66 J. A. Sparenberg 
The remark of Prof. Isay that the theory for the quickly rotating propeller (sections 6 
and 7 of my paper) is too crude may be true. It does not follow, however, from the fact that 
the angle of incidence becomes infinite at the places p = 0 and 9=7. The regions where 
@ is large are so small that they can be neglected by the realisation of a model. As is well 
known in many linearised theories, there occur singularities at points where the theory does 
not hold, for instance, the endpoints of the range of contact by indentation problems and the 
leading edge by lifting surface theory. This does not affect the applicability of these the- 
ories, when the influence of the singularities is restricted within narrow limits. Moreover 
the results of the simple theory are not devoid of sense and show the behaviour mentioned 
by Prof. Isay in his contribution. 
It is, as Prof. Isay stresses, highly recommendable to investigate the interaction of 
turbulence and free vorticity. However, in my opinion this must be done on the basis of 
experiments or theory and not on the basis of a law stated a priori. 
