system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 why do some boats have 4 engines with 300 HP. Does the horse power stack?
gpt
This phenomenon usually happens with outboard motors, where the entire motor pivots and turns. A single 1200 HP outboard motor would be impractically enormous. 4 x 300 HP gives you the same total power but in a much more convenient package. Because you can hang the motor out the back, it also takes up way less space inside the boat hull than one 1200 HP would.

You also get the advantage that if one engine dies, you still have 900 HP to work with. If you have only one 1200 HP engine and one engine dies you have nothing. This advantage drops off really fast in $ terms, so it usually manifests as two motor installations, very rarely three.

More than three is purely for packaging efficiency, not redundancy.

Depending on how fast the boat is and how the drive works, there can also be speed reasons. Going very fast requires a very fast propeller and it's easier to spin smaller props quickly. A 300 HP prop is a lot smaller than a 1200 HP one. Four smaller props putting out the same total power is a lot (lot!) less propeller weight and complexity and draft.