In the first
world war, the K-class submarines had been developed with a
surface speed high enough to accompany the fleet and take part
in a fleet action. They had, however, to be propelled by steam
turbines which made them dangerous and slow diving submarines
with a poor submerged performance. It was now believed that
diesel engines of sufficient power could be designed to replace
the steam turbines and so produce a much more satisfactory fleet
submarine. The design that emerged was a lengthened R-class
with diesel engines as powerful as the steam turbines of the
K-class. The stern torpedo tubes and a deep diving depth, however,
had to be sacrificed. This produced, when new, submarines
of the same surface speed as the slower battleships. The Rivers,
however, had the same troubles as the O, P and R-classes and
their main engines had to be de-rated to 80%. They could no
longer accompany the fleet and became
