Anchors - Bigger is Better ?

[Andrew Troup]

The above post deals with an entirely different question from the load at which the shank will bend.

It seems that Delfin would like to shift the argument in that direction.

Now that ability to resist sideload, for each anchor in the range, is something any decent anchor manufacturer will have decided early in the process.

If a material with higher tensile strength is substituted for mild steel for a given anchor, the thickness of the shank can be reduced for the same resistance to permanent bending. In other words, an increasing sideload will reach the same value for both anchors at the moment where the shank develops a permanent bend.

The thinner shank is good for helping that anchor to dig, but bad news if the wind shifts through 90 degrees when the anchor is well dug in but the tip of the shank is unsupported, and pulls hard enough to bend both shanks (remembering that the thickness is adjusted so they both have the same bending strength), the high tensile shank is scrap whereas the mild steel one can be returned to service after straightening.

If you were to straighten a high tensile shank, the next time it was bent it would very likely snap. A well dug-in anchor with a bent shank generally holds well; such an anchor with a broken shank will generally not hold at all.

JonJo asks for instances of bent or broken hi-tensile shanks. I am aware of a few cases of the former but none of the latter. I am not aware of anyone trying to straighten and reuse one of the former, and I presume that's why I don't know of any cases of the latter.

I rregret that I can't give chapter and verse, JonJo, because I didn't find it particularly interesting or remarkable that high tensile shanks can bend, so I didn't document the instances. (It would be remarkable if they didn't ever bend, but I guess if everyone adopts the BiB doctrine enthusiastically, it's possible to approach that desirable state of affairs)

- - - - -

The question of steel grades used in buildings is a complete red herring, to my mind, because once a building beam or column is bent, all bets are off for future occupation of that building regardless of what grade it's made of.

No-one tries to straighten and reuse structural steel, at least, not in first world countries where higher tensile grades are sometimes employed.

It is typically nowhere near breaking when yielding begins. IOW yielding begins at a much lower proportion of the eventual breaking load than for stronger steels.

The intervening zone absorbs a lot of energy and makes mild steel a very forgiving material: if you want a workboat which can hit rocks without rupture, it's hard to go past mild steel for the hull plating. Copper-nickel would be another good choice.

If OTOH you're prepared to put up with the risk of rupture, but don't want any dents in the meanwhile, high tensile plating would be the way to go.
Having said that, the people who design and build it need to know a lot more about the material to produce a satisfactory result: it's not straightforward in its properties, nor forgiving of a variety of possible insults.

[DumnMad]

Thorough explanation Andrew. As you say some don't understand the words being used, let alone the metallurgy etc.

[JonJo]

Hi Andrew,

I note your comment on not documenting bent Hi-tensile shanks. They are not remarkable in that any metal will bend. They are remarkable in that it would be extreme to achieve such a feat (and thus possibly merits some attention in the future). I suspect, and earnestly hope, that being so difficult to straighten they were discarded.

I think, considering the rarity of the cases, that greater publicity is given to the bending (failure) of hi tensile shanks. The conditions and type and size of anchor would merit mention. It would merit checking what damage was done to the yacht in bending a high tensile steel shank. It particularly merits noting the model as the term 'hi-tensile' lacks any technical meaning.

There is an issue here, you suggest a mild steel shanked anchor could be straighten twice (I'm not going to argue over once or three times) and you suggest Hi Tensile should not be straightened. I wonder how one defines 'mild steel', 250 MPa, 400 MPa, 500 MPa? I would prefer if you consider a slightly more cautious approach and suggest shanks should never, ever, be straightened - except to allow a yacht to anchor en-route to replacement. No-one buying a second hand yacht will know the history of an anchor - and it would be better - as matter of safety - if the recommendation was that all bent anchors were simply scrapped.

I'd prefer a smile on the face of anchor makers than:

I'd hate anyone to buy a yacht with a straightened anchor.

In terms of BIB, I've mentioned this previously but anchor makers are constrained in their shank plate thickness by what is actually made. One anchor maker accuses me of working for a steel maker (in my dreams!, I remain fiercely independent) and with fear of retribution:

Bisalloy plate sizes are 10, 12, 16, 20, 25mm etc. I think these are the same sizes as supplied by Tata et al. The Rocna 20, 25 and 33kg models were (are?) all made with 16mm plate. There is some acceptance of the restriction of plate sizes (by Rocna) and the shanks are wider as the anchors become bigger (but they are also longer). The end result is that the 20kg shank is able to withstand a higher load 90 degrees) than the 25kg which can withstand a higher load than 33kg shank. Rocna are not unique - other anchor makers have exactly the same problem (but I confess not knowing how other anchor makers accommodate the issue). The bottom line is that the 20kg anchor, to be used on a substantially smaller yacht - is stronger than the 33kg anchor. Now I suspect the 25kg anchor is probably about right, the 20kg anchor over engineered - but the 33kg anchor is 'potentially' weaker than it should be.

