Dragging of anchors

[noelex]

I thought it polite to specifically answer your post.

Thank you.

it is sadly not possible to build an anchor such that if you double weight you double hold.

You don't seem convinced by the 2002 data that I posted. This showed for the two anchors tested a (Delta and a Bruce), that as the weight was doubled the holding power was also doubled.

Let's look at a separate and much more comprehensive later study (Practical Boat Owner August 2011). This looked at large range of anchor designs and sizes.

It found that the holding power increased more rapidly than the weight (doubling the weight increased the holding capacity by more than 2x). In fact, in nearly all cases doubling the weight increased the holding capacity by considerably more than 2x. The only exception was the Bruce, which was a bit below double and the Claw (Bruce copies). The different sized Claws were made by two different companies and I think the results can be discarded.

Overall, this shows that it is certainly possible to build an anchor that as you double the weight you double the hold. In most cases this study showed the holding power rises considerably more rapidly than the weight. (I am taking about anchors if the same design made from the same material)

For example, increasing the weight of the Manson Supreme by 1.7x increased the holding power by 2.5x. Increasing the weight of the Rocna by 3x increased the holding power by 5.6x.

Here is the data:

Spade 6 kg 120 kgf
Spade 15 kg 420 kgf

Rocna 5 kg 85 kgf
Rocna 15 kg 480 kgf

Bruce 5 kg 35 kgf
Bruce 15 kg 80 kgf

Claw 5 kg 43 kgf
Claw 15 kg 50kgf

Delta 9 lb 34 kgf
Delta 15 lb 76 kgf
Delta 35 lb 186 kgf

CQR 15 lb 44 kgf
CQR 45 lb 175 kgf

Manson Supreme 15 lb 90 kgf
Manson Supreme 25 lb 225 kgf

 

[Neeves]

Sorry but on balance I am inclined to believe the work conducted by the oil industry who say that the best efficiency they can achieve is 0.9, against 1.0 being perfect. Somehow I have this feeling that if they found that it was possible, by copying, say a Supreme, they could break the magic 1.0 barrier (or even the 0.9 barrier) they would have done so. Part of the fault in the data might be, as mentioned, that small anchors are over engineered and are not scaled versions of large one. It is also dangerous to extrapolate from such small anchors to something larger.

The data in itself is interesting - this is a good clean sand yet a 15kg Bruce can only hold 80kg. 80kg is about the load on a yacht at 20 knots! There is a rule of thumb that a fit male can exert a force equal to his own body weight - I think this indicates that same fit man could pull that Bruce through the seabed. Equally I suspect there would be many that would look questioningly at a 45lb CQR only being able to hold 175kg.

I also recall that the West Marine tests that you praised quite highly tested similarly sized anchors as the larger ones you quote and they achieve holding capacities of around 2,000kg. The discrepancy is quite large and might raise some questions. I have used a 15kg Spade and measured snatch loads on our catamaran, without dragging, of over 650kg and have pulled the same anchor with a winch and achieved 2,000kg, consistent with the WM results. I think anyone using a 15kg Rocna and knowing the best it could hold would be 480kg would be seriously worried, or disbelieving.

This might be a peculiar seabed - in which case the results need quoted with caution.

Jonathan

edit I have 5kg models of Supreme, Rocna, Excel, Mantus and Spade (as well as larger versions), my CQR and Ray are bigger and in the soft sands of Australia (in which you tell me anything will set) I cannot set any of these by hand such that even the fluke is buried - and I'm a relatively fit, sylph like 72kg. I can get the toe to penetrate - but never the heel and it takes 70kg to achieve this poor level of set (they can obviously dive more and achieve higher holding capacity)- so how Professor Knox had these self same, or very similar, anchors to set such that they reached maximum diving depth with a winch and at such low loads is totally outside my imagination. close edit.

 

[Neeves]

No problem, and I agree with you. Heavier of the same design cannot have lower holding capacity that lighter of the same design. Which begs the question, if two anchors are identical in every way other than weight, is it plausible that the heavier anchor would set less quickly than the lighter? Can't think so, which in a nutshell is the argument for bigger is better. A Fortress made of depleted uranium will set quicker than the same Fortress in aluminum. Gravity dictates that outcome. Having said that, the aluminum Fortress is designed so it doesn't require high weight to achieve high holding even if it might benefit from the weight, but for every other example I can think of, if you add more mass without a proportional increase in surface area, the anchor with more weight per square inch of surface area will set quicker and hold better than the same design that is lighter and has a lower weight per square inch of surface area. Physics dictates this outcome.

