Anchor Scope

[prv]

Rare that one can't see the anchor when it comes off the bottom

Where are you sailing, then?!?

I have a series of red stripes painted on the last couple of metres of chain. When this comes over the bow roller I know to start leaning over the bow to keep an eye on the anchor.

Pete

 

[prv]

When I am lifting mine I have my back to the bow.

I'm trying to imagine what kind of odd foredeck layout makes that the best way

Pete

 

[LadyInBed]

I'm trying to imagine what kind of odd foredeck layout makes that the best way

Pete

Nothing kinky
Button is fwd of anchor locker and I have to guide the chain into the locker to stop it stacking under the entry point.

 

[prv]

Nothing kinky
Button is fwd of anchor locker

I think that's the odd bit

Pete

 

[pmyatt]

In most popular anchorages, swinging room rather than depth/scope is the major problem. Let us assume your size of boat requires a 20 kg anchor and you have chosen to carry 75 m of 10 mm chain; all-up ground tackle weight is therefore about 180 kg. Surely the most sensible solution is to get as much of that weight on the bottom as possible - it being of no use to your anchoring situation if most of it remains in the chain locker. Might I suggest a solution would be to double the size of your anchor, and of your chain - up to a total weight of 180 kg. You would need to carry a length of warp to extend your scope, and you would need a bigger windlass; I doubt you would ever have a problem with dragging though.

 

[prv]

In most popular anchorages, swinging room rather than depth/scope is the major problem.

Indeed, that's the counter to all these people who say "I put out all of my 250m of chain even when the water is only three feet deep - doesn't do any good in the locker, you know!"

Then at the turn of the tide they go wandering over to bump into someone miles away, who spotted they were on chain, assumed a reasonable 4 or 5 to 1, and picked their own position accordingly.

Pete

 

[pmyatt]

Indeed, that's the counter to all these people who say "I put out all of my 250m of chain even when the water is only three feet deep - doesn't do any good in the locker, you know!"

Then at the turn of the tide they go wandering over to bump into someone miles away, who spotted they were on chain, assumed a reasonable 4 or 5 to 1, and picked their own position accordingly.

Pete

But if, in the same boat as before, you had doubled the weight of your anchor to 40 kg and upped the size of chain to 16 mm, you would only have about 20 m of chain to put out. With 55 m of warp you are going to be able to anchor on shorter scope than someone who has lighter anchor and chain. You are not going to be wandering all over an anchorage; in fact, you should never need to put out more warp than the depth in which you are anchored because you have a heavy anchor, and all your chain, already on the bottom.

 

[pmagowan]

There is no need. Carry the normal anchor and chain. If you need more weight out but don't have room then just shackle two links together and you can stick out all your chain with whatever swing you like.

 

[noelex]

But if, in the same boat as before, you had doubled the weight of your anchor to 40 kg and upped the size of chain to 16 mm, you would only have about 20 m of chain to put out. With 55 m of warp you are going to be able to anchor on shorter scope than someone who has lighter anchor and chain. You are not going to be wandering all over an anchorage; in fact, you should never need to put out more warp than the depth in which you are anchored because you have a heavy anchor, and all your chain, already on the bottom.

The holding of an anchor is influenced by angle of pull of the shank. The higher this pull the poorer the holding.
In light conditions the angle of pull is strongly influenced by the weight in the rode, as the wind rises the rodes weight becomes less of a factor and the angle of pull is influenced by the scope.


Adding weight to the rode will not compensate for inadequate scope in stronger winds.

Scopes of 1:1 as you suggest, or a more realistic low scope of say 2-3:1 will still have poor holding in strong winds no matter how heavy (within reason) the rode is.

In strong wind a short heavy chain will give poorer holding (usually much poorer) than a long light chain of the same overall weight.

 

[lustyd]

All this talk of marking chain in various places it's worth mentioning Tom Cunliff's advice to mark chain every 3m and rope every 5m so you don't need to do any maths. Obviously change 3 and 5 to whatever barmy nonsense you decide on, but the logic is great as you just count out as many marks as the depth you need at high tide + bow roller This way you also don't give a stuff how much chain is out. It would even work for the lunatics who put 50m+ out in 2m depth, just put the first mark there then the second mark at 100m for when it's really deep

 

[lw395]

I'm curious how you define "locals"? I lived in Portsmouth for many years, perhaps I should have sold the boat when I left? Besides, mine isn't on a mooring, it's in EYH which was closed off for logging or some such many years before boats were put in there so doesn't count

You should just admit that because there are more boats and more people from all over the place using them, that harbours and anchorages, and the way people use them, have changed over the years for a reason.

 

[lustyd]

You should just admit that because there are more boats and more people from all over the place using them, that harbours and anchorages, and the way people use them, have changed over the years for a reason.

I've no idea what your point is?

 

[pmyatt]

The holding of an anchor is influenced by angle of pull of the shank. The higher this pull the poorer the holding.
In light conditions the angle of pull is strongly influenced by the weight in the rode, as the wind rises the rodes weight becomes less of a factor and the angle of pull is influenced by the scope.


Adding weight to the rode will not compensate for inadequate scope in stronger winds.

Scopes of 1:1 as you suggest, or a more realistic low scope of say 2-3:1 will still have poor holding in strong winds no matter how heavy (within reason) the rode is.

