Boating Basics Online - Basic Boating Safety Course
Chapter 3 - The Boat

Propulsion Requirements

Each boat, depending on its design and intended use, will require different types of propulsion. Most recreational boats in the United States today use outboard engines and are less than twenty feet in length.

Outboard

Graphic of outboard motorOutboard motors are popular and quite useful on smaller boats. They are light and powerful, and modern outboards are extremely quiet. The outboard provides a completely self-contained propulsion system from engine to transmission to shaft and propeller. They are most often mounted directly on the transom of the boat, however, you may find boat designs incorporating a motor well or bracket on which the motor mounts. The entire motor swivels to provide easy steering as the turning propeller pushes the stern.

Outboards come in a large range of sizes and horsepower and can use different fuel sources. From small electric trolling motors to gasoline-and-oil-mixture two-cycle engines to gasoline-only four-cycle engines to diesel-powered outboards, the selection is large.

Inboard/Outboard

These are also referred to as I/Os or stern drive engines. Stern drives are generally heavier than outboards. They consist of an engine mounted inboard and a lower unit attached low on the transom. This lower unit resembles the bottom part or lower unit of an outboard motor. The outdrive or lower unit part swivels from side to side to provide for the steering of the boat. It can also be tilted up and down to provide boat trim while underway.

Graphic of an inboard/outboard motor

I/Os come in both gasoline and diesel models, and larger ones generally have more power than outboards. Because the main power supply is similar to a small automobile engine, easily accessible and more powerful, stern drives are often favored over outboards, especially on larger motorboats.

Inboards

These engines are most popular on motorboats over twenty six feet in length. The engine, similar to the inboard/outboard, is mounted inside the watercraft toward the center to give good weight distribution.

Graphic of an inboard motor

The engine connects directly to a transmission, out of which comes a shaft which goes through the hull of the boat as it passes through the "stuffing box." The shaft is then attached to a propeller that turns to propel the boat. (The stuffing box is a cylinder through which the shaft passes. The shaft is surrounded by a stuffing material that, when compressed between the cylinder wall and the shaft, prevents excessive water from entering the boat.) Because the shaft is fixed and does not swivel from side to side, a rudder is mounted behind the shaft and propeller to deflect the flow of water and provide steering direction.

Jet Drive

These propulsion systems do not have propellers, which are a potential danger to people in the water and to marine life. Jet drives are usually inboard engines that take in water that flows through a pump powered by an impeller. The water is then discharged at high pressure through a nozzle propelling the boat forward. The nozzle swivels to provide steering to the boat. Most personal watercraft use jet drives.

Diagram of a jet drive.

Note: When power is not being applied, a jet driven watercraft loses its steering because it is the stream of water that steers the boat. Keep hands, feet and hair away from the pump intake and do not operate in shallow water.

Choosing the right type of propulsion system for your boat is a very important matter. Both its weight and horsepower will both have an impact on the performance of your boat. If your boat is underpowered, its engine will work hard continually and will provide poor performance. Additionally if your boat is overpowered, it may exceed the safe operating speed that was designed for the watercraft.

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