Photos of Anchors Setting

[Llamedos]

I have just found this thread and have looked through it with great interest, so firstly many thanks to Noelex and the other contributors for some very informative photos and observations.


However there has been some mention and discussion on anchoring scope and the factors involved, even though everyone has different ways of determining their ideal scope ratio. So perhaps a little more scientific explanation and discussion may be of benefit.


I have no problem with the concept of high scope ratios, 8:1 or even 12:1, in extreme conditions or deserted anchorages if that is your preference, after all chain does no good in the locker.


We are currently sailing in the same Greek Islands and Turkish Coast as Noelex and NornaByron so are very familiar with the depths and bottom conditions. (Never met Noelex but have shared a few beers with NornaByron) We anchor our 42ft, 15 tonne cat every night in the summer and have done for nearly 5 years so quite a bit of anchoring experience I think!!


In the Greek Islands and the adjacent Turkish Coast the anchorages are generally fairly crowded and many are quite small, this means that long anchor scopes are rarely possible and could probably be considered a bit selfish since you effectively take up more room if everyone else is on lesser scope ratios, and problems will almost inevitably occur if the wind shifts appreciably. Consequently scope ratios of between 3:1 and 5:1 are normal in depths of 4 to 10m of water, longer scopes would be almost impossible in many places.


The biggest impact on the scope required is actually the weight of the chain and the catenary effect of that weight – basically a length of chain will always have a curve in it (catenary) unless you apply huge forces to it (or it is vertical!!). This curve means that the angle of pull on the anchor shank will almost always be less than the angle calculated from the actual scope.


Try this experiment to see how big this effect is: lay out a 6m length of chain along your deck or on the dock and secure one end to a cleat, take the other end and raise it to chest height, this is about 4:1 scope, try and pull the chain absolutely straight. You will find it just about impossible to achieve, there will always be some small amount of curve in the chain due to the catenary effect. If you like you can try with 12m of chain and see how much bigger the curve becomes, hence as the chain length increases the catenary effect becomes more important so the end of the chain attached to the anchor is at a much lesser angle than you might expect, in fact it will likely to be almost lying on the sea bed in almost all conditions – and this is the angle of pull that is being applied to your anchor shank, not the predicted angle. Obviously a yacht in windy conditions can exert much more force on the chain than you can but it will still not be able to actually straighten the chain in anything but the most extreme conditions.


Just for completeness here are the predicted angles between chain and sea bed, at 2:1 scope the angle is 30 degrees, at 3:1 the angle is 19 degrees, at 4:1 it is 14 degrees and 5:1 gives 12 degrees. You should be able to see quite clearly that the improvement as scope increases becomes less and less so higher scope ratios are really of little benefit normally.


As already mentioned in previous posts the modern generation of anchors are designed to function correctly on lower scope ratios, Noelex has already reported good performance with his Mantus on 2:1 and 3:1 scopes. Personally I like to use a 4:1 scope in “normal” depths and have found this to work extremely well with our Rocna style anchor. I would go as far as to suggest to everyone that for “normal” conditions and depths with a modern anchor you will be perfectly secure with a 4:1 scope, longer scope will not increase your security very much at all.


So now to the actual effect of catenary as depth of water increases, most people probably know that you may not need your 4:1 scope of chain in deeper water say 15m deep but how much shorter can you actually go?


First we need a starting point and that is the depth of water at which you are happy to anchor with 4:1 scope. For us that is 4.5m of water, plus 1.5m water to bow roller giving 6m total and hence 24m of chain. Our experience shows that this works very well for winds up to 25knts maybe 30knt gusts and minimal wave action. As I said at the beginning by all means put out more in extreme conditions. At this scope ratio our anchor sets very quickly, digs in deeply and holds extremely well when anchoring on the type of sandy bottom shown in most of Noelex’s photos.


So what happens as the depths increase? The catenary effect of the chain becomes more pronounced and has more effect so you need much less scope than you imagine. The following figures are for 10mm or 3/8” chain and the calculations are fairly straightforward engineering formulae so they are accurate figures not guesswork.


Our starting point then is 6m overall depth (water + roller height) and 24m of chain, the calculations show that for 10m depth you now need 30m of chain (3:1) but the catenary effect of the chain means that it works exactly the same as your 4:1 scope at 6m depth. For 15m depth the chain length required is 38m and at 20m you need 45m to give the same effect as 4:1 scope at 6m. The calcs also show that you need more chain in shallower water so 4m depth will need 19m or nearly 5:1.


The calcs can also be used to show how much extra chain you need to increase your safety by a factor of 3, so at 6m depth you need an extra 6m of chain and at 10m you need an extra 7m to effectively give you an extra factor of safety of 3. This means you should be secure in much stronger winds and/or bigger waves, probably up to 45/50 knts and choppy waves.


