6 Types of Boat Engines

An Overview of Marine Drive Systems

Happy people on a boat

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Boat owners have an array of choices when it comes to picking an engine, otherwise known as a motor or drive. The basic mechanical principle of a boat engine is the same as for any internal combustion engine, such as those that power cars, trucks, or other vehicles. However, whereas a land vehicle moves forward when energy released by the combustion of fuel powers a set of wheels mounted on tires, boats move forward when the drive shaft turns a propeller.

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Inboard Drives

An inboard motor

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The term drive is interchangeable with motor and engine, so an inboard drive is simply a marine engine enclosed inside the boat. With an inboard drive, the shaft, rudder, and props are located on the underside of the boat, leaving the transom clear.

Inboard drives can be powered either by gasoline or diesel fuel, and single or twin engines are available. A marine V-drive engine is a modified conventional inboard drive that is located closer to the stern of the boat than a conventional inboard drive.

Inboard motors can range from 1-cylinder to 12-cylinder models, but because many are derived from automobile engines, 4-cylinder or 6-cylinder engines are most common. Some inboard motors are air-cooled, while others use a water-cooling system—either a fresh-water radiator similar to that in an automobile or a water pump system that brings in lake or sea water to cool the engine. 

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Outboard Motors

Speed boat with large outboard motor

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Outboard motors are the most common type of boat propulsion, found on most freshwater fishing boats and many pleasure craft. They are self-contained engine units mounted to the rear wall, known as the transom, of the boat. Each unit has an engine, propeller, and steering control. In most units, cables attached to the steering wheel actually pivot the entire motor unit to provide steering. To make it easier to move the boat in and out of the water, the entire motor unit can be pivoted up and out of the water.

Two-cylinder and three-cylinder models are the most common, but very large outboard motors are also available, including V-6 and V-8 engines that rival the power available in inboard drive systems. Most motor types drive a rotating propeller, but some are jet-propulsion systems that move the craft by shooting water through the system. 

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Sterndrives (Inboard/Outboard)

Sterndrive boat

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Otherwise known as the inboard/outboard marine motor, sterndrives are thought by some to be the best of both worlds. The engine is mounted inboard forward of the transom, with a shaft that goes through the transom to the drive unit located outside the boat below the water.

Similar to the outboard lower unit, this portion of the engine has a propeller and acts as a rudder to steer the boat. Like an outboard, the lower drive unit on a sterndrive can be pivoted up to facilitate moving the boat in and out of the water.

Engine sizes are comparable to those in larger outboard motors: Four-cylinder and V-6 engines are common.

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Surface Drives

Men Riding in Speedboat
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Surface drives are specialized drives, mostly used by high-performance boats, with an inboard engine that drives a propeller that "pierces" the surface of the water to provide increased thrust. They operate half in and half out of the water in the planing wake of the boat, with a propeller shaft that exits almost horizontally through the transom. These drives are used when drivers want to achieve a high rate of speed. Racing boats, such as cigarette boats, use surface drive systems. 

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Jet Drives

A jet boat

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Most often used in personal watercraft or very large boats, jet drives replace propellers to push a boat through the water using high-pressure air forced out of the stern of a vessel. The water jet draws water from beneath the hull and passes it through impellers and out a moveable nozzle that steers the boat. In smaller boats, jet drives have the advantage of very fast acceleration, but are quite loud and not very efficient when it comes to fuel economy. Jet skis use this kind of motor.

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Pod Drives

A pod drive is a system in which the propeller units extend down directly beneath the engine through the bottom hull of the boat. The best known of these systems is the Volvo Penta Inboard Performance System (IPS), which became available for recreational boats in 2005.

In the Volvo IPS, the propellers are set in front of the drive shaft, so that the boat is actually pulled through the water, not pushed. This increases efficiency and speed by up to 20 percent. Other pod drive models push the boat in traditional fashion, with propellers mounted behind the drive shaft unit. 

Pod drives are usually mounted in pairs, and this allows the boat to be extremely maneuverable. With the pods controlled individually, a boat can literally spin on its axis while remaining in place, a decided advantage for docking or boating in tight quarters.