Anchor Scope - Newbie Question

[Neeves]

Roberto,

I was trying to be too simple, but I was hoping KellysEye would think about it. Of course there is the weight of the chain, in my 80kg example - the chain weighs about 40kg. But to suggest as KE does that there is no load on the anchor is (when the chain is horizontal), basically nonsense (which is what I wanted him to think about). The load might be low, but its still there. And if the anchor was not there you would rapidly find yourself attached to a beach (or rock) If you take 50m of 8mm chain and drag it along the seabed, you can do it by hand easily (and because you are doing it by hand its basically all dragging on the seabed, which seems to be horizontal) - or you can, drag it, if there is not an anchor attached.

To suggest you can carry enough chain such that you basically do not need an anchor is possibly correct - but none of us can carry that amount of chain. Most of us need to live in a reality which means being forced to use, say, a maximum of 50m (because there is no room to deploy more) and given the vagueries of weather you need a good anchor to make sure you do not move. Riding sails are one mechanism to stop snatching, 2 anchors in a V is another, good snubbers are another, an anchor off the bow (dragging on the seabed) is another, a bucket off the bow - all work - just because you do not see riding sails does not mean someone has not found, for them, a better way.

And finally - to carry bigger, heavier, chain is counterproductive - it will stop your anchor setting properly.

If you read any information from the Classification Societies (Lloyds Register etc) you will find their rules on anchors are predicated by a statement that suggests the rules apply to anchors that are to be used under benign conditions prior to entering port. Here, Sydney, we have a queue of ships waiting to enter Newcastle Harbour to load coal. There can be anything upto 40 of them, they are bulk carriers of around 2/300,000t. Some of them stop their engines way north and simply drift down on the East Australian Current (which is a bit disconcerting if you are sailing north - as you can see a vessel, obviously large, moving toward you, broadside on), some anchor and some just motor gently around in lazy circles. Those that anchor are ordered off the anchorage (its really the Tasman Sea, at about 40m deep) when winds here are around 25knots (not sure of the how/when) and are not allowed to sit at anchor (we have had too many on the beaches). We have had 2 days of 40 knots and none are at anchor just now.

 

[jimbaerselman]

So if your cell load on the anchor chain measures a given load, it is very, very likely that the anchor pull will be a lot less than that.
All in purely static terms of course.

Roberto,
Good to see a proper force analysis.
I'd add another sentence:

"And, of course, if you have 5:1 scope or longer, and it is stretched, then the pull at the anchor will be only a very little less than the measured load"

 

[NormanS]

Neeves.
Tell me how using heavier chain is going to stop you anchor setting properly?
I'm sure that a lot of anchoring problems are caused by people using chain which is so light that it doesn't form a useful catenary in strong winds.
8mm chain may well be strong enough for an average 35 ft yacht, but it is nowhere near heavy enough for reliable anchoring in all weather.

 

[vyv_cox]

Neeves.
Tell me how using heavier chain is going to stop you anchor setting properly?
I'm sure that a lot of anchoring problems are caused by people using chain which is so light that it doesn't form a useful catenary in strong winds.
8mm chain may well be strong enough for an average 35 ft yacht, but it is nowhere near heavy enough for reliable anchoring in all weather.

Many people anchor using mixed chain and rope rodes with total success. My Fortress, with 7 metres of chain and Octoplait, sets in extreme conditions and has never come even near to dragging. Indeed, the problem is usually getting it out of the bottom. Alain Poiraud used a mixed rode on his Hylas 46, quite light chain plus rope. He spent a lot of time around Patagonia and further south and seems to have experienced little problem with his anchor arrangement.

 

[Neeves]

Neeves.
Tell me how using heavier chain is going to stop you anchor setting properly?
I'm sure that a lot of anchoring problems are caused by people using chain which is so light that it doesn't form a useful catenary in strong winds.
8mm chain may well be strong enough for an average 35 ft yacht, but it is nowhere near heavy enough for reliable anchoring in all weather.

