Naval Photo Interpretation 
making resistance would not take place if there were no 
skin friction. 
It is proved for any given ship, that the wider the 
frictional wake, the greater will be the speed. 
Stern Wake "1*9 
The disturbed body of water behind a moving ship, 
which is set into motion by the passage of the vessel, is 
called wake, wake current or stern wake. This forward- 
moving water has a certain influence on the effect of the 
propeller. Its characteristic pattern, however, is not 
caused by the propellers, as is often believed, but depends 
on the design of the hull of the ships, especially the stern. 
The wake varies with the speed, every different hull de- 
sign and stern shape, however, will present its own charac- 
teristic wake pattern. This fact is of great importance when 
analyzing aerial photos. By an intimate knowledge of such 
characteristics it is possible to state what ship it is, even 
if the hull perhaps does not appear in the picture, only her 
wake. 
The photo interpreter in World War II seems not to 
have practised such a thorough-going fashion when ana- 
lyzing reconnaissance pictures, perhaps because of lack of 
knowledge. We must remember, however, that the photo- 
graphic emulsion was then not as fine-grained as today, 
and a close study of details of a wake pattern in small scale 
verticals much more difficult. Mostly, distinctive wakes 
were studied for estimating speed by visual reconnaissance 
only. It was stated, that white wake increases with speed, 
further, that for any given type of ships the wider, heavier 
and longer the wake, the faster the ship is going. It was 
observed, too, that at any given speed, the smaller the