not good.
i dont think installing multiple destroyer powerplants will make something as massive as the montana class move very fast.
and installing multiple small powerplants will eat up more space than a singular engine.
you going to have to put an powerplant specifically designed for the ship for it to hit the fast battleship speed and above.
Not... entirely. Actually, for a good length of time in the 20s and 30s, the US Navy had standard "unit" machinery designs for carriers, cruisers, and battleships, reducing cost by making them common. For example, if you had a 180,000 horsepower four-shaft machinery plant for a new carrier, then it would be quite likely that the next class of cruiser would be designed around a 90,000 horsepower two-shaft plant that just happened to be half of the carrier plant--four boilers and two engines instead of eight and four, respectively. This was actually done by BuEng designing their standard machinery units as single-shaft plants that made a specified amount of horsepower (45,000 shp, in my example), allowing C&R's designers to put as many of them as were needed into a given design.
Oh, didn't know. I figured that because a single Shimakaze class Destroyer can produce 80,000 shp that it would make a difference there.
Based on Springsharp's rather optimistic view of the Montanas' performance, wedging three Shimakaze-class machinery plants into it would (just barely) be enough to reach 35 knots; realistically, you'd probably be looking at four of them. Which means you're going to have six to eight screws, depending on which version you use, which is a nightmare designers try very hard to avoid (because more shafts == more shaft alleys == more places that you can suffer flooding in if a torpedo springs the seals, not to mention simply trying to find space for all of them without any interference from their tip vortices).
Of course, the big problem is that destroyer machinery is designed for brief periods of very high output, but very
low output over the long term. To make it small and light enough to fit into a destroyer, they have to make it a sprinter, not a distance runner, whereas battleships need to be able to sail at close to top speed in the battle line for hours on end in a fight. As an example of how low the power is most of the time, Fletcher-class destroyers had about 60,000 horsepower on tap to hit their top speed... but to make their cruising speed of 15 knots? They were putting out about
2700 horsepower.
And to give you an idea of the difference in weight in the machinery plants, the Iowa class made 212,000 shp with a total machinery weight (wet) of 4,835.8 tons, for a power-to-weight ratio of 43.84 horsepower per ton. By comparison, the Fletcher class, designed around the same time, made 60,000 shp with a total machinery weight (wet) of 862.4 tons, for a PWR of 69.57 horsepower/ton. (Figures taken from Friedman, though I calculated the PWRs myself.) And just ask any car guy what trying to force 50% more power per displacement out of an engine does to reliability and durability...