I wondered if this might raise some eyebrows

When a modern anchor sets, and this includes a Delta and Bruce (type) they set with both the shackle end of the shank and the toe of the anchor digging in at the same time. More modern anchors set the same way. More modern anchors will continue to set under load (call it the wind) until they disappear completely. Anchor tend to set with the described orientation such that as/if they disappear the top of the curve of the shank disappears last, toe and shackle being roughly at the same depth. The more load, the more they disappear, as they disappear they drag the chain down with them. Even in 30 knots a well set anchor should have pulled maybe 2m of chain under the seabed (depends on the seabed). The bigger the chain, the bigger its surface area, the more difficult it is for the anchor to pull the chain down - the more difficult to pull the chain down the shallower they will set. Buried anchor chain has a reversed catenary - simply because of this issue. The deeper your anchor sets - the better the hold.

In the case of a CQR and Delta, possibly does not matter, they are not known to dive - more a surfing anchor. and heavier chain might be advantageous, but I suspect marginal. In the case of a Bruce type - they do dive in the right seabed but the heavier the chain the greater the surface area (of the chain) the less they can dive. Similar with new anchors, Spade, Rocna, Supreme etc (and with the latter 2 they need fight the surface area of the roll bar as well).

This is not a major issue with Fortress, or Danforth where the anchor flukes sets long before the shackle end of the shank.

If your anchor is big, you would have a bigger chain - but there is a tendency to think catenary is a yachtowner's right to successful anchoring (and on this basis people buy bigger chain than necessary). Its the bigger chain, disproportionately, that is the issue. You would be better off with 100kg of more chain of a lighter diameter than a bigger chain and heavier diameter (on the basis the lighter chain is of the right size for your yacht). Its more extreme when going from 10mm to 12mm, but still an issue to consider when going from 8mm to 10mm.

Anchors actually set better with a wire strop, than with chain, but there are other issues of maintenance - so not generally recommended.

And strangely lots of people anchor quite successfully with a few 10's of metres of chain, like 20m, and nylon - catenary is not all its cracked up to be.

In strong winds a 8mm chain will lift, completely, not long before a 10mm chain (assuming the same yacht) - in strong winds with both chains lifted - the 10mm chain offers little real advantage - what you need is a decent anchor, snubbers and anything that stops you veering. If your yacht merits 10mm chain and you consider 12mm - the same argument holds, the 12mm will sit on the seabed longer but at 30 knots expect it (for the same length, unless you can carry 200m!) to be off the seabed and offer little 'catenary' advantage.

Kellyseye remarked on how few people use a riding sail - I'm amazed at the numbers who do not use snubbers - and the few that do tend to use a 2m snubber (which offers little elasticity at all).

edited addition

Vyv posted as I was composing - but he seems to echo one of my comments - that catenary is not the panacea some think.

Jonathan

 

[KellysEye]

>But to suggest as KE does that there is no load on the anchor is (when the chain is horizontal), basically nonsense (which is what I wanted him to think about). The load might be low, but its still there.

OK explain the physics of how there is load on the chain when lying flat on the bottom.

>And if the anchor was not there you would rapidly find yourself attached to a beach

Some years ago a gale was forecast and a yacht anchored of Southend pier putting out all their chain and anchor. When they pulled up the chain there was no anchor attached and they hadn't moved. That was written up in one of the yachting mags. I'm not suggesting anyone does that but it does prove that if you have enough chain, which few do, the chain can work alone.

Bigger chain doesn't stop an anchor setting, you would just need less catenary.

 

[Sybarite]

There's always a great deal of 'opinion' bandied about in threads like this, on forums right across the world, from Tasmania to Tobermory. There's usually something along the lines of "This is what I do, and I've been doing it for forty years or more, so it must be right. You, on the other hand, haven't. so you can't be right."