Going BIB to engender strength is not the panacea suggested.

This is not to denigrate Rocna, as I say all anchor makers have the same issue and they all solve the problem (in their own way) - but to increase plate thickness alters balance (so the anchor does not set so well, which is exacerbated because the thicker plate retards diving, or setting), increase plate width, again - balance, decrease length (I'm not sure but I'm sure balance could be a problem). There is no easy answer - but BIB is not necessarily correct.

Jonathan

[Snowpetrel]

Ahh, this is more like an anchor thread I am enjoying dredging up and trying to apply the little knowledge I picked up in my very basic mechanical engineering training.

Seriously I am actually very interested in why they might prefer lower strength steel in buildings over the high tensile grades even though it has little to do with anchors. I vaguely remember something about concrete beam design and catastrophic failure modes but the details are hazy... Maybe the NZ earthquake situation is also a factor?

Originally Posted by Andrew Troup

... which is why it is generally OK to straighten a mild steel anchor shank and carry on using it. You might even get away with doing this twice, if it's not severely bent.

We bent our old Danforth every decent blow. We used to crudely straighten it and kept using it, must have bent it a dozen times at least, one way then the other. It's still going strong as a spare...

Mild steel is a very tough material indeed from the start, in the precise engineering sense of the word "tough". In crude terms, toughness measures the amount of energy a material can absorb after yielding and before breaking.

Is it has to do with the difference between the yield and UTS that determines the toughness? Ie

Say Bis 80 at 750 yield vs 830 UTS has about 80 mpa or about 9% extra after yield and before failure

Mild at 250 yield vs say 400 UTS has 150 mpa or about 67% extra in reserve after yield.

Or is it more complex and to do with the areas under the stress/strain graph or somesuch.. maybe I need to go back to my old textbooks!

[daddle]

Today's anecdotal datapoint on the OP subject: A cruiser just arrived here. Has circled the world in the last few years. Tayana 37, or whatever. Only a very tired 25 pound Danforth on the bow. Reports no particular issues ...

Apparently cheated certain death and destruction a thousand nights.

[DumnMad]

We come from earthquake country where maximum reo strength allowed is 500MPA and then only in grade 500E. No QT, no grade 80 because these can't be bent back and forth without too much risk of sudden failure and building collapse.

[Kettlewell]

So how about spring steel for shanks, like Danforth tried with their Deepset series of anchors? They had extremely thin shanks to aid rapid and deep penetration.

[congo]

Unbelievable some of the comments.

I think some of you guys should invent an anchor design of your own, go through proof testing and do a whole lot of research, it may just give you some idea of what and why we have experts to set out a criteria to make anchors to a standard, H/Holding power anchors, you have to demonstrate in field testing that your design meets the criteria, further, you have to proof test anchors to a standard so as to meet the field test.

Same again, S/H/H/Power is considerably higher holding power over H/H/Power, this means field and proof testing all over again and you have to meet these requirements. Higher loads again mean a stronger anchor regardless of argument if you are to meet these requirements.

Simply if you require certification you have to meet all requirements, S/H/H/Power set deeper to produce this holding power then it has to be built stronger to handle the extra loads to reduce the chance of not just bending the shank but other major aspects of the design. (SAND) I am talking it is the standard test, if you want S/H/H/Power certification in clay, mud, weed over sand, then you must again have to test, prove what you are stating and test in all of those sea beds. We have them all.

After twenty years in anchor design meeting the requirements of the Authorities rarely do we bend shanks, mild steel or other, the following recent post from the home of new gen anchors is rewarding enough and tells us we don’t have to join the BIB band, if the anchor size is boarder line then we will recommend the next, and this post was recent from the home of the new gen anchors that makes it more rewarding.

Kiwikat
Registered User

Join Date: Jun 2007
Location: New Zealand/QLD Australia
Boat: Fountaine Pajot Bahia 46 & Farr 1104 Sport Cruiser
Posts: 193
Re: Anchors, Bigger is Better?

I just found this thread so I have not gone through and read all of the 700 and something odd posts but would like to add my 2 cents worth in as I missed out posting on those other threads before they unfortunately closed them.

The initial reaction is of course Bigger is Better.......... unless you are moving to a more superior anchor. Just my opinion, so shoot me down in flames if you so desire.

Six months ago we switched from a 32kg Trefoil (Bruce lookalike) to a 25kg Super Sarca as our primary anchor. Four of those six months have been out cruising and on the pick full time in a wide variety of anchorages and conditions. We chose a #7 which falls well into the manufacturer's recommendations for our boat length and weight and even though we are a catamaran, we felt we did not need to go one size up as a #8 would have seemed a bit of an overkill.

