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Inboard/Outboard (Outdrive)


A permanently mounted inboard engine connected through the transom of a vessel to an outboard propeller drive assembly that both propels the vessel and is turned from side to side to steer the vessel. This form of engine eliminates the need for a rudder, rudder post, shaft log, stuffing box, stern bearing, coupling, and conventional propeller shaft. The drive unit is known as "sterndriveā€ or ā€œoutdriveā€.

There are several companies manufacturing sterndrive units, but only two brands are commonly found on recent model boats, MerCruiser and Volvo Penta. Both brands have single propeller and dual propeller models, and gas, diesel and high performance models.

MerCruiser Alpha drive unit

MerCruiser Bravo-I drive unit

MerCruiser Bravo-II drive unit

MerCruiser Bravo-III drive unit (dual propellers)

Volvo Penta SX drive unit

Volvo Penta DP drive unit (dual propellers)

Sterndrive unit components differ slightly by model and brand. Part identification terminology also slightly differs.

Large component unit drawings of MerCruiser Alpha and Bravo and Volvo Penta drives for quick reference:

Volvo Penta Transom Assembly

Volvo Penta Transom Shield Assembly

Volvo Penta Intermediate Unit

Volvo Penta Upper Unit

Volvo Penta Lower Unit

MerCruiser Alpha Transom Plate

MerCruiser Alpha Gimbal Ring

MerCruiser Alpha Gimbal Housing

MerCruiser Alpha Bell Housing

MerCruiser Alpha Driveshaft Housing (upper) - U-joint assembly

MerCruiser Alpha Driveshaft Housing (upper) - Upper drive shaft

MerCruiser Alpha Gear Housing (lower) - Vertical drive shaft

MerCruiser Alpha Gear Housing (lower) - Propeller shaft assembly

MerCruiser Bravo Transom Plate

MerCruiser Bravo Gimbal Ring

MerCruiser Bravo Gimbal Housing

MerCruiser Bravo Bell Housing

MerCruiser Bravo Universal Joint Assembly

MerCruiser Bravo Driveshaft Housing (upper)

MerCruiser Bravo I Gear Housing (lower)

MerCruiser Bravo II Gear Housing (lower)

MerCruiser Bravo III Gear Housing (lower)

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