[Hitman]

From the U-Boot Commander's Handbook (Official Kriegsmarine publication, revised 1943, ful text here http://hnsa.org/doc/uboat/index.htm) :

Section IX
Use (Expenditure) of Torpedoes.
391.) Every possibility of attack must be used quickly and resolutely. In most theaters of operation, the opportunities of hitting with a torpedo are rare. Do not, therefore, economize in torpedoes when attacking.
392.) The torpedo must always be ready for action at shortest notice. Always be prepared for unexpected opportunities of attack. A neglected opportunity will not recur.
In foggy weather and bad visibility, the tubes should be ready flooded. If necessary, they should be flooded via the mouth flaps.
393.) As far as the supply of torpedoes allows, several discharges, in the form of multiple shots (double or three-fold shots) should be directed against worthwhile targets, even at short range and when the aiming data are not in doubt. In this case, all the torpedoes should hit the mark, in order to ensure the annihilation of the enemy. This means that the torpedoes should be fired at different parts of the target.
394.) If the range is over 1,000 m, or if there is uncertainty as regards the aiming data (high speed of the enemy), several torpedoes (2, 3 or 4) should be released on the "fan" pattern. The idea is to make sure of one hit. It is better to score only one hit than to miss the target with each of several consecutive shots.
The target should therefore be covered by aiming at the boundaries of the area of dispersion on the target, i.e., the shots should be spread by the width of the dispersion area in relation to one shot aimed on the basis of the estimated data (if 2 or 4 shots are fired), in relation to an imaginary middle shot).
395.) If a final shot is necessary to sink the damaged ship, remember that the number of misses at the kill is proportionately greater than in firing during the attack.
a) At the kill, steer the submarine ahead of the stationary target, in position 90 at range 2,000 to 3,000 m, and approach slowly, carefully keeping the course, to find out whether the enemy is still making headway. When the change of bearing has been measured, the speed of the target should be set on the director angle of the computer, or the enemy should be finished off by the bow or stern torpedo attack proper. The speed of the enemy should be taken into account in determining the displacement of the marking point on the target.
b) Go in as close as possible, range under 1,000 m. On moonlit nights and during the daytime, submerge once more and attack underwater at 400 to 500 m.
c) If, in remote sea areas, an early arrival of enemy defence forces is not to be anticipated, the final shot should not be precipitated. Many ships sink only after 2 to 3 hours.
d) If enemy anti-submarine forces are sighted (naval or air), the final torpedo should be fired at once.
e) In the circumstances described under c), it should be ascertained whether the target can be destroyed by gunfire, instead of by the final torpedo. See also No. 277.