[Jimbuna]

If the outward pressure generated by the pump is greater than that of the water pressure outside the hull then water can/will be expelled from the boat.

Often they would only stop the water coming in and wait until their hunters left the scene before starting the pumps because of the noise they generated.

Of course, if the flooding was too severe they would pump all they could because the weight of the water would make the sub sink beyond its safe diving depth with the obvious resulting consequences.