Questions about anchor chain?

[doug748]

If it helps, I agree with normanS in post 10

 

[Greenheart]

>Doesn't it follow that the depth must be around five meters?

No. As I said the length of chain diminishes as the water gets deeper. In 5 metres I use 10 times chain and in 20 meters 3 X chain. Perhaps GHA will tell us what the 3 to 1 scope depth was.

But wasn't it tacit that the depth in this case was five meters? If it had been anything else, as your sliding scale of increasing depth would necessitate, surely he'd have specified.

 

[GHA]

>In that example it takes just 52kg of force on the boat to lift 15m of 11mm chain off the sea floor at a 3 to 1 scope.

You don't say what the depth is so what you have said is meaningless. I've always said the shallower the water is the more chain you need and the amount decreases with deeper depths which the table clearly shows. What I use is depth 5 metres 10 x depth, 10 metres 7 x, 15 metres 5 x, 20 metres 3x.

Depth H Minimum scope N Minimum chain length Lc
4.6 m (15 ft) 5.6:1 26 m (84 ft)
9 m (30 ft) 4.0:1 37 m (120 ft)
18 m (60 ft) 2.9:1 54 m (176 ft)

Makes no difference. When the forces are high the chain is off the bottom. Even with a chum hanging off it. And the catinery on it's own has little damping effect.

 

[vyv_cox]

Vyv - as an afterthought - is the reason for failure not due to the reduction in cross sectional diameter but small irregular pitting causing stress points from which a fatigue crack will propagate? If that is the case then I can see that any significant corrosion presents a risk.

Nobody has ever posted or sent me a picture of a link from a chain that failed in a real anchoring situation, so I cannot be certain. Fatigue doesn't seem very likely in such a ductile metal as G30 chain. I suspect that the more likely mechanism is overload fracture due to reduction of area. Ready to be proved wrong, though!

 

[vyv_cox]

Ouch, you need 200 to 250 metres of chain for that ......

That post reaches new extremes of nonsense, even for you! Are you seriously suggesting a scope of 100:1? I suggest you take stock of your thoughts on anchoring, do a bit of research and accept that modern maths, equipment and experimentation have superseded whatever rules you apply.

 

[NormanS]

In the example I have quoted so many times, in between 2 and 2.5 metres depth we had 35 metres of 8 mm chain out. In about 30 knots of wind the whole lot was off the bottom and the shank of the anchor was lifting in the gusts. Catenary was contributing zero but my nylon snubber was doubtless taking most of the snatch loading. Cala Volpe, Sardinia. Almost identical situation in Galixhidi, Gulf of Corinth. The water was warm and clear and I dived on the anchor many times to watch what was happening.

One would hardly expect catenary to contribute much in such shallow water.

 

[vyv_cox]

One would hardly expect catenary to contribute much in such shallow water.

I don't expect it to contribute anything in any depth of water

 

[skyflyer]

>Thats exactly my point - the chain is much stronger than needed to take the load because it is over-specified to give a catenary.

I've never known a boat with over-specified chain, chain and anchor size is always determined by the weight of the boat. No one in their right mind would use chain that has lost 10% of it's diameter. It will be stronger than rope but it's not a valid comparison rope doesn't get severe corrosion.

Define 'severe'. In an ideal world the chain would uniformly corrode. In life that doesn't happen, so is it when any part of any link loses 10% of its diameter, even if the original diameter was actually greater than the nominal spec (e.g. 8mm chain with actual diameter of 8.5 mm do you bin it when a the diameter gets to 7.6mm or 7.2mm - all a bit theoretical, isn't it?

Over specced for what? tensile strength or weight (to give that catenary you so desire)? This is precisely what I am asking - do we spec 8mm chain because that is what it needs to withstand the tensile load or because we feel that will give a good catenary whereas in fact 6mm chain (say)would have adequate tensile strength.

"the effect of catenary is hugely over-estimated" - They are completely wrong, catenary weight is the greatest determinant of a safely anchored boat assuming the anchor is set properly. As said they are selling anchors and most boats can't have an oversized anchor because the bow roller is designed for the weight of the anchor determined as I said above.

I really doubt that they make much more profit on a large anchor than on a small one. It is facile to suggest that the small commercial advantage of selling a slightly bigger anchor is worth the risk of persuading yachtsmen to fit a smaller than necessary chain.

With respect I think you are wrong about catenary. You don't need rocket science to prove it yourself, a first floor window or similar) will do. Stake one end of the chain on the lawn and haul away at the chain through window and see how little effort it takes to lift it off the ground at the stake end.

Anyway, all i wanted to know was - forgetting anything to do with catenary - what size chain I need for a 5 tonne 32ft yacht!!

 

[KellysEye]

>With respect I think you are wrong about catenary. You don't need rocket science to prove it yourself, a first floor window or similar) will do. Stake one end of the chain on the lawn and haul away at the chain through window and see how little effort it takes to lift it off the ground at the stake end.

