Anchor question,

[JumbleDuck]

The catenary effect is alive and well in the bosom of Physics.

Unless you are thinking of the weight and perhaps racing, there is little I see to commend long ropes attached to anchors. One of the great pleasures of getting a larger boat is kissing goodbye to lengths of wet, slimy, muddy difficult to stow rope.
Don't do it.

Good point. On the other hand, my crew is very small, and though the current CQR + chain is too heavy, should easily be able to manage an FX-7 plus rope, so having that as kedge and available for occasional main anchor use could be handy.

 

[Babylon]

My YM article was somewhat bigger when I wrote it but in the event they only had space for two pages...

Sums up why I no longer read YM (and most other magazines for that matter): too much real meat aways seems to get stripped out to make way for the bread of advertising, the lard of commercial placements, and much superficial glazing.

 

[vyv_cox]

If I was anchored in a F7 I'd want a considerably greater scope than 4:1. In fact I rarely use less than 4:1 except in very sheltered locations.

There were quite a few of us in there. We had already moved in the dark when the wind shifted and we felt we were too close to another boat. One of the big advantages of NG anchors is that they seem to hold well with small scopes.

I have written several times about our experience in Cala Volpe in Sardinia. We had well over 10:1 there in very shallow water but all the chain was lifting off the bottom in the gusts.

 

[vyv_cox]

Most reports I have seen indicate that a steel spade outperforms an alloy one of the same size.

I have never seen anything that says that and the only user of an aluminium Spade that I know says its holding and setting are just as good as the steel one. Do you have a link to the report?

 

[doug748]

............One of the big advantages of NG anchors is that they seem to hold well with small scopes.
.

Do you find the Rocna sets quickly on a variety of bottoms?

This is the key for me. My Bruce has never dragged when well set but in a couple of locations it has had problem getting a grip, rarely but enough to think it might be improved upon.

 

[adwuk]

I have never seen anything that says that and the only user of an aluminium Spade that I know says its holding and setting are just as good as the steel one. Do you have a link to the report?

Yes - this one: http://www.manson-marine.co.nz/Anchor Tests/Yachting Monthly -Anchor Test Nov09.pdf

 

[NormanS]

The oft repeated notion that in "x" knots of wind, all the chain is off the bottom, while true, doesn't mean that all catenary has disappeared. The catenary is still there, keeping the load on the anchor as close to horizontal as possible, and still giving some "give". The idea that an anchor chain can be pulled into a straight line is a nonsense. In shallow water, its benefit will be small, but in reasonable depth, catenary will help considerably, unless your chain is too light, of course.

 

[jimbaerselman]

One of the big advantages of NG anchors is that they seem to hold well with small scopes.

+1.
One advantage of chartering a lot is that I get the opportunity to try many different anchors in familiar anchorages (5m my favourites). I routinely test to check the scope at which full astern causes the anchor to drag on hard sand surfaces. Older ploughs and claws - setting is less reliable, whatever the scope. Flat anchors - 6, sometimes 7 needed. NG - all less than 6, and down to 4.

 

[GHA]

The oft repeated notion that in "x" knots of wind, all the chain is off the bottom, while true, doesn't mean that all catenary has disappeared. The catenary is still there, keeping the load on the anchor as close to horizontal as possible, and still giving some "give". The idea that an anchor chain can be pulled into a straight line is a nonsense. In shallow water, its benefit will be small, but in reasonable depth, catenary will help considerably, unless your chain is too light, of course.

and still giving some "give"--------very small, do the sums, between say, 200Kg & 300Kg load it's just a few inches. And because there's so little give the shock loads are much higher, pulling the chain well off the seabed.
Forget about catenary doing any real damping when you need it most, get a snubber.

Where light chain does loose out a lot, IMHO, is not slowing the boat sailing around the anchorage so much as its' heavier cousins.

 

[jimbaerselman]

The oft repeated notion that in "x" knots of wind, all the chain is off the bottom, while true, doesn't mean that all catenary has disappeared. The catenary is still there, keeping the load on the anchor as close to horizontal as possible, and still giving some "give". The idea that an anchor chain can be pulled into a straight line is a nonsense. In shallow water, its benefit will be small, but in reasonable depth, catenary will help considerably, unless your chain is too light, of course.

Two benefits are quoted for catenary:
(1) the angle of pull at the shank is reduced - allowing a shorter scope to be used
(2) during surges of stress, lifting the catenary provides a "spring" effect, softening snatch loads.

I'm sure someone here can look up tables for 36m of 8mm chain immersed in 5m water depth, with hawse 1m above water (6:1 scope) with 1,000kgf applied (the sort of stress achieved in surges). The straight line at anchor would be 10 degrees. I'll guess catenary reducing that by 2 degrees.