I not sure that I follow the reasoning. A steel Spade sets to the same depth as an alloy spade (of half the weight)) gravity offers no advantage. I cannot understand why the depleted uranium Spade will set more quickly if the steel Spade does not set more quickly (than the lighter alloy Spade). If gravity offer no advantage for an anchor of twice the weight I cannot see why gravity would help, for a larger or lessor weight difference. Both anchors are set and develop hold as a result of force exerted through the rode, wind or engine. If there is no force exerted by the rode - nothing happens, both anchors sit where they are.

Jonathan

 

[noelex]

Sorry but on balance I am inclined to believe the work conducted by the oil industry who say that the best efficiency they can achieve is 0.9, against 1.0 being perfect.

That sounds much better.

A 1.0 ratio is not "perfect", it just indicates that when you double the weight of the anchor, you double the holding power. Much higher numbers than 1.0 are possible and seen in practice as the study I quoted shows.

Vryhof who seem to supply most of the oil rig anchors, list the ratio for every single one of their anchor models as 0.92. So 0.9 is not the "best", but may be close to the average if you want to use oil industry data.

A ratio of 0.9 would mean if you double the weight you increase the holding power by 1.8x. This is close to my understanding of what occurs, so we are now the same page. Let's consider the debate closed.

 

[Delfin]

I not sure that I follow the reasoning. A steel Spade sets to the same depth as an alloy spade (of half the weight)) gravity offers no advantage. I cannot understand why the depleted uranium Spade will set more quickly if the steel Spade does not set more quickly (than the lighter alloy Spade). If gravity offer no advantage for an anchor of twice the weight I cannot see why gravity would help, for a larger or lessor weight difference. Both anchors are set and develop hold as a result of force exerted through the rode, wind or engine. If there is no force exerted by the rode - nothing happens, both anchors sit where they are.

Jonathan

Sorry to get all pedantic on you Jon, but actually my comment was that the heavier anchor will set more "quickly". Weight on planet earth is a function of gravity. Weight on earth is felt as the acceleration field pulling massive objects to the center of the field, a.k.a. the center of the earth. Unless the laws of physics are suspended, then F = ma and this simply means that the the force (F) pulling the anchor to the center of the earth is a function of the gravitational field (a) times its mass. The more massive anchor will set faster because of this force. It doesn't mean that a lighter doppelganger won't set, nor does it mean that once set the lighter can't present the same resistance to the sea bed, but that wasn't my point.

More importantly to real world anchoring, since an increase in the weight of an anchor goes up faster than the increase in surface area the heavier anchor ends up with greater mass density than the lighter hook, and in anchor sizes we care about that results in the increase in holding greater than the increase in weight Practical Boat Owner measured. The .9 increase in house sized anchors used by oil rigs for a doubling of weight doesn't surprise me since without understanding all the variables there must be some limit to the increase in holding beyond certain physical dimensions. But at the sizes us Dude boaters use, for modern anchors increasing the weight more than increasing the surface area translates to faster penetration of a wider range of sea beds.

None of this pertains to the ease with which an anchor can be set by hand, which impacts your perspective. Mine is slightly different. A 44# Bruce did an ok job of anchoring my 12 ton sailboat, dragging only a few times in fairly light winds. A 176# Claw did a stellar job of anchoring my 65 ton trawler, which has greater windange, never dragging even in winds of 50 knots+.

 

[Neeves]

That sounds much better.

A 1.0 ratio is not "perfect", it just indicates that when you double the weight of the anchor, you double the holding power. Much higher numbers than 1.0 are possible and seen in practice as the study I quoted shows.

Vryhof who seem to supply most of the oil rig anchors, list the ratio for every single one of their anchor models as 0.92. So 0.9 is not the "best", but may be close to the average if you want to use oil industry data.

A ratio of 0.9 would mean if you double the weight you increase the holding power by 1.8x. This is close to my understanding of what occurs, so we are now the same page. Let's consider the debate closed.