So, FWIW, I've been anchoring for over 40 years too, using all sorts of 'ooks from all sorts of boats, and the more I do it and the more I read of others' deep experience, the more I learn about it. I've anchored a cruising trimaran in 4m in Gallanach Bay, by Crinan, with 5x and dragged an ill-set CQR. Another 10m. scope had it hold fine. I've kedged, twice, and successfully in very deep water > 80m. during Fastnet Races various a la Adlard Coles in Cohoe III. I've anchored a 6T Rival with a flying moor at the head of Kilnaughton Bay/Port Ellen, on a 25lb CQR and 30m chain, and it held first time, all right and all night, in a NW gale. I've used Danforth copies successfully ( !! ) around the Channel Isles in borrowed boats to 55'. I've dragged in the mildest of breeze just off Falmouth's Custom House Quay and in Puildobhrain, when I thought I had enough anchor, chain, room and set..... Likewise off the pub at Inverie.

I've always believed in 'big anchors, lots of chain/scope, and a determined approach to getting the thing set'. None of it, not any specific technique, set of tackle, or tricksy ways with 'angels on horseback' is clearly and unarguably reliable. Every last anchoring situation I've been in or can think of needs a suspicious eye and systematic checking, to ensure that one's guests and dependents sleep safely below, every time. Counsel of perfection? My judgement, my choice.

These days, I'm sufficiently convinced of the improved performance of recent designs of anchors - once set - to shell out some shekels on one or other of 'em. But that's only a small part of the process. Setting technique matters, and so also does scope. Especially scope.....

I've explored just about all the learned articles by deeply-experienced types, the research published by the respected mags, the columns of detailed and reasoned opinion on sailing websites world-wide, and even some of the stuff on here, too. I'm more than ever convinced that, when it matters, a large digging anchor, lots of chain PLUS nylon warp, lots of scope, good holding in a shallow sheltered site, is important to making this process work. When it doesn't matter - a gentle summer's evening in the Solent, for example - than any old approximation to the above will probably do just fine.

These days, I'm persuaded in those views by the research and published work of Alain Fraisse, of Dr Knox, of John Barry, and a certain Jonathan Neeves of Tasmania - much of which is convergent. I no longer believe that 'all chain is best', but that 'enough' chain for the circumstances PLUS an added stretchy nylon rode is best when it matters. The elasticity of the nylon absorbs shock 'snubbing' loads from short seas which peak at several times the steady-ish loads imparted by heavy breeze. Not so with all-chain.

It matters that that the pull of the chain on the anchor's shank is horizontal, or certainly as close to horizontal as one can make it, by veering 'enough' scope. New-design anchors are a bit more tolerant of 'angulation' than the older ones, but not by very much - and I can't measure that from the boat. So I want to make sure that I have a 'flat pull' by putting out enough scope to ensure that.

Some are keen to have and use the lightest ( cheapest ) anchor they think they can get away with. More than just understand that, I DID that for some decades. These days, as I have no way of knowing in advance exactly what the quality of 'holding' will be where I place my hook, I use the largest anchor I can reasonably manipulate on the deck. My own experience has shown me that sometimes, perversely, what ought to be good holding and easy ( see above ) turns out to be bluddy-near-impossible. And after a long cold day and a long cold beat from Donegal to Port Ellen, when the engine-gearbox selected 'stuffed' and a sudden NW gale sprung up, having to beat in the night into a dark haven with nowt other than a Reed's chartlet, I wanted that anchor to go down and set, right first time.

Some will argue that a lighter modern-design of anchor can be substituted for a heavier traditional type. But when it matters, it's the area of the fluke - displacing a certain volume of seabed - that matters. Not weight.

I now use Alain Fraisse's interactive website and John Barry's 'goalseeking' spreadsheet to help me determine, for my boat and selected depth and weather conditions, the combination of chain and nylon warp that will ( should ) amply meet the need. I simplify this into 'On my boat, in less than 10m depth and up to 45 knots, my 50m. of 10mm chain plus my 50m. of 18mm nylon should suffice.' I can improve on that should I have, for example 'In less than 5 metres depth and up to 45 knots, then 30m. of 10mm chain plus 30m. of 18mm nylon should suffice.'

