Fully Cavitating Propeller for a Hydrofoil Ship 



30,000 



20,000 



0-6 0-7 



RADIUS FRACTION 



Fig. 27 - Comparison of calculated and measured stresses 



subject to bending, has been fully described by Acum in Ref. 16. When referred 

 to fully cavitating propellers tested in a water tunnel, one of the two additional 

 structural similarity parameters that must be the same for the model and pro- 

 totype is 



n^D^n 



where E is the elastic modulus and cr is Poisson's ratio for the propeller mate- 

 rial. This ensures the equivalence of blade stresses and deflections between 

 model and prototype when they are caused by steadily applied hydrodynamic 

 loads. Then, with this similarity, the stresses vary as p n^D^ and deflections 

 vary directly as the scale. 



When it is necessary to scale stresses and deflections due to inertia forces 

 such as derived from rotation (centrifugal force) and nonsteady hydrodynamic 

 loads, the further requirement of equal densities in model and prototype is also 

 necessary. It is usually impossible to satisfy these two similarity conditions 

 simultaneously, due to restrictions arising from the tunnel operating conditions, 

 model scale, and the limited choice of materials from which models can be 

 made. A means of complying with these conditions is sometimes adopted in 

 structural experiments where the model material and density are kept the same 



995 



