 The Ordnance QF Hotchkiss 6 pounder gun were introduced to defend against watercraft capable of independent operation underwater usually referred to as "boats" rather than what?
Submarine:  The noun "submarine" evolved as a shortened form of "submarine boat"; by naval tradition, submarines are usually referred to as "boats" rather than as "ships", regardless of their size ("boat" is usually reserved for seagoing vessels of relatively small size).
i don't know