“Surveyor’s Comments: According to my analysis of the fracture surface and observations on board, the shaft is broken due to fatigue and not by striking a submerged object or catching a rope etc.
The fatigue fracture surface is characterized by its smooth appearance with almost no plastic strain. Beach marks, indicating the position of the crack front at various stages during propagation, have a typical semi-elliptical shape. The final rupture (low in the image of the fracture surfaces) is ductile. The final fracture surface is very small in comparison to the area of fatigue cracking.
Fatigue cracking occurs under conditions of repeated loading or cyclic loading. It is not the magnitude of the load in itself which causes the failure. It is the cumulative effect of many thousands, often millions, of repetitions of loading, or load cycles.
I note that in this case the propeller’s distance from the strut bearing is with ca. 200 mm unusually large – encouraging severe vibrations. As a rule of thumb this distance should not exceed the shaft diameter, of about 50 mm.
The difficulties in dismantling of the shaft coupling also indicate that the drive system had not been serviced for a long time, possibly for years. As the fracture is vertical to the shaft, I assess that the failure happened due to rotating bending stresses, reason of poorly engineered drive system as well as possible misalignment of the shaft bearings.
It is highly advised that the repair procedure should include shortening the shaft. If not, a similar failure may happen again – possibly with worse consequences.”
From a Report of Cem Baykent, Materials Engineer and member of the marineSOLUTIONS Team
Image: marineSOLUTIONS Archive