The inverted anchor drag

[NormanS]

Because first time it was dropped it landed right way up and did not need to self right.

During the Fortress Cheasapeake tests they could not understand the poor performance of anchors that would have been expected to give at least some hold - these same anchors were developing hold no better than a mushroom weight. They had no diver, and it was mud, and they guessed that maybe the anchors were being dropped and landing upside down and simply 'skating' through the mud. They added a float to one of the anchors to encourage it to land right way up and the anchor developed some hold - suggesting the idea of landing upside down had some merit - but they could never prove the hypothesis. Interestingly, though it did not form part of the debate, the hollow shanked Ultra developed much better hold than the Spade, whose shank is 'open' and thus not a float. Mantus is hydrodynamic - as long as the 'setting' vessel is moving backward (and the anchor is deployed off the bow) Mantus will always arrive at the seabed and land right way up - in the Chesapeake tests the Mantus was the best of a poor bunch. Of course if it then somersaulted and lands on its back - it will not self right, even though it did, in the tests, developed some hold - they did not try somersaulting.

Soft mud occurs in many places, usually where oysters are found. So the idea of an anchor developing some hold and later not resetting - might be unusual - but definitely possible.


Interestingly the only company to test anchors in soft mud is Fortress, whose anchor stands head and shoulders above anything else (which is presumably one reason they chose that bottom type). But other anchor makers have not tried to refute the tests, nor make their own tests in soft mud (I wonder why?)

Jonathan

See post 35.

 

[noelex]

Vliho bay has an easy substrate of medium to soft mud. The bay seems to cop a fair amount of strong wind and I have sat out 50+ knots here using my Rocna anchor (which was my primary anchor when I was in this area) on several occasions. The problem with convex plough anchors like the Delta is that they do not have a lot of surface area and combined with the convex blade design, they do not generate a lot of drag. This can be a problem in softer substrates, especially those which don't become much firmer with depth.

The convex plough anchors tend to be quite reliable in this sort of substrate in light and moderate winds, but in stronger conditions and softer substrates they start to slowly move backwards while remaining set and buried. Often these drags are slow enough that the owner is unaware, but they are quite fascinating to watch underwater. With each stronger gust the anchor will move backwards without diving any deeper. Sometimes the anchor will encounter some debris and breakout with the more common fast drag that is easy to recognise.

In softer substrates the fluke anchors like the Danforth and Fortress provide exceptional holding, but in my opinion they do not cope with a change in direction of pull as well. Vliho bay has a lot of debris on the bottom. There is all sorts of rubbish and any anchor that breaks out with a change in pull runs the risk of encountering debris that will inhibit the anchor setting. An anchor that reliably "shuffles" is I thinkthe best solution, such as one of the modern new generation anchors.

If you want to use a fluke anchor like a Danforth or Fortress I would encourage the use of multiple anchors to keep the direction of pull of each more constant. It is a popular bolt hole for when bad weather is forecast so if you adopt this tactic be aware that with multiple anchors you are likely to swing differently to neighbours lying to a single anchor, especially if they deploy a lot of scope.

 

[vyv_cox]

, but they are quite fascinating to watch underwater.

You, or your mermaid, are very brave diving in Vliho Bay. Holding tanks seem to be few and far between in that almost totally enclosed bay.

 

[noelex]

You, or your mermaid, are very brave diving in Vliho Bay. Holding tanks seem to be few and far between in that almost totally enclosed bay.

Yes, it is not one of my preferred places, but it is one of the few anchorages in the area with protection from all wind directions.

For anyone interested in spending winter at anchor rather than in a marina or tied up to shore, it is a good base. There are a number of nice, but less protected anchorages nearby that can be visited when conditions are settled, but Vliho provides a bolt hole when the winter storms roll in. This is especially true when the wind is predicted to change direction significantly. This is common in winter and always seems to happen at 2am, so an anchorage with 360° protection is sometimes needed.

