Catenary?

[Tranona]

Whilst the point you think you are making may be valid - I'd add a couple of observations about your "evidence":
- there is clearly a catenary in the image, its not as pronounced as the text books show, but at 4:1 with 2500 rpm I wouldn't expect it to be
- the image is taken from an oblique angle, and so determining the shape of the curve is affected by parallax error, its likely that the true catenary would be more obvious if viewed orthogonal to the anchor chain

What would be interesting would be to see carefully taken images with different scopes and different engine RPM. What surely matters though is that the force on the anchor is applied as close to the horizontal as possible. If you drew the straight line in the above image my estimate is that it is something like 20-30 deg above the horizontal whereas the anchor chain is perhaps 5 deg as it exits the stock (all very rough due to the angle of the pic and not actually seeing where your boat is).

Whatever the distortions of the photo, it does not alter the fact that applying a load of around 300kgs straightens the chain. If you want to see proof of this then look at the Practical Sailor tests on anchoring rodes. This is also the sort of load from the wind on a boat like Vyvs in a F6/7 - data from ABYC in their tables or again from PS. What the photo does show is that the load is sufficient to almost completely bury the anchor and assuming the seabed will hold is close to the maximum holding power of the anchor.

As for the angle of pull, once the chain is straight it is a direct function of scope as I explained earlier. Your estimate would suggest a scope of between 3 and 5:1. Before that of course catenary will lower that angle which is one of the claimed benefits cited by catenary believers. I have not found any empirical (or even theoretical) work that explores this. It is claimed catenary by lowering the angle of pull enhances anchor setting, but again there seems to be no work that test and validates this. Indeed some involved in anchor design and testing acknowledge this but consider it not worth taking into account in testing, particularly with NG anchors that in general set very quickly and easily. See Pete Smith's view on the subject in the link Vyv posted earlier.

 

[ylop]

Whatever the distortions of the photo, it does not alter the fact that applying a load of around 300kgs straightens the chain.

That is obvious. It must be obvious to so-called "catenary believers". Are you sure you understand what they think they are claiming? As I've never heard anyone suggest that applying a force (wind/tide/engine etc) to the end of a catenary didn't move the curve closer to the straight line? The only way it could not would be if the anchor dragged or the force was so low it was opposed by the drag of the chain through the water.

As for the angle of pull, once the chain is straight it is a direct function of scope as I explained earlier. Your estimate would suggest a scope of between 3 and 5:1. Before that of course catenary will lower that angle which is one of the claimed benefits cited by catenary believers. I have not found any empirical (or even theoretical) work that explores this. It is claimed catenary by lowering the angle of pull enhances anchor setting, but again there seems to be no work that test and validates this. Indeed some involved in anchor design and testing acknowledge this but consider it not worth taking into account in testing, particularly with NG anchors that in general set very quickly and easily.

There is presumably a breakout angle where the anchor is no longer set. I assume that is different for different anchors. Your aim it presumably that when anchored, you never pull the chain above that angle. (But when weighing anchor can easily get to that angle to recover it).

 

[geem]

That is obvious. It must be obvious to so-called "catenary believers". Are you sure you understand what they think they are claiming? As I've never heard anyone suggest that applying a force (wind/tide/engine etc) to the end of a catenary didn't move the curve closer to the straight line? The only way it could not would be if the anchor dragged or the force was so low it was opposed by the drag of the chain through the water.

There is presumably a breakout angle where the anchor is no longer set. I assume that is different for different anchors. Your aim it presumably that when anchored, you never pull the chain above that angle. (But when weighing anchor can easily get to that angle to recover it).

We often find that even with the chain vertical, the anchor won't break out. The 1750w motor stalls. To break the anchor out, we motor forward with a vertical chain. On our well set Spade anchor there isn't really a break out angle.
We have Inadvertently set the anchor on 2:1 scope for 2 weeks in wind gusting to 35 knot when we end to ended the chain and got the chain markers mixed up

 

[Tranona]

That is obvious. It must be obvious to so-called "catenary believers". Are you sure you understand what they think they are claiming? As I've never heard anyone suggest that applying a force (wind/tide/engine etc) to the end of a catenary didn't move the curve closer to the straight line? The only way it could not would be if the anchor dragged or the force was so low it was opposed by the drag of the chain through the water.

