8mm Anchor chain?

[vyv_cox]

I doubt if there is any catenary with a "rope rode".

Nor with chain in fresh winds.

 

[vyv_cox]

For a 30' yacht I would not be using 8mm chain but 6mm of G30 chain bought from a supplier with a reputation to protect and I'd carry 50m.

J

I drew up a spreadsheet to investigate strength of chain v. the force generated by wind speed, using the John Knox expression. For my 34 ft boat the SWL of 6 mm Grade 30 chain is only exceeded at a wind speed of 43 knots. For a 30 ft boat the figure is 50 knots. SWL with a safety factor of 4 over UTS

 

[NormanS]

I drew up a spreadsheet to investigate strength of chain v. the force generated by wind speed, using the John Knox expression. For my 34 ft boat the SWL of 6 mm Grade 30 chain is only exceeded at a wind speed of 43 knots. For a 30 ft boat the figure is 50 knots. SWL with a safety factor of 4 over UTS

Vyv, just for interest, using your figures, with my 10mm grade 30 chain, holding a 36ft boat, what wind speed would cause a force equal to the SWL with the same factor of safety (4)? We do occasionally get some rather fierce winds.

 

[noelex]

I have some reservations about using 6mm G30 chain on a 30 foot yacht.

The ABYC guidelines indicate that the force at 50 knots is 900 Kg at 43 knots it is 665 Kg. The ABYC guidelines have been criticised as being pessimistic. The Knox formulae for 50k indicates a much lower value of 418 Kg.

6mm chain has a SWL (1/4 UTS) of 400 kg.

I have no idea if the ABYC guidelines or the Knox guidelines are closer to the mark. I suspect the latter, but winds over 50k are not impossible. Forces rise dramatically as the square of the windspeed. 60 knots is 1,300 Kg according to ABYC. Modelling of anchor forces is very dependent on a multitude of factors particularly wave action.

In short, I think if you want to use G30 chain for a 30 foot boat 8mm would be a better choice. The chain is likely to have a bit of wear and tear and even some corrosion before replacement.

Most G30 chain is actually G40 which helps, but if the chain is genuine G30 6mm is marginal.

In practice very few anchor chains break, but cruising boats have been gradually shifting from heavy to lightweight chain and I think those boats pushing the boundaries need to be cautious.

Just my 2c

 

[J Neeves]

Jonathon

Agree with just about all of that, developing a technique whatever anchor you use is important, as in many other areas of what we do. I have never seen a Mantus 'live' so to speak, but as an engineer it looks good from photographs. If it is the bolts you are concerned about then you do not need to be, those four will be every bit as strong as welding IMO, high tensile bolts are good engineering practice, and probably more expensive than welding methods, I feel sure they will have been tested well beyond anything we can do to them.

Cheap HT bolts, or think those bought at low cost, have been found not to be to the specification described. There have been major delays to a new bridge in SF for this reason and major problems here in Oz with a high rise building in Melbourne. I'd dump the bolts supplied and go and buy new ones from a retailer who can offer greater reassurances. HT bolts are not normally subjected to the snatch loads we impose when we anchor, particularly when we lift the anchor in a chop - and because construction HT bolts are meant to be static are not tested by cycling. Moreover HT bolts are seldom subject to constant immersion in seawater - and these bolts on this anchor are 'hidden'.

HT bolts have been available for decades, anchors are welded for a reason and the absence of load bearing bolts in anchors is indicative of the way the industry thinks.

But I am also concerned that the shank of the anchor has less width compared to other anchors of the same size. 15kg Delta, Mantus, Excel, Rocna etc are all made with shanks of 12mm plate. The Mantus shank is 60% the width of the others - giving its shank 60% of the strength - in fact about the same strength as the off spec anchors of the past (not mentioning names).

I stick with my comments, great design, crud engineering and QC.

Jonathan

 

[charles_reed]

Had similar experience with my CQR, but very impressed with what I have seen so far of the Mantus in sand, what size did you purchase.

25 lb - based upon Chesapeake Bay tests and deriving from John Knox calculations, should hold me in <40 knots. Well within the SWL of 8mm G40 chain and 14mm nylon anchorplait. (About 12kN).

