Anchor chain - how Much you use?

[craigsmith]

Re: Oh Dear here we go again ....

[ QUOTE ]
Can anyone point to some body of work that has explored diminishing catenary? There must be some scientific work that has been done on the subject.

[/ QUOTE ]http://alain.fraysse.free.fr/sail/rode/forces/forces.htm

"Rode - Static" is probably what you want. The other pages involve more complicated stuff and his models start to unravel, but that one's okay.

[ QUOTE ]
Logic tells me that there will always be some catenary, albeit much reduced in very strong winds. Even a small catenary will have the effect of reducing the angle at which the chain leaves the seabed - therefore giving an advantage over an all rope rode. As GMac says (paraphrasing) - if your catenary is too little then use more chain.

[/ QUOTE ]Using more chain for a longer length of rode decreases the angle of pull more by way of geometry than by catenary.

[ QUOTE ]
Bar taut is a different matter. But I think the term needs defining before being used in this context. Even allowing for different interpretations of what is 'bar taut' I believe that a chain that can be described as bar taut will still exhibit a catenary - albeit somewhat flattened.

[/ QUOTE ]If almost flat and flat is a difference of half a degree, then a difference which makes no difference is no difference.

 

[duncan]

Re: Oh Dear here we go again ....

ty Craig

are you over for LIBS? happy to buy the guiness!

 

[craigsmith]

Re: Oh Dear here we go again ....

No I'm afraid not, I'll be missing the miserable London weather and enduring the summer down here /forums/images/graemlins/wink.gif

Our guy Steve will be there along with our UK distributor.

 

[claymore]

Re: Oh Dear here we go again ....

Vyv - I thought Jimi and I were short but you diving down to your anchor in 1.5m has got me thinking....Shorty.

 

[mono]

Re: Oh Dear here we go again ....

The Fraysse site is an interesting one - I have looked at it before. Unfortunately he fudges the issue on catenary with the following statement -

"5. If F continues to increase, the catenary reduces to an almost straight line (black curve), and eventually the anchor pulls out."

Unless I've missed something, he doesn't appear to define those forces that will pull the chain almost straight. It's these forces which are crucial to the whole discussion.

[ QUOTE ]
If almost flat and flat is a difference of half a degree, then a difference which makes no difference is no difference.

[/ QUOTE ]
Agreed, and again this is crucial to the whole discussion. What force is required to flatten the catenary to this point? And if we know the point at which this happens we can increase the rode and by using chain the added weight will ensure that the force needs to increase further to lift the chain once more.

 

[Guest]

Inertia ... is the reluctance to move ...

A body has inertia - the resistance to movement caused by pressure or force upon it.

Just thought I'd point it out .....

 

[jimbaerselman]

Re: Oh Dear here we go again ....

Some notes and calculations I made when selecting chain, anchor and warp fifteen years ago for a 12.3m sail yacht, 10 - 12t displacement, fin keel 2m draft.

1. Steady wind drag: 20kt - 225kg, 30kt - 500kg, 40kt - 900kg, 50kt - 1,500kg. (aerodynamic formulae, drag varies as as square of wind speed with some heroic assumptions about the drag coefficient of the yacht)

2. Anchor force: 25kg, good holding, pull parallel to surface, 2,000 kg, pull 10degrees up, only 80% of that. But, 2:1 variation between different anchor types, and modern anchors are more tolerant of upwards pull. Rapid deterioration of holding at greater angles. (Average of an awfully large number of very variable test results, which didn't include more modern anchor types)

3. Chain angles: 8mm chain, 5m of water, 30m chain deployed.
Force to create a given rode angle at the anchor: 100kg -chain lifts from horizontal. 170kg - lifts to 5 degrees. 700kg - chain lifts to 10 degrees. 2,000kg, taut, chain at 11 degrees. (Sorry, I haven't got the catenary calculation formula to hand, but it's in the Admiralty Guide to Seamanship)

That gives you a scale of forces and angles to consider. A few weeks ago I was introduced to Alan Fraisee's site, which has done these calculations with more general coverage. I was glad to see I wasn't too far out.

The interesting points to note are that from 170kg force to 2,000 kg force (anchor break-out) the elongation of the chain through removing the catenary is only a matter of centimetres. ie, there's no spring there.

And, yes, you're right, there is always a catenary, but in the noise of anchor performance, a degree here or there is a trivial matter.

Any help?

 

[Guest]

lack of curve .... (catenary)

Does anyone remember the old playground trick of the length of string .... two kids - each holding an end. Hang a small, lightweight item from the middle .... now try and pull that line straight ... as the line approaches straight - the force needed approaches infinite ........... meaning it is impossible.

A chain rode will always exhibit some curve whatever happens albeit maybe slight. If a chain rode is approaching bar-taut, Then I would be looking to up and go ... as my boats stemhead fittings are being stressed past reasonable.

 

[jimbaerselman]

Re: Inertia ... is the reluctance to move ...

Yes. Force = mass x acceleration.

The trouble is, I can't remember the re-expression of this which tells you how mcuh force is required to achieve a velocity change over a given distance. And I'm past trying to work it out from first principles just at the moment!

 

[Guest]

Not trying to be smart ... just confused ..

Reading through your post - I felt word inertia was being used instead of word force ...
Maybe - I didn't read properly .........

 

[jimbaerselman]

Re: Not trying to be smart ... just confused ..

I see.

I refer to 'inertia forces'. In the context, it would be correct to say 'forces required to accelerate the boat', and perhaps clearer to say 'the forces required to check and reverse the boat's motion'.

