Thanks all for the explanation.

Now, on to the next S Boat-related question: what type of listening equipment did these boats have? I swore I've read somewhere they were not fitted with sonar, but the boats we have show them having the same equipment as the fleet boats. Can anyone shed some light on this?

@LukeFF

"The S-class boats were used as the test platforms for a range of sonar designs tested in the 1920s and 1930s. Higher frequencies and small, trainable transducers allowed for narrow beam width and exclusion of natural sea noises and other interference. "

source: http://www.globalsecurity.org/milita...hip/ss-105.htm