DNV GL has observed an increasing trend in reported incidents involving aft propeller shaft bearing damages. Most of the damages reportedly have developed within a short period of time, typically a few minutes to an hour, under the respective operating conditions. This technical news reflects what is suspected to be one of the contributory factors to some of these types of damages, namely operation involving incomplete propeller immersion. It is recommended that operators undertake suitable precautions and respective mitigating measures in this regard.
Typical damages observed and their consequences
The observed trend is not specific to a vessel type – rather, it reflects operation of the affected vessels in areas with limitations on the draft of the vessel or loading conditions, without suitable precautionary measures to limit the RPM/power of the engine.
Typical damages observed in this regard involve wiping of white-metal-based bearing material in the aft part of the bearing, indicating a complete or partial loss of the hydrodynamic oil film under the reported operating conditions (Figures 1, 2). Physical contact of the shaft with the bearing material is reflected by an elevated bearing temperature and, in most cases, at an exponential rate of rise.
Figure 1
Figure 2
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