Learn how a two-shaft reverse gear for boat engines works with our interactive 3D visualization:
How does the Two-Shaft Boat Gearbox work?
The input shaft, which is connected to the engine, always turns in the same direction.
There are two gears on the input shaft and two gears on the output shaft. On one shaft (in our case the input shaft) the gears are fixed to the shaft. On the other shaft (in our case the output shaft) the gears are mounted on bearings such that they can freewheel around the shaft.
One of the freewheeling gears on the output shaft meshes with the corresponding fixed gear on the input shaft directly; the other freewheeling gear connects with its corresponding fixed gear via an intermediate gear. In the former case, the output gear turns in the opposite direction to the input gear. This is typically the direction of propeller rotation for forwards. In the latter case, because of the intermediate gear, the input and output gears turn in the same direction. This is typically the direction of propeller rotation for reverse. The propeller shaft speed in forwards and reverse can be varied by changing the relative sizes of the various gears.
There is an operating lever on the side of the transmission. Depending on the position of the lever, a clutch mechanism locks one or other of the freewheeling gears to its shaft. