Anchor chain - how Much you use?

[duncan]

upping 30m of 8mm to 40m on a mixed rode isn't going to make any difference to anything so focus on something that is.

I reduced my 30m 8mm to 17m as it was difficult to handle. I have 250m 14mm anchorplait spliced to it. Only the hurd deep within my cruising grounds is a problem generally........... /forums/images/graemlins/wink.gif

 

[woody001]

"upping 30m of 8mm to 40m on a mixed rode isn't going to make any difference to anything so focus on something that is."

How did you work that out? ...?

 

[duncan]

what difference are you expecting it to make and how?

assuming the total rode you deploy is the same length the additional 14kg over 30m from the anchor stock is going to make the square root of bugger all practical difference to the holding of your overall gear

 

[GMac]

Increasing the length of your anchor chain by 50% increases the catenary load by 100%. Adding 10mts would have a noticable effect, in some cases very noticable. Numbers rounded.

That is not a calculated measurement. It was found by using a forklift and load cell (and a fair bit of blood as well, damn it).

The Maggi Aqua7 chain is a Grade 70 finished. A crowd called EyeCo in the UK do Maggi chains. Don't know them from a bar of soap but do know they do Maggi.

If you want to use a G8 chain and galvanise it be very very careful. Most galvanisers would turn this in to a G5 going well or a lot less going badly. Galvanising grades this high is a very very tricky business and at best you will only be left with a G6 maybe 65. Also G8 chains hate shock loads and don't stretch much, if at all i.e no warning of a possible break coming, just 'bang' and it's all over rover. The G8 theory is one of those OK on paper but has many downsides in practice thing.

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Not wanting to be rude but 20mts of 10mm on a 26fter. Someone out to hunt down hurricanes on purpose I suspect

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Not really - when Paul drops the hook he stays put! I've dropped the hook along side Paul on a number of occassions and had to reset my anchor while he stayed put. Oh, BTW I carry 2X80m 5/16" (8mm) chains plus lots and lots of warp, to be sure, to be sure

[/ QUOTE ]

Staying put is fine but at the expense of boat performance. He can stay put just as well with a lighter set-up. Just seems to be a regional differance thing. You guys and gals way up north do carry some monsterous gear compaired to us. A 26 here would be on 8mm max but more likely a 6 or 7mm.

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The weather in the Clyde and along the West Coast of Scotland can change rather quickly and the forecasts are dubious at best, so better to be prepared.

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So our forecasters do your weather as well, didn't know that /forums/images/graemlins/smile.gif /forums/images/graemlins/smile.gif /forums/images/graemlins/smile.gif

 

[gandy]

[ QUOTE ]
Not wanting to be rude but 20mts of 10mm on a 26fter. Someone out to hunt down hurricanes on purpose I suspect /forums/images/graemlins/smile.gif

[/ QUOTE ] Didn't he say 36 foot, not 26?

 

[duncan]

putting a far greater value on your contribution in this area than mine I am seriously concerned I may have been extremely misleading - hence my pushing the point here.

You are seriously suggesting that in say 15m of water and assuming a 16kg delta on the business end and an overall scope of 6:1 ie 30m 8mm chain + 60m anchorplait or 40m 8 mm chain and 50m anchorplait that the latter will deliver an increase in holding power over the former? Add in the assumption that it's a 14mm plait and I would widen the question to 'and that adding another 10m of 8mm chain would be the appropriate enhancement to this particular rode?' in the spirit of balanced ? What about the merits of swapping in 30m of 10mm chain instead? only adding a measily 4kg by going up to a 20kg delta? surely either of these would enhance the holding significantly more than adding that additional 10m of 8mm chain (to the existing 30; if it was being added to an existing 10 that would make more sense) - accepting that moving to 10mm chain adds more weight but the anchor upgrade would be only 1/3rd the increase?

 

[Guest]

Re: Anchor chain - how Much you use? also ...

increasing scope of chain has a reducing effect on pull at end on anchor ... by way of friction on sea bed, inertia of the chain weight. The rise of chain to boat from sea-bed is likely to be same catenary - so no benefit there. Particularly if a rope is added into the equation.
Adding anchor weight / size would have a greater effect IMHO than adding short lengths of chain. But I also believe in chain weight ... in company with anchor weight. The two combined are a formidable anchoring force.

 

[craigsmith]

Re: Anchor chain - how Much you use? also ...

[ QUOTE ]
increasing scope of chain has a reducing effect on pull at end on anchor ... by way of friction on sea bed, inertia of the chain weight. The rise of chain to boat from sea-bed is likely to be same catenary - so no benefit there. Particularly if a rope is added into the equation.

[/ QUOTE ]Wrong.

In the scenario where there is a high load on the system, the chain is not on the seabed, and any minimal friction from it no longer assists.

