Propeller Design 



the paddle wheel is clearly superior. During the last 15 years, sufficient de- 

 sign information has been published on this particular type of propeller. 



The vertical- axis propeller is a propeller type with outstanding maneuver- 

 ing capabilities. Ferries, tugs, and supply vessels are examples of ship types 

 where successful application of the vertical- axis propeller has frequently been 

 realized. In solving problems of dynamic positioning of any vehicle at sea, the 

 vertical-axis propeller has its own advantages. At very high speeds, in which 

 case the blade motions will resemble the motion of a fish, some promises for 

 the future may be hidden in further developments of the vertical-axis propeller. 



The range where the conventional ship screw has never realized itself as 

 a sufficient means of propulsion is that of very-high ship speeds. High-speed 

 vehicles, such as hydrofoil boats, hovering crafts, and hydroskimmers (c.a.b. 

 vehicles), are means of transportation where selection of the propeller type 

 has a dominating effect on the whole design configuration. 



Fully or supercavitating propellers with fixed or adjustable blades have 

 shown successful operation at speeds up to about 50 knots, despite the problems 

 of inclined shafts and partial immersion. Design methods for fully cavitating 

 propellers are approaching a quality where the design requirements may al- 

 ready be satisfied by a first propeller design. Ventilation techniques have im- 

 proved the off-design characteristics of this propeller type considerably. 

 Strength problems have been attacked successfully by proper material selec- 

 tion and original ideas about the geometry of the blade profiles. 



Pulse- jet propulsion and air propulsion are examples that underline our 

 insufficient knowledge to solve the problems of high-speed propulsion in a 

 satisfactory way. In this respect it is worthwhile to mention the development 

 of two-phase hydrojets (water -ramjets) in the U.S.A., the Netherlands, and 

 Italy. This type of high-speed propeller may become the most valuable con- 

 tribution of all our extensive research activities in the field of high-speed 

 propulsion. 



A final remark on the selection of the propeller type may be on a way of 

 representation of the propulsive efficiency which is suitable for comparison 

 purposes. It may occur that for a certain propeller type the usual advance co- 

 efficient J, the ratio between advance and rotational speed of the propeller can- 

 not be determined, for instance, due to the fact that there is no rotational speed. 

 In that case it is recommended to plot against the Froude number based on dis- 

 placement, the ratio between the required power and the product of displace- 

 ment and ship speed: 



SHP ^s 



versus 



A X V^ ^/^ 



In fact, this last ratio is the resistance per ton displacement divided by 

 the total propulsive coefficient and is, as such, qualitative for the propulsion 

 abilities of any ship or vehicle. 



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