Stainless props have a rubber hub that absorbs most of the shock when it hits something. I whacked a long, half submerged 4x4 offshore several years ago with a SS prop on a big Yamaha. No prop damage, but it stripped the hub and left us pretty much without power. I was only about 18 miles out, called the Coast Guard and they were about to send out some reservists on a training cruise, said they'd rather pull somebody in for real than pretend to respond to an emergency, so it worked out well for both of use. Having the hub repaired is a lot less expensive than reworking a prop. My last inboard boat was running 24 x 24 Nibral props when I was about finished with the diesel conversion and upgrades.
I would not use aluminum props in salt water.