The water in Vliho bay is normally not nice, but on rare occasions it goes clear enough to see the bottom from the surface, which we would consider in Australia very good visibility. I try not to think about what is in the water, but at least there is no sharks or jellyfish with stings so bad they can be fatal, as in Australian waters .

 

[Fortress Anchors]

An anchor that reliably "shuffles" is I think the best solution, such as one of the modern new generation anchors.

An anchor that reliably shuffles, as you described, might also lack the holding power in severe wind conditions due to its lack of physical surface area and its lack of ability to bury deeply into a sea bottom.

If you had to rely on an anchor for predicted (ex: hurricane) or even unpredictable (early AM thunderstorms) high wind and surging sea conditions, and you wanted the best holding anchor for the weight, which would you chose?

 

[Neeves]

As you say yourself Noelex, quote 'Modern anchors bury the end of the shank where the chain is attached very early in the setting process'. end quote and this ability to bury the shackle end results in a fluke that sets quickly and deeply. Anchors that do not have this ability will set more shallow (as some of your pictures aptly demonstrate) and though this might, or might not, engender 'shuffling' its not going to enhance hold - as shear strength of seabed increases with the square of depth. Fortress set at an angle of 30 degrees in sand, some anchors (or one specific anchor) only set at 16 degrees - shuffling might be good - but hold? in a decent wind shift??

Interestingly the anchor you champion has never had, independently verifiable, hold data nor setting distance published and has certainly never been Proof Tested. However I am sure it must be an excellent product as you will be using it on your new yacht and if you had any doubts you would immediately express your concerns - and in fact would have done so long ago as a change of heart late in the day would look - peculiar (?).

 

[vyv_cox]

Interestingly the anchor you champion has never had, independently verifiable, hold data nor setting distance published and has certainly never been Proof Tested. However I am sure it must be an excellent product as you will be using it on your new yacht and if you had any doubts you would immediately express your concerns - and in fact would have done so long ago as a change of heart late in the day would look - peculiar (?).

If you follow Andy Marsh (Norna Biron) on Facebook you will find not exactly what you discuss but various comparisons of Mantus with Spade and Rocna. His findings are that in similar seabeds and weather the Mantus appears to be superior. Admittedly in the sandy bottoms of the Mediterranean, although he has carried out some comparisons in weed. As a 365 day anchorer his findings are borne out by valid experience.

 

[Neeves]

And does he have an oversize anchor?

 

[Flica]

And does he have an oversize anchor?

Of course he does. But I have an absolutely standard (25lb) one and can bear out its reliability.

Especially as Spade and Rocna demand almost twice the weight anchor for the same boat.
In about 200 deployments it has only dragged twice, both time due to sea-grass rhizome, though I have never experienced more than 48knots of wind for extended periods.

If there is any criticism, it is of the enormous roll bar, which makes it most difficult to stow. On the other hand its lightness makes up for some of that. It has a larger fluke area than comparable-weight new-age anchors.

Taking it proportionally the Mantus is twice as good (by Rocna's own reckoning) as the Rocna.

 

[vyv_cox]

And does he have an oversize anchor?

It is certainly quite large but then the boat is 50 ft, 20 tons. Not excessively so I think.

 

[vyv_cox]

And does he have an oversize anchor?

48 kg Mantus compared with 45 kg Spade. I think the Rocna may be a size down. His preference is very much for the Mantus.

 

[Neeves]

I hope he does not anchor in stony anchorages or get the toe caught under a ground chain.

The second or third rule given to an apprentice welder - Never make a weld where high stress might occur - it will weaken the underlying steel (its the reason you do not weld the front of one chassis to the back of another - or not without some form of reinforcement). Where is the weld for the little toe plate on the Mantus - what has happened - it snapped off. Rocna also welds on the thickened toe plate but they add strength by welding the fluke bang over the middle

I'd criticise a Kobra for its weak shank - and I'd equally criticise the Mantus for its weak toe - fundamental design flaws.

If the demand is for a bigger anchor in sand or mud -get a Fortress.