We have had a Sarca on our monohull for the last eight years. We were on the borderline spec for that one so we did take the upsize option but we soon realised that we needn't have bothered.

Incidentally, we went out diving for scallops with our cruising buddy and his boat last month. He was very impressed that our Super Sarca was perfectly set whilst his big heavy CQR was lying on it's side - we had not even tried to set our anchor - he had. We sleep very well at night!!

Cheers
Lisa
__________________
s/v Stray Cat
Not all those who wander are lost

Mild steel anchors, we use heavier thicker shanks to meet all requirements,
Bisaloy shanks meet all requirements, stand out features, it is stronger, its flexing properties far out way man=ny of your arguments when you start talking yields, strengths, you can bend - yield mild steel at 5 degrees. Bisaloy regardless of strength can be bent form 15 to 20 degrees and it will simply recoil, incredible properties, so before you start talking yield strength, calculate an impact in to the bisaloys flexing properties together with its yeild strenght and you have got one hell of a strong anchor. note: if you can straighten an anchor on your boat you will sure as hell bend it again, another thing to remember, in wind or change of tide, at 150 degrees your bent anchor will probably not pull out, then the wind or tide changes again, now see how that bent anchor responds. Purchase a certified anchor and you will end up with the better choice in most cases.

Regards Rex.

[Delfin]

Originally Posted by Andrew Troup

It seems that Delfin would like to shift the argument in that direction.

Not really. I was simply pointing out that some statements being made were contradicted by the data. Contrary to MD's continued insistence, HT structural steel can be bent back and forth more times than mild steel, which is why certain grades are ideal for anchor shanks.

Originally Posted by Andrew Troup

If a material with higher tensile strength is substituted for mild steel for a given anchor, the thickness of the shank can be reduced for the same resistance to permanent bending. In other words, an increasing sideload will reach the same value for both anchors at the moment where the shank develops a permanent bend.

Kind of. The HT shank will have the same stress under your scenario as the mild steel when it is thinner than the mild, but its greater elongation value means it will still break after the mild steel given the same force.

Originally Posted by Andrew Troup

The thinner shank is good for helping that anchor to dig, but bad news if the wind shifts through 90 degrees when the anchor is well dug in but the tip of the shank is unsupported, and pulls hard enough to bend both shanks (remembering that the thickness is adjusted so they both have the same bending strength), the high tensile shank is scrap whereas the mild steel one can be returned to service after straightening.

If it were true that HT structural steel cannot be straightened, you would be partially correct. But saying that HT shanks cannot be straightened is simply incorrect. Some cannot, I'm sure. For example, spring steel is high tensile, low carbon steel designed to bend and bend and bend but if deformed resists being returned to its original state. This is why DM's statement that when "high tensile steel is bent its ductility is virtually stuffed" is simply false, as is the statement that HT steel isn't used in high rise buildings. Which HT steel? High carbon knife steel? Maybe. HT structural steel? Clearly not since it is used worldwide for the reasons I provided - you can bend it back and forth through more cycles than you can mild steel of equivalent strength.

And when you say that "both (HT and Mild) have the same bending strength" in your example, you could be right if that is how anchors are designed. But they're not. The use of HT steel in anchor shanks means they can make the shank thinner, but they don't determine the thickness on the basis that they want to have it fail at the same point as mild steel. What would be the point? They use HT because they can save some weight AND have a stronger more resilient shank. When Danforth dealt with bending shanks on its old model by starting to use the right grade of HT steel, they didn't advertise it as "just as weak as the old, but lighter", they advertised it as stronger, which it is.

Originally Posted by Andrew Troup

If you were to straighten a high tensile shank, the next time it was bent it would very likely snap. A well dug-in anchor with a bent shank generally holds well; such an anchor with a broken shank will generally not hold at all.

Again what steel? You are making a generalized statement about a generic group of metals - HT - that is meaningless. It's like asking how long a piece of string is. Which piece of string? You can bend HT structural steel back to its original shape by subjecting it an equivalent force in the opposite direction. It takes more force and is more difficult for a simple reason. It's STRONGER. But it certainly can be done unless it is very brittle, which structural HT most decidedly is not, and HT structural steel is what is used for anchor shanks.

The problem here is that when people make general statements about a subject as complex as metallurgy they make as much sense as someone claiming that the addition of salt ruins a meal. How much salt? What meal? Mild steel is less suitable for anchor shanks that the suitable grade of HT structural steel with the right properties because it accepts bending back and forth better than the mild steel and because the HT steel can be thinner and lighter with improved strength all while providing concentration of weight where it should be.

[Delfin]

Originally Posted by Kettlewell

So how about spring steel for shanks, like Danforth tried with their Deepset series of anchors? They had extremely thin shanks to aid rapid and deep penetration.

