Excerpt From In Harm's Way: Pigboats

Attack
Once at the point where the best shot would occur, the submarine can Attack. The decision on whether to shoot single torpedoes or spreads, and how many torpedoes in the spread, would be made and the torpedoes launched. If circumstances permitted it, multiple Attacks could be launched - especially if the submarine got right in among the ships of the convoy or battle group, for instance in a night surface action, but more usually it was a single attack on one or more ships made all at once, with the submarine then attempting to Escape the escorts.
Single Shot
On a Single Shot, one torpedo is shot from a tube. The firing officer rolls a Torpedo check, modified by the Range and Target Bearing.
Spreads
On a Spread, multiple torpedoes are shot at a single target. The firing officer rolls a Torpedo check, modified by the Range and Target Bearing, plus a Small Modifier Bonus per torpedo after the first - plus 1 Small Modifier for the second torpedo, plus 2 Small Modifiers for the third, etc. If the roll is a success, successively add a Small Modifier Penalty for each extra torpedo to see if it hit.
StarPerc Spread Example
The officer fires a spread of three torpedoes at a target Bearing 25 degrees right and at Short Range. Bearing modifier is -10, and Short Range is +0. Three torpedoes in a spread gives a +20, and adding the -10 Bearing penalty gives a net of +10 to the TN. The firing officer has Torpedo+2 and a COOR of 9, giving him a TN of 55%. This is modified to 65% because of the +10 from Range and Bearing. The officer’s player rolls a 47, so one torpedo hit. Adding +10 for the next torpedo gives a 57, still under 65, so it is also a hit. The third torpedo adds another +10 to give 67, over a 65, so it’s a close miss. Roll Quality for 2 torpedoes.
StarZero Spread Example
The officer fires a spread of three torpedoes at a target Bearing 25 degrees right and at Short Range. Bearing modifier is -1, and Short Range is +0. Three torpedoes in a spread gives a +2, and adding the -1 Bearing penalty gives a net of +1 to the roll. The firing officer has Torpedo+2 and a COOR of 9, giving him a bonus of +3. This is modified to +4 because of the +1 from Range and Bearing. The officer’s player rolls a 2, for a total of 6, so one torpedo hit with 2 successes. Adding -1 for the next torpedo gives a 5, still a hit, but with one success. The third torpedo adds another -1 to give 4, also a hit with once success.
StarNova Spread Example
The officer fires a spread of three torpedoes at a target Bearing 25 degrees right and at Short Range. Bearing modifier is -1, and Short Range is +0. Three torpedoes in a spread gives a +2, and adding the -1 Bearing penalty gives a net of +1 to the roll. The firing officer has Torpedo+2 and a COOR of 9, giving him 3 dice and a bonus of +1. This is modified to +2 because of the +1 from Range and Bearing. The officer’s player rolls a 2+4+2, for a total of 10, so one torpedo hit. Adding -1 for the next torpedo gives a 9, still a hit. The third torpedo adds another -1 to give 8, a miss.