Comparison of two-stroke and four-stroke cycles

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The main difference between the two cycles is the power developed. The
two-stroke cycle engine, with one working or power stroke every
revolution, will, theoretically, develop twice the power of a four-stroke
engine of the same swept volume. Inefficient scavenging however and
other losses, reduce the power advantage to about 1.8. For a particular
engine power the two-stroke engine will be considerably lighter—an
important consideration for ships. Nor does the two-stroke engine
require the complicated valve operating mechanism of the four-stroke.
The four-stroke engine however can operate efficiently at high speeds
which offsets its power disadvantage; it also consumes less lubricating
oil.

Each type of engine has its applications which on board ship have
resulted in the slow speed (i.e. 80— 100 rev/min) main propulsion diesel
operating on the two-stroke cycle. At this low speed the engine requires
no reduction gearbox between it and the propeller. The four-stroke
engine (usually rotating at medium speed, between 250 and 750 rev/
min) is used for auxiliaries such as alternators and sometimes for main
propulsion with a gearbox to provide a propeller speed of between 80
and 100 rev/min.

Reference : Introduction marine engineering