Is it safer to anchor in deep water or shallow water?

[Deleted User YDKXO]

Say you had a choice of anchoring in one of two bays. They are both exactly the same in every respect (ie size, shape, orientation, wind speed/direction, seabed type and every other characteristic) except one bay has a depth of 5m and the other a depth of 10m. The accepted wisdom seems to be that the minimum scope for an all chain system is 3:1 which means that in the first bay you are allowed to pay out 15m of chain and in the second bay, 30m of chain. Which bay do you choose to anchor in and why?
Not a trick question. I'm genuinely interested in replies because I think there are arguments on both sides and certainly, when I've put the question to my boaty friends, there doesn't seem to be one single point of view

 

[noelex]

Scope sould be calculated from the seabed to the bow roller, not just using the water depth.

If the scope is the same the holding power is almost identical, but is slightly better in deep water due to weight of the additional chain providing some, slight extra dampening.

However, your example is unrealistic why not let out 30m of chain in the shallow anchorage and that will make the shallower anchorage a much better choice, as it is in real world.

 

[benjenbav]

If the tide was going to drop by 4m during my sojourn there, I'd definitely prefer the 10m bay cos I'd be aground in the other one.

If tidal height wasn't a consideration and there was plenty of room in both I'd probably let out 35-40m of chain in the 5m bay so I'd got a 6 or 7x scope and I would prefer that.

If I really had to restrict myself to a 3 or 4x scope I don't think I'd be that bothered albeit that 5m would probably be nicer for swimming.

 

[Baddox]

I could swim down to the anchor in 5m of water if there was a problem, 10m would be too deep. However, waves in the shallower bay would steepen more than in the deeper bay. Overall, I'd go for the shallow bay with the extra chain out unless waves were a concern.

 

[Nick_H]

In bad weather i'd far sooner have 30m out in 10m of water, than 15m out in 5 m water, though of course in bad weather i'd put more out in each of them. I use a kind of mental sliding scale to calculate scope, so in 5m depth i'd never put out less than 20m for an overnight, but in 20m depth i'd be happy enough with 50m, though in each case i'd put out more if feasible, or if bad weather expected.

 

[mjf]

I think a lot depends on duration of stay and what swing is expected to tide/wind etc.

 

[MapisM]

Which bay do you choose to anchor in and why?

If really AOTBE (and tide isn't a factor, as I guess you are assuming), the shallower.
It's much nicer to swim in 5m of water, rather than 10.
And if the anchor gets stuck, It's still within easy reach for attaching a line to its crown and use the winch to release it.

But I'm with Nick on not considering the scope as a constant - i.e. I never use less than a 4 scope in shallow water.
I wouldn't go below 3 also on deeper water though, i.e. I wouldn't be so happy with 50m chain in 20m water.

 

[jfm]

Depends on lots of things esp wind strength but if AOTBE and no bad weather expected I'd go in the 5m becuase it is nicer for swimming and I would use 5x not 3x chain. I don't agree at all that 3x is "accepted wisdom". I'd categorise it merely as a "some people say"

 

[Deleted User YDKXO]

Depends on lots of things esp wind strength but if AOTBE and no bad weather expected I'd go in the 5m becuase it is nicer for swimming and I would use 5x not 3x chain. I don't agree at all that 3x is "accepted wisdom". I'd categorise it merely as a "some people say"

I agree entirely but you'll find a lot of advice all over the internet referring to 3x, 4x scope or whatever so I decided to say 3x to make all things equal. I can see arguments for anchoring in both bays. In the shallow bay, if the wind did get up and the chain become taut, the pull on the anchor will be closer to the horizontal and therefore, arguably, the holding power of the anchor will be greater. In the deeper bay, if the chain become taut, the angle to the horizontal would be greater and therefore the holding power of the anchor reduced. On the other hand, of course, in the deeper bay, the greater length of chain would mean that the chain would be less likely to get taut because of the greater catenary effect and also transmission of shock loadings would be reduced.
Yes I do the same as you. In a 5m bay, I'd put out 20-25m of chain if I was overnighting but in a 10m bay, I'd be happy with 30-40m. We were anchored in a 35m deep bay the other day and because swinging room was tight and because we were just lunching, I only put out 50m of chain or so. The boat stayed put all afternoon without problem

 

[kid's inheritance]

plus the swinging circle is smaller on a shorter length of chain, one less concern if the wind turns
Regards
Rob

 

[MapisM]

In the shallow bay, if the wind did get up and the chain become taut, the pull on the anchor will be closer to the horizontal and therefore, arguably, the holding power of the anchor will be greater. In the deeper bay, if the chain become taut, the angle to the horizontal would be greater and therefore the holding power of the anchor reduced

Pardon?
A straight chain would make a 20 degrees (actually, 19.5) with the anchor in both cases. Basic trigonometry.

 

[Deleted User YDKXO]

Pardon?
A straight chain would make a 20 degrees (actually, 19.5) with the anchor in both cases. Basic trigonometry.

Mmm, yes you are quite right. Brain fade brought on by too much Croatian wine. You told me only to drink the Italian stuff

 

[MapisM]

LOL, yeah, but it ain't just a matter of quality.
I can't swear that IT wine wouldn't have the same effect, if drunk in the typical Croatian quantities...

Anyway, if considered strictly from a holding viewpoint, I would rate as slightly better the deeper of your examples.
If nothing else, you'd have a lighter boat held by more weight on the bottom.

 

[Deleted User YDKXO]

LOL, yeah, but it ain't just a matter of quality.
I can't swear that IT wine wouldn't have the same effect, if drunk in the typical Croatian quantities...
.

Btw we found an excellent restaurant in Korcula http://www.lesic-dimitri.com/terrace-en.html where they bring you 2 or 3 glasses of different wine to drink with every course in order that you decide which one relates best to the food you are eating. Of course you drink all the wine and by the time you are finished, you are in no fit state to drive a boat and you have a big dent in your wallet

 

[Kawasaki]

Of course you drink all the wine and by the time you are finished, you are in no fit state to drive a boat and you have a big dent in your wallet[/QUOTE]


No 1 Daughter is off to Croatia in Oct
Thanks for the Link
Sorry
Bit dwifty thread thinghy

 

[Deleted User YDKXO]

No 1 Daughter is off to Croatia in Oct
Thanks for the Link
Sorry
Bit dwifty thread thinghy

Is she boating? If she needs any more advice on places to visit/restaurants/anchorages, PM me