Ventilation or cavitation An experimental study to determine dynamic loads on controllable pitch propellers

The load variation on a propeller depends mainly on the change of the inflow velocities on the propeller. And it is essentially the non-uniform velocity distribution in the wake of the ship that causes the thrust and torque to significantly vary throughout one revolution. This load variation, but especially the pressure distribution on the blade in the tip area and consequently the blade spindle torque is affected by the occurrence of cavitation and sometimes ventilation. These variations in tip loading are important because the life expectancy of the propeller system is a direct consequence of the dynamic loadings.

Ventilation may occur at reduced draught conditions but may also occur when the ship operates in heavy waves. Not much is known about the conditions for limited ventilation and its effect on propeller loading. For that reason, a series of exploratory tests have been carried out to study the occurrence of cavitation and ventilation and to determine its effects on loading, especially on blade spindle torque.

The experiments were carried out in a depressurized towing tank which is a suitable facility to study cavitation and ventilation. A high speed video camera was used for visualization while force transducers were mounted in the propeller to measure instantaneous propeller blade forces (propeller thrust and torque and individual blade spindle torque). The visualization comprised the observation of ventilation as well as of its interference with cavitation..

For ventilation, a small formation of air entrainment in the tip region of the propeller was observed, just beyond the propeller’s top position. The appearance of the ventilation was very similar to cavitation on the blade. The experiments showed that ventilation may readily occur without the blade tip protruding the water surface. The blade loading, especially the blade spindle torque was heavily affected and was found to depend on the amount of ventilation. The spindle torque variation was increased while the blade was already over the top, increasing the cyclic loading on the propeller and its components. Therefore, the experiments show that the ventilation changes the dynamic loads considerably. As such, this phemenon should be considered in the lifetime prediction for the controllable pitch propeller.