Similarly inertia is utterly negligible.

Catenary does not reduce the pull on the anchor, only the angle of it.

 

[Guest]

My post was additional to anothers post ...

wher they had considerable scope out already and were adding another 10m chain ...

In terms of short or adequate scope - I would generally agree with you but not entirely. Chain does exert inertia by virtue of its weight ... when chain lays on sea-bed it must add friction - laws of physics have to apply.

Please note that I base this on Ships Training to anchor ships and the acts of increasing scope of anchor cable out and also the folly of chasing cable (the act of just paying out as wind etc. increases without attention to area anchored in).

So to repeat ... taking care to be relevant to what I was posting to ... Adding scope will only increase inertia and friction given that considerable scope is already out and that anchor weight increase may be more benefit if at all.

I'm not interested in anchor tests / BS that gets run out again and again on the subject of rodes and anchors .. sorry but it spoils a good thread ... I don't think I'm the only one to think it also ... or maybe I am ????

 

[craigsmith]

Re: My post was additional to anothers post ...

The anchoring of ships does not scale to the anchoring of small yachts.

When the chain is lying on the seabed, it is not under (much) strain.

Catenary lowers the angle of pull on the anchor, it does not reduce that pull.

 

[Guest]

Oh Dear here we go again ....

Catenary is a result of three factors :

a) Depth of water
b) Length of rode paid out
c) Weight of that rode over its length.

Catenary should allow a length of that rode to lay on sea-bed to direct pull to anchor as horizontally as possible. Second length of rode should be enough to ensure as much as possible that a reasonable length of rode stays on seabed cont. that horizontal pull. The force exerted on the seabed length is same at point where it rises of bed ... but the actual amount reduces due to the friction / inertia as you get nearer to the anchor. If it did not then chain / rope etc. would be totally useless and no need to lay out such scope .. the anchor would be carrying entire force of the boat trying to drag.
Physics dictate that any body lieing against another body will exert friction in any attempt to move either. Second that given that normally that length will be stationary for most of its length there will be inertia to resist its movement.
Physics continues in that increased weight increases inertia and the force needed to move or displace that inertia and in so doing move that rode.

Ships, yachts, basketball players, dog-turds ... Physics doesn't care.

As I said - I was posting regarding another INCREASING his already substantial rode deployed by adding a short length of additional chain.

Lets just say - I never had any of my ships drag, and I have yet to drag anchor on my boat ..... one day it must happen - cause no-ones perfect ... till then I believe Physics rules.

Darwin anyone ?? /forums/images/graemlins/tongue.gif /forums/images/graemlins/wink.gif /forums/images/graemlins/tongue.gif

 

[vyv_cox]

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

My experience doesn't agree with your statement. This summer we anchored for four nights in 1.5 metres during wind of anything from 25 knots. We had 25 metres of 8 mm chain deployed. The weather and sea were warm and I dived on the anchor and chain many times. On many occasions during gusts there was no chain on the bottom, indeed sometimes it was bar tight. I don't believe that veering more chain, up to my maximum of 50 metres, would have made a lot of difference. I would assume that if we had been in deeper water there would have been even less chain on the bottom.

Where we did gain, and I know we've had this discussion many times, is that we were virtually stationary whereas other boats around us on rope were careering all over the anchorage.

 

[duncan]

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

absolutely vyv - people generally have a distorted view of life down there and 'can't believe' how often the rode is bar taut and straight - especially in shallower water.

ineviatably it's larger vessels in deeper water, and they then behave in a similar way due to the additional windage etc

until, as Craig says, you get up to 'ship size'...........where te principle may well be the same but the size and weight of the anchor gear (inc the chain) changes it's proportions.

scope gives the angle of pull on the stock under 'worst case' ie straight pull; which is more often than many think.

elasticity removes shock loads

cateneray starts that pull off in the horizontal, and adds an additional force pulling the load back to the horizontal all the time - if the other forces permit! - with most anchors working better as the force apporaches horizontal

and chain is more abrasion resistant in contact with the seabed

 

[RupertW]

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

If you held with 25 metres then obviously 50m would have made no difference, but perhaps 24 metres might, or 20, or 10 or wherever the unsafe point would have occurred.

The point to me is that you had chain, so could take advantage of the catenery effect, and you had over 10 times the depth in chain so physics ensured that even bar taut the anchor pulls at 3 or 4 degrees from the horizontal. Chain ensures the the force to achieve as much as 3 or 4 degrees is enormous and unlikely.

Rope would reach 3-4 degrees much more easily, but as the angles are small anyway I'd expect it to mostly hold if they really had that much rope out.

Where the weight on a chain really comes into its own, surely, is where 10 times the depth isn't possible as the "bar taut" angle at 3:1 is almost 20 degrees which can pull an anchor out much more easily, and any weight you can get to pull the rope/chain down and reduce that angle under load will pay off enormously.

