A new mechanic for Pigboats, to reinforce that claustrophobic feeling - Stress Checks. Up to four times during each session, when the GM feels it is warranted, he/she can call for a Stress Check. A Stress Check is simply a roll of 1d10. Any 1 rolled means the character is getting stressed. This means they mark off one Stress check box on their character sheet, indicating a new Stress Level.
If another Stress check is called for before they can relieve it, the Target Number for the Stress Check is increased by the current Stress Level. Thus a character with two Stress Levels has a TN of 3 or less, etc. Each Level of Stress incurs a Small Penalty to all rolls until that stress is relieved, so the character with two Stress Levels has two Small Penalties to all rolls.
Stress can be relieved by several means. The R&R between missions relieves all residual Stress. Getting stinking drunk relieves two Stress. The celebration after a successful attack relieves one Stress. Cracking up relieves one Stress. Being really nasty to others relieves one Stress. Going catatonic for a day relieves three Stress. The GM should approve all Stress relief as appropriate. Using these listed reliefs as guidelines, other types of Stress relief should be allowed.
Severe depth charging, tense combat, prolonged silent running, bad surprises, and the like can call for Stress Checks.