We now have a 'type' classification of SS, which indicates a diesel 'fleet submarine'.

Submarines have either nuclear or conventional propulsion, and their roles can be broadly defined as strategic strike (e.g. nuclear ballistic missiles or high volumes of conventional cruise missiles) or tactical (e.g. nuclear attack submarines, diesel patrol submarines).

Nuclear submarines are much faster--with some reaching in excess of 35 knots at flank speed--and have essentially unlimited range, but are typically larger and have a higher baseline level of noise from reactor pumps and other support systems which cannot be turned off. A major advantage of nuclear powered submarines is that they can remain submerged indefinitely, as they produce their own oxygen to support the crew and have no need to recharge batteries using diesel engines.

Diesel (also called conventional) submarines are much slower than nuclear submarines and are limited in their range by the amount of fuel they can carry. When operating on the surface or 'snorkelling' at periscope depth, conventional submarines use their diesel engines to recharge an array of batteries that power the submarine when it is completely submerged. Running the diesel engines is noisy, and the exhaust plume from the snorkel can be detected from quite a distance by aircraft with infra-red sensors. When running on battery power, however, a conventional submarine is extremely quiet and therefore very difficult to detect. This, combined with their small size and low cost makes conventional submarines a popular choice for many of the world's navies.

Continue to close with the submarine and hold contact until you are able to make a positive identification with passive sonar.