Why are submarines called boats rather than ships?

Why are submarines called boats and not ships?

Early submarines were small craft intended to be carried or deployed by larger ships, so naval usage classified them as boats; the term then became traditional.

The traditional distinction is that a boat can be carried aboard a ship, whereas a ship cannot be carried aboard a boat. Early submarines had small crews, junior commanders, and plans for transport aboard larger vessels. The same logic appears in torpedo-boat destroyers, whose longer name marked their development beyond the carried-boat role. The submarine name persisted even after those vessels grew far beyond their original scale.