Rod Gillies (2 Much Caffeine) tells us:
The only Midget Submarine to see action was the “Little Pig”, captained by Jock “Stumpy” McGurk. In a daring incursion into the Vulgarian fjords, the small craft used its single Explosive Harpoon to sink the battleship “Dragon”, bringing an abrupt end to Vulgarian naval ambitions in the Baltic.
Read more about O’Neill’s Midget Submarine on Empire of Steam.