Problem with Stern Tube – Water Coming Inside

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

We received similar questions. Our Service Engineer was sent to perform vibration and displacement measurements of the shaft during berthing operations. To evaluate whether shaft movement if it is the cause of leakage two places were chosen to perform displacement measurement, close to the stern tube and close to the intermediate bearing. The shaft in the first place was found in a very rusty

Fig 1. Condition of propeller shaft near stern tube seal.

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

Results of the vibration measurement showed that the source of the problem was not coming from the interaction between the crankshaft and the engine’s bed plate. A comparison of measured vibration near the 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 colour) and ahead (blue colour) which suggested a problem with shaft alignment in stern tube/intermediate bearing.

Fig 2. Velocity RMS results from ME
Fig 3. Velocity RMS results from Stern tube

Results from shaft displacement measurement showed that shaft movement in relation to an intermediate bearing is smaller than in relation to a stern tube. It was also noticeable that shaft centre movement for the similar revolution of the main engine maybe even 3 times bigger when we compare results from going astern and going ahead. After obtaining these results we recommended recalculating the type and design of stern tube seals and repositioning stern tube seals to compensate for their deflection during astern movement.

If the 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.

 

Fig 4. Comparison of shaft centre orbit at forward and astern–stern tube