What made a bomb vessel different from an ordinary warship?

What exactly was a bomb vessel?

A bomb vessel carried mortars on specially strengthened beds and decks, often using the robust hull of a converted collier and reducing its forward rig before firing.

Unlike a cannon, a mortar throws its projectile upward and transfers a different load into the deck. Bomb vessels therefore needed strong internal support beneath the mortar bed, which made stout merchant hulls such as colliers attractive conversions. The forward mast could be removed for firing, and their siege-support function more closely resembled First World War monitors than the low-freeboard monitors of the American Civil War.