On my offshore vessel when one of both bow thrusters are running close to maximum speed they start to vibrate heavily and effectuating vibration of whole superstructure. Can you help?

In September 2014 we confronted similar situation in Singapore. At first our service engineer performed vibration measurement and cross-check of alignment for both el. motors.

Results of alignment were within manufacturer limits, however vibration results were over limits given by Class society and ISO standards. They were indicating problem with blades as one of the source of vibration was coming from blade pass frequency.

After measurements it was decided by ship’s superintendent to check condition of the blades during dry dock and if necessary level the gap between blades and tunnels.

During dry dock it was found out that the gaps were uneven and few cracks on the surface were found.

Repair of tunnels was done and additional brackets for el. motors and tunnels were installed. Repetition of vibration measurement showed decrease in vibration velocity RMS value by 30% for BT1 and 50% for BT2 and both are now within Class society and ISO limits.

In this case it was decided to put both bow thrusters under constant vibration monitoring.