 

[rogerthebodger]

Re: Inertia ... is the reluctance to move ...

! newton is the force required to accelerate 1 Kg mass at the rate of 1 metre/second/second

 

[mono]

Re: Oh Dear here we go again ....

[ QUOTE ]
1. Steady wind drag: 20kt - 225kg, 30kt - 500kg, 40kt - 900kg, 50kt - 1,500kg. (aerodynamic formulae, drag varies as as square of wind speed with some heroic assumptions about the drag coefficient of the yacht)

[/ QUOTE ]

Interesting calculations Jim - thanks for that. I went through a similar exercise for my 10m boat using the formula on the Anchorwatch website. For 30kts of wind it is estimated that the force will be a maximum of about 180kgs - considerably less than your estimate for your 12.3m boat. It's surprising how different are the forces for the two boats, assuming that the formulae used are similar. Did you build in a margin for error?

 

[mono]

Re: lack of curve .... (catenary)

[ QUOTE ]
Does anyone remember the old playground trick of the length of string .... two kids - each holding an end. Hang a small, lightweight item from the middle .... now try and pull that line straight ... as the line approaches straight - the force needed approaches infinite ........... meaning it is impossible.

[/ QUOTE ]

Yes...and to all intents and purposes the line could be described as bar taut. In other words, it's perfectly possible for a rode to have both catenary and be bar taut at the same time.

But...I do acknowledge that this catenary may be so flat in extreme conditions that it serves little purpose.

Good thread.

 

[Roberto]

Re: lack of curve .... (catenary)

[ QUOTE ]

But...I do acknowledge that this catenary may be so flat in extreme conditions that it serves little purpose.

Good thread.

[/ QUOTE ]

yep, the chain will go beyond its breaking strength before being "flat", then it will suddenyl recover its full catenary /forums/images/graemlins/crazy.gif

 

[Roberto]

Re: Not trying to be smart ... just confused ..

[ QUOTE ]
'forces required to accelerate the boat', and perhaps clearer to say 'the forces required to check and reverse the boat's motion'.

[/ QUOTE ]

Jim,

IIRC
consider a boat veering at anchor: if it moves with a speed V, (say it drifts to one side at 2kt) its energy will be
0.5 x M x (V^2)
where M is the mass of the boat

If you want to stop the boat, you have to absorb that energy, as E= F x D
where D is the distance it takes to stop the boat

then F = 0.5 x M x (V^2) / D

this means that the force on an almost bar taut chain which can elongate 10cm is 10 times bigger than the force applied to a length of rope that can elongate 1m, hence the appreciable quality of rope in rough conditions..

otoh, you could always stop the boat with a veeeery long piece of string /forums/images/graemlins/wink.gif

 

[jimbaerselman]

Re: Oh Dear here we go again ....

Two possible differences -

First, my boat had a deck saloon plus a spray hood, which adds about 30% to the profile area; second, I allowed for yawing which presents almost double the area to the wind direction.

And I really can't remember what drag coefficient I used - another heroic assumption I suppose which could easily throw up 30% difference from your website.

It's a very approximate game really, isn't it, based on some wonderful assumptions. Better than pure guesses though . . .

Especially when you do the inertia sums, which pop out at the odd thousand of kg (sorry! N)

And, yes to Roberto, I never use less than 20m of nylon snubber when anchoring and there's the possibility of an increase of wind.

 

[Guest]

Re: Not trying to be smart ... just confused ..

"this means that the force on an almost bar taut chain which can elongate 10cm is 10 times bigger than the force applied to a length of rope that can elongate 1m, hence the appreciable quality of rope in rough conditions.."

But in real terms the force applied to a rode makes no acount of whether rope or chain and also a chain rode will have the buffering effect of its weight ... IMHO valuable dampening agent ... IMHO better than elasticity of rope ...

I find it interesting when people ask whether to go for chain etc. - many post about weight of chain carried in fwd section of boat etc. But when it comes to anchoring that same weight that was mentioned is forgotten ... /forums/images/graemlins/tongue.gif

I think a look around at any anchorage and what happens to various boats .... Compare the motion of rope rode and chain rode boats ... most chain rode boats are sitting quieter ... less snatch and less dancing around - due to the weight of chain hanging of the bow ...

I'm not trying to bury head in sand ... just having difficulty after years of throwing chain over to use rope ...

As another says - good thread !!

 

[duncan]

Re: lack of curve .... (catenary)

you are sort of argueing against yourself here Nige............

[ QUOTE ]
Does anyone remember the old playground trick of the length of string .... two kids - each holding an end. Hang a small, lightweight item from the middle .... now try and pull that line straight ... as the line approaches straight - the force needed approaches infinite ........... meaning it is impossible.

[/ QUOTE ]

or put another way - long before the chain is bar tight the forces acting along it will be huge therefore, as has been said by many, the last bit of the catenary isn't doing a lot of damping and is already transmitting significant force.

 

[jimbaerselman]

Re: Not trying to be smart ... just confused ..

If you're talking about all rope, versus all chain, I understand your point. No contest.

The problem is: how to lower the extreme stresses created when a chain already under some tension suddenly has to cope with the additional snatch of a boat sailing around it's anchor (or pitching in a wave). So we're talking 30kts wind plus . . . when there's not much catenary, and what catenary there is left (your expression - buffering?) will only give you centimeters of spring.

By all means use an all chain rode, but add a suitable length of nylon snubber. Some say 10m, I use 20m. That gives you metres of elastic, against centimetres. Crudely, one tenth of the stress. Good for winches, bow rollers, and nerves.