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Old 23-09-2008, 18:54   #1
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Anchors- shorter heavier chain better

Am a newbie to this forum, and have been browsing old anchor threads (morbid curiosity and entertainment value. . . ).
I have a question- mostly aimed at GMac, since he was the one that offered original opinion, but anyone else can chime in too.

Forget the exact thread reference, recall statement that some people go for a heavier chain leader in the belief that this allows them to get away with shorter length of chain.
GMac refuted the utility of this logic, without going into any details. I would like to know more about whats behind this opinion, as I would think that is just the right direction to go. FWIW, here is my logic: if we put aside the chafe resistance function of chain, and any ideas about catenary, then the one other major function is to put some weight on the shank to keep anchor horizontal. Weight near the boat end of a rode is carried mostly by the boat, so is no use for this purpose- so better concentrate it towards the anchor, the closer the better. long lengths are not the way to go as when rode tightens, a greater proportion is carried by the bow and therefore "wasted" in its "keeping the angle more horizontal" purpose. taken to its logical conclusion I'd reckon the ultimate would be only one or two REALLY massive links (like from a smallish ship's chain) attached to the shank, and your rope onto that.

Following the oft repeated statement (from the anti-all chain mob) that you are best to put the weight saved into a bigger anchor (ie one the wt of big links and anchor combined) I think to be false logic here. Reason is that the anchors are very sensitive to pulling out at short scope even when large and all-rope will never cure that situation. Therefore an adequate size anchor with a big weight attached would seem to be superior to all other setups. (You would of course now have to address chafe issues- but you get that to some degree with any rope set up).
Comments?

A little anchoring anecdotes: following years of anchoring small commercial boats around coral reefs we used picks(grapples)/ploughs about 50/50 depending on locality. Always put a trip on the ploughs as didn't want stuck anchors in dodgy places on reef fronts, etc. In deeper water they dragged very often, but no real drama in that as when did hook up it meant there you had found a bit of decent hard among the sand and thus fish. . . Funny thing was that they often held after the trip had broken.

I did buy a SARCA for my own boat (only a little Hartley 18) as it was the most spade-like thing then available. The slot was an attraction, but don't think it ever really came into play. Did like it though- never failed- and much superior to (smaller) danforths aboard.
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Old 23-09-2008, 20:22   #2
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What actually works on a Hartley 18 is not the same as on boats much heavier. The general cruising rule would be no less than 1.5 times the LOA of the boat for the chain portion of a rope / chain rode. The chain should be sized appropriate to the boat. Using a larger anchor would not normally be a good idea instead of the chain. It wouldn't be cheaper either. Using a larger sized anchor is not uncommon.
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Old 23-09-2008, 21:12   #3
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As I hope GMac is busy working so's he can pay his taxes to help keep me in comfort I will give you my view which can be seen as another's "chiming in" or as another's opinion -

I agree with GMac if he said, as you say he did, that the claim of going for a heavier chain leader in the belief that this allows them to get away with shorter length of chain is a claim of poor utility.

You cannot forget the catenary as you say you can as it is only the catenary of the chain (assuming the rope part of the cable is weightless in water which it is nearly so) that keeps the shank of the anchor down - the catenary force resists the force of the boat and the shank lies horizontal until the boat force overcomes the catenary force enough so that the bottom end of the chain is no longer tangential with the seabed and the anchor shank lifts (eventually leading to breakout).

At the point which the force from the boat (Fb say) overcomes that of the catenary enough so that the shank starts lifting Fb is proportional to the weight per unit length of the chain x (chain length squared - the height from the seabed to the top of the chain part of the cable). You will see from that that the force from the boat pulling the chain out and so ending up lifting the anchor shank is much more dependant on the length of the chain than on its weight per unit length. That is you need to add a very much heavier length of chain to make up for the shortening of a lighter one.

Hope that makes sense.

Regarding putting weight into the anchor rather than the chain, I disagree with the claim of the others you relate but will leave that else this will be too long - later perhaps .
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Old 23-09-2008, 22:16   #4
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Lets say for example you have two lengths of chain with each length weighing 500 pounds. Lets say one length is 100 feet and another is 200 feet. Which length is going to help you hold ground better? Or is there no difference? My gut feeling is there would be no difference. Is that right?
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Old 23-09-2008, 22:44   #5
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G'day Carlo, Always nice to see someone with a similar morbid curiosity of anchoring and welcome to CF.

One quick thing, If Alain saw you drawing comparisons between a SARCA and Spade, he'd die... err or you would They are very different and close to being on opposite ends of the holding power graph.

Basically the shorter the length of chain the quicker you straighten it out and once that's happened you have lost all centenary (as used in anchoring discussions) effect. It is easy to lift a very short length of even very big chain.

