Anchor Scope - Newbie Question

[Sinnot]

From what I've read, an Anchor scope of 5:1 / 7:1 is what should be normally used to anchor safely and thats fine at night. But during the day in a busy anchorage what should I be putting out ?. If I did a scope of 7:1 and the wind shifted I'd hit every other boat in the Anchorage. To me it seems like people are putting out a scope of 3:1 or 4:1 which is what I currently do and then when everyone moves off in the evening I extend that to 7:1.

 

[onesea]

From what I've read, an Anchor scope of 5:1 / 7:1 is what should be normally used to anchor safely and thats fine at night. But during the day in a busy anchorage what should I be putting out ?. If I did a scope of 7:1 and the wind shifted I'd hit every other boat in the Anchorage. To me it seems like people are putting out a scope of 3:1 or 4:1 which is what I currently do and then when everyone moves off in the evening I extend that to 7:1.

I have always taken the view if it did not drag I had enough scope

Depends on the weather, ground tackle the holding etc... But sounds abut right to me.

 

[dunedin]

Depends on a range of factors - notably whether all chain or chain / rope mix, and weather expected.

With all chain I tend to work on the basis of
- 3:1 for lunch stop staying on board
- 4:1 to 5:1 for night stop or going ashore
and usually round up a bit for luck

Remember scope measured from waterline not the cleat - that's a 3 metre difference on my boat - and allowing for high tide.

 

[photodog]

For me it depends on two things....

How much room, and do I hold....


Ergo... Put out as much as you can get away with.
That's how I reckon.

 

[cliff]

Short stop onboard > 3Xdepth at high water + an additional 10' (3m) so in 10' I would drop 3X10+10 =>40' - the additional 10' is from waterline to the winch/chain cleat
Over night or leaving boat > 5Xdepth at high water + an additional 10' (3m)
Getting bumpy > 7Xdepth at high water + an additional 10' (3m)
Getting really bumpy > ALL out (270') plus rope rode ready to attach if needed.

 

[vyv_cox]

Depends on a range of factors - notably whether all chain or chain / rope mix, and weather expected.

With all chain I tend to work on the basis of
- 3:1 for lunch stop staying on board
- 4:1 to 5:1 for night stop or going ashore
and usually round up a bit for luck

Remember scope measured from waterline not the cleat - that's a 3 metre difference on my boat - and allowing for high tide.

That's about what we use in average conditions, although we are now in the fortunate situation of having no tides to worry about. In stronger winds when we have had to sit at anchor for several days we typically use far more. In the past couple of years we have had 35 metres of chain out in 2 - 2.5 metres water depth. The biggest problem when anchored close off the shore is the varying wind direction in gusts, which causes to boat to sheer about, working the anchor from one side to the other. This seems to be one of the bigger contributory causes of dragging, which is reduced by putting lots more chain out. Ultimately we use two anchors in a fork moor, which about halves the sheering angle.

 

[Kukri]

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!

In very shallow water a boat can drag, even with a very big scope, because not enough chain is clear of the bottom to form a catenary, so a snatch load starts the anchor (it helps to draw this to scale, to see the point!) In such a situation the best thing to do is to use a "kellet" or "angel" - a big shackle which slides down the chain with a weight attached (with a line to your bow!) and drop that underfoot.This is a good way to stop charging about the anchorage anyway.

 

[jimbaerselman]

For a lunch stop, often you're just putting a weight on the ground so you don't shift in the probable circumstances.

Assuming you are a sail boat. For a good sleep, or to leave the boat for a period, you'll want the anchor to be able to develop its best holding power. That usually means it should be able to hold full astern (a steady force of course!). Different anchor designs have different critical angles of pull - the critical angle at which it will start to dig out, rather than dig in further.

You can find your critical angle by experimenting, re-anchoring using your usual "dig in" techniques each time. Try 3:1 first . . . did it hold? Almost certainly not. Now 4:1 . . . hmm. Not likely. Now 5:1 . . a few modern anchors will dig in fine. 6:1 . . . you're on the margin for some flat anchors, such as the Britany or a Fortress set shallow. 7:1 seems to be a winner . . . unless it's a CQR in soft mud, or a fisherman which can't find a rock.

Sound like you've got it right! But you can remove your uncertainty by testing, and it'll be valid for your kit. You can then ignore other's advice. You know!

