Length of anchor chain to properly secure your boat?

[Nostrodamus]

I don't know how long ago the guide to how much anchor chain you need out to secure your boat was devised but with the advancements made in anchor design have things changed.
The idea of having chain out is to stop your anchor lifting and pulling out by making the scope less. This is often accomplished by the weight and length of the chain you put out in relation to your boat. Obviously if you have the room you put out as much as you can get away with but when you don't have the room do you still get good holding with shorter than recommended chain lengths?
I know a lot of people have oversized anchors from those specified for the boat and the new breed of anchors give better holding than some older designs.
So, do oversized anchors, the new designs make any difference to how much chain you need to deploy?

 

[macd]

So, do oversized anchors, the new designs make any difference to how much chain you need to deploy?

Probably, but not if you want them to work optimally.
All hooks are designed to work with a pull which approaches horizontal. As has many times been reported on here, in a proper blow, even a chain rode can be pulled bar-tight. In the absence of a catenary the only thing that can contribute to a near-horizontal pull is greater length of rode.

 

[Nostrodamus]

Probably, but not if you want them to work optimally.
All hooks are designed to work with a pull which approaches horizontal. As has many times been reported on here, in a proper blow, even a chain rode can be pulled bar-tight. In the absence of a catenary the only thing that can contribute to a near-horizontal pull is greater length of rode.

Mac, by catenary I presume you mean a weight on the chain away from the anchor?

 

[macd]

Mac, by catenary I presume you mean a weight on the chain away from the anchor?

No, by catenary I mean the natural curve of the chain from bow-roller to sea-bed. This acts as a shock-absorber in the system. But, as said, when the chain approaches a straight line there is no catenary and thus no shock-absorbing properties. (Which is why a long, elastic snubber is often recommended for extreme conditions.)

 

[Nostrodamus]

Thank you Mac.. I wonder if someone has ever though of designing an anchor for storms with a higher shank so it digs in when the chain goes tight.?

 

[macd]

I wonder if someone has ever though of designing an anchor for storms with a higher shank so it digs in when the chain goes tight.?

Not someone, but pretty well everyone who's ever designed an anchor, I'd have thought (other than a grapnel or fishermans). It's an essential quality of the geometry of most of them. Whatever anchor you have, see how it sits in the yard/car park next winter. The shank will be somewhat horizontal but will point up at...20, 30 degrees? Any pull on this that is closer to horizontal will tend to dig it in.

Shank angles seem to be quite critical. That said, there is no 'ideal' figure for all situations...viz the recent Fortress test in very soft mud in Chesapeake Bay. With the shank adjusted to it's maximum setting (45 degrees), the Fortress out-performed every other anchor by some distance, including an idential Fortress with a shallower shank angle (32 degree). But that's a very specific characteristic of the soft bottom: Fortress themselves strongly advocate you not to use the higher angle routinely.

It's worth adding that the 'geometric' shank angle of a hook and the effective shank angle when it hits the bottom are not necessarily the same: neither you nor the designer have much control as how it will lie on the bottom, particularly if the bottom is very uneven. Equally, anchoring on a steep upslope (as is common with stern lines ashore in Turkey, for instance) must make a difference, although I've never seen it quantified.

 

[RichardS]

I don't know how long ago the guide to how much anchor chain you need out to secure your boat was devised but with the advancements made in anchor design have things changed.
The idea of having chain out is to stop your anchor lifting and pulling out by making the scope less. This is often accomplished by the weight and length of the chain you put out in relation to your boat. Obviously if you have the room you put out as much as you can get away with but when you don't have the room do you still get good holding with shorter than recommended chain lengths?
I know a lot of people have oversized anchors from those specified for the boat and the new breed of anchors give better holding than some older designs.
So, do oversized anchors, the new designs make any difference to how much chain you need to deploy?

If you mention weight of chain you must know what will happen next! :-(

Richard

 

[KAL]

Peter Smith (Rocna inventor) recommends a scope of at least 5:1, as do the Blackwells (The Art of Happy Hooking), but more (up to 8:1) in a big blow. After that, it apparently doesn't seem to make much difference...

Please note: this is not intended to start a big slanging match. It's just what I've read. FWIW I tend to go for 5:1 as a minimum, with an all-chain rode. I'm sure others do it differently.

 

[GrahamM376]

Peter Smith (Rocna inventor) recommends a scope of at least 5:1, as do the Blackwells (The Art of Happy Hooking), but more (up to 8:1) in a big blow. After that, it apparently doesn't seem to make much difference...

Please note: this is not intended to start a big slanging match. It's just what I've read. FWIW I tend to go for 5:1 as a minimum, with an all-chain rode. I'm sure others do it differently.

5 : 1 chain + the snubber line works for me with the Manson but over 18 - 20 kts constant or gusty conditions I chuck some more out.

 

[jimbaerselman]

If you're expecting some wind, you let out enough scope (in a typical sail cruising boat) to hold your boat in full astern.

That test will take account of all factors - anchor type, rode type (rope or chain or a combination), bottom slope, bottom type, height of hawse over water, depth.

After many such tests, you'll conclude there's never any point in using more than ( x/1) rode to depth scope, nor using less than (y/1). You'll settle for (z/1), which will work for you in most circumstances (level bottom, average holding).

Then, when you drag in the test, you'll know:
1. Go find a bit of better bottom, or,
2. use (z+1)/1 and test again

Simples, eh?