Hopefully if you have managed to follow the details this far you can see that you actually need much less chain than you probably thought to anchor safely and securely. The catenary effect is a huge factor and reduces the angle of pull on your anchor. Next time you are anchored, preferably with 4:1 scope in 6m of water, go for a swim when you have a good strong breeze blowing or even better some stronger gusts blowing and observe the action of the chain close to the anchor – you should be able to see that even very big gusts hardly lift the chain off the seabed close to the anchor. It is very informative to actually have a good look at this.


I hope that this helps to demystify the effect of anchor chain scope on anchor performance and the effect of deeper water. Obviously the above discussion only applies to all chain rode and smaller chain size will have a lesser effect, thicker chain will have a bigger effect. Rope rode makes hardly any difference so stick to all chain if you can.


In conclusion I believe a modern anchor will perform as designed with 4:1 scope in 6m of water and longer scopes make little difference to holding power in “normal” anchoring conditions and good holding on the sea bed. Poor holding is a different matter entirely and Noelex has already said that an oversize anchor can give acceptable (maybe marginal) performance in poor holding conditions or you just have to hunt around for some better holding.

[ImaginaryNumber]

Noelex,
How do you estimate (measure?) water depth, scope, and length of drag marks on boats other than your own?

[SVNeko]

This might be asking a bit much and I don't know if you have the gear, Noelex, but it would be interesting to see how a deployment of inline tandem anchors set.

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[noelex 77]

Originally Posted by ImaginaryNumber

Noelex,
How do you estimate (measure?) water depth, scope, and length of drag marks on boats other than your own?

Yes, that's a good question. When quoting distances, depth and scopes on other boats these are only estimates. It gets a bit cumbersome to write "in my estimation" or "approximately" on every occasion, but I hope people realise these are potential errors in these numbers.

Depth is reasonably easy. The bottom clarity from the surface is great clue as is the effort and difficulty to dive down. I know the depth of my anchor (from the depth sounder) and this is always the anchor I visit first. I use this as a calibration. A bit deeper takes a lot more effort diving down so I think the depth estimations are reasonably accurate.

For the length of chain it is usually a matter of just of estimating the distance like a golf shot, but I do sometimes used a timed swim. This is a useful trick if you are anchored with a potential hazard in your swing circle. Swim at a steady pace from your anchor to the back of the boat. Swimming for the same time, at the same speed, shows where your stern will finish if the wind swings around. (Assuming the chain is reasonably stretched out). It is surprisingly accurate. After a while you learn your swimming speed and can use this to measure distance instead of having to swim to the back of the boat each time.

Bow rollers heights are reasonably easy to estimate based on the size and type of boat.

I don't think the errors in my estimates are high enough to be significant with the exception of short scopes. There is not much impact on the anchor from a change in scope from 5:1 to 4.5:1 but there is a significant difference between say 3:1 and 2.5:1. Fortunately humans are better at estimating short distances so the error in my estimation of the scope goes down as it becomes more important. I take particular care at short scopes to get it as right as possible, but there are obvious limitations.

My mermaid provides a second opinion and we all know they are never wrong .

[noelex 77]

Originally Posted by SVNeko

This might be asking a bit much and I don't know if you have the gear, Noelex, but it would be interesting to see how a deployment of inline tandem anchors set.

I did try tandem anchoring and all its variations 6-7 years ago when I had a rather unreliable plow, a similar CQR and a much better Fortress. I was diving on the anchors just as frequently in those days, but it was before the camera, so sadly no pictures.

I came away not very impressed with any of the options deploying multiple anchors. This cemented my belief that a big good single anchor was usually the best option.

So sorry you will have to twist my arm hard to give it try again .

[noelex 77]

Originally Posted by Sulaire

I would certainly take a mantus as a second anchor, if i could convince myself I could do without the roll bar ( loooooong bowsprit) don't suppose you fancy whipping off the roll bar and giving it a few goes?

Thanks for the comments Charlie.
There have been a few calls for this so I will try and get around to it, but no promises.

I always watch the anchor land on the bottom if possible (and it usually is possible in this clear water). If you free fall it down, it lands with a satisfying puff of sand like a mini mushroom cloud .

Anyway on every occasion I have observed it, the Mantus has landed upright (not even on its side). It seems to naturally fall this way. However, it is very hard to know if different sized chain, or even a big swivel or removing the rollbar would alter the "flight characteristics" due to a different balance point or hydrodynamic drag.
I doubt it, but it is impossible to test all the combinations.

Slowly lowering the anchor obviously does not produce the same result. Although if traveling backwards (as you should do) it should also land upright but perhaps not with the consistency of the drop method.

If the anchor is dropped the risk of it landing upside down seems small at least in my configuration. This does not answer the question, but that's all I can offer at the moment.

[kas_1611]

Sulaire:

Not sure it would be possible but have you considered having your bow roller rotated so it is set off to one side like some of the modern smaller yachts do? I know they do this to accommodate a furling genoa but it might be an option to deviate by a couple of degrees to allow the roll bar to miss your bowsprit.

Just an idea to throw out there and I understand it may not be physically possible.

