Why submarine diesels do not scale directly into a battleship

Could enlarged Type XXI U-boat diesels power a battleship from two engine decks?

Not sensibly through direct drive; a diesel-electric system would be needed to distribute engines while delivering battleship-scale shaft power.

The enormous difference in displacement and horsepower between a Type XXI submarine and Bismarck makes a direct scale-up impractical. Multiple direct-drive engines on different decks would create severe gearbox and shaft-angle problems. Diesel-electric drive offers the credible alternative because generators can be distributed while electric motors drive the shafts, as in American turbo-electric concepts. The numerical comparison, roughly 4,000 horsepower per Type XXI diesel against about 148,000 shaft horsepower for Bismarck, shows the scale of machinery required.