Chain dragging trials

[vyv_cox]

The boat was hauled out today, just putting the finishing touches to laying up until next year. One of the last jobs was dropping the chain over the bow to hang it on lines attached to the cradle. While doing so I was reminded of the many threads in which people claim that the anchor does no work, the boat being held only by the chain. I decided to see how much chain I could drag over the yard.

The surface of the yard is rolled aggregate, mostly crushed rock that came from the hillside that was blasted away to create the marina. So probably not too different from many seabeds, except that it is sunbaked and totally dry.

My chain is 8 mm galvanised, 64 metres in total. I could drag 30 metres easily with one hand. The full 64 metres needed two hands and some effort but I was quite able to drag it a few metres. What was surprising to me was that when I pulled one end in a curve back upon itself it was relatively easy: i expected the friction of the lateral pull on the curve to make this movement very difficult.

So what load was I exerting? I am no muscleman and will see my threescore years and ten this year. At a rough guess I would say no more than 100 kg, so not very much in windspeed terms on a reasonable sized yacht. Not much more than 15 knots according to Neeves' recent Practical Sailor article. Of course, in water the load needed to drag the chain would be even less.

 

[bigwow]

Was this test before or after your spinach?

 

[noelex]

so not very much in windspeed terms on a reasonable sized yacht. Not much more than 15 knots according to Neeves' recent Practical Sailor article. Of course, in water the load needed to drag the chain would be even less.

Thanks for the report.
Its fits in with my observations moving thicker 13mm chain. I could drag 30-40m lengths reasonably easily.

I do however think the drag in water would be a little higher rather than lower. Although the chain weight is less in water the liquid softens the sustaraite allowing the chain to sink/ bury increasing the friction coefficient.

With one end of the chain fixed by the anchor a boat will normally drag all the chain across the bottom in 15-25k. With no anchor the chain alone will be lower and I think you assment of 15k is about right.

I hope we can dispel, forever, the idea that the chain holds the boat and the anchor is a backup (for small boats)

 

[sarabande]

the only way this can be resolved is to put a load cell between the chain and the boat. Anyone got a spare unit lying around ?


I can understand how a chain moves easily at the start when the tide turns and the boat swings, but there are so many degrees of freedom in the real life scenario of a chain used without an anchor.

 

[Stemar]

the only way this can be resolved is to put a load cell between the chain and the boat. Anyone got a spare unit lying around ? /QUOTE]

The interesting thing would be to put one cell between the chain & the boat and another between the chain & the anchor then compare the two in different conditions.

 

[Avocet]

Laying up at the end of July??!!

Anyway, that aside, I agree with noelex - a wet bottom ought to make things much harder. Also, gut feeling is that it would be very hard for a person standing on a loose surface and pulling something to exert a 100kg force. As a complete guess, maybe half that?

 

[KellysEye]

>While doing so I was reminded of the many threads in which people claim that the anchor does no work, the boat being held only by the chain.

Nobody has claimed that I've always said it should be viewed as back up when the wind picks up and the forecast says a gale is coming while you decide whether to put out more chain or a second anchor or both.

On the subject of pulling chain along the ground as you said it is easy but it's the weight in the catenary that mainly holds the boat, so find a 64 meter high building and try lifting it.

 

[macd]

gut feeling is that it would be very hard for a person standing on a loose surface and pulling something to exert a 100kg force. As a complete guess, maybe half that?

Ah, but Vyv's spent so long working with the stuff, he's now mostly steel, heat-treated...annually

 

[vyv_cox]

Laying up at the end of July??!!

We have been in Greece since the end of April, most of it on a cruise from Leros up to the north of the Aegean and back. It is now very hot (35C in the saloon): most Brits and other northern Europeans go home for July and August to avoid it. Many come back in September but we have decided to enjoy the English summer (?) on our other boat this year.

The estimated pulling force is based on a bit of trivia I have never forgotten. Apparently Graham Hill could exert 400lb force on his brake pedal. In fact the surface of the yard is not at all soft. It has been rolled many times, the part I was on is frequently traversed by many vehicles, and it is sprayed with seawater several times every day. This has given it a finish not far off the consistency of concrete, although with some loose stuff in places.