Easy example. Get a 20ft length of 3/8" chain and fix one end. Stand on a 10ft tall box and lift the other end. You'll be able to straighten it easily. Now get a 57ft length of 1/4" and do the same. You'll find the load required to straighten this will be a lot higher. Both lengths of chain weigh the same. Numbers swapped from metric to that old imperial thing so 'close as'.

Increase the length of a chain by 50% and the load required to straighten it roughly doubles, not quiet but close. The load required to straighten say 100ft of 5/16" is big very big. Also with more scope you have the loads increase over short scoping. Try the above example at say 3:1 and then again at 8:1 and you'll see what I mean.

So the longer the length the more load the boat has to apply to the chain before it starts throwing it's whole weight onto the anchor. Until you have maxed out the chain it will still be 'bending' so the anchor end will still be providing some horizontal pull on the anchor, all good.

People will argue against that and there is tht spreadsheet thingy drifting around suggesting all rope is best but sadly real life doesn't seem to agree with them/it.

We often get people in here with 'crap anchors' that are not holding only to find they are running short lengths of chain. Rather than swap anchors we add chain and suddenly the anchor is wonderful. The chain added is sometimes only another 20% sometimes more.

Actually SARCA owners are great for this as they actually believed the stupid SARCA recommendations, which are OK in the 10ft deep flat water they anchor in but not so once out of sheltered inland waterways. It's an Aussie thing.

Not to mention we have used heavy gear and load cells to find out real numbers.

And of course you have the other chain bonus's being chafe resistance, drag thru the water which reduce shock loading and in some bottom types the drag co-efficient of the chain itself is surprisingly high so yet more added holding grunt.

The, mainly US, theory of tiny chains and massive anchors really just doesn't stack up well. By doing this you are basically taking the load off the rode and anchor and transferring it to the anchor. So your anchor is doing a pile more work and the rode's doing less. Why not make both work and reduce the stress's on both is my thoughts. And we are back to my favourite saying "Think System, not anchor only".

We run from a base of 1.5 to 2 times boat length and then tweak to suit the individual situation. Considerations we use are - the anchor, the boat shape style and windage, anchor winch or not and the biggest wildcard of it all - 'the dick on the stick' known by some as the boat user

For you and the mighty Hartley I'd be starting at 10mts of 6mm and then adding to that if you or we think you should (sometimes we have to gently save the odd owner form them self). Behind that some 12mm warp. And a Stainless shackle to go on the SARCA. Don't use a galv one on the slot, they sometimes jamb up too easy.

The slot on the SARCA amongst many others, is a good idea. It has saved some anchors from spending eternity with the fishes. Not all of them but at least some of them.

ML1 - just work harder yourself. I'm paying for too many lazy bums now, there is 120 of the in Parliament alone

David M - I'd say the 200fter by quite a margin.

Just try the example I used above. Even use a bit rope. Tie one end off and grab it a little way back and pull straight. Then go further back and pull straight again. You should notice it takes a lot more the longer you are from the end. Then imagine the difference with something many times heavier. Maybe tie a brick in the middle to simulate the more weight you'd have with chain.

This whole this is very simple really, how the hell do you think I cope with it otherwise I think many are just trying to over calculate or analyse it, which makes it look a lot more complex than it really is.
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Old 24-09-2008, 03:48   #6
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And on top of G-mac’s excellent experiment explanation...... Water is denser that air, so the catenary effect is even greater.

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Old 24-09-2008, 05:43   #7
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It can be surprising how effective long chain is. You can also learn a lot about what works by spending time diving your anchor and rode in clear water. On my last boat I carried an overkill chain on my 30' ketch. 200' of 3/8" chain and a 35# anchor. Since I had no windlass it was a real chore in deep water, but amazingly effective at holding the boat. I spent lots of time in the water so I was always watching how the anchor and rode sat,dug, pulled, and spun around. I eventually learned that it took 25 knots of wind and associated chop to pull the chain back into a straight line after a wind shift. Up to 20 knots of wind and in shallow water, I didn't even need an anchor on the end of the chain to hold the boat in place.
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Old 24-09-2008, 09:20   #8
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I am an advocate of a shorter heavier chain. But before you jump at me - you need to define what you mean.

I originally had a large quantity of 5/16" chain, most of which rarely saw the light of day.

I changed to 30m of 3/8" chain, then 14mm octoplait. I rarely used the rope element in 21 years of frequent anchoring.

My revised system had less overall weight.

The effect when anchoring was dramatic, not only with significantly more holding power, but the extra weight reduced the yawing as well.


