321 



Figure 5. Data collecting memory. 



Figure 5. Desk calculator with printer and plotter. 



second degree of a propeller revolution in digital 

 form in a RAM semi-conductor memory controlled by 

 the desk calculator. With this instrument, instan- 

 taneous or time averaged test results can be stored 

 and are readily available for printing, plotting. 



transformation to full scale, and harmonic analysis 

 as well as integration of resulting hull surface 

 forces and similar calculations with the aid of 

 the desk calculator. 



ERLING HUSE 



The authors in their presentation draw atten- 

 tion to the problem of calculating cavity geometry 

 and thus the excitation force due to cavitation. 

 At the Norwegian Ship Model Tank in Trondheim we 

 are at present developing a procedure to overcome 

 this difficulty. In the cavitation tunnel we 

 measure the propeller-induced pressure at only 4 

 positions on the hull model above the propeller. 

 The measurements are made for non-cavitating as well 

 as cavitating propellers. From the results of these 

 measurements we calculate an equivalent singularity 



distribution to represent the propeller. This is 

 next combined with a theory similar to that of 

 Dr. Vorus to obtain the excitation force on the hull 

 referring to any given vibratory mode of the hull. 

 As a second comment on the paper I notice in 

 Figure 4 integration areas extending up to 30 pro- 

 peller diameters upstream. This is, in my opinion, 

 not very realistic because one is then passing one 

 or more nodal points of practically occurring modes 

 of vibration. 



O. RUTGERSSON 



First I would like to congratulate the authors 

 on this interesting paper. The possibility of cal- 

 culating hull forces and moments and their distri- 

 butions directly on the body without the roundabout 

 way over freestream pressures and solid boundary 

 factors is especially elegant. Being somewhat in- 

 volved in calculations and measurements of pressure 

 fluctuations (with and without cavitation) at SSPA* 

 I would like to ask if the authors intend to use 

 this new method also to calculate solid boundary 



*Statens Skeppsprovningsanstalt, Goteborg, Sweden 



factors for different afterbody shapes and propeller 

 configurations? 



Unfortunately the authors ' investigation is 

 limited to non-cavitating propellers. This is a 

 severe limitation as the contribution from the 

 transient cavitation often is of a much higher mag- 

 nitude than the contributions from blade loading 

 and thickness. When discussing this subject the 

 authors declare that methods "for predicting trans- 

 ient blade cavity geometry and the attendant pres- 

 sure field" are not available. I would like to ask 

 why the methods developed by Huse (1972) , Johnsson 

 and Sjzfndvedt (1972), and van Oossanen (1974) have 



