Rig Tuning: Difference in breaking strength between 302 vs. 316 wire

Hi,

In trying to refine the rig tuning on our 1984 E30+ I borrowed a friend's Loos gauge today and found that starboard and port shroud tensions were very similar )) so that was nice. However, this evening I checked breaking strength of the different size shrouds/stays on several websites and found a substantial difference between the breaking strength of #316 1 x 19 wire vs. #302 1 x 19 wire. For example, for 1/4" wire the #316 is 6,900 lbs. compared with 8,200 lbs. for the #302--a substantial difference! The implication of this is that a certain tension would be fine for #302 wire, but could over-tension #316 wire.

I'm guessing that most riggers use #302 for normal boat standing rigging, but does anyone know this for sure? Is there a way to tell the difference between 316 and 302 by looking at it (eg. colour, shininess, etc.)?

Before using the Loos gauge I worked carefully to ensure the mast was straight and the shrouds tight enough so they would begin to feel slightly slack but not look loose on the leeward side, in about 15 knots of wind, which I think is close to correct.

Any comments or suggestions on any of this would be much appreciated!

Frank

Hi Frank,

I looked through my Navtec catalog and didn't see a single piece of rigging hardware made with 302 stainless, everything in wire and fittings looked like it was 316.

http://www.lewmar.com/

Mark

Hi Mark,

Thanks for your reply. I did look at the Navtec link you included, and that was helpful. However, although I'm not a huge fan of West Marine, their catalogue lists both 302 and 316, and their West Advisor discusses advantages/disadvantages of each. I'm guessing that different riggers use one or the other or both, depending on budget and customer preference.

But it does make a significant difference, as the West Advisor confirms that the 316 is about 15% weaker than the 302, which would affect shroud tension accordingly.

So I'm still trying to determine if their is a way to identify one vs. the other--any ideas?

Thanks,
Frank

Easy

Take a magnet, 304 is slightly magnetic, (I use a small magnet on a string, it sticks to the wire, just enough to move the string etc).

316 is non magnetic.

Guy

Thanks Guy.That's what I needed.
I have since also learned that 304 was commonly used until about 15 years ago, but now 316 is used almost exclusively.
Frank.

Hi Frank,

My materials and corrosion guy at work (US Navy Ship and Sub Engineering) says 302 is the "least" stainless of the stainless steels. It would be slightly magnetic too. It didn't sound like 302 would ever be used for rigging, 304 sounds more like it.

Here is a Navtec tech document that talks about inspecting and servicing rigging:

http://www.pcmarinesurveys.com/Rigging Navtec service.pdf

Other than corrosion issues, the primary cause for failure is the number of cycles and the degree to which the rigging is loaded relative to breaking strength. Since the typical cruising boat is designed for 15-20% loading of the rigging, during "normal" conditions, the ultimate breaking strength of the wire is not that critical. In any case, your rigging was probably designed to the lower strength of 316 wire.

Mark

Thanks, Mark!

I checked my rig with a magnet yesterday, and found it to be not magnetic at all, not even a little bit, so I'm thinking as you suggested that the wire is 316, not 302/304. The only reason breaking strength is important is that one needs to know it to know what 15 - 20% of that number will be. A rigger I spoke with yesterday laughed and said that even though the breaking strength differs by about 15%, the actual difference in tuning and rig tension is almost negligible--he said there were lots of other factors that would have a greater effect on the actual tune.

So I'm back to just trying to do the best tuning job I can do, which after all these years is not too bad. You can probably tell that I'm a bit fussy when it comes to our boat!

Thanks again!
Frank