212 



FIGURE 5. Rotor-type vortexir.eter . 



PHnTn-TRANSi.';inR 



J" 



\miniature 



BEARING 



mean value of the vorticity in a path of a wing 

 element. 



Q(r) = 55p«.UC r (D(r)dr 



2ir 



0(r) 



2lTv 



Q(9,r)de, io(r) 



(4) 



2Tf 



2ir> 



(u(6,r)de 



ment, at the same conditions there was no indication 

 of this phenomenon during the tests concerned in 

 this report. The body plan of the 4 and 7m geosim 

 models are shown in Figure 7 and the principal di- 

 mensions are summarized in Table 1. 



The intent of the experiments was twofold: first 

 to determine the structure of the stern vortices 

 using the rotor-type vortexmeter, and secondly, 

 to investigate the performance of the propeller 

 working in the presence of these stern vortices. 



On the other hand, concerning the vorticity gradient 

 influence on the radial portion of the rotor, from 

 the following equation it can be understood that 

 the tip of the rotor has a higher sensitivity: 



Q(r)dr = p«.UC ^f I r^uCrjdr 



(5) 



-R 



In practice, using only large models, the error due 

 to the finite diameter of the rotor can be eliminated. 

 Such a problem is also present when determining 

 the mesh interval in the vorticity measurement 

 by a five-hole Pitot tube. 



3. EXPERIMENTS AND RESULTS 



The ship models used in the experiments exhibited 

 an unstable propulsive performance in ballast 

 condition. In recent studies, it has been recog- 

 nized that the limiting stream line around the stern 

 and the pressure distribution change along with the 

 thrust fluctuation in the self-propulsion tests 

 of the model ship. The influence of these 

 phenomena on the ship design has been reported by 

 Watanabe and Tanibayashi (1977) and Watanabe et al . 

 (1972) . 



A special feature of this phenomenon was that it 

 appeared only in the self-propulsion tests and was 

 not observed on the towing tests. Thus, while this 

 phenomenon easily appeared in the self-propulsion 

 tests at Froude number 0.18 and 65% full displace- 



FIGURE 6. Rotor-type vortexmeter and stern of model. 