Noelex,

I am not on the same page - and I suggest you move onto the next page as well. We then do not need to have this ridiculous discussion again

The 0.9 efficiency is for the best anchors the oil industry can produce, anchors that we recognise (like some Danforth) have an efficiency of 0.66 (and many think the Danforth a superb anchor - and has been 'copied' to make the Fortress or Brittany). The original cast Bruce, that looks identical to the original Bruce we see, but for the oil industry so much bigger is considered grossly ineffincient now and was superseded in efficiency decades ago. So thinking that our anchors are as good as the best the oil industry can produce might suit your ideas - but is wrong and badly wrong as the best anchors in our the leisure market, and Fortress are considered the best, only have an efficiency of 0.83 and all the others are lower, Rocna, Spade etc. Vryhof have tested a Fortress and there is a host of consistent data on the Fortress from tests in a variety of seabeds and right through the range of their sizes. Looking at six different sizes in tests conducted in entirely different seabeds, by different people comes up with a 0.83 efficiency. Suddenly your willingness to accept 1.8 becomes needing to accept 1.66, 2 x 0.83 (as the most efficient of the leisure anchors). And if you are cautious and consider that many anchors are only 1.32 (2s 0.66) - then your desire to accept 1.8 looks, well, as I said - wrong. An efficiency of 1.32 means you need to more than treble weight to double hold, and in the case i quoted trebling weight doubles, surface area and doubles hold - exactly what I quoted.

But if you are willing to accept the data that the industry produces this puts into question your posts based on Professor Knox data which is giving efficiencies of 3 times and more - maybe those posts should be deleted.

I had a look at Professor Knox technique and he used nylon as his rode for some of the tests, he also used chain and wire cable - but does not define when. You cannot conduct this sort of test using different rodes, especially one that is elastic. This might explain why his holding power data is so low, compared to everyone else, but also puts into doubt much of the data anyway. I am sure the data is totally accurate - but I think the use of nylon makes the results impossible to interpret.

Vryhof only supply some oil rig anchors the other big player is Bruce. Bruce' website also has some interesting information, particularly the videos (they have? had a great little video of a model anchor setting in a test tank). If you are interested more information is offered if you work with the research technicians in both companies who can be free with their data (but they sometimes request confidentiality). You will also find, if you search, that they add further information that is relevant to our industry when they make Industry presentations at the oil exploration/development conferences.

I appreciate you want your monster anchor to be double the hold of the one recommended (maybe 1/2 the size of the one you have) for your yacht. If I had invested that sort of money I would also have that desire. If I had hung my hat on Bigger is Better and had thousands of post to support it - I would also want my anchor to reflect what I am posting. It might be better, safer and more responsible, to consider that at worst it (at 2 times the size) is only 1.32 times better than the smaller one (until there is evidence to show otherwise) - and to bear this in mind when suggesting to people they should buy bigger anchors. Basically buying one or 2 sizes bigger makes virtually no difference to the hold of the anchor - but does increase the cost and also increases the inability to change the anchor (because its heavy). There is considerable merit in considering that if you want to double hold then you might need to go to 3 times bigger - or set 2 anchors in a 'V'. The exception is a Fortress but it is exceptional. As has been mentioned in this thread - many people are very happy with the recommended sized anchors and there is evidence that having an anchor that is too big (for the setting ability of the yacht) can result in a poor set - and a poorly set anchor cannot be a safe anchor.

Finally efficiency (as the oil industry define it) does not mean good (or bad) - to us. It is a test in a straight line pull only. it does not need to fit a bow roller, it does not need to be cheap (the oil industry will pay anything for better). High efficiency does not mean it will work in weed, turn with the tide - it simply means it is good in a straight line pull. Our anchor makers (including Fortress) had to make compromises, to fit the bow roller etc, and that is why our anchors are not as efficient -

Our anchors are more efficient at doing what we want and need.

Now, hopefully we have all of that out of the way, maybe we can go back to dragging of anchors?

Jonathan

 

[Neeves]

In order for an anchor to develop any hold it has to set first:

http://www.cruisersforum.com/forums/f118/experience-with-rocna-vulcan-153055-3.html

Post No 35 makes some sobering reading. By my definition its a modern anchor, I think it might also be called a New Generation anchor, it does appear to be suffering from dragging (and not a mechanism reported by anyone on YBW).

Jonathan

edit - It illustrates an issue, possibly with locked up shackles - but the anchor does not appear too forgiving of the development (which might happen to anyone). close edit

 

[JumbleDuck]

JD - enjoy the sail and the weekend.

It's been fantastic. Clear blue skies, hot days ... not exactly what one expects of a late September weekend in Scotland. The anchor performed flawlessly!