The cannon is attached to a rope which can be hauled in to save ammo. The Harpoon Cannon is mainly fired at the rear of an enemy ship, piercing the very contents of the vessel. It was acquired in order to destroy Rhaknam. This S. Cannon requires 15 spirit points to be executed and can only be fired at specific moments during battle.
Vyse and Aika first heard about the Harpoon Cannon from the Ship Parts Merchant in Sailors' Island. By using this information, the two were able to convince Captain Drachma to take them to the Valuan Capital. Apparently, Drachma was searching for a weapon powerful enough to destroy the arcwhale Rhaknam. Drachma had the cannon installed in Valua's Lower City.
The Harpoon Cannon had many other purposes besides inflicting massive damage on an enemy vessel. It was first used on a Valuan blockade near the Grand Fortress' gates. It also served as a crucial part of survival during the encounter of the Red Gigas. The Harpoon Cannon must be fired before Recumen attacks in order to knock its head off balance, thus allowing the player to avoid the destructive force of the 'Red Ray'. Later, the Little Jack got the chance to devastate 4th Admiral Belleza's Flagship with the S. Cannon when Vyse suddenly stopped the engines and maneuvered behind the Lynx. Then when Vyse attempts to confront Grendel, the Green Gigas, Drachma devises a plan to momentarily stun the giant: use the Harpoon Cannon to knock the Gigas into a canyon of Ixa'Taka.
The cannon is last used in the lands of Valua when the Purple Gigas appears. After the 6th Fleet's front lines have been eliminated, Drachma fires the Harpoon Cannon at Rhaknam, attaching to the Gigas. Unfortunately, Ramirez takes aim and burns the party's ship by cannonfire from behind. It is later revealed in the game that Captain Drachma survived the assault when Rhaknam pulled him along back to its home at the ancient capital of Glacia, in the Lands of Ice.
While in Deep Sky, De Loco challenges Vyse and crew. After losing the first battle, he fires a large grappling hook that he calls "The De Loco Harpoon Cannon" to latch onto the Delphinus and ensure his shots hit. This works both ways, however, meaning that the closeness enables the Delphinus to fire just as accurately.