 

[Flica]

I hope he does not anchor in stony anchorages or get the toe caught under a ground chain.

The second or third rule given to an apprentice welder - Never make a weld where high stress might occur - it will weaken the underlying steel (its the reason you do not weld the front of one chassis to the back of another - or not without some form of reinforcement). Where is the weld for the little toe plate on the Mantus - what has happened - it snapped off. Rocna also welds on the thickened toe plate but they add strength by welding the fluke bang over the middle

I'd criticise a Kobra for its weak shank - and I'd equally criticise the Mantus for its weak toe - fundamental design flaws.

If the demand is for a bigger anchor in sand or mud -get a Fortress.

Whilst I'll not disagree with your final conclusion, I note that you do not dismiss, out-of-hand, those anchors where the shank is welded to the fluke?
A curious dichotomy in thinking - or is the shank/fluke join far less stressed than the toe?
Or maybe you are referring to embrittlement caused by local heating? More an error by welder than designer?

 

[Neeves]

Interesting you point it out - whereas it is possible that the same fault in the toe might also exist in the fluke to shank joint. - fortunately I suspect the toe is welded by one group and the shanks by another group. The shanks are an entirely different design as the shank is welded to its base plate from both sides.

I don't find any contradiction - its the toe that has failed, not the fluke/shank bolted joint. As I point out the shank joint is an entirely different design. Knowing that embrittlement might be possible one would think particular care would be taken with the patch welding and some form of testing introduced. One might also have though that to reduce problems occurring the design might have been altered.

As I suggested in an earlier post - none of these faults would exist (or at least they might have been noticed early and Quality Control introduced) if the anchors had been subjected to a Proof Test.

This is the same anchor that was introduced with a mild steel shank, subsequently improved to a 800 MPa steel. But you would think that after all the brouha ha over shanks no-one, except someone who had never heard of Rocna, would have considered that design in the first place. It was also suggested that the anchor could be used without its roll bar - but quickly withdrew that idea when it was demonstrably shown to be untenable. This is also the same company that reduced their snubber hook specification by 60% when it was pointed out that their test procedure was nonsense.

However as the anchor and Company appears to have such overwhelming and unqualified support any issues are obviously of no consequence and can be ignored, or shuffled under the carpet. I suspect also that many wish such issues were not aired. Knowing that Noelex is to use the same anchor as his primary on his million Euro yacht is reassurance of the highest calibre and I look forward to following his exploits with his Mantus anchor in colder waters than the Med

 

[vyv_cox]

I don't think embrittlement is an issue when welding the low carbon content of the steels used in anchor manufacture, although I don't know a lot about the boron hardened steels used by some manufacturers. The Rocna shank is welded to the flukes with a single pass of a very large MIG electrode, so I was told several years ago. The analysis I carried out on their current shanks showed them to have very low levels of any alloying elements (HSLA), making them excellent to weld without problems. According to Wiki this also makes them less prone to corrosion. https://en.wikipedia.org/wiki/High-strength_low-alloy_steel

No idea how Mantus do it.

 

[Neeves]

I don't know why the plate is there at all but assume it is simply to strengthen the toe. HT steels, including some 800 MPa steels, but there are others are specifically used as abrasion resistant components in large pieces of mining equipment. If Mantus simply welded a long narrow piece of plate from the toe back to the bolt holes it could act as the toe (and be much stronger than currently) and strengthen up the whole of the forward part of the fluke.

The problem being - as soon as you make any such changes you need to explain why leaving yourself open to the need to replace existing product. The other weak part of the design is the large hoop - which will (does) bend. SARCA overcame this issue, their roll bar is simply thin reo, by welding a small plate from shank to roll bar and Knox took this step further by passing the roll bar through the shank itself. A simple brace, ring at one end, fork at the other (to bolt into the shank) would suffice - and still be demountable (but might be difficult to retrofit).

Of course all of this is extra expense - detracting for the original low cost.

There are other improvements - but they are another story!