Boat Propulsion

Inboard-Outboard Motorboat
Motorboats are popular recreational vehicles, present on many lakes and oceans throughout the world. Inboard-outboard (I/O) motorboats such as this Donzi Z-25 are capable of reaching speeds of 80 kilometers per hour (50 miles per hour).

Until the 20th century, boats were propelled by oar and sail (see Sailing), just as they had been since ancient times. Paddles and poles (for pushing against the bottom of the waterway) were variations on the oar suitable for small craft. The rapid rise of the steam engine to dominance among ships did not affect boats until the late 19th century, when steam engines and boilers had become compact enough to fit into a small hull. The age of steam-propelled boats was brief, however, for the advent of commercial internal-combustion engines came within a few years of boat-size steam machinery. By the late 20th century, the internal-combustion engine, either in diesel or spark-ignition (gasoline) form, had become almost universal in boat propulsion, save for sail, oar, and paddle that survive with many types of recreational boats (see Motorboat).

I. Inboard Motors

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.


Cabin Cruiser
Powerboats are popular as a leisurely form of yachting. A modern yacht may include a kitchen, sleeping cabins, and other conveniences below decks.

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.

II. Outboard Motors


Outboard Motors
Outboard motors, like the ones mounted on these skiffs, are the most common means of propulsion for boats. Because it is mounted external to the hull, the motor is adaptable to almost any boat.

The outboard motor is probably the most common means of propulsion for boats. This machine is nearly always a two-stroke spark-ignition (gasoline) engine, mounted vertically at the stern of a boat in order to drive a shaft that in turn drives a conventional screw propeller through right-angle gears. It has the great virtue of being mounted external to the hull, thus being easily adaptable to almost any boat. Outboard motors operate on a mixture of gasoline and marine oil.

The development of the outboard motor was rather slow in the early years of the 20th century. After World War I (1914-1918), the popularity of the outboard motor grew steadily, and as a result its power gradually was increased, and it was made more reliable. The popularity of the outboard motor increased tremendously after World War II (1939-1945), and small cruisers, runabouts, utility boats, and various classes of racers became available. By the late 1970s, powerful outboard motors, ranging up to 200 horsepower rating, were available, as was special equipment for handling such engines at the water's edge. The use of two outboard motors on cruisers and runabouts was common.

Because of the great power available and because the low transom required by this type of motor creates the danger of the boat being swamped, the safety of the outboard motorboat has become a matter of public and governmental concern. The popularity of this type of boat also has produced crowded waters at lake and seaside resorts. In many areas measures have been adopted to prevent accidents caused by excessive speed, careless or reckless operation, and poor boat design.

III. Water-Jet Drive

Several innovations in propulsion became important during the late 20th century. One is propulsion by a high-speed jet of water ejected from the stern by a pump within the boat hull. The pump is driven by an engine that is identical to the one that might drive the conventional propeller. Water-jet propulsion is most suitable for high-speed boats, since for good propulsion efficiency the forward speed of the boat must be close in magnitude to the afterward speed of the jet. Regardless of boat speed, water-jet propulsion is often used for craft that must operate in shallow or rocky waters, where a conventional propeller might be damaged.

IV. Surface-Piercing Propeller

An innovation of great interest to designers and users of high-speed boats that was introduced in the 1970s is the surface-piercing propeller, in which the propeller is mounted with its center at the water surface. It is most commonly seen in the form called the Arneson surface drive. The drive shaft projects through the stern of the boat, supported by two hydraulic cylinders that adjust the vertical position of the propeller, a feature of great importance because the angle and level of the boat change as the boat speed changes. The engine is fixed in its position, but two universal joints accommodate changing shaft angles.