There is presumably a breakout angle where the anchor is no longer set. I assume that is different for different anchors. Your aim it presumably that when anchored, you never pull the chain above that angle. (But when weighing anchor can easily get to that angle to recover it).

Yes. repeatedly one or 2 dismiss the idea that the chain straightens at such "relatively" low wind speeds and that any benefits of catenary are lost.

As for breaking out then you need a just about vertical pull (or 1:1) for a well set anchor. Of course anchors break out at different scopes - known as dragging! or when subject to sideways pull. any of the tests show that some anchors are better than others at resisting this.

 

[vyv_cox]

That is obvious. It must be obvious to so-called "catenary believers". Are you sure you understand what they think they are claiming? As I've never heard anyone suggest that applying a force (wind/tide/engine etc) to the end of a catenary didn't move the curve closer to the straight line? The only way it could not would be if the anchor dragged or the force was so low it was opposed by the drag of the chain through the water.

There is presumably a breakout angle where the anchor is no longer set. I assume that is different for different anchors. Your aim it presumably that when anchored, you never pull the chain above that angle. (But when weighing anchor can easily get to that angle to recover it).

I think the point about 'catenary believers' is that, as Peter Smith's article clarifies, it used to be believed that catenary provided some elasticity to the rode. We now know that catenary in yacht all-chain rodes only exists in light winds, when it has no use. In stronger winds < F6 it ceases to exist in practical terms.

So far as the setting of the anchor is concerned it has been shown many times that new generation anchors will set with the rode at considerable angles to the seabed, 25 - 30 degrees being easily accommodated.

 

[lustyd]

Yes. repeatedly one or 2 dismiss the idea that the chain straightens

Straightens but doesn't become straight. It's an exponential increase in effort required to make it straight, so catenary will always exist regardless how straight you perceive it to be. Whether that helps anchoring or not is a different question.

 

[[2574]]

Straightens but doesn't become straight. It's an exponential increase in effort required to make it straight, so catenary will always exist regardless how straight you perceive it to be. Whether that helps anchoring or not is a different question.

Straight for all practical purposes? Acknowledging that you are geometrically correct. We recently anchored with 45kt gusts when, at maximum extent of yaw, the boat end of the chain was well out of the water and appeared pretty straight to the eye.

 

[Neeves]

I'm posting this with limited internet access - and don't have the specifications at my finger tips

So.....

If you have 10mm G30 chain then why not change to a 8mm chain. The 10mm might be a G30 (very likely), as this G30 was never tested (it never deformed and certainly did not fail) - so why change the strength spec. But if you are cautious you might change to a 8mm x G40.

Why stop at 8mm - no belief in catenary, nylon (a snubber or bridle) will offer all the elasticity you need.

What is wrong with a G80 x 6mm, or (if the maths does not stack up) a G100 x 6mm?

Down sizing will save weight but also volume in the chain locker. The windlass might be cheaper and its power demands less, (smaller house bank and smaller cables). You might want to think more deeply about your snubber.

Anyone doing the maths : you will lose about 30% of UTS of the chain, G100, when you galvanise - but if you choose your galvaniser with care - the galvanising will have a longer life.


I know its, replacing 10mm with 6mm, a step too far - but why.

And as I have mentioned on another thread - with a contemptuous post in reply - why not 4mm HT chain? Not a sensible development, there are no 4mm gypsies AFAIK - so stick to 6mm G100 as the ultimate.


I do accept that if you have an older yacht then weight in the bow might not be important for performance but most modern yachts are weight sensitive. I also accept that many do not care about performance, they are not racing, they have no schedule - they just like being 'on the water' - its about relaxation - cannot knock it.

But if you accept that catenary and anchor weight as safety features are questionable - why not carry downsizing to the limit (which is 6mm x G100) and light weight anchors.

