Fully Cavitating Propeller for a Hydrofoil Ship 



REPLY TO DISCUSSION 



B.V. Davis 



I am indebted to Mr. S. Morita, who has provided the following answers to 

 Mr. W.B. Morgan's questions: 



1, The methods applied to analyse the hydrofoil propeller blades included 

 the simple beam theory for determining spanwise bending stresses, and tor- 

 sional stress analysis of the noncircular blade sections. The combination of 

 bending and shear stresses can give principal stresses in directions which are 

 not normal to the section nose-to-tail line. This feature was clearly demon- 

 strated during extensive structural testing of a full-scale aluminum blade model. 



2. With regard to the need for a large annex to the blade section, this was 

 considered for early versions of the design. It was found that when applying the 

 simple beam theory, the section modulus at the leading edge was reduced in 

 spite of the increase in the moment of inertia. This is due to rotation of the 

 principal axes, giving rise to increased fibre distance at the leading edge. The 

 conclusion was that an annex at the trailing edge does not always provide a re- 

 duction in the peak stress levels, and has the disadvantage of additional weight 

 and hence higher centrifugal stresses. 



(Author's Note) - For the final design it was found necessary to trim off the 

 upper rear section surfaces, to obtain small tension stresses in the thin blade 

 leading edges, i.e., by rotating the section principal axis slightly above the sec- 

 tion leading edge. A computer programme was used to obtain section properties 

 at various blade stations, and a trial and error method used by removing trail- 

 ing edge upper surface material until the desired principal axis orientation was 

 obtained. 



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