Performance of Partially Submerged Propellers 



Appendix A 

 MEASUREMENT OF TRANSVERSE FORCES 



DESCRIPTION 



As previously discussed in this paper, propellers which operate partly out 

 of the water cause the effective center of thrust to be applied below the shaft 

 centerline. The imbalance of the torque force results in a transverse force in 

 the plane of the propeller. In order to determine the resultant force and mo- 

 ment, a four-component dynamometer was designed and built. The four com- 

 ponents were the horizontal and vertical components of the resultant moment 

 and the horizontal and vertical components of the transverse force. 



Li principle, the dynamometer acts as a cantilever beam of hollow cross 

 section with a concentrated load at the free end. Two reduced-area sections, 

 designated Plane A and Plane B, are strain gaged. The strain gages are wired 

 into 4 four-arm Wheatstone bridges. Each bridge is wired so that it reacts to 

 the bending moment at a given plane in a given direction. For example the 

 vertical moment at Plane A is proportional to the signal produced by a Wheat- 

 stone bridge where one side (two legs) measures the strains in the top of the 

 flexure while the other two legs measure the strains in the lower sides of the 

 flexures. Hence, the output is the difference between a compressive and a ten- 

 sile force in the material. Figure Al shows a schematic of the dynamometer, 

 and Fig. A2 shows the dynamometer assembly and the strain-gage wiring 

 diagram. 



Fig. Al - Schematic diagram of four-component dynamometer 



PRINCIPLE OF OPERATION 



Consider a coordinate system where the shaft is coincident with the x axis, 

 the positive y axis is vertically upward, and the positive x axis is horizontally 



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