So, for benefit (1), catenary suggests you can use 20% less scope, say 5:1 rather than 6:1 for nylon (this is dirty maths, after all!)
And for benefit (2), lifting the chain; the stress/strain curve is asymptotic. Lifting a 12 degrees droop to 2 degrees occurs over 50cm, about 1.5% strain extension.
Nylon, on the other hand, extends linearly with stress up to 10% - 3.6m. A very much softer ride . . . much kinder to your deck fittings when you're checking a boat's inertia arising from yawing or vertical pitching. Hence the use of snubbers, of course, and the longer the better!

 

[vyv_cox]

This is the photo from the YM article. Taken in around 5 metres of water with a scope of about 4:1, engine pulling astern at 2500 rpm. I think most will agree that the chain is as close to straight as makes no difference. I know from many underwater expeditions that this engine speed equates to about force 6, although of course without gusts and lulls so the chain angle is unvarying.

 

[NormanS]

I don't see how you can say that the chain is straight, as the chain is disappearing into the distance, and obviously rising. You would have to show two photos taken from the side, at the same time. One down at the anchor, and one at the bow roller. They would show a difference in angle, illustrating catenary.

Unless you are out in the open sea, if you are anchored properly, you should not be getting snatch loads on your chain.

 

[Neeves]

Yes - this one: http://www.manson-marine.co.nz/Anchor Tests/Yachting Monthly -Anchor Test Nov09.pdf

Based on many anchorages with a cross section of bottoms I would not agree. Blindfold I could not tell the difference, and nor could anyone else. I'd hesitate to rely on one test to determine anchor choice - I would prefer a summary of a number of tests. In another test conducted by the same magazine, 2 years earleir or later, the Kobra did not do so well - but the conflict was never discussed. I simply would not carry the steel Spade - its no better than the alloy. The downside of the alloy Spade is that its shank might be weaker, from some tests - it is weaker, its a compromise we accept (and importantly know of).

Jonathan

 

[Neeves]

This is the photo from the YM article. Taken in around 5 metres of water with a scope of about 4:1, engine pulling astern at 2500 rpm. I think most will agree that the chain is as close to straight as makes no difference. I know from many underwater expeditions that this engine speed equates to about force 6, although of course without gusts and lulls so the chain angle is unvarying.

Vyv's image shows the anchor just set, with a bit more tension, some wind, the anchor will set more deeply - maybe Vyv can confirm - I am sure that often the anchor has disappeared completely along with a couple of metres of chain. Of course a 'disappeared' anchor is not very photogenic - all you see is chain disappearing into the seabed - not enough to excite anyone

The angle of the chain at the seabed is then determined by the catenary, or lack thereof - but the angle at the shackle is different, because it enjoys a reverse catenary imposed by the seabed. The angle at the seabed might be 8 degrees, the angle at the shackle of a well buried anchor can be 15 degrees. The angle of 15 degrees is only slightly influenced by the angle at the seabed - it is strongly influenced by the depth the anchor has buried. The other factor influencing the angle of the shackle is the size of chain, size of shackle and swivel if any. Anything big and chunky increases the angle at the shackle and reduces the anchor's ability to be buried further. It merits mention - rollbars, once they have done their work to self right an anchor, are a restriction to burying (ideally they should be streamlined and as small a surface area as possible).

So the focus on catenary and its impact on the angle of the chain at the seabed is only relevant to an anchor whose shackle is still visible. Once that shackle disappears (and its the angle of the shackle that determined how any tension is applied) then the angle of tension increases quite rapidly - and has little relationship with the chain angle. In fact a chain lying on the seabed, so 0 degrees, can still drive an anchor under the seabed and can still result in a 15 degree angle at the shackle. So the chain apparently is acting to best advantage in reality its no different to one acting at 8 degrees but producing that same 15 degrees at the shackle.

If your anchor does not bury, most images of CQRs shown them on the seabed - then catenary angle is important. Most, all?, modern anchors dive, or bury (unless they are too big). Many pictures of anchors that are big never show them buried. To me a well set anchor is one you cannot see.

The more chain you have buried the more tension is required to impact the shackle end during veering - as the chain has to be pulled through the seabed in which it is buried.

So another benefit of smaller chain - it allows your anchor to set more deeply and has a positive benefit if the yacht is veering.

Jonathan

 

[noelex]

Surely the hammer is the yacht and the nail the anchor? The weight of the anchor is not relevant - most people who use them would say the same sized Spades (talking anchors now, so not the garden variety ), alloy and steel perform similarly.

I have never seen anything that says that and the only user of an aluminium Spade that I know says its holding and setting are just as good as the steel one. Do you have a link to the report?

The steel Spade is an excellent anchor. Unfortunately the aluminium version, while still a good anchor, or even a very good anchor, is not as competent in hard substrates in my view.