No-elex pics only show his oversize Mantus setting in sand - the CQR has served me well in sand, mud and shingle. Big benefit with Mantus (and Spade, Rocna, Manson) is that they set in about 1m instead of the 4 needed for CQR/Delta and need much shorter scope.
Having said that I've been anchored for 4 days in 25knots gusting 30 winds on a 14lb Danforth with 25m of 7mm chain and 30m of 12mm 8-plait. That's only rated @ 4-5kN.
I do agree with Vyv about the lack of hysteresis in all chain rode and the body of informed opinion supports this view. I'd never anchor, in earnest, on all chain, but equally would not anchor on all textile.
There are diehards to maintain the superiority of all chain, we must agree to differ.
Jonathan's comment about the duff engineering in the Mantus is down to the original mild-steel shank bending (on a well-oversize anchor) - now replaced with an ASTM 514 high-tensile steel.
The anchor continued to perform OK with the bend and I've twisted both the CQR and Delta shanks - one of the inherent characteristics of a deep-digging, large fluke anchor is a tendency to overload the shank - witness Rocna. Fortunately easily replaced and free under the terms of the Mantus warranty.
Whilst I admire Sarnia's faith in the infallibility of engineers, I retain a degree of scepticism - why do cars have recalls?
As to scope - the shallower the water the larger the # needs to be. If you're worried about swinging, just put out a 2nd anchor - for me the stern Delta. Different anchor, different scope, in 6m the Fortress is OK (I'm told) @2:1, the Brittany needs 6:1. Then take into account tidal rise and fall (even in some parts of the Med) swell action and you soon come to the conclusion that 3:1 is overly optimistic
The big problem is yottie psychology - how often do you sail into a large open anchorage and find all the boats anchored in an heap in the corner. Sheep...
As to ships anchoring in one spot - just watch them - every few hours - they move back up to windward again.

 

[charles_reed]

Cheap HT bolts, or think those bought at low cost, have been found not to be to the specification described. There have been major delays to a new bridge in SF for this reason and major problems here in Oz with a high rise building in Melbourne. I'd dump the bolts supplied and go and buy new ones from a retailer who can offer greater reassurances. HT bolts are not normally subjected to the snatch loads we impose when we anchor, particularly when we lift the anchor in a chop - and because construction HT bolts are meant to be static are not tested by cycling. Moreover HT bolts are seldom subject to constant immersion in seawater - and these bolts on this anchor are 'hidden'.

HT bolts have been available for decades, anchors are welded for a reason and the absence of load bearing bolts in anchors is indicative of the way the industry thinks.

But I am also concerned that the shank of the anchor has less width compared to other anchors of the same size. 15kg Delta, Mantus, Excel, Rocna etc are all made with shanks of 12mm plate. The Mantus shank is 60% the width of the others - giving its shank 60% of the strength - in fact about the same strength as the off spec anchors of the past (not mentioning names).

I stick with my comments, great design, crud engineering and QC.

Jonathan

Maybe your oratory has borne fruit - the shank on the 25lb Mantus delivered to me in Nea Perama is a larger X-section than that on the 6kg Delta - the first being allegedly 103ksi and the latter supposedly 47ksi. Immaterial as the second has a 4/5 degree bend.
I will, however, report back as to how the Mantus behaves in practice, but can't provide photos or mermaids.

 

[J Neeves]

As to ships anchoring in one spot - just watch them - every few hours - they move back up to windward again.

We have bulk carriers, 100,000t-300,000t, anchored north of Sydney waiting in a queue to get into Newcastle to load coal. Its a recognised anchorage for the ships., its open sea, Tasman, about 40m deep. We have had a number of ships on the beach, the most recent was the Pasha Bulker (I think that was the name) which was eventually refloated (and then maybe floated to Korea and scrapped). Now - new rules, as son as wind speeds are forecast to be over 25 knots or thereabouts all ships must leave the anchorage and sit out whatever that forecast is (until winds drop) offshore.

Big ship anchoring and our anchoring - are worlds apart. We expect our, that personal' choice of, anchor(s) and ground tackle to work whatever - though you are most unlucky if you actually are at anchor and actually measure wind speeds of over 45 knots on your wind gear. Hopefully most of us anchor in anchorages sheltered from 45knots + and though it might be 70 knots outside - its actually a benign and calm 30 knots inside (even though when we recount the tale in the pub - we omit to mention the highest wind speed on our wind gear was a 30 knot gust!)

A steel shank that is 60% of the width of the industry norm - and the Excel, Supreme and Boss use ASTM 514 (and Delta and Rocna near ASTM 514), is only 60% of the strength. If the industry norm is say 100mm - its a brave man who says I'll buck the industry norm and go for 60mm, especially after shanks of about 60% of the strength of industry norm were shown to bend - and were the catalyst for an interesting long and passionate Forum debate - if 60% of industry norm is sufficient then much of the rhetoric of those passionate threads was grossly unfair.

