Torpedo power

[kabex]

Hi all, been playing my first real campaign since a few years ago...

1939, Type IIA, 1st Flotilla u-21

As you know, the type II only carries 5 torpedoes and has no deckgun, so I've been having problems getting a decent tonnage in patrols; first patrol I sank one ship after hitting it 3 times if I remember correctly(having failed 1 torpedo) it was a medium cargo.

2nd patrol I spotted a Passenger/Cargo and it took me also 3 torpedoes to bring it down, after 1 having misfired/missed(not sure)... this is a big problem since I used up 4 out of 5 torpedoes on a single vessel and had to go back to base.

Also in my first patrol with my last torpedo I hit another merchant ship and followed it for about 4 hours to see if it went down and it didn't.

So how many torps do I need?! Or is something wrong with my game? GWX3.0 + a few mods.