Delfin
Well-Known Member
I had the opportunity to stop by West Marine and measure an FX-11 Fortress weighing 7 pounds and compared it to a 22# Rocna. The shank thickness of the Fortress at the mid point of the shank measured 11/16", while that of the Rocna 3/8", or about half that of the Fortress.The strength of the material used by yourselves, extracted from your website, is 385 MPa. The strength measured on the Rocna anchors by the Manson tester is about 500 MPa, to take a mean value. Anybody can see that the Rocna figure is higher than yours, which means that its strength is higher.
That isn't the whole story of course, as the strength needs to be applied to the cross-sectional area. As I have said in a previous post, if we assumed the shanks of Rocna and Fortress anchors specified for my 35 ft boat to be the same, that makes the Rocna something like 4/3 times stronger. My memory says that my 15 kg Rocna has a shank significantly larger in CSA than my FX16 Fortress, so I think the margin is actually larger than 4/3.
I also note that the Fortress shank is hexagonal giving six angles to transfer lateral loading and representing a stronger cross section vs. 4 for the rectangular, and thinner Rocna shank.
If you start with incorrect or incomplete data, the conclusion will not be worth much, even when an engineer makes the conclusion.
While at the WM store, I decided that I would go ahead and purchase the 22# Rocna, shown in the first picture, since I need an easily handled stern anchor and one that isn't too out-sized for the tender. I figured that I could give the Rocna the punch test, and if it looked like it was made of mild steel, I'd decide what to do next. The pictures are below.
I used a standard steel center punch, brand new, and a ball peen hammer. I tapped the underside of the flukes, which is supposed to be made of mild steel and made the dimple shown in the second picture. I then applied the same force to the shank, making the dimple shown in the third picture. I then took one of my Japanese wood chisels as a control, because I can look up in the catalog for these tools that they have a Rockwell hardness of 63, which is the equivalent of a Brinell hardness of 255, which is the same hardness as Bisalloy 80, which has tensile strength of 800 MPa. This is the material Rocna says they make their shanks out of because as Peter Smith has noted, you wouldn't want to compromise on materials in something of as advanced design as the Rocna anchor.
My initial concern was that I try to replicate the exact same force I used on the shank and fluke of the Rocna so the test would be accurate. I needn't have worried, because the 800 MPa metal showed no visible indentation although it did scuff off some smudge, but the center punch is ruined. The chisel is shown in the fourth picture and the now flattened center punch in the fifth.
At this point, I guess I could take this anchor back to WM and get a refund, since it is clearly defective by Rocna's definition. However, I think I will go ahead and pay a few bucks to find out what this unit is made of. When I get those results, I'll let you know.
p.s. I have added a couple more pictures I forgot. The sixth below is the dimple made in a piece of A36 channel I had lying around, also shown in the seventh picture. This provides a reference for the Rocna and the chisel. Based on this, if I had to choose a type of steel for the Rocna shank, it looks like it would be mild, A36.
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