 

[SeamanStaines]

I was going to try really hard to keep out of this one

OK, I think we are all agreed that anchors in general are designed to have a horizontal pull, not one at an angle. The catenary of the chain provided that by using the weight of the chain to convert the pull from an angle (as you would have with a rope for example) to horizontal This is a calculation, I found a couple of links that explain this well.

http://www.petersmith.net.nz/boat-anchors/catenary.php
http://www.anchorwatch.co.uk/index-page04.htm

Obviously, if you extend the chain in very high winds to a its maximum pull then you lose the catenary effect. It also explains why you need a smaller scope to depth ratio in greater depths than in shallower ones. up to the point at which all the chain is lifted from the bottom in which case there is no subsitute to incrasing the scope. You can of course keep the anchor pull horizontal by adding a weight to the chain at some distance from the anchor, this is simply increasing the caternary effect.

So.... you end up saying that, sensibly, in say three meters of water you need about twenty/twenty five meters and in ten meters at least forty five metres. Given the proximity that people want to anchor to you in many places, some times that is just not practical so you will drag in strong winds, then the perenial debate about anchor designs. Its worth mentioning that when a big ship anchors it lays out so much heavy chain out that a crochet hook would probably hold it! I think what I am trying to say is that to get the optimal pull on your anchor you need to have at least a five to one scope and as the water shallows, more. The difficulty is that is not always practical. We find ourselves constantly in the position that we have laid the scope I think is appropriate and then the wind changes and we find ourselves in conflict with a boat that is just bouncing its anchor of the bottom.

Finally, can someone explain to me why people feel (we are in the Ionian at the moment so we are an excellent 'muppet' area) why a skipper feels twenty metres of chain is perfectly appropriate to anchor in ten meters of water, yet then wants to lay sixty metres when med mooring onto a harbour wall in three metres?

 

[Resolution]

S/S
Re your last question, the answer could relate to the consequences of dragging. In ten metres in a bay they may feel relaxed about dragging a bit, perhaps they are only there for lunch, perhaps they are staying on board and will be ready to react if necessary. Whereas on the quayside there is no doubt that if the anchor drags you are highly likely to damage your boat against the quay. And perhaps you will be spending longer away from the boat as well.

 

[SeamanStaines]

It does create two further problems though (which I see constantly here). Firstly, the more chain you let out the greater the chance you don't maintain and exactly ninety degree angle to the quay so greatly increasing collecting half a dozen other anchors on the way in and secondly you end up not really digging the anchor in and laying to the chain (snagged on another anchor?) and when it straightens out you hit the wall anyway. For what its worth we tend never to put out more than two boat lengths which gives us twenty five meters or a 7:1 scope in three metres. We all know that past about a 10:1 scope it makes no difference anyway (unless you count snagging any other obstacles in the way). a 20:1 scope is pretty pointless and just asking for trouble.

 

[noelex]

An anchor's performance will always be better at a longer scope, but it is a very useful attribute for an anchor to retain as much of its holding power at short scopes as possible.

This opens up anchorages and situations that would otherwise be unusable. In addition, the bottom slope can alter the effective scope considerably so good short scope performance helps provide a buffer for those times when your effective scope is less than you think.

New generation anchors have improved most aspects of anchor performance, but unfortunately one area that has not universally improved is the short scope performance.

For example the old generation Bruce is still one of the best models at a very short scope.

I have been very impressed with the Mantus at a short scope. This is only @ 2:1 in 7m. It is quite amazing to watch underwater when the chain pulls up at the anchor shank in a seemingly impossibly steep angle. As the anchor digs in, the shank is forced down although not at its normal angle. It is hard to image the fluke managing to dig in at all, but you can see the anchor has managed to bury all of its fluke and held full reverse in this position. This is the equivalent of about 25-30 knots of wind (but bear in mind it is a large anchor).

By virtue of its greater holding power a big anchor can lose a higher percentage of its holding power when used at a short scope and still hold the boat successfully. So going big is a help if you need to anchor at a shorter scope. However, be aware that as the scope is shortened, an anchor will reach the stage where it refuses set at all. When this happens the holding power will be very poor and this change can occur quite abruptly with only a minor change in scope. The point at which this occurs varies from one model of anchor to another.

Always remember using a larger scope where possible will improve your grip. The rode in the locker does no good. However, with some judgment, suitable equipment and an eye on the weather, you can on some occasions anchor on quite a short scope safely. It is almost like getting a new boat with a shallow draft. It opens up a lot of possibilities. If you do this consider you may swing quite differently to other boats on a longer scope.

 

[vyv_cox]

Additional to noelex' post and photo, when Mantus announced their anchor they claimed that it would set at a scope of 2:1, to much derision in these pages. It is quite clear that they did not exaggerate. I regularly anchor with a scope of 3:1 with my Rocna, even in fresh winds, with no problems. I would prefer more but it is not always possible.

Several years ago in Sardinia I was forced to anchor in a depth of 19 metres. At that time my chain had a total length of between 50 and 55 metres, with a Delta on the end. We held successfully for several days in a great deal of wind.

 

[Nostrodamus]

I find this thread very interesting and informative so thank you all for your sensible answers.
We have just got new anchor chain. The old chain was starting to drop apart in big flakes and was far to old. The new chain in heavier and gives us more confidence (weakest link and all that). An oversized modern anchor also makes a difference.
It is nice to talk about ideals but certainly in the med with crowded anchorages and people laying out all different lengths in different directions it is not always possible.
I just wondered really about minimum lengths rather than maximum.
I suppose it just comes down to laying your anchor the best you can and making sure it is well dug in and you are not moving.

 

[Pyramid]

So, how much cable should I have on board then?
Beneteau 36, sailing East Coast where depth rarely exceeds 5m.