Lindgren, Johnsson and Dyne 



The maximum speed has increased considerably. For both types of ships, en- 

 gine power has been doubled many times over. 



For propellers, this development has caused remarkable changes in loading 

 conditions and cavitation numbers (Fig. 1). The upper left corner of the diagram 

 indicates the cavitation-free region for SSPA 5.60 propellers (1) in homogeneous 

 flow. For tanker-ship propellers the danger for cavitation problems has in- 

 creased due to increased load coefficients 



Vr)'"=(T„V.V/)' 



while for dry-cargo-ship propellers decreased cavitation numbers have in- 

 creased the cavitation problems. The trend towards higher propeller load for 

 the tankers means decreased propeller efficiency, while for cargo -ship propel- 

 lers limited draft, and thus limited propeller diameter, may cause similar ef- 

 fects. Figure 2 exemplifies the situation. In this diagram a scale of B is also 

 included. 



Contra - rot. 

 76 rim 



Fig. 1 - Cavitation number for propellers. Trend curves. 



2. EARLIER SELF-PROPULSION STUDIES 



A great number of reports and papers presented in different modern publi- 

 cations deal with the propulsion of merchant ships with contrarotating and ducted 



1266 



