In recent years, a number of propeller shaft sleeves have been damaged due to circumferential groovings which are produced at the fore part of sleeve supported by stern bush bearing. The appearance of the groovings suggests erosion and it should be noted that they are usually confined to segmental areas approximately midway between the corresponding propeller blades. In order to investigate the cause of such groovings, the author has made (a) the statistic analysis of eroded shafts, (b) the microscopic observation of eroded parts, (c) the measurement of lateral vibration of shaftings, (d) investigation on the growing mechanism of cavitation erosion by models and, (e) the study on the load distribution of plummer blocks and stern bush bearing, whereupon he presumes the cause of grooving erosion as follows : (1) The lateral vibration of shafting does not attribute to resonance of natural frequency of shafting, but to forced vibration caused by exciting moment on screw propeller produced by wake fraction. (2) The amplitude of the shaft vibration at the fore part of stern bush bearing map increase remarkably, where the load on aftermost plummer block is very small or negative. (3) The lateral vibration of shaft in stern tube induces the pulsation of pressure of sea water between shaft and stern bush bearing. (4) Such pulsation of sea water pressure may originate erosion due to cavitation, the pro gression of which will be greatly intensified by the corrosion of bronze sleeve by sea water.