Ducted and Contrarotating Propellers on Merchant Ships 



(Forw prop.) 



Fig. 22 - Container vessel project. Incipient cavitation curves. 



7.2. Cavitation in Irregular Flow 



Most of the contrarotating propellers used in connection with the tanker and 

 container projects have been tested in two different wake distributions in the 

 cavitation tunnel. The model wake for the two projects was simulated by the aid 

 of a combination of transverse wire meshes and a longitudinal centre line plate. 



The results of the experiments are exemplified in Figs. 23 and 24, repre- 

 senting the tanker and container vessel projects respectively. In both figures, 

 the cavitation patterns of the set of contrarotating propellers having the best 

 efficiency are compared with those obtained with a conventional propeller from 

 the SSPA propeller family. From these studies it can be concluded that no bub- 

 ble cavitation or any other kind of cavitation, which is supposed to cause ero- 

 sion, was observed on any of the propellers. The only exception is the cavita- 

 tion which occurs where the tip vortex from the forward propeller hits the aft 

 propeller. Other studies in the cavitation tunnel have shown that this may cause 

 serious problems. It seems to be necessary to eliminate this risk, either by a 

 further decrease of the diameter of the aft propeller in relation to the forward 

 propeller, or by further unloading of the propeller tips of the forward propeller. 



1295 



