Mortars were the only kind of naval armament to fire explosive shells rather than solid shot until the invention of the Paixhans gun. Since it was considered dangerous to have large stocks of shells on board the ships that were firing them, and because the reinforced mortar platforms occupied so much space below decks, bomb vessels were usually accompanied by a tender to carry ammunition as well as the ordnance officers in charge of firing the mortars. However, as naval warfare became more advanced, bomb ships were also accompanied by frigates to protect them from direct assault by faster, smaller vessels.