On my vessel only when the ship’s engine is going astern the crew observes water coming inside through stern tube. Can you help?

In April 2014 we received similar question. Our Service Engineer was sent to perform vibration and displacement measurements of shaft during berthing operations. To evaluate shaft movement if it is the cause of leakage two places where chosen to perform displacement measurement, close to the stern tube and close to intermediate bearing. Shaft in the first place was found in a vary rusty condition.

For vibration measurement number of measuring points were extended by horizontal measurement of main engine bearings and axial measurement of driving end side.

The results of vibration measurement showed that source of the problem is not coming from interaction between crankshaft and engine’s bed plate. Comparison of measured vibration near stern tube and intermediate shaft showed that even though the level of vibration is lower there is a bigger difference in values for the same RPM when going astern (red color) and ahead (blue color) which suggested problem with shaft alignment in stern tube/intermediate bearing.

  

Results from shaft displacement measurement showed that shaft movement with relation to intermediate bearing is smaller than with relation to stern tube. It was also noticeable that shaft center movement for similar revolution of main engine may be even 3 times bigger when we compare results from going astern and going ahead. After obtaining these results we recommended to recalculate the type and design of stern tube seals, reposition stern tube seals in order to compensate its deflection during astern movement.  If actions mentioned before would not bring satisfactory results, we recommended to recalculate weight and force distribution during astern movement bearing in mind slopes and clearance.

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