I'm happy to be shown that Manson, Anchor Right, Rocna, Lewmar are over engineered and have a far too high safety factor - but this has not been forthcoming. And for the difference in cost I like the 40% extra in strength.

I like the idea of a 100% no questions asked guarantee of free replacement. - how long does it take to get a new shank to Ardnamurchan (is that how you spell it) and what do you do whilst you wait? Personally, again, I like the idea of a shank that is over engineered.

Jonathan

 

[NormanS]

It must be great to have these anchorages which shelter you from the actual wind strength. On the West Coast of Scotland we are blessed with many sheltered anchorages, where there is no "sea" from any direction, but few of them stop the wind, and some indeed suffer from accelerated winds, coming down glens and off the hills. The Sailing Directions are full of warnings about severe gusts. To be fair, I've never had more than 74knots, but quite often in the mid 60s. The good thing is that midges can only fly at 4mph.

PS In terms of a recent post here, I'm a diehard. (Unless I put out a second anchor).

 

[J Neeves]

NormanS,

I was a bit simplistic.

We actually have the same issues here in Tasmania, with bullets coming down the valleys - sometimes at 180 degrees to each other. It can be most odd in an anchorage with flat, almost glassy, water but with gusts than lift the water off the sea. In the grand scheme of things few actually travel to these areas, a couple of years ago the numbers of yachts visiting the west coast of Tassie was the grand total of 60! per annum (and hopefully we, who do go, know what we are doing).

We are wimps and set a second anchor and tie to trees or rocks - and are all geared up for same.

Fortunately Tassie does not have midges!

Jonathan

 

[charles_reed]

<<I like the idea of a 100% no questions asked guarantee of free replacement. - how long does it take to get a new shank to Ardnamurchan (is that how you spell it) and what do you do whilst you wait? Personally, again, I like the idea of a shank that is over engineered.>>

One big problem with a massive shank - it upsets the balance that these new age anchors are so dependent upon. Probably why the Ultra uses a hollow shank.
Your concern, I assume, is not about the shank breaking but rather bending. In that case there is no replacement for X-sectional area and all the new age anchors have tat Achilles heel.
Having distorted both Delta and CQR shanks I suspect that it's an inherent problem with all deep-setting anchors.

PS. The lifetime warranty is void if you remove the roll bar on the Mantus - if in terror of the deeps (and shank-bending beasties) one could always carry a spare. :ambivalence:

 

[noelex]

I am not sure why so many discussions about anchors, or even chain end up discussing shank strength.

It is far more likely for a boat to drag than to ever damage an anchor shank. Dragging carries a real risk of damage, whereas if an anchor shank bends it nearly always occurs on recovery.

There never seems to be any discussion about anchor fluke and tip strength. I have seen more bent anchor tips than bent shanks. These are some of the puzzles of anchor threads.

Unfortunately, shank weight effects the centre of gravity of the anchor. Making the shank heavier decreases anchor performance. I think it is reasonable to influence manufacturers to use high quality steel in the shanks of anchors. This carries no penalties of weight or performance (only cost). However, threads like this have the danger that we are forcing anchor manufacturers who already use the best quality steels to also try and design heavy shanks that are unbendable in any circumstances (if this is even possible).

Manson boast that they have never had a bent shank on a Manson Supreme. I have personally seen a bent fluke on this model of anchor. The fact that the shank is extremely unlikely to bend does not make the whole anchor bulletproof.

I think Manson have realised this is not a sensible strength/performance compromise and that by making the shank lighter anchor performance is helped. On the newer Manson Boss anchor they have used thinner steel in the shank.

The Mantus shank is stronger than the Kobra, Delta, Manson Boss, Ultra, Super Sarca and many others if this computer finite element analysis is correct:
http://mantusanchors.com/anchor-shank-strength-for-lateral-applied-loads/

Mantus use the same strength high tensile steel in their shanks as the Manson Supreme. The Mantus shank is same thickness as Manson Supreme but less deep helping to give the anchor slightly better performance. It is not quite as indestructible as the shank of the Manson Supreme, but the Mantus is covered by a lifetime warranty against bending, unlike the Manson which is not covered at all.

 

[J Neeves]

Noelex, There a number of errors of fact in your post

There are HT steels stronger that ASTM 514A, the Knox anchor uses a 900 MPa steel.

The Super SARCA has been re-engineered to use HT steel, I believe stronger than ASTM 514a.