That bears no relation to an anchored yacht because the window doesn't move back. As the boat moves back the catenary weight increases to a level only a winch can lift. The catenary is as important as the anchor and the objective of the different amounts of chain for different depths is to ensure there is a fair amount of chain (we aim for 50% in shallow water and 15% in deep water ) lying on the bottom in the prevailing wind. That covers you for an increase in wind, if it increases even more and the chain near the top stops having a curve then let out more chain until the curve and catenary comes back.

On the chain front I would use 4/5mm depending how much weight you can carry in the bows and the size of the anchor locker. Also work out your anchoring depths to decide how much chain to buy.

 

[Greenheart]

On the chain front I would use 4/5mm...

Where can one buy 4mm chain? I've never seen lighter than 6mm for sale in chandleries. I'd like small stuff for the dinghy...unless chain generally is really of no benefit...

...I feel like I know slightly less now, than before I began reading this thread!

 

[Tranona]

Define 'severe'. In an ideal world the chain would uniformly corrode. In life that doesn't happen, so is it when any part of any link loses 10% of its diameter, even if the original diameter was actually greater than the nominal spec (e.g. 8mm chain with actual diameter of 8.5 mm do you bin it when a the diameter gets to 7.6mm or 7.2mm - all a bit theoretical, isn't it?

Over specced for what? tensile strength or weight (to give that catenary you so desire)? This is precisely what I am asking - do we spec 8mm chain because that is what it needs to withstand the tensile load or because we feel that will give a good catenary whereas in fact 6mm chain (say)would have adequate tensile strength.



I really doubt that they make much more profit on a large anchor than on a small one. It is facile to suggest that the small commercial advantage of selling a slightly bigger anchor is worth the risk of persuading yachtsmen to fit a smaller than necessary chain.

With respect I think you are wrong about catenary. You don't need rocket science to prove it yourself, a first floor window or similar) will do. Stake one end of the chain on the lawn and haul away at the chain through window and see how little effort it takes to lift it off the ground at the stake end.

Anyway, all i wanted to know was - forgetting anything to do with catenary - what size chain I need for a 5 tonne 32ft yacht!!

8 mm and 14 mm rope. Now all you have to decide on is the anchor and windlass if you have not got them already.

 

[skyflyer]

That bears no relation to an anchored yacht because the window doesn't move back. As the boat moves back the catenary weight increases to a level only a winch can lift. The catenary is as important as the anchor and the objective of the different amounts of chain for different depths is to ensure there is a fair amount of chain (we aim for half) lying on the bottom in the prevailing wind. That covers you for an increase in wind, if it increases even more and the chain near the top stops having a curve then let out more chain until the curve and catenary comes back.
.

Sigh!
Right - fix one end of the chain to the bedroom window and go down on the front lawn and walk backwards to see how little force is required to lift the entire chain off the ground. I'm going to try it on my back lawn next week as we are on a slope with a lower level at the rear so I can create a 'depth ' of about 6 metres, although I'm not sure the garden is long enough for a 4:1 scope now I think about it!

 

[GHA]

3 4 5 6 7 8 9 10 scope
3 24 45 72 106 145 190 242 299 chain weight 2.013
4 32 60 97 141 193 254 322 398
5 40 75 121 176 242 317 403 498
6 48 91 145 211 290 380 483 598
7 56 106 169 247 338 444 564 697
8 64 121 193 282 386 507 644 797
9 72 136 217 317 435 571 725 897
10 81 151 242 352 483 634 805 996
11 89 166 266 387 531 697 886 1096
12 97 181 290 423 580 761 966 1195
13 105 196 314 458 628 824 1047 1295
14 113 211 338 493 676 888 1127 1395
15 121 226 362 528 725 951 1208 1494
16 129 242 386 564 773 1014 1288 1594
17 137 257 411 599 821 1078 1369 1694
18 145 272 435 634 869 1141 1449 1793

Quick spreadsheet came up with the above. 1st column down the side is depth of water, top column is scope.
So 4m depth with 20m chain out (4 to 1 scope) it takes 75kg to lift the chain off the seabed.

Anyone care to check the maths?
Cell B2 is "=$L$2*(($A2*B$1)*($A2*B$1)-($A2*$A2))/(2*$A2)"
Which should be =chain weight * (lenght^2 - depth^2) /2 * depth

So even at a 10:1 scope in 3m of water it only takes 300kg to lift all the chain off the seabed.


Edit. Blimmin formating makes it almost unreadable

 

[GHA]

0.000 3.000 4.000 5.000 6.000 7.000 8.000 9.000 10.000 scope
3.000 24.15 45.28 72.45 105.7 144.9 190.2 241.5 298.9 chain weight 2.01
4.000 32.20 60.38 96.60 140.9 193.2 253.6 322.0 398.5
5.000 40.25 75.47 120.8 176.1 241.5 317.0 402.5 498.1
6.000 48.30 90.56 144.9 211.3 289.8 380.4 483.0 597.7
7.000 56.35 105.7 169.1 246.5 338.1 443.8 563.5 697.3
8.000 64.40 120.8 193.2 281.8 386.4 507.2 644.0 797.0
9.000 72.45 135.8 217.4 317.0 434.7 570.5 724.5 896.6
10.00 80.50 150.9 241.5 352.2 483.0 633.9 805.0 996.2
11.00 88.55 166.0 265.7 387.4 531.3 697.3 885.5 1096
12.00 96.60 181.1 289.8 422.6 579.6 760.7 966.0 1195
13.00 104.7 196.2 314.0 457.8 627.9 824.1 1047 1295
14.00 112.7 211.3 338.1 493.1 676.2 887.5 1127 1395
15.00 120.8 226.4 362.3 528.3 724.5 950.9 1208 1494
16.00 128.8 241.5 386.4 563.5 772.8 1014 1288 1594
17.00 136.9 256.6 410.6 598.7 821.1 1078 1369 1694
18.00 144.9 271.7 434.7 633.9 869.4 1141 1449 1793


depth



try again

 

