A new special stainless steel which is very suitable for propeller material has been developed. The steel has the corrosion fatigue strength in sea water larger than 40 kg/mm², twice as large as that of nickel-aluminum bronze. It has also excellent anti-erosion property about thirty times as strong as that of nickel-aluminum bronze. Using this steel for propeller, many merits will be realized as follows :
(1) Lightening.
(2) Improvement in propelling efficiency.
(3) Decrease in vibrational troubles by increasing the number of propeller blades and/or by reducing the blade thickness.
(4) Easiness in making huge propellers. (building up by welding)
This steel has the material features just appropriate for large castings, since the slower the cooling velocity is, the larger its strength and elongation become. There is no problem to make the large casting of this steel of sound structure, and the weldability is fine in spite of its high hardness. The propeller of this material can be cast as a whole, but for the saving of machining hours which are liable to increase due to its high hardness, it is desirable to cast blades of accurate dimension separately to make them into a complete propeller by welding.
The unlikable characteristic of stainless steel, susceptibility to pitting corrosion resulting in deterioration of fatigue strength, is avoidable by using cathodic protection. The propeller of 1. 8m in diameter used for testing purpose for about one year showed the effect of cathodic protection under -0.8V (saturated calomel electrode scale) was perfect.
To know the method of manufacturing the large propeller by welding, a propeller of 5. 7 m in diameter was made tentatively. The proper control of chemical composition in the melting process was rather easy, and the castings were perfectly sound even in the inner side of massive part. It is presumed that the dimensional deviations concerning pitch, rake and inter-blades angle due to casting, welding and heat-treatment can be corrected without any difficulty by taking countermeasures against the deviation previously at the time of moulding and by optimum setting in the building-up process.
The propeller of this steel will presents marked contributions to the improvement in propelling efficiency and to the prevention of troubles concerning vibration and propeller shaft system of huge tankers.