Over a period of 1.5 years roughness measurements ani other observations were made on more than 100 used marine propellers. The most surprising result was to find that the surfaces of the majority of these propellers had become smoother in service than they had been when originally delivered. In sea water propellers made on Mn Bronze produced clearly different, and less satisfactory, results than those made of Ni-Al Bronze. In fresh water, on the other hand, Mn Bronze propellers produced results that were virtually equally as good as those achieved by Ni-Al Bronze in sea water.
Investigations of the surfaces of new and used propellers by means of a scanning electron microscope revealed that a definite change occurs in the surface structure. The scratched finish on the surface of a propeller that is produced by grinding during manufacture disappears, leaving a surface having a smooth appearance.