Bow thruster propeller damage – vibration detection

Bow thruster is a very important machine for each vessel. Without working bow thruster, berthing in port is more costly or can even be impossible due to safety rules. Taking above into consideration, the technical condition of BT is very important and should be maintained on good level. The most effective way to maintain good condition of machinery is CBM. Implementation of CBM gives information about actual condition of machine based on measurements. The three main methods are: vibration measurements, oils analysis and temperature measurements. More about implementation of CBM on thrusters can be read here:

During our standard analysis we discovered high vibrations on bow thruster el. motor.

The highest overall amplitude of the signal was 76 mm/s (velocity RMS) observed on Non drive End with 80% of thruster’s load. Precise locations of measurement points and overall vibration signals are shown below:

The vibration signal was significantly above the ISO limit and source of the problem was discovered during FFT analysis. The highest peak was 21Hz (corresponding to revolution speed of electric motor), second highest peak was 16,5Hz and third highest was 5,5Hz.

According to technical information about the bow thruster, we knew that gear ratio is 1:3,81 and that propeller has 4 blades. This information was used to determine that peak on 5,5Hz is corelated to shaft propeller speed. Still correlation about peak on 16,5 Hz wasn’t known. It was noticed that 16,5Hz is exactly 3 times the propeller shaft frequency and the blade pass frequency (which would be 4 timed propeller frequency) wasn’t noticed. Conclusion was that probably one of the blades is damaged or missing.

The diver’s photos confirmed our assumptions.

After this discovery, it was important to the owner of the vessel to extend working condition of bow thruster till dry dock.

Additional measurements were done to estimate what load is appropriate for manoeuvring with bow thruster. The results are shown below.

Bow thruster 50% load

Bow thruster 0% load

It was decided that bow thruster should be working up to 50% of load and measurements interval needs to be shortened to maximum 2 weeks.

The essential information for the owner before dry dock was if the other parts (expect propeller) were damaged. Luckily, our diagnosis confirmed that gearbox condition was good.