The M-class is a type of British submarine armed with a battleship cannon. Ammunition for such a gun is much cheaper and compact than a full-sized torpedo, while allowing the M-class to bomb the enemy's coasts. One of them, M1, was crewed by British sailors loyal to the royal family who received news of the revolution at home, and orders to return. Instead, they sailed towards the close-by Oceanyka, and settled. Their submarine was studied and copied by local shipworkers, unleashing the terror of the cruiser submarine upon Oceanyka.
As a cruiser submarine, the M-class can engage ships with far cheaper cannon shot, as well as bomb surface targets. In a close range ambush, it will decimate even fairly heavily armoured ships. However, as it must surface to fire, it will become vulnerable to returning shots, as submarines are unarmoured.