Learn how a planetary reverse transmission for boat engines works with our interactive 3D visualization:
How does the Planetary Reverse Gear work?
The input shaft (bronze color) is connected to the so-called sun gear in the center of the transmission. It is connected to the engine and always turning in the same direction. Two sets of planet gears are mounted on a shaft in a gear carrier with one set in contact with the sun gear and the other set in contact with an outer ring gear.
When the transmission is in forward gear, a clutch locks the sun gear to the surrounding sets of planet gears and to the outer ring gear - they are all locked up together and turn as one unit with the sun gear. The output flange (black), which is attached to the planet gear carrier and to which the propeller shaft is attached now spins at the same speed as the input shaft and sun gear.
When the transmission is in neutral, the sun gear is still turning at the same speed as the engine. The planet gear carrier is stationary and as such so is the output flange. Let's assume the sun gear is rotating in an anti-clockwise direction as is shown in the illustration. The inner planet gears will rotate clockwise, and the outer planet gears anti-clockwise, turning the the ring gear anti-clockwise. None of this motion is transmitted to the output coupling and propeller shaft.
When the transmission is in reverse, the ring gear is locked in place by a clutch. The sun gear is turning in the same direction as before. Instead of the ring gear turning, as it does in neutral, the planet gears drive the planet gear carrier around in the opposite direction to the sun gear. The output flange and the connected propeller spin in reverse. The speed ratio of reverse to forward can be adjusted by varying the sizes of the sun gear and planet gears.
We also have an animation of two-shaft reverse gears.