 

[Kukri]

For a lunch stop, often you're just putting a weight on the ground so you don't shift in the probable circumstances.

Assuming you are a sail boat. For a good sleep, or to leave the boat for a period, you'll want the anchor to be able to develop its best holding power. That usually means it should be able to hold full astern (a steady force of course!). Different anchor designs have different critical angles of pull - the critical angle at which it will start to dig out, rather than dig in further.

You can find your critical angle by experimenting, re-anchoring using your usual "dig in" techniques each time. Try 3:1 first . . . did it hold? Almost certainly not. Now 4:1 . . . hmm. Not likely. Now 5:1 . . a few modern anchors will dig in fine. 6:1 . . . you're on the margin for some flat anchors, such as the Britany or a Fortress set shallow. 7:1 seems to be a winner . . . unless it's a CQR in soft mud, or a fisherman which can't find a rock.

Sound like you've got it right! But you can remove your uncertainty by testing, and it'll be valid for your kit. You can then ignore other's advice. You know!

Your figures surprise me. Assuming reasonably heavy chain, then in my experience, over 40 years, provided the chain nearest the anchor is on the seabed, the anchor will set, unless something has fouled it

 

[RichardS]

Remember scope measured from waterline not the cleat - that's a 3 metre difference on my boat - and allowing for high tide.

Are you sure? I always measure my scope from the point where the chain / snubber is secured to the boat.

Richard

 

[binch]

I was taught that one was made fast by the chain on the bottom and that the anchor was there to hold the end of the chain.
So much depends on where and what bottom and the constancy of the pull.
In lower Danube, anchored in current measured on log as 7 knots, sandy bottom at 10 feet. Lay out 610 fathoms chain. Ship laid as quiet as a lamb.
Another night, anchord in Vlikho in 3 fathoms depth. Put out 20 fathoms of chain. Wind gusty. At sunset, veered another 20 fathoms.
Duringthe night a violent gust came down the mountainside. Boats all round dragging in wind off the anemom. Let go secondanchor wwith forty fathoms half-inch chain cable and third anchor with 20 fathoms of 3" circ nylon.
Many boats dragged. Several sunsail boats hauled out for winter were blown off their chocks (Chocked up on oil drums, which is stupid) Everything obscured by wind, spray and rain. Thank God for a wheelhouse, for GPS and power windlasses.
Two extremes. Between these there are as many permutations and combinations as anyone can imagine..
Lessons: anchor prudently for the conditions but have spare capacity for mishap. In a serious squall, you may not be able even to stand up in the bows.

 

[oldbilbo]

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.

'Take a step or two closer, lads.....' Erskine Childers

 

[Fascadale]

Lay out 610 fathoms chain. Ship laid as quiet as a lamb.

610 fathoms of chain: about 1,125metres in the new money; that really is impressive. What sort of ship was it?

 

[Richard10002]

Two things to correct a few people's misconceptions:

Scope is measured from the anchor roller.... Often about a metre or so from the surface of the water - the anchor chain doesn't know where the water surface is, it knows where it's attached to the anchor, and it knows where it makes contact with the boat.

If it is windy enough, or choppy enough to make the chain taught, none of it will be on the bottom, it will make a straight line from the connector to the roller. In these instances, the only thing holding the boat in position is the anchor dug into the bottom. The longer the chain, the shallower the angle of the chain to the bottom, and the more chance of the anchor remaining dug in. IIRC the returns from a scope greater than 7:1 diminish quite dramatically.

 

[photodog]

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.

Add more pictures please. Too much copy makes me sleepy.

 

[AMCD300]

Are you sure? I always measure my scope from the point where the chain / snubber is secured to the boat.

Richard

+1

 

[onesea]

Scope is measured from the anchor roller.... Often about a metre or so from the surface of the water - the anchor chain doesn't know where the water surface is, it knows where it's attached to the anchor, and it knows where it makes contact with the boat.

If it is windy enough, or choppy enough to make the chain taught, none of it will be on the bottom, it will make a straight line from the connector to the roller. In these instances, the only thing holding the boat in position is the anchor dug into the bottom. The longer the chain, the shallower the angle of the chain to the bottom, and the more chance of the anchor remaining dug in. IIRC the returns from a scope greater than 7:1 diminish quite dramatically.

Nothing like a good anchor debate

The chain/ warp does know where the water is, they are lighter in water (although probably not significantly).