The Mantus shank uses 40% less steel than the Supreme making it, the Mantus, 40% weaker - if this is your idea of 'not quite as indestructible' you appear to have still 'sold that soul'.

The reason shank strength is a common topic for discussion is that an anchor maker stated innumerable times that a ASTM 514a quality was essential and then did not use it. A number of the shanks - with the lower quality steel bent, some when the yacht was at anchor (not when the anchor was retrieved). That anchor maker set the benchmark which others met - and is now exceeded by both Knox and Anchor Right (for both of their anchors). To not meet that benchmark, by 40%, looks questionable.

I have a 15kg Mantus shank (and anchor) and I have a 15kg Anchor Right Excel - there is a 40% difference in shank depth (same thickness of plate) - how one can compute that this 40% does not result in a 40% difference in strength is quite clever.

Jonathan

Interesting that Mantus, having said you could remove the roll bar and the anchor still operate (this was treated with much derision at the time) now say that warranties fail if you take the roll bar off - one wonders what other changes they might want to make

Great design - terrible engineering (and marketing).

 

[JumbleDuck]

NThe Mantus shank uses 40% less steel than the Supreme making it, the Mantus, 40% weaker - if this is your idea of 'not quite as indestructible' you appear to have still 'sold that soul'..

Could you explain, please, what exactly you mean by "weaker" here? Unless it's a simple tensile failure (and I would be very surprised if the cross sectional area of a Mantus shank was less than the cross sectional area of the chain attached to it), strength generally depends on how material is used as much as the amount and properties. For example, strength in bending depends on the elastic section modulus, Z_e and - for example - a rectangular hollow bar with the middle 40% removed only loses 16% of Z_e.

I have a 15kg Mantus shank (and anchor) and I have a 15kg Anchor Right Excel - there is a 40% difference in shank depth (same thickness of plate) - how one can compute that this 40% does not result in a 40% difference in strength is quite clever.

In that case there will - assuming materials with the same yield point - be a 40% reduction in bending strength for the long (vertical) axis and a 64% reduction for the short (transverse) axis, though the later will probably start an order of magnitude higher os so anyway. But does it matter? Looking at amount of material alone is just not enough. Is there any evidence to suggest that Mantus anchors are failing?

 

[vyv_cox]

Vyv, just for interest, using your figures, with my 10mm grade 30 chain, holding a 36ft boat, what wind speed would cause a force equal to the SWL with the same factor of safety (4)? We do occasionally get some rather fierce winds.

A hell of a lot! My initial spreadsheet goes to 57 knots, when the load on the chain is 773 kgf. The SWL of Grade 30 10 mm chain is 1275 kgf.

I have extended the data and find that 1275 is exceeded at 73 knots. No idea whether the Knox expression has ever been tested with winds as high as this but I do know it holds true to around gale force, measured with load cells.

 

[J Neeves]

Could you explain, please, what exactly you mean by "weaker" here? Unless it's a simple tensile failure (and I would be very surprised if the cross sectional area of a Mantus shank was less than the cross sectional area of the chain attached to it), strength generally depends on how material is used as much as the amount and properties. For example, strength in bending depends on the elastic section modulus, Z_e and - for example - a rectangular hollow bar with the middle 40% removed only loses 16% of Z_e.



In that case there will - assuming materials with the same yield point - be a 40% reduction in bending strength for the long (vertical) axis and a 64% reduction for the short (transverse) axis, though the later will probably start an order of magnitude higher os so anyway. But does it matter? Looking at amount of material alone is just not enough. Is there any evidence to suggest that Mantus anchors are failing?

There is no evidence at all that Mantus shanks, with the new high tensile steel are failing. However the fact there are no reports of failure might mean there are very few sold and or that those that have bent are rapidly replaced.

Rocna had the same argument when they were exposed to have been using a lesser strength steel - they said something like 'the lower strength steel was perfectly safe etc etc' - and then contradicted themselves by replacing it with a better steel.

You appear to have answered your first question/paragraph with your second - and with more economy than I would have achieved

You do tend to independently emphasise the idea that 'not quite indestructible' is not quite correct?

However I do wonder why Rocna who have been around now for 14 years, Manson for much longer, Anchor Right since the early 90's etc all have shanks on their anchors of very similar dimensions yet a recently introduced company/product knows better (and thinks shank strength less than critical) - given they started off with a mild steel shank.

We buy chain with a 4:1 safety factor, I'm not sure what safety factor, if any, anchors are built to but reducing an industry norm, by 40% and 64%, looks 'unusual' and hardly one with which I sit comfortably.