Kettlewell, spring steel takes the ability of HT steels to flex and return to the extreme by the addition of a lot of chromium and nickel. It would probably be nearly impossible to permanently deform it but once deformed the same characteristics that allow it to resist permanent deformation would prevent it from being "de-deformed". Plus it would be hideously expensive and welding it would probably be impossible. On the Danforth, no welds (as I recall). But I don't think Danforth actually uses spring steel. From the yield strength, it is stronger than ASTM A514 like Bisalloy 80, but has less than half the yield of spring steel.

[Graceunderway]

Originally Posted by Pelagic

I understand there are now new regulations for barbless anchors.

Whale Steals Anchor From Boat and Gets Caught in Rope | RTM - Right This Minute

Wow....that was incredible!

[DumnMad]

Delfin: You are right of course, spring steel and high tensile steel can bend back and forth millions of times - that is if they don't bend beyond their fatigue strength. What it can't do so well is bend back after being deformed like the one in the picture.
I was talking about the "plastic yielding" of the shank in the picture, you are talking about "elastic bending" which is a very different thing.

[Delfin]

Originally Posted by DumnMad

Delfin: You are right of course, spring steel and high tensile steel can bend back and forth millions of times - that is if they don't bend beyond their fatigue strength. What it can't do so well is bend back after being deformed like the one in the picture.
I was talking about the "plastic yielding" of the shank in the picture, you are talking about "elastic bending" which is a very different thing.

Thank you DM. My quibble was the idea that HT structural steel would take less cyclic loading than mild, which I think we both agree isn't correct. But cyclic loading isn't the same thing as exceeding the yield strength and inducing plastic deformation. It is this increased ability to withstand cyclic loading without weakening that is the reason HT structural steel is used in tall buildings and why it is pretty good material for anchor shanks.

No argument that once deformed, HT steel is harder to return to its original shape, but the difficulty of this isn't the same for all chemical compositions of what is broadly classed at "high tensile". Generalizations about something as variable as metallurgy are likely going to be wrong.

[JonJo]

This appeared on another forum:

It has some nice home truths!

It is slightly disingenuous in that they omit to mention they are sponsored by Spade and have, or had, a very detailed comparison of Spade and Rocna on their website - so I'm sceptical of their having heard 'good things' about Rocna but omit to mention, by name, other new designs.

There seems a slight, I emphasis the 'slight'. change of their recommendation of BIB toward primarily larger anchors for larger yachts - they are choosing a 45kg boundary. The thing that is most odd is

'We are convinced that the increase in anchor holding as size increases is far more than linear.'

Now there are some engineers on this thread - surely someone can come up with a sensible technical reason for this. This, to me, lacks logic - there must be a technical reason. Knox suggests holding capacity is linear, at the other extreme Vryhof suggest holding capacity is linear (both by weight as a proxy for surface area).

I recall there has been a similar comment of the Bruce anchor (that it worked better in larger sizes), I think based on the genuine model but perhaps true of the myriad of copies, that it worked 'better' at over 45kg (I'd heard variously 50kg and 75kg for the Bruce).

Interesting recommendation on use of G7 chain.



The quote from YBW:

Here's some anchor philosophy from the "Attainable Adventure Cruising" site(FWIW) :

Lessons Learned:

1.Don’t accept that this is as good as it gets.
In offshore sailing we often go along for years with poor gear without ever questioning the prevailing wisdom that this is as good as it gets. Does anyone else remember that incredibly dangerous machine, the halyard reel winch? For years after the drum winch became readily available, we were told that we had to put up with this wrist breaking monster on the main halyard because it was unseamanlike to use rope halyards or have a rope to wire splice take the load when reefed. It’s hard to believe now.

2.Be open to new technology.
We still see the majority of cruisers out there with old style anchors when there are several much better modern designs available. The SPADE is only one option; we have heard good things about the Rocna and several other new designs.

3.Up-size your anchor—a lot.
We are convinced that the increase in anchor holding as size increases is far more than linear. We also think that once you get over 45kg (100lb) you enter a new world of holding and setting capability. I can hear many of you now, “I’m not putting all that weight on the bow!” But have you thought of going to high tensile chain—G4 or even G7—which will allow you to reduce at least one size on the chain and more than make up for the weight of a bigger anchor? This has been Steve Dashew’s approach for years.

4.Don’t be fooled by the tests.
The CQR and other old design anchors often do relatively well in them, but we, and many other experienced cruisers, have not had those results in the real world.

5.Don’t let inconvenience stop you from doing the right thing.
Don’t let the hassles of modifying your boat’s bow roller to fit a new style anchor stop you from getting one. It will be worth the expense and aggravation, believe me.

end quote.

I confess not to have looked at the quote in the original and there may be other comment or qualification.

Jonathan

[Kettlewell]

Here's a link to the Attainable Adventure Cruising site.