1:600 HM Submarine Clyde ( Severn similar ). For their day, these were very large and fast submarines, and were very handsome ships. © Andrew Arthur

1:600 Thames. She had a different conning tower and gun arrangement, the latter being enclosed by a revolving breastwork. © Andrew Arthur

The River class were big and fast - very fast for their day, 22 kts, world record at the time - submarines for their day, if underarmed for their size.
They were yet another attempt ( "K" class anyone?!? ) to produce a "fleet" submarine performing silent scouting and attacking ahead of the fleet, but at the time, even some of the largest fleet units were bettering 30 knots, and this was a clearly unatainable goal.
Thus, the navy were left with 3 very large and fast submarines, which turned out to be very useful; their relative bulk and great performance found them jobs running supplies to beseiged Malta in relative security.
They had several other noteable features; welded feul tanks prevented oil escaping and sea-water entering the tanks, a comprehensive asdic set ( Asdic Type 120, Type 710 hydrophones ); seperate diesels for high powered running and battery charging, and in 1935, Severn mounted a heavy 4.7" gun.
Clyde was sent to South Africa for scrapping, because her powerful machinery was needed by the South African industry, there being no supply of such machinery internally, and no way Britain could afford to send any...