In strong wind a short heavy chain will give poorer holding (usually much poorer) than a long light chain of the same overall weight.

The scope of 1:1 is only the warp - you can always let out more warp to increase the scope if necessary; you already have 20 m of very heavy chain on the bottom which has got to be lifted.

 

[noelex]

In strong winds it will be lifted and straightened (and what happens in light winds is not very important) , then you are back to primarily the scope determining the angle of pull, and therefore the holding of the anchor.

In strong winds scope is king, rode weight plays very little role.

 

[pmyatt]

In strong winds it will be lifted and straightened (and what happens in light winds is not very important) , then you are back to primarily the scope determining the angle of pull, and therefore the holding of the anchor.

In strong winds scope is king, rode weight plays very little role.

Somehow I don't seem to be explaining this very well. Let's assume you have a boat with a 20 kg anchor and 75 m of 10 mm chain - all-up weight is about 180 kg. Then assume you change it for a 40 kg anchor and 23 m of 16 mm chain - all-up weight is 180 kg, same as before. If you then add an anchor warp of 52 m, you have the same overall length of ground tackle, so can anchor at the same maximum scope as before; the difference is that your scope is front-loaded so, surely, is similar to having an "Angel" parked at the 23 m point - surely this must be an advantage, particularly as the warp would act as a shock absorber in strong winds at maximum scope?

 

[noelex]

Somehow I don't seem to be explaining this very well. Let's assume you have a boat with a 20 kg anchor and 75 m of 10 mm chain - all-up weight is about 180 kg. Then assume you change it for a 40 kg anchor and 23 m of 16 mm chain - all-up weight is 180 kg, same as before. If you then add an anchor warp of 52 m, you have the same overall length of ground tackle, so can anchor at the same maximum scope as before; the difference is that your scope is front-loaded so, surely, is similar to having an "Angel" parked at the 23 m point - surely this must be an advantage, particularly as the warp would act as a shock absorber in strong winds at maximum scope?

The drawback with set up is the chafe and possible abrasion on the rope rode as well as swinging differently to other boats on an all chain rode.

The holding in strong wind will be barely any better than using 23m of 10m chain so if rope rode is satisfactory in your cruising ground (from an abrasion and swing point) than a better strategy is to use the lighter weight chain and take advantage of the weight savings, or add a bit more weight (and therefore size) to the anchor and improve your holding.

Weight in the rode improves holding in light wind, but this effect disappears in stronger wind when its required.

If you have all chain rode a snubber is required to introduce the desired stretch. The snubber is thinner and can be made out of stretchier materials than rope rode. So has superior stretch to a much longer length of rope rode.

 

[GHA]

Somehow I don't seem to be explaining this very well. Let's assume you have a boat with a 20 kg anchor and 75 m of 10 mm chain - all-up weight is about 180 kg. Then assume you change it for a 40 kg anchor and 23 m of 16 mm chain - all-up weight is 180 kg, same as before. If you then add an anchor warp of 52 m, you have the same overall length of ground tackle, so can anchor at the same maximum scope as before; the difference is that your scope is front-loaded so, surely, is similar to having an "Angel" parked at the 23 m point



- surely this must be an advantage, particularly as the warp would act as a shock absorber in strong winds at maximum scope?

Possibly the other way round.



First off with 10mm chain @ 2.35Kg/M & 16mm chain @ 6.1m it would be 68m of 10mm chain/20Kg anchor and 23m of 16mm chain/40Kg anchor to wiegh the same.
So....
With 10mm chain at 2.04Kg/m under seawater (87%) approx 6.25 scope (50m from anchor in 8m water) a horizontal force of 227KgF should just lift the chain off the seabed.
https://www.desmos.com/calculator/dqqrwqopig

With 16mm (5.3Kg/m in seawater) chain I'm not clever enough to model it with warp, so if we assume the warp to be weightless and look at what's going on where the chain meets the warp, then for the same scope 23m/68m = ratio of 0.33, then the chain end will be in 2.64m of water 16.5m away from the anchor, same 277KgF force...

https://www.desmos.com/calculator/hp0ojiv3cn

Your heavier shorter chain actually starts lifting the chain off the seabed compared with the lighter longer chain, you are worse off.

But gain lots from having a heavier anchor

 

[GHA]

But gain lots from having a heavier anchor

Or another route, same weight overall, go down to 100m of 8mm g70 hi tensile & 40m anchor. Would a 40kg anchor with chain about 4deg off seabed hold better than 20kg anchor with chain just touching?
I certainly hate having all the chain out and relying on just nylon. 100m would be nice. As would being able to afford it

 

[KellysEye]

>Weight in the rode improves holding in light wind, but this effect disappears in stronger wind when its required.

No it doesn't, you have to put out more chain the stronger the wind gets, then a second anchor. Also more lighter chain doesn't improve the holding because it lighter, the heavier the chain the better the anchor will hold because of the catenary weight. Anchor size should be one up from the size recommended for the boat.

 

[GHA]

>Weight in the rode improves holding in light wind, but this effect disappears in stronger wind when its required.

the heavier the chain the better the anchor will hold because of the catenary weight. ....

Just not so. With large forces the catenary from heavy chain may help a little but becomes fairly insignificant. That's very well established now. Both from observations and physics.