Unsteady propeller forces and hull pressure pulses in waves

Ship propellers are traditionally designed for calm water conditions. Propulsion factors and wake distribution are considered time-invariant and speed loss, ship motions and propeller rpm fluctuations are simply neglected. This can be explained by insufficient tools to optimize propellers in waves and by the general lack of knowledge regarding operating conditions in waves.

Nevertheless, results of experiments carried out by Moor and Murdey (1970) showed that propulsion factors change significantly in the presence of waves. Unsteady RANS simulations performed by Guoet al. (2012) indicated that axial velocities at the propeller disk can increase up to 35% of the ship forward speed.

Speed loss (caused by added resistance, thrust loss and change of propulsion point), propeller rpm fluctuations (due to the time-varying wake in waves) and motions of the ship (due to the interaction between ship and surface waves) influence the propeller performance and ultimately cause an increase in propeller-induced hull pressure pulses (Taskar, 2017).Taskar et al. (2016) carried out an investigation regarding the effect of waves on the propeller performance of the KVLCC2 tanker.

It was found that the time-variation of wake distribution has the largest impact on the propeller performance. The entire analysis was carried out by treating wake fields in aquasi-steady manner. This means that for each time instant, the flow field entering the propeller disk was treated as time-invariant.

The underlying assumption is that the ratio of propeller blade frequency and wave encounter frequency is sufficiently large. Ideally, however, the propeller performance and resulting hull pressure pulses should be analyzed with fully unsteady calculations.The purpose of this paper is to investigate if the quasi-steady approach predicts the propeller performance in waves with sufficient accuracy.

Therefore, the KVLCC2 propeller is simulated both usinga quasi-steady and an unsteady approach in full scale in waves behind the ship. The unsteady approach takes into account the time-variation of the wake field. The two approaches are compared by observing the differences in unsteady propeller forces and hull pressure pulses.