Quick and Dirty Anchor Shank Strength Test

Posted by at 11:36AM - Comments: (7)

Jonathan Neeves
Jonathan Neeves

The original Mantus anchor shank is tested on a Brinell Hardness tester. Mantus recently upgraded the metal used in its shanks.

So you’re sitting around this winter with time on your numb-cold hands and a half-dozen old anchors cluttering up the basement. You’ve read our many reports on anchor shanks, and you’re thinking, “I wonder what kind of steel my anchor shank is made of?” You could go to the maker, but you might find, as we did, that some manufacturers consider this proprietary information—as if the strength of the steel is not worth sharing with the consumer. And then there’s the quality-control issue. In the past, some makers claimed they did not know their anchors were being made with steel that did not meet advertised specifications.

So you decide to find out for yourself.

The only way to truly define steel quality is to use the services of a metallurgical lab, but there is a “quick and dirty” test that will allow you to roughly estimate the tensile strength and to compare two or more anchors. To do the test, you need five things: a big powerful vice, a pea- or marble-sized ball bearing, a collection of bolts, a micrometer, and a friend. Between the local automotive store and the home-improvement store, you should be able to find the first three. If you own a boat, the last should be pretty easy to come by, too.

In this test, you are comparing the hardness of a shank to that of steels of a designated hardness used in the sample bolts. The tensile stress of a bolt is defined by markings on the bolt head. These “codes” are well documented on the websites of most major fastener manufacturers. One we refer to, Appendix A-1 of the Fastenal Technical Guide offers a guide to identifying bolts and their respective tensile strength by using the head markings, but there are plenty of other websites with photos and illustrations. Another option is to obtain scrap steel plate—of a known grade with a hardness and yield strength that is well documented.

To determine the grade of steel, you’ll need to remove a very small area of the galvanizing coating. (Although you can protect these areas with spray-on zinc paint, corrosion can occur more quickly in these areas, which is why I suggest doing this with only old anchors you are not using regularly, or ones you have serious concerns about.) The ball bearing is then sandwiched between the shank in query and the “known” standard bolt. The three items are then squeezed, with as much load as is possible, in a bench vice. The indentations on the shank and standard sample bolt are compared with a micrometer. The “softer” metal, with lower yield stress product, will have a larger indentation. If you have enough bolts of varying hardness, you can eventually narrow down the grade of the steel and its tensile strength.

For example, if your anchor shaft loses a battle with a bolt marked 307A on the head, its minimum tensile strength is probably less than 60,000 pounds per square inch or 413 megapascal (MPa), which is the minimum strength for that bolt. This is one of the lowest grades of steel acceptable in construction bolts, and about as strong as the steel used in some of the weakest-shank anchors on the market.

Jonathan Neeves
Jonathan Neeves

An anchor shank's hardness is tested using the "ball-bearing" method.

Remember tensile strength is the load at which failure can be expected to occur, noticeable bending could occur at the point of yield stress, which is lower. For comparison sake, the steel alloy metal plate used in the stronger high-tensile shanks on the market today (ASTM 514) have a minimum nominal tensile strength of about 790 megapascals. Because every anchor maker seems to have a different interpretation of what “high-tensile” means, there is a wide variation in shank strength, even among high-tensile anchors.

We tried this test with one of the original Mantus anchors on the market. Surprised by the results, we took the anchor to a Lloyds Register-approved metallurgist, who used a Brinell Hardness Tester to determine the tensile strength, which was estimated to be about 530 megapascal, well below what some other makers are using. In the wake of our findings, Mantus expedited its program to upgrade the material used in its shanks. (This is why we do what we do.) It also posted a report, complete with finite element analysis plots of various anchor shanks, on its website comparing the strength of its shafts to those of others. (We have not verified this analysis, but it appears to be carried out without bias.) Owners of the original anchors can contact Mantus about replacement shanks for the cost of shipping. More details on this will appear in the February 2014 issue of Practical Sailor.

We’re still collecting bent anchor stories and following up on consumer complaints about anchors. If you have a story to share, comment below, or send it by email to practicalsailor@belvoirpubs.com.

Comments (5)

Most ball bearings we are familiar with are made of harder steel that the steel used in anchor shafts. If your ball bearing dents while testing, you shouldn't have to look to far for one that doesn't.

Posted by: DARRELL N | January 10, 2014 3:58 PM    Report this comment

We are not saying that the ball bearing test would meet the demands of a Classification Society. Its a crude test but will sort out whether the metal you are testing is 'near' what its meant to be. We think if you have enough 'standards' you can get within +- 50 MPa. Hardness is not the same as Tensile Strength - but it is related and if you indent a known steel (and we are talking in the range, say 200 MPa to 800 MPa) the same as the steel you 'question' then you can be fairly comfortable the tensile strengths are similar. In reality you will find it very difficult to indent anything over 800 MPa with this test. We can just indent A514, maybe we need to spend more time in the gym!

It does not matter the strength of the ball bearing - it bears on both surfaces equally. We do not know of ball bearings that would be 'crushed' by an 800 MPa steel. We know about variation of bolts, it is a major issue in the construction industry and anything fastened, including an anchor, with questionable looking bolts is probably - questionable! This is why we suggest collecting a cross section of bolts, ours come from a retailer of fasteners, also a hardware store and we have some steels pieces of known hardness and tensile strength.

If the steel you are questioning falls well outside the range you expect you have conducted a very cheap test to indicate you have not got what you thought. You are now in a position to share your knowledge (maybe get your money back - no further questions asked) and/or invest in professional testing.

Posted by: Jonathan N | January 8, 2014 4:41 PM    Report this comment

This assumes that the bolts you buy are of the strength indicated. Having worked in an industry where the strength of bolts (of all sizes) were critical to the mission, I know that your standard hardware store bolts may be of a much inferior grade than marked. This is a major problem in industry.

Posted by: Marcus L | January 8, 2014 12:08 PM    Report this comment

It is very important to be sure one is measuring the right characteristic of the steel. "Hardness" (measured by the size of the indentation made by a hardened ball at a known pressure, e.g. Rockwell or Brinell number) is not the same thing as "tensile strength," although they are related. A quick web search led me to a conversion table (at www.ruukki.com) which warns that the table can be used only for approximations and that "Hardness conversions are no substitute for real measurements." An extreme example would be a cast-iron or glass bar; its hardness would be very high, and its theoretical tensile strength might even be very high, but its use as an anchor shank would be a, well, shattering experience.

Posted by: Tom | January 8, 2014 11:35 AM    Report this comment

Can we conclude that all ball bearings are made from high tensile strength steel? That is, what if the randomly chosen ball bearing dents before the shank or the bolt?!?

Posted by: Mike W | January 8, 2014 10:25 AM    Report this comment

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