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  #1  
Old 09-08-2011
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Stainless steel anchors

Stainless steel anchors are a beauty, they are expensive but most of all I thought that they were weaker than their steel model. I thought that a good high grade steel was always more resistant to breaking and torsion than any stainless steel but then I saw this add on the net:

" Breaking loads 50% to 100% higher than galvanized anchors of competitors."

Stainless Steel Anchor Suncor CQR Danforth Bruce grapnel plow swivel

Just one more silly add or there is something I don't know about Stainless steel versus high grade steel?

Regards

Paulo
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Old 09-08-2011
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I am just a humble backyard SS mixer repiar hack and Bicycle frame builder BUT all one would need to do is increase the metal thickness and hence overall weight in key areas to get a MUCH stronger unit




I fillet braze Columbus nivacrom bicycle tubing together and while some might thing brazing is to weak for such a strong steel you just need to use the correct method to make a large enough fillet and the tube will fail before the brazing
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Old 09-08-2011
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They're as beautiful as those hood ornaments on classic cars and too pretty to dip in the mud! I can't imagine that people are buying them because of their function,- they are "bling". Take care and joy, Aythya crew
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Old 09-08-2011
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Gotta agree with CaptainForce on this one. Even if SS were stronger, I can't think of any time I've worried about my anchor failing, only whether I set it well.
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Old 09-08-2011
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300 series stainless steel is indeed 'weaker' than an equivalent plain high strength steel.
Stainless steel has the same tensile properties as a high strength steel BUT is more subject to 'fatigue failure'. 300 series Stainless (90,000 psi tensile) has an 'endurance limit' of only 30,000 psi for cyclical stress applications. Cyclical stress is loading/unloading and if the stress level go much above that endurance limit, the material 'cold works' forms micro cracks and rapidly fatigues. Usually 300 series stainless can only withstand ~1 million load cycles above 30000 psi ... then falls apart!!!!!

Once the micro-cracks begin to form, & water enters the cracks and you then have an additional failure mode: crevice corrosion. Fatigue Failure + Crevice corrosion failure = catastrophe.

So, is stainless steel a good material for anchors? ... absolutely NOT!
Is stainless steel a good material for sailboat rigging? ... absolutely NOT! and thats why you have to replace your rigging so damn often.
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Old 09-08-2011
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It all depends what grades you are comparing. They say that their anchors are made of 316 which is good for corrosion but not super strong. If they compare to a low grade cast steel, the yield strength will be higher. However, if you compare to a high strength steel like that used in several of the modern anchors, 316 is very weak. There are much stronger types of stainless than this out there but I don't know of any that is used for anchors.

If you want to compare yield strength or tensile strength only, you can easily find information on many different types of steel online. There is an incredibly wide range depending on the steel.
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Old 09-10-2011
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I don't think stainless steel is any weaker than normal steel. If anything it is stronger.

Someone mentioned a load cycle limit of 1,000,000 cycles over the fatigue limit (typically 1/2 of yield strength for steel). The fatigue limit is the point where stress cycles below that point do not cause failure of the material ever. Steel is one of the few materials that has a fatigue limit. The limit for Aluminum is like zero (it always fatigues). No anchor should be approaching 1/2 of the yield strength of the material on a regular basis.

I don't see how yield strength or the fatigue limit would be an issue for an anchor. But having a grade of stainless steel that does not hold up in saltwater would be kind of a waste of money. Also, your chain and all the parts that are electrically connected to the anchor must also be made out of stainless steel else the stainless will rapidly increase the rate of corrosion of the non stainless steel parts.

Last edited by steel; 09-10-2011 at 02:29 AM.
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Thanks to all.

It seems that there is not a problem with stainless steel anchors but I would have to have also a stainless steel chain. I have looked at the prices of one of the best performance/price anchors, the Kobra II:

The 16kg stainless steel Kobra costs €1298.65. The Galvanized Steel 16kg Kobra costs €138.14


Plastimo Kobra Anchor Stainless Steel | GulfStreamShop.com

Plastimo Kobra Anchor Galvanised Steel | GulfStreamShop.com

I knew that the Stainless steel one was more expensive but I had no idea that would cost almost 10 times more

Regards

Paulo
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Old 09-10-2011
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Quote:
Originally Posted by RichH View Post
300 series stainless steel is indeed 'weaker' than an equivalent plain high strength steel.
Usually 300 series stainless can only withstand ~1 million load cycles above 30000 psi ... then falls apart!!!!!

Once the micro-cracks begin to form, & water enters the cracks and you then have an additional failure mode: crevice corrosion. Fatigue Failure + Crevice corrosion failure = catastrophe.

So, is stainless steel a good material for anchors? ... absolutely NOT!
Is stainless steel a good material for sailboat rigging? ... absolutely NOT! and thats why you have to replace your rigging so damn often.
So, if I use a stainless anchor I should become concerned after about my 500,000th storm while anchored in an exposed location...................... ?
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Old 09-11-2011
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If you look at an SN curve for steel, you will see that there is a given stress level below which an infinite number of cycles are possible and at higher stress levels, fewer and fewer cycles are possible. Ideally, the anchor will be designed so that the stress never exceeds the value for infinite life.

Comments like stainless is as strong as regular steel are not fair because there are too many grades. There are strong stainless steels like 17-4 and there are weak ones (316 being relatively weak). The same thing is true for steel with steels like 4140 being pretty strong and something like 1018 being relatively weak. How the material is heat treated also makes a huge difference. A materials engineer could go into much greater detail but the gist of it is that the stainless anchors are not made out of very strong material.
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