GarryB Sat Jun 04, 2022 1:11 am
Ships propulsion systems are a bit like car propulsion systems... it is not like you have an engine and an accelerator pedal that goes from naught to top speed.
The engine of a car has an efficient running speed and a power running speed... most 2 litre cars 2 to 3 thousand revs for travelling at 100km/h on the motor way, 4-5 thousand revs for accelerating or climbing hills with loads on.
A ship generally has quarter, half, three quarter and full flank speeds forward and a reverse speed or two. The hydrodynamic shape of the hull and the propeller means it wont be able to go backwards as fast as it can go forward.
Most naval diesel engines can be stopped and then run backwards so no special gears are needed to run backwards.
A combined gas turbine and diesel powered ship the quarter speed power setting is usually running on diesels and it most efficient but also rather slow.
A ship with diesels and gas turbines use the gas turbines for sprints at max speeds...
The point is that for different jobs different speeds are required or desired... if you look at the Udaloy and the Sovremmeny class ships the former is a sub hunter and the latter is a destroyer so they have different roles with different speed requirements so they have quite different propulsion systems too.
For the new ships they need to be flexible because they will be used for a range of different roles for which different speed capabilities will be valuable.
For short range light transport aircraft sometimes a turboprop is more efficient than a low bypass turbofan... if you get it wrong you end up with a plane that burns too much fuel for what it does, or you end up with a plane that is too slow or too short ranged to be useful.