EXPERIMENTAL DETERMINATION OF 

 UNSTEADY PROPELLER FORCES 



Marlin L. Miller 

 Naval Ship Research and Development Center . -- 

 Washington, D. C. 



ABSTRACT 



Unsteady propeller forces were measured on propellers in a water 

 tunnel in a nonuniform flow produced by wire screens. In order to 

 make these measurements, dynamometers, instrumentation, and testing 

 techniques had to be developed. The effect of blade width and skew of 

 single propellers and the effect of changing the angle between the pro- 

 pellers of a tandem set on the unsteady forces were deternained. 



INTRODUCTION •-• - 



One of the principal causes of vibration of ships is the unsteady hydrody- 

 namic action of the propeller. Due to its periodic structure, the propeller can 

 excite severe vibrations in the ship's structure in two ways. The nonuniform 

 pressure field of the propeller rotating past nearby portions of the ship's hull 

 will excite them into vibration and the propeller moving through the spatially 

 nonuniform flow behind the ship will produce unsteady forces that are trans- 

 mitted to the hull through the propeller shaft and bearings. Since it is usually 

 more convenient to measure these forces separately, it is important that phase 

 angles, as well as amplitudes, be considered, since the net excitation is the 

 vector sum of these two types of forces. In this paper, only the experimental 

 determination of the unsteady forces and moments and their phase angles that 

 the propeller transmits through the shaft will be considered. 



In order to be able to reduce these vibratory forces to a minimum, it is 

 necessary to know how they are related to the propeller design parameters and 

 the characteristics of the flow into the propeller. A number of theoretical 

 methods for calculating these forces have been developed, ranging from simple 

 quasi-steady methods based on open-water characteristics to highly sophisti- 

 cated methods using three-dimensional unsteady propeller theory. Application 

 of these theories produce widely differing results, and, in order to evaluate 

 them, it is necessary to have experimental results for comparison. A number 

 of propeller dynamometers for unsteady forces have been developed at NSRDC 

 and elsewhere (1-8). The early ones measured only thrust, or thrust and torque, 

 and had questionable dynamic characteristics or were limited to low frequen- 

 cies. Some more recently developed dynamometers have highly improved char- 

 acteristics, and some are able to measure the six components of the propeller 

 forces and moments. At NSRDC three unsteady — propeller dynamometers are 

 in current use. The MK-III dynamometer, for thrust only, uses a capacitance- 

 type gage between the propeller and an inertial mass [8]. The "Bass" six- 

 component dynamometer was developed for use with ship models in the towing 



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