I think the sbc "physics" line feels true about applying to all scalse from ships to dog turds and he must be right that friction and inertia are involved. However I'd question whether friction and inertia are significant at all compared to the holding power of an anchor at different angles of pull.

 

[duncan]

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

[ QUOTE ]
Where the weight on a chain really comes into its own, surely, is where 10 times the depth isn't possible as the "bar taut" angle at 3:1 is almost 20 degrees which can pull an anchor out much more easily, and any weight you can get to pull the rope/chain down and reduce that angle under load will pay off enormously.


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if you are saying that more chain will assist in maintaining a better angle (than rope) when anchoring at lower scopes in more general (lighter) conditions then I for one will have no problem agreeing. How much chain contributes how much additional holding at what scope in what conditions is however a little dificult to fathom ( /forums/images/graemlins/frown.gif) but as a principle I agree - and use this often when out fishing - 90ft Sunday off the Needles in a snotty swell and F4 but 3:1 was fine for us.

Would I anchor overnight like that? Nope.

 

[Pye_End]

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

If the ground tackle is holding with a chain which has lifted off the sea bed then really the holding discussion here is purely on the anchor not on the rode. Under these conditions an all rope rode would have held the same.

Of course there then is the question of shock loads and abrasion etc. so would not do this myself!

Really for higher windspeeds it is about angle of rode as much as anything, so in these conditions rope may well be better as the top part of the rode as it will reduce shock.

There is of course the assumption in all the answers that the boat has a windlass - pulling up an all chain rode in deepish water is not for the fainthearted!

 

[GMac]

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Not wanting to be rude but 20mts of 10mm on a 26fter. Someone out to hunt down hurricanes on purpose I suspect /forums/images/graemlins/smile.gif

[/ QUOTE ] Didn't he say 36 foot, not 26?

[/ QUOTE ]

No that comment was about one of the posters not the orginal post.

Re Rodes and lengths of components there in:
My comment saying the increased holding that is gained by increasing your rode, in this case of adding 10mts to the 30mts, by 33% comes about by this train of thought and a lot of 'playing'. The term 'holding' is all encompassing not just what load the anchor itself takes to move, more the loads required to move the entire boat from a stay put into a dragging situation.

Well more than 50% of the time an anchor does sweet bu**er all, it just sites there at the end of the chain. The wind comes up a bit and the boat leans back on the rode. Before the anchor does anything the boat has to 'lift' the rode. A XXmt length of chain takes YYkg to lift it. Increase the XXmts by 50% and it takes YYkg x 2 to lift to the same point. I don't have the numbers with me but it would surprise many how little chain it takes to require 100kg to straighten it out. Actually it is damn near impossible to get a chain directly straight anyway, the anchor gets dragged into the mix well before that.

By increasing your chain length (using an anchor buddy or similar does pretty much the same thing but less efficiantly) you increase the load at which your anchor comes under stress and the stress applied to it. The less work your anchor does the better as when that lets go your off across the bay. The anchor is a last resort bit of equipment meaning that is the end of the line, when that gives up you have issues. Like the shackle on your mailsail, when that lets go the main comes down.

We have done it on so many occasions I would not even try to count up. Custy comes in "Me anchors dragging and I'm pis*ed off, it's a bit of sh*t". This is usually quite wrong and when questioned "what is your rode?" we get things like "30fter with 10mts of 8mm chain backed by rope, I want a new anchor". We normally say use a longer length of chain. Custy leaves with another 10mts of 8mm and a C Link (we use very good safe ones and know they are perfectly fine, flame if you like but your wasting your time /forums/images/graemlins/smile.gif ). Punter calls back saying all is now good. This is a very very common situation especially when some belive one or 2 anchor makers (none seen here recently by the way).

What has happened is that the punters boat now has to put something like 300% more kilos into the rode before the anchor does much. Problem solved.

Obviously this does not cover the situations where your stuck out the back of Kiribati in 60knots of brezze. But equally as obviously, 99% of boats will never be in that situation. If you're in that situation I would suggest lots of big long lengthed gear, good anchors, bloody good snubber/s and a good supply of underwear and/or Rum.

Some things we have found out by actual measurement i.e no calculations just reading numbers on load cells. In practice this has been well and truely confirmed.

Increase your chain length by 50% and you will require twice the kilos to straighten it. Round fgures.
XXmts of 8mm changed to XXmts of 10mm takes less load to straighten than XXmts of 8mm plus 50%.

Damn I wish I had my numbers and I could quote them. Hopefully I will remember them and post them tomorrow, quite interesting really.

Basically length is better than girth (some women would dispute this I'm told).