Jonathan

 

[lustyd]

Straight for all practical purposes? Acknowledging that you are geometrically correct. We recently anchored with 45kt gusts when, at maximum extent of yaw, the boat end of the chain was well out of the water and appeared pretty straight to the eye.

I think whether it's straight or not is irrelevant. The real outcome of catenary is pull on the end points whether it's straight or very curved. Once all chain is off of the ground all force is against the anchor and the boat, and before that there's a bit of friction from the seabed. You're then down to whether the anchor will hold at the angle it ends up at, and whether everything is strong enough. The way catenary works is that the force on the endpoints rises exponentially with the straightening of the curve, so it will definitely give way before it's actually straight because the anchor cannot take infinite load.

Back to the original picture, the setup will fail due to fouling on the anchor buoy way before it gives way due to the anchor anyway

 

[westernman]

I think whether it's straight or not is irrelevant. The real outcome of catenary is pull on the end points whether it's straight or very curved. Once all chain is off of the ground all force is against the anchor and the boat, and before that there's a bit of friction from the seabed. You're then down to whether the anchor will hold at the angle it ends up at, and whether everything is strong enough. The way catenary works is that the force on the endpoints rises exponentially with the straightening of the curve, so it will definitely give way before it's actually straight because the anchor cannot take infinite load.

The real issue is jerking. It is the peaks of load, the jerks which will make the anchor drag.

As the catenary gets less and less, the absorption of an increase in load has to happen over a much shorter distance. This results in higher peak loads than if there was more elasticity or more catenary which would mean that the load would only increase more slowly over a longer time. Enough time to allow the peak windage load on the boat to decrease a bit. The easy thing to do is to either let out a lot more chain (if enough room and you have enough chain), or to add a shock absorber to the system. The latter is easy to do with a stretchy bridle.

Of course if the conditions are constant, then no amount of elasticity is going to help. Then anchor then has to be quite simply big enough (enough area) to handle the constant pull.

You can reduce the pull in such conditions using the engine to provide a forward thrust to reduce the load on the anchor.

 

[[2574]]

The real issue is jerking. It is the peaks of load, the jerks which will make the anchor drag.

As the catenary gets less and less, the absorption of an increase in load has to happen over a much shorter distance. This results in higher peak loads than if there was more elasticity or more catenary which would mean that the load would only increase more slowly over a longer time. Enough time to allow the peak windage load on the boat to decrease a bit. The easy thing to do is to either let out a lot more chain (if enough room and you have enough chain), or to add a shock absorber to the system. The latter is easy to do with a stretchy bridle.

Of course if the conditions are constant, then no amount of elasticity is going to help. Then anchor then has to be quite simply big enough (enough area) to handle the constant pull.

You can reduce the pull in such conditions using the engine to provide a forward thrust to reduce the load on the anchor.

I've often thought that using the engine to reduce the pull on the anchor might make the situation worse as it would take a skilled helmsman to keep the boat pointing at the anchor. Loads applied randomly by a yacht motoring might well upset the anchor. It'd be interesting to hear from someone who has done it as to whether it was easily done and whether it was a real benefit.

 

[vyv_cox]

Back to the original picture, the setup will fail due to fouling on the anchor buoy way before it gives way due to the anchor anyway

The 'anchor buoy' is a small fishing float just out of shot. The line is about 1 metre long. Many Med sailors use them as it helps to locate the anchor in weed. Not much use in murky waters

 

[westernman]

I've often thought that using the engine to reduce the pull on the anchor might make the situation worse as it would take a skilled helmsman to keep the boat pointing at the anchor. Loads applied randomly by a yacht motoring might well upset the anchor. It'd be interesting to hear from someone who has done it as to whether it was easily done and whether it was a real benefit.

No need for any helming if the load on the anchor line is always higher than the thrust you are applying with the motor.

