Big low speed diesels as used in container ships are the wrong tool for a warship. Number one, the accelerate and deccelerate very very slowly. A Nimitz class can usually out accelerate it's escorts! Such diesels turn at or below 100-105 rpm and are bolted to the shaft with no reduction gearing. The prop turns at crankshaft speed. To reverse one you have to stop the engine, and a tech has to reverse a gear on the cam shaft, then the engine is restarted. This process takes a couple of minutes to do. Big marine low speed diesels are fine for commercial ships. They are fuel efficient and work just fine for medium speed ships making long open ocean passages at a constant speed and constant shaft RPM ( in effect the power plant is optimized for a single speed and that's it ), but they do not have the sort of acceleration or flexibility needed for combat ships. Also, only two engines means limited redundancy. Last, they are noisy, meaning even an old sub will hear it coming miles away. Steam or gas turbine propulsion can be made much quieter than a big low speed diesel. You will also find that big combat ships have higher propeller rpms for the speeds they require than those diesels can deliver ( keep in mind that giants like that do not use reduction gears at all and adding a set would make the size and weight prohibitive ). The rpm of a Nimitz class is classified, but if you look back a bit you will see an Iowa class battleship made top speed at shaft speeds over 200 rpm. 30 kts came up at 205 rpm. Combat ships typically use higher shaft rpm than merchantmen. All of this ignors the huge internal reconfiguration needed to make a four shaft steam design accomodate a two shaft low speed diesel power plant.
Re-fitting the Russian boilers makes a lot more sense if that is what is happening.
