Effectiveness of anchor chain weight versus, length!

[Zagato]

Having been completely confused by Peter Smiths graphs of Rocna repute ( but actually very interesting) I wondered in real life wether it would be more advantageous to have say a boat length of heavy chain plus rope or twice the boat length of chain which Totals the same weight. This is concentrating on holding, comfort at anchor not sailing at anchor etc discounting arguments of all chain etc.

For instance would the boat hold a better position with heavier short chain in tight anchorages rather than double the lighter chain the same weight?

The scenario might happen to pertain Ahem! ....a 2.5 tonne IF Boat in 5 meters of water on a 5.1 scope in let's say casual conditions (What! I have to weedle out the answers from you somehow, it's hard being me you know Lol!)

10m of 8mm 14kg chain versus 20m of 6mm 16KG chain. Both would then have rope bringing the total rode to 30 meters in all including the chain.

I presume half the 6mm chain would be off the seabed and most of the 8mm would remain on it. Gut feeling is 8mm chain would be better, although harder to pull up by hand.

Catenary & Scope In Anchor Rode: Anchor Systems For Small Boats

 

[thinwater]

Presumably you took a basic physics class in school. Approximate the chain as a few blocks of weight and do the math. Your question does not have a simple answer, but for short scope anchoring, generally a short length of heavy chain is better, though the difference is small... but...

A longer length of lighter chain provides a lot more chafe protection, so I think given the choice, most will opt for longer, lighter chain at the same weight. Much of the time you will reep the benifits of all-chain... but...

Then you have more chain to pull, hand over hand, which is less fun.

So yeah, choices, dependent on you, your boat, and your sailing area. I would lean toward a longer, light chain. It's more versatile.

 

[jdc]

I think that heavier chain, albeit shorter, is better. Not everyone agrees., but none of the dissenters have found errors in my algebra, see fig 6 in An Analysis of Anchor Rode Length versus Depth

 

[differentroads]

There are people on this forum who are brighter and more experienced than me, and who have researched and thought about this topic more than I have. That said, an equal weight of 6mm or 8mm chain on a rope rode is just that. An equal weight that sits in front of your anchor, arguably helping it to do its initial set until the rode is in a straight line when the question is moot.

The catenery principle on light gear as you describe works only when there is relatively light wind and current loading on the boat and anchor. The weight of the anchor and chain alone is often holding you then. I've often dove over my anchor to find that after a couple of windshifts, its still facing the way I set it, the chain is in a couple of loops on the seabed and the boat is holding position merely with the weight of 20m of chain on the seabed.

When setting the anchor using the engine in reverse or when the wind picks up sufficiently, my experience is that the rode will be straight or nearly straight, not in the deep catenery that books usually depict, unless you have a lot of very heavy chain let out. A modern anchor on a scope that its desgned for will dig itself in without the theoretical horizontal pull of catenery. But that's an opinion not universally subscribed to ☺

 

[differentroads]

I think that heavier chain, albeit shorter, is better. Not everyone agrees., but none of the dissenters have found errors in my algebra, see fig 6 in An Analysis of Anchor Rode Length versus Depth

See, Zagato, I knew that someone brighter than me would soon come along.
Interesting info JDC. Thanks. I need more time, a pencil and paper and it not to be so late at night to understand it and reflect on what it means to my catenary doubts. But its already taught me the correct spelling is not 'catenery'! ?

 

[RichardS]

In my experience, for a given anchor, one can faff around for the rest of eternity with chain weight, % chain lengths, % rode lengths etc but, the only paramter that really matters is scope. Use a scope of 5:1 or greater and everything else recedes into the minutiae.

Richard

 

[Neeves]

A good snubber will replace some chain

The art of snubbing, in the nicest possible way - MySailing.com.au

How to: Dealing with Snatch Loads in an Anchorage

Anchor Snubber Tips

Anchor Chain Calculator - SAN

5:1 (or more) is fine, cannot argue with it - but if you are anchored in some location you cannot deploy a 5:1 scope, you don't have the chain, the anchorage is too small, there are too many people seeking the same shelter for the same reason.