Should there be a forecast of wilder weather, and I have nowhere else to go, then I can consider an additional anchor, taking lines ashore, running the engine ( 'dodging' ) to help take the strain in gusts.

However mild the weather, I am unlikely these days to veer less than 6x the total depth. That excellent chain does me no good lying in the chain locker.

I agree with everything you say with one slight nuance. I prefer an all chain rode but with nylon or mooring rubber snubbers added to absorb the shock loads. It's perhaps pyschological but I feel a join somewhere is a source of weakness.

 

[NormanS]

I am happy in my belief that heavy chain gives a useful catenary, in a reasonable depth of water. Obviously, if anchoring in shallow water, the only advantage of using chain, is in preventing chafe, and so if I was in the habit of anchoring in shallow water, I would probably use a nylon rode with a few metres of chain next to the anchor.

I find it incredible that the slight increase in surface area of heavier chain prevents any anchor from setting. After all, the increase in surface area is matched with the greater weight of the chain, so I wouldn't have thought that there would be any measureable difference. If this was seen as a problem, it would be common practice to have a length of wire next to the anchor. I have seen no evidence of this.

Re anchor riding sails, last week I carried out some experiments with a twin riding sail, hoisted on my mizzen mast. Although it was just a temporary lash-up, I was impressed with its ability to stop yawing. I am now asking some sailmakers for quotes. I can see problems in that, when it is really windy, when the sail would do most good say 50 knots plus, do you really want the extra windage?

 

[Otter]

If you have a heavier anchor and chain than the pundits suggest for your boat, you can get away with 3:1 in fair weather. This is a useful trick if, like me, you spend your time anchoring on the edge of channels on the East Coast where you are between, not the devil and the deep blue sea, but the saltings and the big ship channel!

Agreed, we've had more problems anchoring from having two much scope than too little. Several times we've run aground because a tide change in a channel has swung as round on a big scope and we've stuck on the bank. We never use more than 4:1 now but haven't anchored in a blow in water with a big fetch which would require more.

 

[windfan]

In all the discussion on chain and nylon rode there has been no mention of breaking strain. In over 20 years of anchoring in various locations I have witnessed several snapped anchor chains in violent conditions. My solution 25m 10mm chain and 150m of 22/25mm nylon Octoplait. The nylon has over twice the breaking strain of chain and can stretch to double its original length much more effective than the effect of catenary IMHO.

 

[RichardS]

Some years ago a gale was forecast and a yacht anchored of Southend pier putting out all their chain and anchor. When they pulled up the chain there was no anchor attached and they hadn't moved. That was written up in one of the yachting mags. I'm not suggesting anyone does that but it does prove that if you have enough chain, which few do, the chain can work alone.

Bigger chain doesn't stop an anchor setting, you would just need less catenary.

Do you have a reference to this as I simply don't believe it!

You seem to be suggesting that friction alone between a long chain and the seabed can resist a pull of hundreds of kgs and I just cannot see it. The anchor works by physically digging itself into the seabed rather than relying on surface friction. If it were relying on surface fricition you could use a weight of the same mass as the anchor and achieve a similar effect. Whilst this would be a slight improvement on chain alone, I firmly believe that a 25kg weight on the end of my chain would still drag hopelessly in anything above 20 knots.

Richard

 

[westernman]

Kellyseye remarked on how few people use a riding sail - I'm amazed at the numbers who do not use snubbers - and the few that do tend to use a 2m snubber (which offers little elasticity at all).

I often use a couple of meters of nylon warp (22mm - the same big chunky stuff I use when I tie up in a marina).
In a decent bit of wind it will easily stretch to twice its length - that is enough give to take the shock load of the chain.

I would have thought that too much elasticity is not good either - you would end up see sawing all over the place.

 

[Neeves]

I often use a couple of meters of nylon warp (22mm - the same big chunky stuff I use when I tie up in a marina).
In a decent bit of wind it will easily stretch to twice its length - that is enough give to take the shock load of the chain.

I would have thought that too much elasticity is not good either - you would end up see sawing all over the place.

