Anchor shank strengths

Curves,

If I had known you were going to provide so much detail I would certainly have hesitated in asking.

Taking bignick's example of the nail in a piece of wood - load it at the head and it bends, where it enters the wood, as an almost neat right angle. There is a curve but its small, or short and most of the shank of the nail is straight. I have seen danforth copies with similarly bent shanks. , the bend is a neat right angle and the rest of the shank remains relatively straight. looking at images of bent Spades, Ultras and Fortress they seem to bend evenly, might be parabolic - you would need to measure it to find out. But certainly different from the bent nail. The suggestion is that the designer put sufficient thought into the shank that it bends evenly (or more evenly than the nail) and they seem to have designed the shank with some skill such that the stresses along the shank are relatively even and when it reaches its failure point - it all fails at once.

So my question was - is it the same formula?

Jonathan
 
I'm seeking clarification:


Quote Originally Posted by Neeves

The formula I have used is as follows:

The yield force or distortion load is

1/3 Y x b d^2/L in Newtons or 1/30 Y x b x d^2/L in kg



This derived as follows:

Section Modulas, Z = b x d^2/6

etc.


JumbleDuck you have said:

OK for a rectangular section being bent around the b direction - my question is: does this statement, 'OK for....' cover both equations?


On units, measurements in metres, sadly Tensile and Yields strengths are quoted in MPa (or they are on the data sheets I see), so somewhere this needs to include 10^6 so one can convert to Pascals or stick 10^6 in the equation.

I accept you do not like my fudge factor (however scientifically derived :)). Consequently you are suggesting simply use the whole of the lever arm length. Would you hazard a suggestion on how to measure that length as it impinges on the vertical beneath, which has a finite width. So the maximum length, including the width of the vertical or the minimum length excluding the vertical completely.

I am impressed and, as mentioned, humbled with the ease and comfort with which you use these equations. You are not wasted here, hopefully I am not the only one sufficiently impressed! I have been relying on a rather dated text, though I cannot believe the theory has changed. I do not think I can aspire to your depth of knowledge.

Jonathan
 
All the above formulae are for linear elastic 'small' deflection. This assumes that the deflection is small relative to the length of the beam. As load, stress and strain increase this 'assumption' loses its validity and you move into large deflection theory where the stress through the cross section is no longer linear. This is now a non-linear problem so the maths starts to get a bit funky and its better to solve numerically using Matlab or some such other tool, though if you're going to do this then you might as well go straight to FEA.

When you see a piece of material with permanent set then it has passed its yield point. Different materials behave differently past this point. Mild steels creep and stress relieve, high strength steels often go straight into plastic mode but can still sustain increasing stress, which is why they are often quoted by 0.2% proof stress rather than yield. There are others on here much better qualified than I to talk about this though.... (Vyv cox etc)

After yield, even large deflection theory goes out of the window. What would normally happen is that the yield stress would be exceeded at a particular location and the material starts to deform in a plastic manner. This redistributes the stress around it, and a plastic hinge forms and then progresses through the thickness of the beam or the plate. The beam thus 'fails' at this point, though it will be intact, but permanently distorted.

Have you some pics of a bent Spade, Ultra or Fortress to illustrate what you're talking about? It sounds like some strange behaviour is going on, or someone has been trying very hard to bend it using something other than a tip load.

If you want your anchor shank to stay straight then you don't want to be anywhere near the yield (or 0.2% proof stress) point though. Linear elastic theory is good enough to do basic calcs with, and should let you size things accordingly.

For a point loaded cantilever beam, the bending moment varies linearly from the load point to the wall. It wouldn't make sense to have a constant cross section for this purpose, and the only reason to do so is so that it can be cut from a sheet. The thickness would have to be sized for the max bending moment, which as JD said is at the wall. I would expect a more advanced anchor to have a tapered shaft, as that would be more suitable for the load pattern. It does come down to how the designer expects their anchor to behave though. If they feel that it will change its set to align with the load then why would they design to withstand huge sideways load? And how big a load do you design to? If the anchor is stuck so fast that it's never going to move then you'll bend it at some point..... Obviously the shaft should be designed to withstand the max holding power of the anchor in good ground at any angle, but in that case the anchor would re-set anyway....

Ps yes I do sleep, but we're on holiday in the world of the giant mouse at the moment, hence the interesting posting times... :-)
 
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I remember buying a calculator that would do natural logs, specifically for calculating plastic deformation above yield point (it was a very long time ago). It gets very complicated above yield.

Only mild steel has the classic yield point forming a marked change from elastic conditions to plastic. Most other metals, including hardened steels, do not have an obvious yield point, which is why the 0.2% proof stresss figure is quoted.

I do not believe the Mantus figure for delta shanks is anything like correct. AFAIR the measured strength, by the Smith family, was what they based the requirements on for the original rocna. I don't have the info with me but I think it was a YS of 750 Mpa.
 
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Thanks bignick and Vyv,

I recall you had posted about Delta shank steel in the past but could not recall what figure you had suggested. 750 MPa seems high, the data I have from the Smiths is 700 MPa but I did not know how they derived it. I have all the Smiths data - but not how they derived the YS of the Delta steel nor the formula they used (I'm sure thei did not do so - but they could have plucked their 700 MPa out of the air) The Mantus shank has a width of 60% of the Delta - which would then put the Delta (as the steels seem similar) at 40% stronger than the Mantus based on A514. This suggests that the data they have posted is erroneous (in their favour) and a grossly misrepresentation of a Delta. One has to question the accuracy of any other data on their spread sheet. I am sure it was a simple and natural mistake.