The argument about a straight anchor rode goes on, am not convinced. What I would say even if its 1/4 of a degree catinary still count.

But IMHO it makes little difference, you either drag or you do not....
Simple facts do count, the larger the anchor the heaver the rode and the more you have out will help you not to drag... probably.

So anything less than the biggest and longest is a compromise, I am sure the females on here will agree ;D

 

[oldbilbo]

Add more pictures please. Too much copy makes me sleepy.

You're quite right, so here's a pic of a collection I made earlier.

The one on the right is the 'Belfast' by Ian Nicolson RINA. It's for kelp and mangroves. It makes a real mess of eel grass.

The one in the middle I gave away. The one on the LEFT - I mean 'Left' - I can't give away.



( Sheesh! Can't tell my right from my 'other right' now..... think I'll go back to bed for a while)

 

[Neeves]

Nothing like a good anchor debate

The chain/ warp does know where the water is, they are lighter in water (although probably not significantly).

The argument about a straight anchor rode goes on, am not convinced. What I would say even if its 1/4 of a degree catinary still count.


;D

I'm not sure if anyone has said the chain is straight (as in a straight line) but it has been said the chain is off the seabed.

Try this as an experiment, take your chain off the yacht and fix one end to a tree (use a Dyneema strop and a big tree). Take the biggest motor vehicle to which you have access and securely fix the chain to the vehicle (decent shackle to the tow ring). The experiment is easiest if you have short chain and its not too big, 8mm or 10mm (its then easy to lug about). I'd suggest a 30m length and start the vehicle so that it runs unrestrained for about 15m (you are trying to simulate a yacht veering). Then drive the vehicle as slowly as you can, aim for 2mph, the snatch you feel when the chain restrains you is but a fraction of the snatch the anchor feels. There is still catenary (so what) and if you are lucky you will not pull the tow-ring off the vehicle. If you are using a car, it will weigh between 1t and 2t a yacht weighs slightly more 5t -10t (of if your lucky 20t). If you have a 2t car and your yacht weighs 10t then the snatch will be 5 times greater. - and that is what the anchor (and wherever the chain is attached to the bow) feels. You can do the same experiment with your yacht, set your anchor, wait till the wind hits 30 knots, motor forward until you are sitting roughly over the anchor and then let the wind drive the yacht back, there might be still some catenary left, but the shock will be sufficient to knock you (and anything loose in the cabin) over. It might pull your windlass out, depends on the yacht. And that is only 30 knots - now imagine 40 knots. The fact there is catenary does not stop the snatch load. This is not actually something I recommend anyone does, its also not an exact simulation but its a close as I can think up quickly (maybe someone can polish it a bit). You can reduce this snatch, it almost disappears, with snubbers - but they need be long, say 10m plus, nylon and maybe 10mm diam (depends on vessel size), for a beamy yacht I would suggest a snubber down each side deck (both 10m minimum). I'd go with Vyv, deploy 2 anchors (as a 'V') and you will find this reduces veering. An alternative is to deploy a second anchor just under the bow with sufficient scope to allow it to drag on the seabed at high tide, this will act as a friction brake (and reduce veering). I find kellets, or anchor angels, a waste of space (you'd be better with the same weight of more chain) - and if the anchorage is tight try the anchor off the bow trick or tie to a tree. Its not a competition, do any and everything if the forecast is such that you worry, there is no need to prove your anchor or your technique is better - its all about pleasure and having a decent night's sleep.

Any one with an idea that the existence of a catenary will stop snatch loads should try the experiments and then argue their case.

Water has an SG of 1 and steel about 8, consequently in water the chain weighs 7/8th of its weight in air. But at around 25 knots 30m will be all off the seabed (chain size does not matter as bigger chain will be on larger windage yachts) and the yacht will be free to rush of unrestrained (until it, or you, reaches the end of its tether).

But there are no hard and fast rules, seabeds are often unknown, anchorages can be prone to swell - as Oldbilbo suggests - each anchorage needs to be looked at critically and anew each time. A comforting thought is that few yachts are actually lost through a dragging anchor (anchor alarms are de rigour). Oldbilbo, thanks for the kind words, but I find the more I know the more there is to learn!

Jonathan

 

[oldbilbo]

This is NBG! I'm finding myself in agreement with peeps on here! Harumph!!