loading between 90 and 135 degrees (also see Figure 11) , which may be 

 related to the outward turning of the vertical velocity due to the hull. 



Table 3 compares the results presented here for hull pitching in 

 calm water with the same type of results presented by Boswell et al. 

 (1976a, 1976b) for a single-screw transom-stern configuration, and with 

 results presented by Boswell et al. (1978) and Jessup et al. (1977) for 

 a twin-screw transom-stern configuration. The results presented in 

 Table 3 indicate that the experimental results on these three configu- 

 rations are consistent. The unsteady loads presented in this paper 

 increased by smaller fractions of their values without hull pitching 

 than did the unsteady loads reported by Boswell et al. (1976a, 1976b, 

 1978) ; however, this results from the smaller fractional increase in the 

 vertical velocity component relative to the propeller with hull pitching 

 of the present model than with the models reported by Boswell et al. 

 (1976a, 1976b, 1978), The estimated increase in vertical velocity com- 

 ponent due to hull pitching was larger than the measured increase in 

 unsteady loading with hull pitching for all three configurations. 



Hull pitching was the only one of the six components of ship mo- 

 tions (surge, heave, sway, roll, pitch and yaw) for which blade loads 

 were measured. These experiments showed that hull pitching affects 

 primarily the peak and unsteady blade loading and that this effect 

 appears to be controlled by the ratio of the maximum vertical velocity 

 of the propeller to the ship speed. It appears that the increases in 

 the peak and unsteady blade loading due to the vertical velocity compo- 

 nent of the propeller are independent of the type of ship motions pro- 

 ducing this vertical velocity. Heave and roll (for propellers off the 

 ship centerline) also produce velocities in the vertical plane of the 

 propeller. Therefore, the effect of heave and roll on the peak and un- 

 steady blade loading can be deduced from the experimental results with 

 hull pitching by calculating the equivalent hull pitching required to 

 produce the same vertical velocity component of the propeller as pro- 

 duced by the specified heave and/or roll. 



Surge, sway, and yaw do not significantly alter the flow relative 

 to the propeller in the vertical plane, therefore it is expected that 

 these ship motions would have an insignificant influence on the peak or 

 unsteady blade loading. The primary cause of this unsteady blade load 

 in calm water without ship motions for hulls of the type under consider- 

 ation here is the upward vertical wake velocity component relative to 

 the propeller plane, therefore any transverse velocity which is small 

 relative to this vertical wake velocity is insignificant when vectori- 

 ally added to the vertical wake velocity component. 



Blade loads were measured for only one pitching frequency. However, 

 any realistic hull pitching frequency is small relative to the propeller 

 rotational frequency; therefore, pitching frequency should not signifi- 

 cantly alter the trends of the experimental data. The magnitude of the 

 maximum vertical velocity for a given pitch amplitude is directly pro- 

 portional to pitching frequency; therefore the peak and unsteady com- 

 ponents of blade loading tend to increase as the pitching frequency 

 increases. 



16 