As a quick aside - It is interesting to see the regional differances and what is used. From what I have gleened and speaking very generally:
Up UK way size is king i.e put on the biggest you can and take little account of boat performance and even if the boat can actually set the rode. Back to the 26fter and the big pile of 10mm, I doubt very much if the boat could actually set this well and I'm thinking it stays there just because of the very big physical weight of it. Not knocking them as they seem very happy which is very very important but the negitive effect of boat performance must be getting up there. Maybe it's just my racing side taking over /forums/images/graemlins/smile.gif.
As mentioned earlier, in the US it seems to be get the biggest anchor possible and the smallest rode between that and the boat by using materials not well suited for the job i.e Grade 80 chains. When you have a close look at this theory it is full of holes.
Austraila - any old bollix seems to be OK i.e Silver rope (polyprop is crap and silver is polpyprops evil cousin). Mind you they don't enjoy quite as good cruising ground easily got to in many places and tend to be more close in-shore type stuff.
NZ - All sorts of stuff but generally lighter and longer than the UK. Mind you we do tend to have a huge pile of yachts that race and cruise rather than one or the other. Here the general absolute minimum of chain is 1.5 to 2 times the boat length backed by rope. Most carry 3 time boat length some a tad more. An average 40fter here is on 8 or 9mm, 50fter 10mm, 30 and below 6 or7mm with a few on 8mm. Boats dragging does happen but it is not that much of a major thing really. Unless in the Sounds or the Gulf most boating would be regarded as coastal to semi-exposed with a nice bit of open water (next stop is Chile). We do have a few boats here, 1.6 per household if I remember right. Anyone anchoring on anything at less than 3:1 scope is known as a Nut-job. Rope to chain 5:1 minimum, all chain 3:1 in nice weather more as it goes bad.

All trying to do the same thing but in quite differing ways. Damn interesting stuff really and by saying that I think I need to get out more /forums/images/graemlins/smile.gif

I'll drag up those numbers.
Gezz I hope that made sense.

 

[Roberto]

thanks a lot Gmac, very interesting !

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XXmts of 8mm changed to XXmts of 10mm takes less load to straighten than XXmts of 8mm plus 50%.


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I haven't got any numbers either, but as catenary depends roughly linearly on specific weight and to the square of cable length, to increase the force by the same amount one has the choice between lengthening a little or increasing the weight a lot...
a not too difficult choice I suppose /forums/images/graemlins/smile.gif

 

[jimbaerselman]

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

[ QUOTE ]
people generally have a distorted view of life down there and 'can't believe' how often the rode is bar taut and straight - especially in shallower water.


[/ QUOTE ]
Even in deeper water.

If you've set your anchor properly, and you're checking it by going astern putting a good tension on the rode, it's very obvious (by looking along the line of the rode) that there's hardly any catenary. And that's before the anchor drags.

With a well set anchor, there are only two reasons why it will drag.

First, it's being pulled at too steep an angle relative to the bottom, so it pulls out (note - relative to the bottom - so if you're on a sloping bottom your scope may need to be modified). On level sites, you'll need 5:1 to 6:1 scope (varies a bit by anchor type) to ensure you're not going to pull out due to angle . . . on yachts, forget the catenary effect . . . In the astern case for testing your hold - equivalent to a beefy 30kts or so - it's hardly measureable.

Second, you've exceeded your anchor's ability to hold on that particular bottom and in the local circumstances. Now if it held well in astern, that implies local circumstances are different. Particularly, inertia forces are coming into play, so the plain wind effect (simulated by going astern) actually comes and goes creating motion surges. Many boats also sail around their anchor, and the jerk as they tack creates high inertia forces. This jerk is also at an angle to the original setting angle. Add waves in some conditions.

My conclusion; if you've got the scope right, the inertia forces (on top of wind force) are what will initiate an anchor drag.

Recommendations? Reduce the inertia effects - stop the sailing (if it occurs), add better springing. Your catenary can't give more than a foot or two of spring. 30ft of nylon will give you three times as much.

Now, (keeping to the spirit of the earlier discussion) my physics formulae became hazy with age, but I seem to remember that a square law comes into this, and if you accelerate a mass from one velocity to another, the distance over which you do so will tell you the force implied.

Anyone out there with the formulae in their brain?- say, 4fps change, mass of 2000 kilos (only a quarter of the whole), and a distance of .5m as against 1.5m? What are the two forces?

 

[mono]

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

[ QUOTE ]
...people generally have a distorted view of life down there and 'can't believe' how often the rode is bar taut and straight...

[/ QUOTE ]

I have to confess to being one of those people who 'can't believe' that the rode (chain) is ever straight, except in the most severe circumstances but, in the interests of self preservation, I'm prepared to be convinced otherwise. And to convince me I'm going to need more than anecdotal evidence. 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.

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.

Rope, unlike chain, has negligible weight in water, and therefore needs very little strain on it to become straight.

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.