[skyflyer]

Isn't 8mm chain 1.35kg /metre though, so my 6m @ 4:1 experiment will take 90.6kg based on your 2.01 weight/m or presumably 60kg of force based on 1.35kg/m- stuff all for a 100kg bloke!

 

[GHA]

Isn't 8mm chain 1.35kg /metre though, so my 6m @ 4:1 experiment will take 90.6kg based on your 2.01 weight/m or presumably 60kg of force based on 1.35kg/m- stuff all for a 100kg bloke!

Well spotted it's 10mm chain, x 0.875 as it's a bit lighter in water.
this comes up for 8mm chain..

0.000 3.000 4.000 5.000 6.000 7.000 8.000 9.000 10.000 scope
3.000 14.18 26.58 42.53 62.00 85.10 111.6 141.8 175.4 chain weight 1.18
4.000 18.90 35.44 56.70 82.7 ......113.4 148.8 189.0 233.9
5.000 23.63 44.30 70.9 ......103.4 141.8 186.0 236.3 292.4
6.000 28.35 53.16 85.1 .......124.0 170.1 223.3 283.5 350.8
7.000 33.08 62.00 99.2 .......144.7 198.5 260.5 330.8 409.3
8.000 37.80 70.90 113.4 165.4 226.8 297.7 378.0 467.8
9.000 42.53 79.70 127.6 186.0 255.2 334.9 425.3 526.2
10.00 47.25 88.6........141.8 206.7 283.5 372.1 472.5 584.7
11.00 51.98 97.5 ........155.9 227.4 311.9 409.3 519.8 643
12.00 56.70 106.3 170.1 248.1 340.2 446.5 567.0 702
13.00 61.4 ........115.2 184.3 268.7 368.6 483.7 614... 760
14.00 66.2 ........124.0 198.5 289.4 396.9 520.9 662 ........819
15.00 70.9 .........132.9 212.6 310.1 425.3 558.1 709... 877
16.00 75.6 .........141.8 226.8 330.8 453.6 595.... 756... 936
17.00 80.3 .........150.6 241.0 351.4 482.0 633 ........803. 994
18.00 85.1. 159.5 255.2 372.1 510.3 670 ........851. 1052


depth hope those numbers are right, they seem a bit on the low side...

 

[vyv_cox]

Isn't 8mm chain 1.35kg /metre though, so my 6m @ 4:1 experiment will take 90.6kg based on your 2.01 weight/m or presumably 60kg of force based on 1.35kg/m- stuff all for a 100kg bloke!

I took my 60 metres of 8 mm chain to BE Wedge to be regalvanised. It was in one of those builders' plastic containers with two handles on the top. The foreman came to the car and lifted it out, carrying it across the shop floor. While we were marvelling at this feat of strength, one of the handles broke off and he continued walking carrying it one handed!

 

[Neeves]

30m of 8mm at 5:1 scope took around 80kg to lift the last link off the ground (in air). We did not have enough space to do a longer length. Chain winch, load cell.

In water, same conditions, less than 20 knots is our reckoning and all the chain is off the seabed - thereafter as the wind increases the shank is being loaded above the horizontal and the catenary is disappearing. The idea of a chain offering friction resistance on the seabed looks pretty remote.

One of the advantages of ignoring any advise on the advantages of catenary is that anyone could carry the rode one handed!

Jonathan

 

[Greenheart]

One of the advantages of ignoring any advise on the advantages of catenary is that anyone could carry the rode one handed!

That's the feeling I'm getting! I'd planned to get maybe 10 meters of 6mm for the dinghy for next year, on the basis that my little danforth needs a horizontal pull...

...but if it won't be horizontal, why bother carrying any excess weight? Although, I'd only anchor the dinghy in light winds when it might make a difference.

 

[vyv_cox]

That's the feeling I'm getting! I'd planned to get maybe 10 meters of 6mm for the dinghy for next year, on the basis that my little danforth needs a horizontal pull...

...but if it won't be horizontal, why bother carrying any excess weight? Although, I'd only anchor the dinghy in light winds when it might make a difference.

Thousands (millions?) of boats anchor all over the world on rope with very little or no chain. A properly designed anchor will still hold with its shank at an angle to the sea bed. In the examples I gave earlier, to which a certain poster advised that I needed 250 metres of chain, we sat for four or five days in winds reaching 42 knots or maybe more. Didn't move on either occasion, first time with a Delta, second with a Rocna. The chain was all off the bottom for the majority of that time.