Str^m-Tejsen and Chislett 



65 70 75 



FVopcUer Revolutions /Minute 



Fig. 25 - Relationship between propeller 

 revolutions and propeller torque as a 

 function of speed reduction encountered 

 while manoeuvring 



Measurement of Model Polar Moment of Inertia 



The term N^ - l^ measured in the "pure yaw" test contains the effect of 

 model moment of inertia l^. For prediction of ship-scale manoeuvres, it is 

 necessary, as mentioned above, to correct for the difference between model and 

 ship inertias. 



The inertia of the model, including instrumentation and equipment, is found 

 by oscillating the model in air on a torsional pendulum. The torsion rod is of 

 6-mm piano wire, 2 m long, clamped at the top to a stiff beam and at the bottom 

 to a fitting which can be rigidly fastened to the yoke normally used for lifting 

 models. Four wooden struts are used to prevent movement of the model rela- 

 tive to the yoke (Fig. 26). 



The torsion rod is enclosed, as a safety precaution, by a brass tube which 

 will restrain the rod if it should break. A steadying support is used to prevent 

 the model from swinging. 



362 