 

[lustyd]

The 'anchor buoy' is a small fishing float just out of shot. The line is about 1 metre long. Many Med sailors use them as it helps to locate the anchor in weed. Not much use in murky waters

I'll let you off then

 

[Neeves]

It merits note that most anchor tests, as a result of which a holding capacity of specific anchors are derived, are commonly conducted at scopes we would recognise as being common or acceptable in real life. Scopes are usually at 5:1 , sometimes 3:1 and sometimes 7:1. Tests with the chain parallel with a dead flat sea bed are like hen's teeth.

The hold data from these tests, commonly finding modern anchors (Rocna, Supreme, Spade et al) of 15kgs have a hold in a consistent sand seabed of around 2,000kg - well beyond the hold such anchors are likely to be subjected to in real life.

It also merits note that posts of chains failing are now also like hen's teeth - which suggest to me chains are well made and the spread sheets for chain vs vessel (size/weight) might be overly cautious.

Jonathan

 

[lustyd]

As the catenary gets less and less, the absorption of an increase in load has to happen over a much shorter distance. This results in higher peak loads than if there was more elasticity or more catenary

This isn't how physics works. The loads will be identical regardless of catenary, the only thing changing is how far the boat moves

 

[westernman]

This isn't how physics works. The loads will be identical regardless of catenary, the only thing changing is how far the boat moves

One case has high peak loads, the other doesn't. The average load is the same.

 

[Tranona]

I'm posting this with limited internet access - and don't have the specifications at my finger tips

So.....

If you have 10mm G30 chain then why not change to a 8mm chain. The 10mm might be a G30 (very likely), as this G30 was never tested (it never deformed and certainly did not fail) - so why change the strength spec. But if you are cautious you might change to a 8mm x G40.

Why stop at 8mm - no belief in catenary, nylon (a snubber or bridle) will offer all the elasticity you need.

What is wrong with a G80 x 6mm, or (if the maths does not stack up) a G100 x 6mm?

Down sizing will save weight but also volume in the chain locker. The windlass might be cheaper and its power demands less, (smaller house bank and smaller cables). You might want to think more deeply about your snubber.

Anyone doing the maths : you will lose about 30% of UTS of the chain, G100, when you galvanise - but if you choose your galvaniser with care - the galvanising will have a longer life.


I know its, replacing 10mm with 6mm, a step too far - but why.

And as I have mentioned on another thread - with a contemptuous post in reply - why not 4mm HT chain? Not a sensible development, there are no 4mm gypsies AFAIK - so stick to 6mm G100 as the ultimate.


I do accept that if you have an older yacht then weight in the bow might not be important for performance but most modern yachts are weight sensitive. I also accept that many do not care about performance, they are not racing, they have no schedule - they just like being 'on the water' - its about relaxation - cannot knock it.

But if you accept that catenary and anchor weight as safety features are questionable - why not carry downsizing to the limit (which is 6mm x G100) and light weight anchors.

Jonathan

As you will see from my earlier post my dilemma is whether to upsize from the 6mm that came with the boat to the 8mm norm for the size of boat. It is all chain so I have the benefits of catenary and it is clear that my new Epsilon will hold in situations where the chain is at its limit (which the boat cannot achieve anyway). What are the measurable benefits of catenary and how much do they change by increasing from 6 to 8mm?

Answers on a postcard please.

 

[geem]

Cantenary in shallow water and deep water is totally different. In shallow water the snatch loads can be alarming. Rarely so in deep water.
In deep water you need a lot of chain out. Not so in shallow water. The amount of heavy chain you need to lift to get it straight in deep water is huge. In shallow water the weight of chain is tiny by comparison, hence the large snatch loads in gusts.

 

[Tranona]

The real issue is jerking. It is the peaks of load, the jerks which will make the anchor drag.

This seems to be an unexplored area. If the anchor drags then the dynamic load must be greater than the holding power of the anchor. given the high holding power of many anchors that would need peak loads approaching the limits of the chin - at least in terms of stretching the chain. This does not seem to happen. If the anchor holds the boat stops with a bang. If it drags it is acting as dampener absorbing some of the dynamic load.