Rough seas and damaged ships.
 

Do rough seas make a damaged vessel go down faster than if the sea was calm?

Yes it does.

exactally how so?

I don't know about stock but IIRC in GWX flooding is a major part of how damage to another ship is assessed over time from the initial attack. And if you punch a hole in a ship in rough seas where it is pitching up and down (and perhaps getting waves over the bow/stern/sides) it is probably going to take on more water in a shorter period of time than it would if it was sitting on a relatively flat, calm sea.

Yes. In fact, there is a shooting tactic for this: aim your torpedo for just under the first or second crane of a heavily pitching merchant.

Aiming for just under a crane helps you break two compartments, and the pitching and momentum of the ship helps flood it faster.

However, this does not apply to passenger/cargos, which routinely carry ping-pong balls crated in balsa wood.

Quite correct and one of my favorite tactics. Few things are more satisfying than watching a merchant's own engines do the dirty work for you. :up:

I have even (in stock SH3) experienced that a british fishing boat sunk completly by itself in heavy weather! :D

Intresting As I noticed I tend to sink more destroyers in heavy roling seas than when it's calm.