Well, it was no secret back in my day.
If you have a steam ship, it takes a while whether it's nuclear or boiler-fired. My ship used to light boilers two or three DAYS before getting underway.
As someone mentioned, of course, there's more to it than just the power plant. Are their divers working over the side? are their majoe pieces of equipment under repair? Is the ship moored in a nest inboard of another ship that has some issues? Etc....
A gas-turbine ship that's otherwise ready to go? Stone cold to underway in no time flat. We used to joke that when you became the CHENG on a steam ship, they turned over the logs, the samples, the steam tables, the piping diagrams, and a pair of coveralls. On a gas turbine, they just toss you the keys.
BTW, as mentioned above, at least one ship at Pearl Harbor went from nearly stone-cold to underway in less than an hour, which is AMAZING for a steam plant. They even managed to bag one of the bastards while fighting their way out of the harbor.