10m of chain is for lunch hooks and kedges

5m is for 16 ft speed boats.
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Old 24-09-2008, 11:35   #9
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I'm with fishspearit. I had almost the identical "overkill" on a 33' sloop--200+ feet of 3/8 chain--and even after backing down hard to set on short scope, then paying out the balance of the chain in reverse, I would never straighten the chain out again except in strong wind. If you're around coral, then all chain with snubber is required.
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Old 24-09-2008, 15:31   #10
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Quote:
Originally Posted by GMac View Post
Basically the shorter the length of chain the quicker you straighten it out and once that's happened you have lost all centenary (as used in anchoring discussions) effect. It is easy to lift a very short length of even very big chain.
I've been wondering about this argument for a long time. I need an answer that had some math behind it, because if there are no supporting math the result is really just an opinion.

In looking into it, I found the following website:

Rode - Static Behavior

That's were the 'spread sheet thingy' is. It lets you plug numbers in to find the amount of load needed to lift all the rode off the ground. By the way, it doesn't even hint at using all rope rode.

Quote:
Originally Posted by GMac View Post
Easy example. Get a 20ft length of 3/8" chain and fix one end. Stand on a 10ft tall box and lift the other end. You'll be able to straighten it easily. Now get a 57ft length of 1/4" and do the same. You'll find the load required to straighten this will be a lot higher. Both lengths of chain weigh the same. Numbers swapped from metric to that old imperial thing so 'close as'.
So let's look at GMac's example more closely. (Please note; I'm not trying to get in anyone's face. It's just that I like to use tools like this spread sheet ...)

First, no one anchors with 2:1 scope. Using 20 ft of 3/8" chain in 10 ft of water plus waterline to roller is bound to cause you problems. Second, you can't really expect to compare anchoring on 2:1 scope with anchoring on 5.7:1 scope. So the 3/8" chain gets to have a rope tail on it to give it the same scope as the all 1/4" chain rode.

Lets make our 'depth' 14 ft (10 ft of water plus 4 ft from waterline to bow roller). That gives us about 4:1 scope when we use 57 ft of rode.

Question: How hard do we have to pull to lift the entire rode from the bottom, the Critical Tension (CT)?

Case #1
0 ft line
57 ft 1/4" chain
CT = 62 lbs

Case #2
37 ft 5/8" line
20 ft 3/8" chain
CT = 84 lb

The 3/8" chain works about 30% better.

Note, even when all the rode is lifted from the ground it's not 'straightened out' (unless it's all line) it's just not touching the bottom any more. So you can argue that the 1/4" chain rode holds it's 'catenary' longer, and you'd be right. But absorbing shock is not what all chain is for, that's the job of a good snubber.

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Old 24-09-2008, 16:05   #11
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I have no opinion on this, but I will mention that I was forced to anchor in a less than ideal spot on Sunday night. I was anchored in about 8m (25') of water. I had a 45 lb plough and 8m (25') of heavy chain and about 40m (125') of rode out. We say up to 59 knots on anchor, in an area that is not known for good holding, and we never budged an inch. Ever since I bought that big ol' plough, I always used to grumble about its size and weight... not any more
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Old 24-09-2008, 23:49   #12
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[/quote] Question: How hard do we have to pull to lift the entire rode from the bottom, the Critical Tension (CT)?

Case #1
0 ft line
57 ft 1/4" chain
CT = 62 lbs

Case #2
37 ft 5/8" line
20 ft 3/8" chain
CT = 84 lb

The 3/8" chain works about 30% better.
[/quote]

And note that for the same all up weight as these chain options, but attaching a single big weight to the anchor shank, then all rope line to boat of same total length, the CT becomes ~ 112 lb. That is a further 30% improvement again over the heavy chain- and roughly double the lighter all-chain set up!

This 'spreadsheet thingy' is undoubtedly right- but I reckon GMac's disparaging comments are more related to some extra factors coming into play in the real world- like dynamic issues also mentioned in that website. . . (couldn't open the 'dynamic spreadsheet to play with that unfortunately- might be more enlightening, or at least entertaing).

And I apologise for comparison of SARCA's to superior bretheren- they were the only 'improvement' around when needed! My background is bias toward heavy commercial workboats, 100ton+, full of hydraulics/electricity with massive anchors and chain. The pursuit of minimilist gear was mainly an academic exercise for my contrasting weekend lightweight boating times. . .
As an aside, I have done much freediving and spearfishing in spare time while on the hook from trawlers and other fishing boats- and can say that their anchors (usually plough) are almost never buried- so much for the theory that heavy gear encourages that. Often the chain isn't even straight. Coming unstuck was pretty rare though. Trust (or lack of) in these conventional anchor designs is reflected in general skipper behaviour when sheltering from expected really nasty weather in coral waters- you would drop the anchor on lee side of a house-size bommie and steam the chain in a circle right around it. A wonder some of those outcrops are not sawn off at the base by now, but at least the ploughs never get the chace to furrow accross the lagoon.
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Old 25-09-2008, 02:29   #13
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Quote:
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I am an advocate of a shorter heavier chain. But before you jump at me - you need to define what you mean.
Lets all jump on Talbot - PILLOW FIGHT

To me 30mts isn't 'short' and if the boat was the 9mt Cat I'd say that was a good length almost a longish length. And if that boat again, 3/8" isn't that small either. By short I mean a boat and a bit length or less.