Nylon, has a maximum elasticity of between 30% and 40% - it depends on the method of manufacture - it then stretches inelastically and fails (at not much more than the 30% and 40%). To have 22mm nylon stretching 100% looks unusual(?).

I would say, unless the yacht is heavy then 22mm is too thick (you seldom get any stretch - but Michael your experience suggests otherwise) and for a 35' yacht around 10/12mm would be good - but 10m - 12m length. 10mm breaking strain about 2.7t, 22mm breaking strain about 12t (not sure why you need 22mm mooring lines!)

And no you do not go see (nor sea?) sawing all over the place. It takes the snatches and gently puts you back where you were.

The only problem with mixed rodes is wear, or chafe. In Oz many yachts are equipped for the reef and coral even dead coral pieces (ghoulish though it seems) has an insatiable appetite for nylon. Living coral will eat nylon overnight (or so it is said - and I'm not willing to test it out). Consequently we nearly all use all chain.

Jonathan

 

[Neeves]

>But to suggest as KE does that there is no load on the anchor is (when the chain is horizontal), basically nonsense (which is what I wanted him to think about). The load might be low, but its still there.

OK explain the physics of how there is load on the chain when lying flat on the bottom.

>And if the anchor was not there you would rapidly find yourself attached to a beach

Some years ago a gale was forecast and a yacht anchored of Southend pier putting out all their chain and anchor. When they pulled up the chain there was no anchor attached and they hadn't moved. That was written up in one of the yachting mags. I'm not suggesting anyone does that but it does prove that if you have enough chain, which few do, the chain can work alone.

Bigger chain doesn't stop an anchor setting, you would just need less catenary.

Kellyseye,

I will not explain the physics - but let you prove there is no load.

Take some of you chain off your yacht. A 30m length is manageable. Drop it as a big heap on the ground under the highest window in your house, drop a rope out of the window - you sit on the window sill with the rope. 2 storey house should be about 4m. Have your wife walk about 20m away from the wall (with the window in it). You lift the chain up with the rope, have you wife stand on the other end of the chain at 20m. Wonder why the chain slips? Take a video and post it.

A different experiment, take 50m of 8mm chain off your yacht, lay it in the sea next to the wave line (so its in the water). I'm a 74kg weakling and I can drag it along the beach, its a struggle to get it started - but once it moves its easy. I guess the scope is around 30:1. Pulling sideways is a different issue. A wind generating 70/80kg - summer breeze of 20-25knots, 35' - 40' yacht.

Oddly - you do need an anchor.

Jonathan

 

[Deleted member 36384]

....... chafe. In Oz many yachts are equipped for the reef and coral even dead coral pieces (ghoulish though it seems) has an insatiable appetite for nylon. Living coral will eat nylon overnight (or so it is said - and I'm not willing to test it out). Consequently we nearly all use all chain.

Jonathan

There is a possible solution to the risk of chafe in shallow water. The nylon snubber can made off to an aft cleat and run along the side deck and connected to the chain just over the stem head fitting. This way the stretch is all on the boat and with no nylon rode in the water. Of course deck related chafe needs to be addressed.

 

[westernman]

Nylon, has a maximum elasticity of between 30% and 40% - it depends on the method of manufacture - it then stretches inelastically and fails (at not much more than the 30% and 40%). To have 22mm nylon stretching 100% looks unusual(?).

I would say, unless the yacht is heavy then 22mm is too thick (you seldom get any stretch - but Michael your experience suggests otherwise) and for a 35' yacht around 10/12mm would be good - but 10m - 12m length. 10mm breaking strain about 2.7t, 22mm breaking strain about 12t (not sure why you need 22mm mooring lines!)

And no you do not go see (nor sea?) sawing all over the place. It takes the snatches and gently puts you back where you were.

The only problem with mixed rodes is wear, or chafe. In Oz many yachts are equipped for the reef and coral even dead coral pieces (ghoulish though it seems) has an insatiable appetite for nylon. Living coral will eat nylon overnight (or so it is said - and I'm not willing to test it out). Consequently we nearly all use all chain.

