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Lower Units

Written by Nicholas Kamuda
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Lower halves of outboard engines are called lower units. The upper half, or the power head, is the heart of the engine. The lower unit is the limbs of the engine, and also contains important cooling system and exhaust system components.

Some Important Elements of Lower Units

Lower units contain 6 important parts, not including the gears. The prop shaft and propeller is the propulsion system of an outboard engine. The propeller is connected to the engine through the prop shaft, which is driven by the drive shaft. The drive shaft travels up through the lower unit and enters the engine's power head through the bottom.

The anti-cavitation plate helps protect the propeller from cavitation--when vacuums form in a liquid due to the movement of a solid passing through that liquid. In motion, it is possible for a slightly damaged propeller to cause water to vaporize due to the reduction in pressure on the back of the blade. Poor design and too much power for the propeller type can also cause cavitation.

An outboard engine's lower units also contain the water intake and the output of the engine exhaust. The water intake is an array of small holes that is connected to the water pump of an engine. The holes are kept small to reduce the amount of flotsam that can enter the cooling system. The engine exhaust is generally funneled out the back of the prop to reduce noise.

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