For marine use, it is not the strength that matters as much as rust resistance. Plain carbon steel, even "soft" free machining steel, will make a screw that will hold parts together. But unless they are immersed in oil, rust will destroy 304 stainless in salt air. They call it stainless, but check it with a strong magnet before you buy! If a magnet has no attraction to it, it will probably last; if the magnet will pick up a screw, it probably will rust. That's a heck of a lot cheaper and more convenient than spectrographic analysis!
After reading this thread, I tried the magnet trick last night on my pile of spares and was rather horrified by the results - more than half of my fasteners were picked up, including:
- A Ronstan swivel fitting!
- Boat nails!!
- All my "stainless" spring clips, sister clips and split pins!
- A large variety of screws, nuts, bolts and washers - some bought from the local hardware store, some from who-knows-where.
Without the magnet, it didn't seem possible to tell to them apart - so I know the test was working. Some of the larger stainless bolts seemed to be weakly attracted, hanging on by the head but falling off without much force (are these an issue? don't know..) and some larger pulleys and hooks were not attracted at all.
I didn't try my stainless rivets, but by my reckoning nearly 2/3rds of the "stainless" fittings on my boat are magnetic.. but have yet to show signs of rust. A bit of a worry!!