Thrust reversers are commonly used on aeroplane jet engines, to generate backwards thrust. They work by directing the hot, high speed exhaust forwards instead of backwards. At a basic level, the air has a net momentum gain forwards because the incoming air is slow (and dense), but the outgoing air is fast (and less dense), so the engine has a net momentum gain backwards. This makes sense to me (and I hope it's right).
Boats also apparently have the same system, however I don't understand the physics. Since water is basically in-compressible, the water coming in cant be less dense than the water going out. This must mean that the water coming in is travelling at the same speed as the water going out, so there is no net momentum change. Am I being dumb? It seems like you could just connect a pipe directly from the exhaust to the intake and it would be the same. Is the main slowdown from something different, like the drag inside the intake or something?
edit: is momentum the wrong thing to be thinking about here? maybe its to do with the fact that at the inlet, the water is coming from all directions on the surface of the hull, but at the outlet it's all going in one direction?