But if you think, without reservation, this reduction of the industry norm is acceptable - I would certainly want to factor your views into my thoughts.

Jonathan

 

[sighmoon]

Kelp grows on rock and will defeat most anchors, avoid like the plague - unless you carry an old fashioned fisherman's and to be useful it will be so heavy as to be unmanageable by a small crew. As NormanS suggests get a fish finder or in shallower clear water look and anchor in the sand patches.

20m anchoring depths are basically outside the safe anchoring depth for most small to medium yachts - we cannot carry sufficient rode for anything but a lunch time stop and then we, personally, would not leave the yacht. 20m implies having a 100m rode to get anywhere near a 5:1 scope and often 20m means isolation and exposure to swell. An alternative is to moor to shore in a cleft in the shoreline but this needs a very good forward looking depth sounder (or clear water), good coordination between deck and shore and a 'user friendly' cleft. Carrying short lengths of chain, 3m, ensures there is no chafe, chain round rocks and shackle.

We have done it but find its too much like hard work, physical and mental, and avoid 20m anchorages completely now this might be a sign of maturity or increasingly becoming more wimpish.

Jonathan

Good advice, but where the OP intends to sail, it is often the reality. The CCC directions are on the boat, but I'm pretty sure it even said 'anchor in thick kelp' in there somewhere.

Same with the depth - 20m opens up the choice of where you can anchor, and on the West coast of Scotland, that generally doesn't mean exposure to swell. Off the top of my head, Tobermory bay, and Loch Dram na Buidh were two places where the extra chain would have meant a more comfortable night's sleep for us.

 

[oldbilbo]

I'm just wondering whether the experience, and the techniques, illustrated here carry any weight in this discussion....

Then there's the corollorary vid ( which IPC/Time/Warner won't permit, despite it being their own production )....


https://www.youtube.com/watch?v=pjZVVstkdQU


It sometimes seems, in these circular discussions, that opinions derive from two groups of people.... Those who seek the smallest and lightest gear they believe they can get away with, and those who want their safety gear to be 'bombproof'.

In my value-system, where I am likely to have responsibility, in a blow, for others' safety and security - friends, family, other rabid forumeers - there just ain't no such animule as 'too big and strong' in ground tackle.

 

[lw395]

A hell of a lot! My initial spreadsheet goes to 57 knots, when the load on the chain is 773 kgf. The SWL of Grade 30 10 mm chain is 1275 kgf.

I have extended the data and find that 1275 is exceeded at 73 knots. No idea whether the Knox expression has ever been tested with winds as high as this but I do know it holds true to around gale force, measured with load cells.

In flat water.
If you have a few big ripples accelerating your 5T boat at 0.3g (peak), that'll be 1500kgf.
At which point one might like the cqr to drag a few inches rather than the chain to snap?

 

[JumbleDuck]

Rocna had the same argument when they were exposed to have been using a lesser strength steel - they said something like 'the lower strength steel was perfectly safe etc etc' - and then contradicted themselves by replacing it with a better steel.

Rocna rather shot themselves in the foot: their rather aggressive representative/cheerleader here regularly stressed the importance of the steel they used and sneered at others for using different grades, so when it turned out that they were not, in fact, using that steel they were left with little wriggles room. I was and remain unconvinced that the grade of steel is much more than a marketing ploy and that the material properties (yield / UTS / toughness) of anchors matters terribly much in a reasonable design.

However I do wonder why Rocna who have been around now for 14 years, Manson for much longer, Anchor Right since the early 90's etc all have shanks on their anchors of very similar dimensions yet a recently introduced company/product knows better (and thinks shank strength less than critical) - given they started off with a mild steel shank.

I can think of two plausible reasons, neither of which may be accurate.

(1) The other manufacturers all copy each other out of innate conservatism and/or the knowledge that steel is cheap and customers tend to buy the beefiest looking

(2) The Mantus is designed to rotate more easily when a sideways load is applied, so doesn't required the same stiffness/strength in ending of the shank.

But if you think, without reservation, this reduction of the industry norm is acceptable - I would certainly want to factor your views into my thoughts.

Absent evidence of a problem, I wouldn't worry about it too much. I'd be much more concerned with the fluke materials. These NG anchors all seem to depend on digging in well, and the pictures which circulated of Rocnas with their fronts turned up like the toes on a jester's shoes after hitting a rock concerned me, I would never buy one, in any case, because of the attitude of the cheerleader mentioned above.