Firing a deck gun mounted on a U boat deck manned by four men with the movement of the waves and taking into account every grenade has to come from within the ship and given through a small hatch near the deck gun?
I think that one grenade every three seconds is completely unrealistic. There are some good original documentaries about U boats on youtube including an original German one which shows the operation of the deck gun in quite some detail. The short barreled 8.8cm deck gun is not a belt feeder ya know...
OLtzS Ashikaga
1.
Part 2.
https://www.dailymotion.com/video/x74dadg