In "Inchcape Rock," why and how did Sir Ralph's ship sink?

Sir Ralph's ship sank because it crashed onto the Inchcape Rock. As the ship filled with water, it sank.

Every ship floats because the buoyancy force (the force that pushes up the ship) is greater than the gravity force (the force that weighs the ship down). In Sir Ralph's case, as the ship filled up with water, the extra weight increased its gravity force relative to its buoyancy force. That's why it sank.

In the poem, the Abbot of Aberbrothok had put a bell on a buoy to warn sailors about the "perilous" Inchcape Rock. However, Sir Ralph had effectively "cut the bell from the Inchcape Float" so that "“The next who comes to the Rock,/ Won’t bless the Abbot of Aberbrothok.” Sir Ralph had been intent upon causing others harm, but he himself later fell prey to his own machinations.