The War to End All Wars 3rd Ed.
Complete Game Rules
Convoy Battles
During an enemy's collect resource phase, if you have submarines in the same sea zone as Naval Trade Resources belonging to that player, then you may chose to have your submarines attack that shipping and try to destroy those resources. For each submarine you have in the sea zone roll 1D10. On a roll of 1-3 the submarine fails to destroy any Naval Trade Resources, on a 4-6 it destroys (ONE) Naval Trade Resource, 7-8 it destroys (TWO) Naval Trade Resources and a 9-10 it destroys (THREE) Naval Trade Resources.
Of course, whenever submarines attack merchant shipping like this, the merchant ships will attempt to fight back and any combat vessels the enemy has in that sea zone will attempt to hunt your submarines. First you must roll to see if the merchant ships themselves can successfully fight back against the subs. The player owning the Naval Trade Points gets this defensive roll regardless of weather they have actual ships in the sea zone or not. Roll 2D10 and if you roll a 20, one submarine get destroyed.
If the nation owning the Naval Trade Resources or any of its allies have ships in the sea zone and the submarines chose to attempt to attack that trade this turn, then those ships will have a chance to hunt and attack any submarines participating in the convoy attacks. Each ship attempting to detect enemy subs must roll equal to or lower than it’s search capability (see below). Additionally, fights and bombers (assuming they have the range: fuel) may be flown into a sea zone with enemy subs and also attempt to locate and destroy those submarines.
All ships and aircraft that successfully detect the submarines are then engaged in battle against all of the enemy submarines in that sea zone (see Naval Combat). Ships and aircraft that failed the detection attempt remain in that sea zone, but may not participate in the battle.
All Convoy Battles only last one round. The Submarines get a defensive roll, but they can not possibly hit any enemy aircraft. After one round of combat the submarines manage to retreat from the battle, though the subs will remain in the same sea zone after the Convoy Battle.
