[mookiemookie]

Historically, it was not uncommon to have merchant ships take more than 2 torpedoes to sink. Especially if it was a highly compartmentalized ship (more compartments = more hatches that can be closed to control flooding) or carring cargo like lumber, or a tanker (meant to carry liquid, so flooding is much harder!).

Some general tips are to make sure you're aiming your torpedoes at different parts of the ship in order to create flooding in more compartments. Impact shots are good for targeting specific parts of the ship such as engines/fuel tanks or ammo bunkers under the guns in warships. Magnetic detonators are good for "breaking a ships back" which means that you've broken the keel and the ship has cracked in two and looks like a "V" shape.