See this thread. Paticuarly post 28:

http://www.ybw.com/forums/showthread.php?359258-Anybody-got-a-Spade-A80/page3

Maine Sail produced a video showing the aluminium Spade refusing to set where the steel version easily penetrated. To address criticisms from users Spade increased the ballast in the subsequent aluminium versions. I suspect this was a token amount as observing these anchors underwater there is still a considerable gap between the performance of the Steel and aluminium versions.

It should also be noted that the aluminium version suffered problems with galvanic action between the aluminium and lead. These problems seem to have been solved or at least reduced with later versions.

 

[RichardS]

I don't see how you can say that the chain is straight, as the chain is disappearing into the distance, and obviously rising. You would have to show two photos taken from the side, at the same time. One down at the anchor, and one at the bow roller. They would show a difference in angle, illustrating catenary.

Unless you are out in the open sea, if you are anchored properly, you should not be getting snatch loads on your chain.

I agree with Vyv that the chain is as straight as makes no difference. Any "slack" in the chain would be easily noticeable at the anchor end where the slack would show itself as the chain lying close to the seabed .... and it clearly isn't.

As for snatch loads? You can be anchored in what appears to be the most sheltered bay on earth but if a gusty wind and/or or a swell gets up then you will get snatch loads. I simply do not think it possible that you could always anchor somewhere where there is no snatch unless you only ever take the boat out of the marina and stay out in perfect settled conditions.

Richard

 

[Neeves]

I suspect this was a token amount as observing these anchors underwater there is still a considerable gap between the performance of the Steel and aluminium versions.

It should also be noted that the aluminium version suffered problems with galvanic action between the aluminium and lead. These problems seem to have been solved or at least reduced with later versions.

If an anchor maker has taken note and improved their model or design I fail to see why the original version with its faults continues to enjoy your attention. Most of the posts and threads are of people wanting to choose a new anchor - they want to know about today's quality. We all know of the faults of Rocna, and most of us agree this is history. Some of us also know that the Mantus had a mild steel shank - but this has been rectified. So why this constant denigration of faults that have been over come? Give credit where its due, they had faults they rectified them. Possibly in your Mantus thread you constantly remind people of the mild steel shank - but somehow I doubt it.

We have had our alloy Spade for 5 or 6 years and have no sign of corrosion between lead and alloy. Our alloy Spade was subsequent to the modification of lead in the toe and works a dream. My comments are not anecdotal nor pre- modification. My comment refer to the current model, as fas as I know, and based on our own usage - its as good as the steel version.

I recall your constant and repetitive denigration of convex anchors even though you had never used the model on which you focussed and certainly never seen one in use. I now note there are videos showing it to be one of the best anchors available. If you actually compared a steel and alloy Spade and used them yourself in a number of different seabeds - your comments would have merit. You are showing partiality based on anecdotes.

Jonathan

 

[NormanS]

I agree with Vyv that the chain is as straight as makes no difference. Any "slack" in the chain would be easily noticeable at the anchor end where the slack would show itself as the chain lying close to the seabed .... and it clearly isn't.

As for snatch loads? You can be anchored in what appears to be the most sheltered bay on earth but if a gusty wind and/or or a swell gets up then you will get snatch loads. I simply do not think it possible that you could always anchor somewhere where there is no snatch unless you only ever take the boat out of the marina and stay out in perfect settled conditions.

Richard

Nobody is suggesting that there is "slack" in the chain, but it is impossible for the chain to be straight. A photo from one end of a chain disappearing into the distance shows that there is tension in the chain, nothing more.

I don't do marinas.

 

[JumbleDuck]

The angle of the chain at the seabed is then determined by the catenary, or lack thereof - but the angle at the shackle is different, because it enjoys a reverse catenary imposed by the seabed.

A catenary is the curve you get when a flexible member is loaded with a constant load per unit length (if it's a constant load per unit span you get a parabola), so while this certainly applies well to a chain suspended in seawater I am not at all sure that it would apply to a chain buried in the seabed. In fact I am pretty sure it wouldn't.

The angle at the seabed might be 8 degrees, the angle at the shackle of a well buried anchor can be 15 degrees. The angle of 15 degrees is only slightly influenced by the angle at the seabed - it is strongly influenced by the depth the anchor has buried.

Has anyone actually measured these figures?

 

[RichardS]

Nobody is suggesting that there is "slack" in the chain, but it is impossible for the chain to be straight. A photo from one end of a chain disappearing into the distance shows that there is tension in the chain, nothing more.

I don't do marinas.

You don't do marinas and yet you believe that "if you are anchored properly, you should not be getting snatch loads on your chain."

Please tell us all your anchoring secret? :encouragement:

Richard