24 J. D. van Manen 
horsepower is the normal case, and frequently 20,000-shp installations per shaft are encoun- 
tered. Ship speed for tankers increased at the same time from 10 to approximately 17 knots. 
The development of push boats has led to a large application of this type of transpor- 
tation on inland waterways. 
The rising importance of the hydrofoil boat is undeniable. The application of the 
hovercraft principle to marine vessels at the present time offers unknown prospects. 
These imposing developments have induced several investigators to make speculative 
examinations of future possibilities. Gabrielli, Von Karman, Davidson, and others have 
kept themselves busy with more or less philosophical considerations in the field of 
transportation. 
Representative of their ideas is the relationship between the dimensionless constant 
P/V,A and the speed coefficient V;/A!/® for various means of transportation. The various 
ranges of this relationship are clearly presented in Fig. 1 for displacement ships, sub- 
marines, hydrofoil boats, hovercraft, and so forth. 
The philosophy behind this presentation in Fig. 1 is that there will always be a require- 
ment for new developments in a vehicle whose characteristics permit it to be placed in a 
blank area of the plot. In accordance with this thought process both hydrofoil boats and 
hovercraft are always meaningful. They may be expected to fulfill already existing and yet 
to be established requirements. 
Supertankers 
Supertankers form a type of ship which is indeed one of the most spectacular examples 
of increases in displacement and speed of ships. In Figs. 2a, 2b and 2c, several charac- 
teristic properties of supertankers are given on the basis of cargo carrying capacity or 
deadweight tonnage. The various points on these plots are derived from model tests con- 
ducted at NSMB; the curves presented are faired lines drawn through these points as 
accurately as possible, without any correlation analysis. Noteworthy in Figs. 2 are the 
following: 
The increase in the beam-draft ratio, B/T, with increase in deadweight tonnage. The 
limitation in draft to a maximum of about 10 meters in connection with the expected depth 
of water in harbors thus plays an important role. 
The increase in the block coefficient 51; with larger deadweight tonnages is alarming 
by comparison with standards which still were valid 15 years ago. 
Finally, the power required for ships’ speeds between 15 and 17 knots and deadweight 
tonnages above 50,000 metric tons deserves consideration. Considered from this point of 
view, the development of diesel engines of 20,000 to 22,000 hp in one 10- or 12-cylinder 
installation is not amazing. 
The large power per shaft in these extremely full ships has caused serious vibration 
phenomena and has led to damage. Experimental research has shown that extreme after- 
bodies, for the obtaining of a uniform circumferential velocity field in way of the propeller, 
and propellers other than the conventional screw must be taken into consideration. 