Jonathan

I can't swear it was 100% - it looked like it. The wind was strong and very gusty. In the gusts the boat moved back a good way taking up all the catenary in the chain and stretching the warp so that the chain was straight under that too. Mind you that particular night bent the substantial stainless steel double hook used to attach the warp to the chain.

I use three strand 22mm nylon warps. It is the biggest size I can conveniently handle on my boat. I have broken quite a lot of old 18mm sheets which the previous owner used as warps due to snatch loads.

The biggest problem I have now with warps is chafe. I have firehose around them for protection, but I chafe through the firehose fairly rapidly as well - but then the marina I in regularly gets hurricane force winds during winter.

The boat displaces about 32T with full water and fuel tanks.

 

[Neeves]

Sorry Michael, I had assumed you had a 'normal' (whatever that is!) yacht - not a 32t tonner. I understand why you might use 22mm warps! But if they stretch 100%, they are not nylon! It does depend on construction, but even 3 strand will not stretch 100%. Sheet rope has very limited stretch (or none if its Dyneema) - which is possibly why they broke. We have broken snubbers as well, but not quite your size - they simply snapped, no wear (just overuse). We have slowly moved up in size, for snubbers, but are still only using 2 x 14m x 11mm (its a 6t cat).

Most rope makers have elasticity charts, or limits - and I am sure Marlow have information on their nylon. Locally (to Oz and NZ) Donaghys' quote 40% dry and 35% wet for nylon and Liros quote a bit less (but again it depends on construction.) Bungy cord (for jumping off bridges etc) is built to 2 standards either 100% or 200% elasticity (not sure what they are built from).

I'd be guessing but suggest that longer lines, as long as you can accommodate as both mooring lines and snubbers will help (as it spreads the loads). Chafe is the big issue and I guess you have worked through most of the options (and fire hose is at the top of my list).

Really your best option is migration - its really nice here!

Jonathan

 

[oldbilbo]

I have the suspicion that 'Sinnot', the original poster, may be wondering why on earth he bothered! Nonetheless, here's another perspective to add some more 'salt to the porridge'....


http://cic.oceancruisingclub.org/publications/1019


To counter the problem of drifting away while rowing out the line ashore, I developed the habit - while barebumboating in the Greek Islands - of dropping the hook a little way out then sending the First Mate to swim ashore with a long line in a bag, and with a small fender. The task was to tie the line to a tree, then swim the end back to the boat . That saved me the effort of pumping up the inflatable....

One problem I had to overcome was that she could rarely remember any suitable knots-for-trees. 'Three or four round turns and a kerfankle' seemed her preferred technique, so I always had to check and reset when I went ashore later... after the 'cold, sharp Harp!'

The fender? That was to keep her afloat when she swam back, if I had forgotten to lower the swim ladder and had fallen asleep below...

 

[westernman]

I have the suspicion that 'Sinnot', the original poster, may be wondering why on earth he bothered! Nonetheless, here's another perspective to add some more 'salt to the porridge'....


http://cic.oceancruisingclub.org/publications/1019


To counter the problem of drifting away while rowing out the line ashore, I developed the habit - while barebumboating in the Greek Islands - of dropping the hook a little way out then sending the First Mate to swim ashore with a long line in a bag, and with a small fender. The task was to tie the line to a tree, then swim the end back to the boat . That saved me the effort of pumping up the inflatable....

One problem I had to overcome was that she could rarely remember any suitable knots-for-trees. 'Three or four round turns and a kerfankle' seemed her preferred technique, so I always had to check and reset when I went ashore later... after the 'cold, sharp Harp!'

The fender? That was to keep her afloat when she swam back, if I had forgotten to lower the swim ladder and had fallen asleep below...

A real gentleman.

 

[Sinnot]

I got my answer in the first couple of replies Thanks to everyone who has replied, its been an interesting debate. (but note to self, no more new threads about Anchors)