However it does raise the question

Can an anchor shank be too strong, or maybe put another way - has the search for strong shanks gone too far. Might we now have anchors whose shanks are too strong (we might have got fixated by strength) and maybe a Mantus shank of 60% of say a Delta strength is more than adequate? If a Delta is 700 MPa then a Rocna, Supreme, Excel - all have similar shank strengths, similar dimensions, similar steel etc. Are we paying too much for anchors because the manufacturers have gone 'too far'?

Personally I'd rather have overkill and a bullet proof shank - but I also want the anchor to 'work' and have high holding capacity.

Jonathan

Edit

To put into context, if the Mantus A514 is 600lb then the Delta would be around 840lb, not the 300lb they quote! see chart posted by Delfin above. The Smiths when they calculated a Delta shank based on an A514 steel, so same as Mantus (this was presumably an academic exercise - not what was used) resulted in a shank of strength 769kg for the 44kg model.
 
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When you talk about width of the shaft, are you talking about the width when viewed from the side or the thickness, as seen from above? The thickness is the key dimension here, as that's the one to the power of three in the calculation of I.

I'm curious as to where you're getting the info from for these "mantus" anchors. I can't find any tech info on their website about grades of materials, except that it says "In the mild steel version every part of the anchor is hot-dipped galvanized for corrosion protection".

If they are saying it is mild steel why do you assert that its A514 as I can't find the info anywhere on their website?

In answer to your question, there are limits to material properties which are desirable. Consider what happens to this piece of uber steel when you weld it together. Do you maintain those über properties at the most highly loaded junction? Or do the properties reduce significantly and need recovering (if possible) through post treatment? What about damage tolerance? If you have a very high strength steel, that's loaded to within a whisker of its limits, what happens if you sustain any damage? Stress concentrations at geometric discontinuities can be huge...
 
I try to refer to plate 'thickness' and 'width', as from the side.

They have publically stated on various forum that they have been using a mild steel designated A36 but are to upgrade (are upgrading) to one called A514. My information is that A36 has a minimum YS of 250 MPa and A514 has a minimum YS of 690 MPa.

Manufacturers of high tensile steels, Corus/Tata in the UK, Bisalloy in Australia (and there are suppliers in Finland, Japan, China etc), offer full technical data sheets and recommendations for welding these products - including post treatment. Certainly Corus and Bisalloy suggest that if these Q&T steels are welded correctly then there are no siginficant issues and the weld will be stronger than the steel. I cannot comment as to what might happen in China - but the Chinese can make anything as carefully as we can, if the correct specifications and QC are in place. If you contact either Corus or Bisalloy they bend over backwards to help (and I assume their competitors will do the same).

Hopefully if the products are bullet proof and over engineered then you will never load them to a whisker of their limits. However cutting back on shank thickness (particularly) or width will allow one to getting closer to that whisker distance - so your implications are clear and are not that different to mine - except you articulate them with more effectiveness.

For you background the Mantus shank is of the same thickness, plate, as their peers - they do appear to have reduced width significantly (40%) and if you look at the profile the shank is longer (than say a Delta - who developed the initial geometry). A Rocna would essentially mirror a Delta.

The Mantus did not need the same profile as the Delta as it has a roll bar.

Jonathan
 
bignick said:
When you talk about width of the shaft, are you talking about the width when viewed from the side or the thickness, as seen from above? The thickness is the key dimension here, as that's the one to the power of three in the calculation of I.

I'm curious as to where you're getting the info from for these "mantus" anchors. I can't find any tech info on their website about grades of materials, except that it says "In the mild steel version every part of the anchor is hot-dipped galvanized for corrosion protection".

If they are saying it is mild steel why do you assert that its A514 as I can't find the info anywhere on their website?

In answer to your question, there are limits to material properties which are desirable. Consider what happens to this piece of uber steel when you weld it together. Do you maintain those über properties at the most highly loaded junction? Or do the properties reduce significantly and need recovering (if possible) through post treatment? What about damage tolerance? If you have a very high strength steel, that's loaded to within a whisker of its limits, what happens if you sustain any damage? Stress concentrations at geometric discontinuities can be huge...
Click to expand...

The information on the Mantus came from a thread on CruisersForum. In that Mantus variously stated that they had replaced the mild steel in their shank with A514; that they intended to; and most humorously indicated that the time spent answering questions on their product was interfering with their ability to make the change to stronger shanks. So as of now, it appears the shank is made of mild steel.

The chart I posted showing Mantus' calculations was from a post on the CruisersForum thread prepared by the engineer at Mantus. That post has been excised from a thread already closed by the moderators without explanation, which means a moderator had to go in and remove it themselves. This may have been done because the data appears to be intentionally bogus in an effort by Mantus to defend their choice of design materials and the moderator wanted to lend a helping hand. Beats me, but one would think that in the interest of boaters, CruisersForum would post a retraction to correct the prior distortion rather than assist Mantus in maintaining the charade by simply disappearing the now inconvenient post.

While hucksters seem to appear in the anchor manufacturing space frequently, it is unusual for a forum like CruisersForum to assist the huckster to the degree they have. The sanitizing of a thread they closed rather than continue to allow Mantus to be subjected to public inquiry on their products and claims is a sad commentary on the ability of boaters to get accurate information from sites like that one.
 
Neeves said:
+1

Is this an impact of advertising?

Jonathan
Click to expand...
I have no idea. It's hard to imagine that the admittedly copious advertising Mantus does on the CruisersForum site would influence moderators I assume are volunteers. So why moderators on that site would behave as if they were in the bag for a manufacturer, covering up what certainly looks like an intentional attempt to mislead the public is a mystery to me.
 
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