Quote:
This 'spreadsheet thingy' is undoubtedly right- but I reckon GMac's disparaging comments are more related to some extra factors coming into play in the real world- like dynamic issues also mentioned in that website.
Not so much disparaging, more 'just use with caution' as part of the total 'think' package. I see many who think it's the be all to end all and I think that is very dangerous. It possibly is correct considering, as you mention, that big lack of many of the variables that do actually exist. At 3am in 50kts the missing variables could easily kill those who take it as gospel.

But when we have tried to replicate the numbers in real life we can't and get quite different ones. This is done by using heavy machinery (the loads are large) on land.

There is also the fact that I sussed it for one of the boats I play on and it says that in 25kts of wind it should have over 1200lb on the anchoring system. I'm either a massive pile stronger than I think I am or the SS has a issue somewhere. I regularly pull the anchor up by hand in winds of that and on occasion above 25, which involves me puling the boat to the anchor first, it has no winch and I don't find it that hard really. My wife even pulled it up one day in 25kts and 3-4 kts of tide with waves. She is into power lifting but she ain't no spring chicken no more (repeat that and I'll hurt you )

svnakia - maths is good very good but being a sceptical bastard as I am I like to see it physically happen. Calculations showed the Space Shuttle safe as houses, the world being hit by a big asteroid in the 1950's, financial derivatives to be the greatest thing since canned beer and chinese milk to be perfectly fine. Reality has proven to be a tad different.

When it comes down to the nitty gritty there is no good solid way to replicate all the variables that are involved in an anchoring system when in use. Anchor tests, this test, that test and even our love of big mechanical toys with loads cells can only give us an idea of specific bits only. They are also handy to see trends and for head to head comparisons in a specifc situation.

Hence we put a huge pile more credence in what the users actually find themselves. I actively ask everyone I come across 'what do you use, how do you use it, how do you find it and so on. I've been doing it for over 15 years and have built quite a substantial database. This does show certain things, one being a longer bit of chain worked better than a short one. And as mentioned above when I say short I mean short to the boat it's on, it's all relative.

Due to this we use the findings on our customers boats and as yet they haven't proven to be incorrect.

My boat is one of a class so it's handy to use for comparison purposes. Last Xmas we were away with 2 others. I have 15mts of 1/4" then to rope, it is smaller than I would recommend but I know how to use my gear to it's best advantage so I'm happy with it, I also come from a strong racing background so have personal issues I need to deal with regarding weight One of the other boats had 5mts of 3/8" then to rope, the other had 7mts of 3/8". On a couple of occasions we were having evening Church meetings and having a fair whack of late arriving big nasty fizz boat slop, the inconsiderate bastards. Everyone commented on how my boat didn't snatch on the anchor when the other 2 did. The only differences was the chains we all used. Since then one of the other guys has swapped to a similar sized system as mine and he often remarks on how much smoother his anchored ride is.

It's all a seriously grey science and I mainly go on what I physically see happen and have found out with my own paws in over 35 years of anchoring myself and 15 in the game of selling and playing with the gear. All that has me convinced that in most occasions longer is better than shorter. This is all talking Cruising type boats not the off the beach guys which can get away with just about anything often.

But and it's a fun but, I do love my toys and I have just built a new one that has got us more than a little excited. An anchor system load susser, yeah, I'm not to flash on making up good names. Just finishing final testing and all looks good for it to go into the field for this season. Soon I'll be able to verify both my thinking and those of others, inc the SS. Maybe I'm right, maybe I'm not. Soon we'll know, for better or worse.

It's a great subject and I suspect no-one has it quite right, me included. But still fun to ponder, discuss and learn from each other.

And no need to worry about upsetting me. Specialists have tried without joy so unless you're a bit of a legend at it you're all only learners. I'm surprisingly thick both in the head and the hide
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Old 25-09-2008, 02:44   #14
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....

And no need to worry about upsetting me. Specialists have tried without joy so unless you're a bit of a legend at it you're all only learners. I'm surprisingly thick both in the head and the hide
So you really are a KIWI
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Old 25-09-2008, 03:17   #15
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