Trans RINA, Vol 153, Part A1, Intl J Maritime Eng, Jan-Mar 2011


and stern-up), and the minimum load is near the zero pitch value (zero pitch angle). It is noted that there is a small phase difference in maximum loads between different components.


It is worth noting that the load fluctuation is not only the consequence of ship motion. The non-uniform inflow into the propeller disc due to the induced wave particle velocity also leads to variation in the propeller loads in a periodic manner.


When a propeller works in waves without presence of ship hull, and being fixed vertically, the propeller loads change periodically in correspondence with the wave elevation at


discussed extensively in [2],


the position of the propeller. This is in which the experiments


done in open water conditions in waves without the ship hull are reported.


thrust are observed when the propeller is located at X=0, L/2, and L relative to the wave (see Figure 13). In other words, the propeller


gives the maximum torque and


thrust when the averaged velocity induced by wave particles in the propeller disc is purely axial and in the opposite direction of the incoming flow leading to the lowest effective advance coefficient (X=L/2). The propeller gives the minimum torque and thrust at X=0 and X=L; because in these points the purely axial velocity induced by wave particles is in the same direction as incoming flow leading to the highest effective advance coefficient.


Generally, waves cause a non-uniform inflow into the propeller which causes bending moments and side forces on the propeller shaft. The peaks in side forces and bending moments are observed at times corresponding to X=L/4, X=3L/4 when there is purely strong in-plane velocity into the propeller disc generated by the wave particles as in the same way as for oblique inflow leads ( see for instance the horizontal bending moment form the experiment in Figure 14) .


Figure 14: Variation of the loads on the propeller in open water condition


With a view to the practical use of the results obtained in waves in the strength and vibration of the propeller-shaft system, the results are summarized in Figure 15


Figure 20.


The loads are presented as the ratio between the max values found in waves and the mean load with no waves for different advance velocities and wave amplitudes. It is seen that the bending load fluctuation increases with the increase in the wave amplitude. The bending load fluctuation is larger for the head sea condition compared with the following sea for the particular wave condition being tested. (In following seas, it is normally required to apply higher azimuth angles to keep a straight course compared to the head sea condition. In this study, the effect of higher azimuth angles in following seas will of course not appear since the thrusters are fixed.)


The amplitudes are made non-dimensional by the


obtained behind the hull in calm water. The results show that the forces and moments increase considerably due to waves and ship motion.


Figure 13: Wave particles velocities in the propeller plane at different locations


The interaction of the wave induced velocities, the velocities induced by the hull motion, and the steady wake field creates a complex velocity field which we


Figure 21-Figure 26 show the propeller loads in waves divided by the ratio of the wave amplitude to the propeller diameter in order to present the results as a kind of transfer function.


The results obtained for the loads in waves are summarized as below:


1.


The load fluctuation increased with an increase in the wave amplitude.


A-16 ©2011: The Royal Institution of Naval Architects The peaks of the propeller torque and


know will not be homogeneous over the propeller, and thereby will create side forces and bending moments.


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