Jonathan

 

[TernVI]

A lot of people delight in publishing technical looking formulae about a situation where the wind force is constant and the boat stays still.
I've noticed the real world is not like that.
The wind gusts and lulls and is turbulent.
The boat moves
The tension in the rode varies with even small waves

It's easy to 'prove' with irrelevant steady-state formulae that chain weight is insignificant, but in the real world, adding some mass along the rode can change the motion of the boat.
Having some elasticity in the rode makes a difference.

The real physics of a boat at anchor are very complex. If you don't like the motion of the boat, try changing the parameters.

As well as the dynamics of the rode, consider that thinner chain will cut into the mud better and allow the anchor to bury more effectively. In some seabeds off course.

IMHO, the OP's 30m is too short for the Solent area, we have 5m of tide, so unless you want to be in the surf at LW, you want to be equipped to anchor in 8 or 10m at HW. I have a loose view of the required scope:depth ratio, I think 5:1 in 5m can be relaxed to maybe 4:1 in 10m for similar holding? 50m is not excessive. For sitting out any real weather I want a fair bit more readily available.
If you aspire to the Foreign side of the Channel, you might be anchoring in a lot more water.

 

[thinwater]

You will find that Neeves' writing is based on testing using a load cell. The theory only supports the data.

 

[MathiasW]

Hello everybody. I am new to this forum, but found it to touch an interesting topic now...

I answered the question for myself a little while ago, as to what is better, a thicker but shorter chain, or a longer but thinner, with both weighing the same in total.

The maths (of steady-state, I do agree) say clearly, that it is beneficial to use longer but thinner chains.

The reason is as follows: The chain absorbs shock energy by getting raised from the seabed. It is simply the potential energy of a chain at a certain height. Now, when I double the chain length but reduce its weight per unit length by a factor two (meaning same total weight), it turns out that the total potential energy of the chain is twice of what it was before. This means it can absorb twice as much energy as before.

Thus, the conclusion is that a longer but thinner chain is better at absorbing shock energy. BUT: You do have to be at twice the anchor depth then to take advantage of it. It will not work to stay at the same anchor depth as with the thicker chain and have half of the thin chain lying on the seabed. In that case you are better off with the thicker chain.

And of course, you will have twice the swinging radius...

If you are interested in the modelling and maths of chains, have a look here: Catenary Anchor Chain Length - Die Kettenkurve - Fun Facts - SAN

If you are interested in an app that allows you to play around with a number of parameters in that model, have a look here: Anchor Chain Calculator - SAN

In my modelling I do go beyond the steady-state situation by allowing for swell and work out how this can be absorbed by chain and snubber.

BTW - the model very clearly supports what many have found out already - snubbers are a vital part of your anchor gear.

Cheers

Mathias

 

[MathiasW]

I think that heavier chain, albeit shorter, is better. Not everyone agrees., but none of the dissenters have found errors in my algebra, see fig 6 in An Analysis of Anchor Rode Length versus Depth

Not sure what situation you are actually referring to. I had a glance at your web page and that looks all ok and great. I am using the same maths for steady state as far as I could tell from that quick glance. But I cannot find a reference there to a comparison of chains of different thickness but same total weight and their ability to absorb shock loads... Did I overlook something?

Cheers

Mathias

 

[MathiasW]

You will find that Neeves' writing is based on testing using a load cell. The theory only supports the data.

Indeed!!!

And other measurement data exist es well - for instance the late Robert Smith's measurements summarised here: http://www.plaisance-pratique.com/IMG/pdf/30-17-Smith.pdf

 

[pawl]

I think that the OP asked for "real life" advice, so here is what I think, for what it's worth. Unless you are an intrepid world girdling sailor intent on anchoring near a dangerous reef in a hurricane, we are looking at simply getting a peaceful nights sleep in a relatively sheltered anchorage.

Therefore, I would have thought that there are only two things to worry about regarding the anchor rode, firstly that the working load limit of the weakest part of the rode exceeds the anchor's holding power. Secondly, the appropriate scope. For the rode I would use whatever is most convenient or easiest to handle. If you feel that you need extra weight why not use an "angel" (aka anchor chum)? At least then the weight can be stored away from the bow when not needed, also it saves heaving up a whole lot of extra weight every time you pull up the anchor for the one time in a hundred when you might benefit from the weight.

As for the scope I prefer the old RYA advice which, as you all will know, is 12 times (for chain, 20 times for rope) the square root of the water depth at HW. This has always worked well for me, however, if you have a lot of rope in the rode you may find it hard to find sufficient swinging room at some of the more crowded anchorages. If you're using all chain you will want to add a snubber to it for comfort.

