Mercury produced the Alpha One, Bravo One, Bravo Two, and Bravo Three outdrives beginning in 1972. Later known as the Generation I Alpha One, this outdrive was replaced by the Alpha One Generation II outdrive for the 1984 model year. Production on the "Gen II" Alpha One continues as of 2011. Because the Alpha One outdrives were limited to relatively low-horsepower applications -- 300 horsepower or less -- Mercury introduced the Bravo series of outdrives in 1986 for engines of up to 600 horsepower and speeds of up to 100 mph.
Alpha One Generation I Outdrive
The Mercruiser Alpha One Generation I outdrive dating from 1972 through 1983 has a square upper gearcase cap with an eye attached to the 4 1/2-by-4 1/2-inch cap. The Alpha One dating from 1984 to 1992 has the same size upper gear case but lacks the lifting eye on top of the gear case cap. The Alpha One Generation I outdrive is limited to engines of 300 horsepower or less.
Alpha One Generation II Outdrive
The Alpha One Generation II upper gear case has a rectangular top cap that's 4 1/2 by 6 1/2 inches, 2 inches larger than the Generation I outdrive. The "Gen II" Alpha One also does not offer a lifting ring on top of the drive cap. The cap has a lifting notch at the center-rear that does not extend as far across the top cap as the rear bolt holes. Like the Generation I Alpha One outdrive, the Generation II Alpha One is limited to applications of 300 horsepower or less.
Bravo One and Bravo Two
The Bravo One outdrive, for a single engine application, has a lifting notch at the center of the rear of the top cap that extends to the rear bolt holes, almost halfway across the 4 1/2-by-6-inch cover. The Bravo Two outdrive, while physically similar, is not found on engines of greater than 450 horsepower or boats that operate at speeds of greater than 55 mph. For lower-speed or lower-horsepower applications, a Mercury dealer can identify the drive by its serial number.
Bravo Three Outdrive
The Bravo Three outdrive overcomes the tendency of Mercury outdrives to pull the boat to the left. Mercury drives rotate counterclockwise. The torque from this rotation pulls a boat somewhat to port along its course when moving forward. When backing, the torque vigorously pulls the boat to port, even after the engine stops. The Bravo Three outdrive uses a propeller shaft-within-a-shaft driving a twin propeller. The outer propeller shaft turns the propeller nearest the outdrive counterclockwise. The inner propeller shaft -- the shaft-within-a-shaft -- turns the propeller farthest from the outdrive clockwise. These "counterrotating" propellers cancel out each others' torque to eliminate pull to the left or the right when the boat moves.
- Mercruiser Service Manual Number 3A -- Sterndrives; Mercury Marine
- Mercruiser Service Manual Number 11 -- Bravo Sterndrives; Mercury Marine