Propulsion Requirements

Chris Riley by Chris Riley Updated on August 21, 2019. In

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

outbd.gif (6795 bytes)Outboard – Outboard 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 about 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.

io.gif (5649 bytes)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. The outdrive or lower unit part also 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. 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 vessels.

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

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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 which turns to propel the boat. (The stuffing box is a cylinder through which the shaft passes. The shaft is surrounded by a stuffing material which when compressed between the cylinder wall and the shaft prevents excessive water from entering the boat.) Since 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 to provide steering direction.

Jet Drive – These propulsion systems have the advantage of having no propeller to cause potential danger to people in the water and marine life. They are usually inboard engines that take in water which flows through a pump powered by an impeller. The water is then discharged at high pressure through a nozzle that propels the boat forward. The nozzle swivels to provide steering to the boat. Most personal watercraft use jet drives.

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


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