theboat stays on the same tack then even in a fierce gust it's not too bad but when the boat gets some way on is when the snubber explodes water as it stops it. I've never dived on the anchor for a look after a blow but from from transits suspect it never moves much. Riding sail is a definite before the next big cruise :cool:
 
NormanS said:
Pulling anchors along a beach, using a tackle bears little resemblance to real life anchoring.
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That was my point. The anchor tests are not to simulate real life anchoring they are to test a very specific thing about various anchors. Dragging along wet sand as an example will show in controlled conditions that given the same bottom (flat sand) a given anchor will consistently dig in better. Because there are no snatch loads and no outside influences this test can be relied upon to show whether a CQR will set as well as a Delta. This specific test says nothing about ultimate holding power. Another test in controlled circumstances will show that thin chain or wire will allow the anchor to dig deeper when already dug in. This says nothing about the initial bite or the type of anchor. With these various tests and research read, the smarter posts on this thread have agreed that a modern anchor with snubber etc. will provide a better experience overall than an old design with massive chain and no snubber. If you read carefully the whole thread then you'll see various tests and research linked to. You'll also notice that all of the evidence in favour of catenary, old designs and heavy chain is in fact not evidence at all but rather old chaps saying they have seen it work so it must be ok. Real science doesn't rely on anecdote :)
 
lustyd said:
That was my point. The anchor tests are not to simulate real life anchoring they are to test a very specific thing about various anchors. Dragging along wet sand as an example will show in controlled conditions that given the same bottom (flat sand) a given anchor will consistently dig in better. Because there are no snatch loads and no outside influences this test can be relied upon to show whether a CQR will set as well as a Delta. This specific test says nothing about ultimate holding power. Another test in controlled circumstances will show that thin chain or wire will allow the anchor to dig deeper when already dug in. This says nothing about the initial bite or the type of anchor. With these various tests and research read, the smarter posts on this thread have agreed that a modern anchor with snubber etc. will provide a better experience overall than an old design with massive chain and no snubber. If yoOld carefully the whole thread then you'll see various tests and research linked to. You'll also notice that all of the evidence in favour of catenary, old designs and heavy chain is in fact not evidence at all but rather old chaps saying they have seen it work so it must be ok. Real science doesn't rely on anecdote :)
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You can carry on with your "real science" on dry land. I'll carry on anchoring (successfully ) in the real world, in the sea.

Cheers, from this Old Chap. Is that not rather ageist?
 
NormanS said:
You can carry on with your "real science" on dry land. I'll carry on anchoring (successfully ) in the real world, in the sea.

Cheers, from this Old Chap. Is that not rather ageist?
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You missed the point again unfortunately. I personally don't need to do science because I can read the studies done by others which allows me to sleep well at anchor knowing I won't drag. My main worry at anchor is people like yourself who choose to ignore the massive body of evidence showing a better way to anchor dragging and hitting my stationary boat!
 
lustyd said:
You missed the point again unfortunately. I personally don't need to do science because I can read the studies done by others which allows me to sleep well at anchor knowing I won't drag. My main worry at anchor is people like yourself who choose to ignore the massive body of evidence showing a better way to anchor dragging and hitting my stationary boat!
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Err, much as I love science and clever people carefully finding things out, Google scholar seems empty of any proper scientific studies on small boat anchoring, some stuff on mooring lines with interesting bits about friction etc but looks like we're stuck with magazine reports or even worse anchor manufacturers reports.
 
If you start with the length in metres of this thread in pages as a start point, you shant go wrong in 4/5 anchorages for a 30ft boat.:-))
 
As an aside the French long distance cruisers and American charterers are to be avoided at all costs in anchorages. The French come close alongside your boat turn right in front of you and drop their anchor then fall back to about 20 feet from your bows without enough chain out. It's difficult to move because they are over your chain and often won't move if you ask. The solution is to start raising the anchor and make it clear that you intend to ram their stern, they move fast. Every French boat that anchored in front of us dragged, one night in English Harbour, Antigua the wind got up and the French were off their boat, we stood watch and fended it off as it went by. Another time it happened was in Virgin Gorda, BVI's. I could go on.

The American charterers are generally inexperienced sailors or have a boat and only use marinas thus they have no idea how lay or set an anchor, often the chain is piled up over the anchor. To their credit you could see they were worried about the anchor not setting. Going over offering to show them what to do was always politely received and often ended up with a gift of a bottle of rum. Another American classic is because they haven't tied up to a buoy before they leave the engine in gear at low revs and sit in the cockpit wondering why they are going around in circles. It dawns on them eventually, it's very funny to watch.
 
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