CONTAINERSHIPS
starboard turns when increased bearing load and slope mismatch between the journal and bearing surfaces has adversely affected the development and maintenance of a satisfactory oil film. Although the shaft alignment design
calculations normally consider the hydrodynamic forces and moments generated by the propeller, Lloyd’s Register has found that overly simplistic assumptions are often made which have led to poor alignment designs being submitted for class approval. It must be remembered that the propeller
hydrodynamic forces and moments are predicted for the zero rudder condition only and often at one vessel draught condition. When the vessel turns or operates in different ballast conditions, the propeller hydrodynamic forces and moments can change significantly due to wake field variations. This causes the centre of propeller thrust to move from its predicted position, thereby invalidating the values assumed in the calculations. Lloyd’s Register has recently amended its shaft
alignment to require a sufficient static load on the sterntube forward bearing to prevent it from unloading in all operating conditions, including the transient conditions experienced during manoeuvring turns. The amended Rules also introduce formal
requirements for bearing load measurements to be performed at the verification stage of the alignment process. When applied to container vessels, this will mean that shipyards will need to conduct jack-up
In a number of cases, overheating damage has occurred during starboard turns due to increased bearing load and slope mismatch between the journal and bearing surfaces.
load tests on the sterntube forward bearing, all the plummer bearings and the aft three main engine bearings at acceptance sea trials. Final adjustments to the plummer bearing
height will be made, where necessary, to optimise the alignment. The final bearing load
measurements required by Lloyd’s Register are to be conducted in a sailing draught condition, with the engine warmed through in order to minimise the uncertainties of hull deflections, machinery thermal rise and propeller buoyancy effects.
Enraf Tanksystem
Enraf
THE NAVAL ARCHITECT FEBRUARY 2007 61
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