Choosing warship propulsion without the Washington Treaty

Which 1920s propulsion systems would be preferable without the Washington Naval Treaty?

Oil-fired steam plants remain the main choice, using geared turbines for efficiency or turbo-electric transmission for control and machinery flexibility. Battleship preferences differ, while diesel engines offer a separate option for auxiliaries and reserve craft.

Oil-fired boilers and geared steam turbines provide the initial conventional choice for a 1920s fleet. Turbo-electric transmission had offered a way around the inefficiency of connecting turbines directly to propellers, but it demanded more machinery weight and volume. A treaty displacement ceiling therefore penalized it strongly even when its operating qualities were attractive. Removing that ceiling would reopen the choice while leaving geared turbines an efficient option. The preferred machinery then depends on the ship's role, with disagreement over battleships and a separate case for diesel machinery in reserve vessels.