 

[vyv_cox]

On the subject of pulling chain along the ground as you said it is easy but it's the weight in the catenary that mainly holds the boat, so find a 64 meter high building and try lifting it.

How can a chain hanging vertically hold anything in place? Or if it comes to that, how can a chain hanging in a near horizontal curve hold anything in place?

 

[Deleted member 36384]

.... I do however think the drag in water would be a little higher rather than lower. Although the chain weight is less in water the liquid softens the sustaraite allowing the chain to sink/ bury increasing the friction coefficient. ....

Bentonite based clays are used as lubricants in the oil well drilling industry, so, intuitively an anchor chain in water, covered in a saturated clay would not have more friction than an anchor chain on dry, compacted gravel. However, who knows for sure!

 

[cryan]

I think the confusion comes from that the theory of chain holding the vessel at anchor comes from the fact that the manuals were written for bigger vessels or at least bigger chain. I know that a couple of years ago I was the Chief of a 28 metre tug. During refit I had to stretch the chains out on the quayside for survey. I can't remember the exact size but the links were forged of bar not much more than an inch in diameter and three men could not pull them off the pallets they had been coiled onto. we could pull off a couple of metres but that was it, we had to use a forklift after that. But on the type of chain yachts use one man could probably carry 20-30 metres of the stuff coiled on his shoulders.
So while on bigger vessels it is definitely the weight of the chain that holds the vessel on yachts it probably isn't.

 

[VicMallows]

Last week I anchored (KOBRA 2) in Chichester Hbr for 5 days at a spot where the tide runs at about 3 kt maximum, drying out for about 1hr each side of low water springs. At each low water I observed the anchor. Every tide it had buried deeper, always (not surprisingly) in the ebb direction. It took considerable effort to eventually motor it out. The question is: did the anchor stay put in the direction I initially set it - which was to the ebb flow - or did it indeed reset on each tide but burrow deeper each time because it had loosened the immediate substrate? Winds were up to Force 5.

 

[PaulJS]

Just playing devil's advocate here, but what about mud anchors as used in the Norfolk Broads? Surely there it must MAINLY be the weight of the chain, and "stiction" between it and the bottom that holds the boat?

 

[Leighb]

Just playing devil's advocate here, but what about mud anchors as used in the Norfolk Broads? Surely there it must MAINLY be the weight of the chain, and "stiction" between it and the bottom that holds the boat?

I have only been on a Broads yacht once, but the mud weight anchor was only attached with rope. I do recall that one evening we heard a forecast of strong winds overnight so "anchored" at the windward end of the broad behind some trees. Come the morning we were halfway across, and some other yachts had gone ashore at the leeward end.

 

[Modulation]

But isn't water a lubricant?? So IN the water there will be even less resistance.

Laying up at the end of July??!!

Anyway, that aside, I agree with noelex - a wet bottom ought to make things much harder. Also, gut feeling is that it would be very hard for a person standing on a loose surface and pulling something to exert a 100kg force. As a complete guess, maybe half that?

 

[Avocet]

But isn't water a lubricant?? So IN the water there will be even less resistance.

YEs, but in such conditions, it wouldn't sit ON the surface of the sea bed (like the compacted stuff in the yard) but would sink (to a degree) into it - especially once moving, so you'd not only be shifting the weight of the chain, but also dragging it through some sand / mud / gravel

 

[tokoloshe]

How can a chain hanging vertically hold anything in place? Or if it comes to that, how can a chain hanging in a near horizontal curve hold anything in place?

That's it. Sod the chain, next time out I'm just going to throw the anchor over the bow and retire below!

 

[Keen_Ed]

The estimated pulling force is based on a bit of trivia I have never forgotten. Apparently Graham Hill could exert 400lb force on his brake pedal.

That's just quad strength braced against something. When I was rowing competitively and doing strength weights, I could leg press something like 2.5x my bodyweight. I would think that the force you could exert by pulling would be much less than this.

 

[vyv_cox]

That's it. Sod the chain, next time out I'm just going to throw the anchor over the bow and retire below!

The only possible argument that the chain contributes to holding the boat in position is the friction between it and the bottom. To state that catenary is what holds it doesn't make any sense at all. Catenary provides a little elasticity at low wind speeds but none over around F5-6.