The internal-combustion engine was applied to boats as early as the 1880s, and many two- and four-cycle engines of one cylinder or more were developed. The engine, permanently mounted inside the hull, drives a screw propeller by means of a horizontal shaft. Today many of these engines are automotive or truck engines that have been altered superficially to make them suitable for marine service.
A variation on the inboard engine arrangement is to place the engine horizontally at the extreme stern of the boat, connected through a watertight aperture in the stern to a lower unit that is similar to the housing for the right-angle gears of an outboard motor. Craft with such engines are called I/O (inboard/outboard) boats.
Powerboats are popular as a leisurely form of yachting. A modern yacht may include a kitchen, sleeping cabins, and other conveniences below decks.