Originally Posted by Pikey Dave
It's something to do with the crystaline stucture of the steel
Now for the science bit
Iron (plain carbon steel) is allotropic which means that at low temps it has a Body Centred Cubic (BCC) structure which is magnetic and at high temps it has a Face Centred Cubic structure which is non magnetic.
Stainless steel (austenitic) props have on average 20% Chromium, 10% Nickel and 3% Molybdenum which produces a FCC - non magnetic structure at low temperatures - that also why austenitic S/S fittings do not produce compass deviation
Pheeeeeew glad thats over