Studies of Dynamic Fluctuations in Propeller Thrust and Torque
Time series of dynamic propeller torque measurements during a ventilation event.
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Propeller dynamic loading fluctuations have caused failures of mechanical components of azimuth and tunnel thrusters. MARINTEK has developed procedures and instrumentation for investigating these phenomena. The main fluctuations are caused by ventilation due to waves or relative vertical motions of the vessel.
Dynamic fluctuations in propeller torque and thrust can be divided in two main categories:
- Low-frequency fluctuations in the range of encounter frequency
- High-frequency fluctuations in the range of propeller-blade frequency.
Low-frequency fluctuations are caused by waves or vertical relative motions leading to partial or full ventilation. The range of variation can be up to 100%. For a vessel operating in rough seas, these fluctuations will result in fatigue and failure of mechanical parts, while the mean available thrust may be considerably reduced.
Blade-frequency fluctuations can be caused by inhomogeneous inflow. These fluctuations usually have much lower amplitudes and do not lead to mechanical failure though they can cause other problems such as cavitation, noise and vibration.
Another important, and more serious, cause of blade-frequency fluctuations is partial emergence of the propeller. High blade-frequency torque and thrust variations are generated every time a blade hits the free surface of the water after emergence. A combination of these high-frequency fluctuations and low encounter-frequency fluctuations is believed to be the cause of mechanical failure of power transmission components such as gears and bearings. In fact, partial ventilation is much more harmful than full ventilation.
During the past few years, MARINTEK has developed procedures and instrumentation for dynamic tests. Studying dynamic problems at model scale is complicated and requires special care. Scaling laws are not always mutually consistent. Models need to be self-propelled and free to move in all six degrees of freedom and tests should be carried out in well-simulated environments.
A small segment of a time series of dynamic measurements of propeller torque of a thruster model in wave is shown in the figure. The tests were carried out in the MARINTEK Ocean Basin. It is clear that more severe fluctuations in blade frequency appear when the propeller is partially ventilated, starting at 10 seconds.
Encounter-frequency fluctuations appear to be less harmful, since their cycle is longer, but the range of fluctuations is much higher. If the ship is operating in harsh seas where such an event (on and off of torque) is repeated frequently, the consequences for the mechanical transmission components will be fatigue, wear and tear and possibly failure, especially since this is combined with the large blade-frequency fluctuations. Similar trends have been observed for propeller thrust.
Contact at MARINTEK: Kourosh Koushan
(Article in MARINTEK Review No 2 - 2004)