If your anchor is adequate and well set I would have thought that up to the point where conditions cause the boat to apply "snatch" loads to the anchor then you're not likely to have any problems. For me once the wind gets up much above 30 knots I'm off to a sheltered harbour.

 

[RichardS]

As for the scope I prefer the old RYA advice which, as you all will know, is 12 times (for chain, 20 times for rope) the square root of the water depth at HW.

I can't say I've ever heard that before.

So, with rope, if you were anchoring in 4 metres, the square root would be 2 metres and you would let out 20 times that, so 40 metres, so a 10:1 scope?

Are you sure about that as it seems a massive scope?

Richard

 

[MathiasW]

I think that the OP asked for "real life" advice, so here is what I think, for what it's worth. Unless you are an intrepid world girdling sailor intent on anchoring near a dangerous reef in a hurricane, we are looking at simply getting a peaceful nights sleep in a relatively sheltered anchorage.

Yes, I am (trying to, given Corona) sail around the world So that is for me real life...

As for the scope I prefer the old RYA advice which, as you all will know, is 12 times (for chain, 20 times for rope) the square root of the water depth at HW. This has always worked well for me, however, if you have a lot of rope in the rode you may find it hard to find sufficient swinging room at some of the more crowded anchorages. If you're using all chain you will want to add a snubber to it for comfort.

This British Admiralty rule is, in fact, a conservative approximation to the standard catenary result that jdc was also referring to. It holds for deep water and really is VERY conservative for most cases. It will get at its limits in shallow water with a lot of swell and wind.

And beware! Your factor 12 is only correct when you use feet! It is a different, smaller factor when you use metres. The British Admiralty rule states L = 1.5 √Y, where Y is measured in metres and chain length L in shackles (90 ft).

 

[MathiasW]

If you're using all chain you will want to add a snubber to it for comfort.

Not only for comfort. Far more important is that a snubber will absorb a good part of the shock load and thus reduce the peak load the anchor sees, which makes it much more likely for the anchor to hold to begin with...

 

[MathiasW]

I can't say I've ever heard that before.

So, with rope, if you were anchoring in 4 metres, the square root would be 2 metres and you would let out 20 times that, so 40 metres, so a 10:1 scope?

Are you sure about that as it seems a massive scope?

Richard

The British Admiralty rule (and with it the RYA rule, which seems derived from that) was designed to work for deep water, not for shallow water.

 

[GHA]

The British Admiralty rule (and with it the RYA rule, which seems derived from that) was designed to work for deep water, not for shallow water.

Looking only at the force required to lift the last link off the seabed the equation is pretty much bang on! Assuming the first equation top left is correct below, think it is, or maybe as good an approximation as it needs to be.

So 14 x square root of depth (plus to the bow roller for the pedants) will just lift the last link on 10mm chain with a 200Kg force.

15m plus 3 times depth is close as well and much easier to work out in your head

Equal force scope

 

[MathiasW]

Looking only at the force required to lift the last link off the seabed the equation is pretty much bang on! Assuming the first equation top left is correct below, think it is, or maybe as good an approximation as it needs to be.

So 14 x square root of depth (plus to the bow roller for the pedants) will just lift the last link on 10mm chain with a 200Kg force.

15m plus 3 times depth is close as well and much easier to work out in your head

Sorry, you are absolutely right. My memory had not served me well. The British Admiralty approximation is valid for shallow water and / or strong wind. Not for deep water. Your factor 14 will be larger in deeper water. And in any case, one needs to be careful with these factors, as they depend on the metrics chosen: feet, fathom, metres... Always another factor, because it is non-linear.

And if you use my App, you would not have to work out anything in your head and it would be tailored to the wind and swell conditions...

 

[RichardS]

The British Admiralty rule (and with it the RYA rule, which seems derived from that) was designed to work for deep water, not for shallow water.

So in 9m it would be 3 x 20 = 60m rode, so almost 7:1. It still seems a very large scope unless you are expecting a hurricane.

In 16m it would be 4 x 20 = 80m rode so 5:1 which is the scope I use, so the formula does work but I would have to say that I very rarely anchor in 16metres / 52 feet.

Richard