Keiron

[Pete7]

When you say "drop" how are you controlling the fall? presumably on the winch but doesn't that produce the same effect as slowly lowering?

I have always lowered by hand, but then I am using anchors 1/4 of the weight that I can lift with one hand and swing under the pullpit before lowering to the sea bed. Even so I did have a runaway in the early days and withdrew fingers and feet rather quickly as 30m disappeared into the marina never to be seen again.

Pete

[Sulaire]

Kas

Thanks for the idea , much appreciated,

I have a hans christian 41 , the bow rollers are integrated into the bowsprit about half way along, so no room for roll bars.

I have an ultra on one roller and a fortress fx-37 on the other, but i could be persuaded to swap out the fortress for a roll barless mantus if it would still work and i can find one for sale in agean greece or Turkey. The fortress would then go back in its bag in the bilge along side the "Kraken" ( 80 lbs luke anchor) for emerencies!

Ps i have told my wife if she ever sees me bring out the luke anchor in anger she best man the life boats!!

C.


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[noelex 77]

Today's anchors.

First, a typical Delta the same heaped up substrate, long drag mark (this was a bit shorter than most at 4-5m).

[noelex 77]

Next a claw.

I do have a bit of soft spot for a BIG Bruce (it needs to be oversized and the design seems to work better is sizes around 35+ kg), however, the Bruce copies (claw) anchors don't have much to recommend them.

This one had been given every chance dropped close to shore in softer sand in 6m of water with a very long scope @ about 10:1. It was a Med-moored boat. The drag mark extended over 10m and it is still on its side. Pretty pathetic really.

[noelex 77]

A Rocna

This is a typical excellent performance from the Rocna. It has set very quickly (18 inches or so)
There is a very slight amount of heaping up, a tiny list and some of the underside of the fluke is still exposed, but this is very much nit picking. The latter two problems would be fixed with a little more setting force. 7m @ 4:1.

[noelex 77]

Originally Posted by Pete7

When you say "drop" how are you controlling the fall? presumably on the winch but doesn't that produce the same effect as slowly lowering?
Pete

For many years I slowly lowered the anchor using the "power down" feature of the windlass.
Jedi suggested on an old thread on Cruising Forum that allowing the anchor to free fall by releasing the clutch meant the anchor hit the bottom with a thud, helping to break up a hard substrate.

After this suggestion it has become my preferred method of dropping. The size of the little mushroom cloud as the anchor hits the bottom gives some indication of the substrate. It is difficult to know if the proposed "breaking up of the substrate" has any merit. It will make little difference to anchor designs that take many metres to set and I think the effect (if there is any) is also likely to only be a factor when dropping larger anchors.

It does have the drawback that it is harder to hit a small patch on the bottom. Sometimes in the Med there are only small patches of sand a few metres in size and to hit these you need to lower the anchor almost to the bottom and drop as soon as you are over the top. If you drop from the surface it is hard to hit a small target. This is especially true of the Mantus which tends to fly away from the boat when dropped. The Rocna tended to drop more like a stone.

Please don't do this if you arrive at an anchorage in the middle of the night. It makes a hell of a din as the chain clatters out .

[forsailbyowner]

Ive been anchored out way more than not for the last 10 yrs. Some observations. Backing down seems just plain silly to me. Most places tides change direction 4 times a day and the anchor must reset constantly not to mention sudden changes due to storms, then there are the times the tide holds the boat perpendicular to the wind and there are many factors at play. My favorite setup for maximum security is setting anchors 180 deg apart and pulling both deep with the windlass. When the wind or tide is pulling both anchors which i try to set up for prevailing conditions I sit rock solid like on a mooring. When the conditions pull on one anchor the other stops the boat from sailing around and jerking the anchor as it hits the end of a swing and reverses position. I used to be a proponent of 1 oversized anchor until I was anchored in a place of strong tide on a very gusty night. The gusts would push me over and past the anchor then the tide would pull me back as the gust eased. the net result was that the chain looped around the anchor all nite until it was one big ball and dragged. Never seen a delta foul the chain till that night. ended up under a bridge and it was so close to disaster that I started the motor and motored off just as the masthead contacted the bridge. Only loss was a broken windex. Gave quite a show to the customers at a restaurant at the bridge. Im buck naked in panic mode mindless of the staring crowd and got a standing ovation when I motored off. From then on I used the 180 deployment tactic in high tide movement areas.

[noelex 77]

This was a rather sad and rusty plow.

It had dragged about 20m. The last metre through weed. I don't even think it was touching the bottom. The whole anchor was just in a great big weed ball.

In this case the weed actually stopped the boat and the skipper thought the anchor had "grabbed".

It is a danger with weed. The weed itself can provide enough grip to hold the anchor, even in moderate winds. People think their anchor has done a good job holding in the weed, because the boat did not move overnight. The reality with this sort of example is the grip is very poor. Without digging into the substrate the anchor will let go as soon as the wind reaches a higher level.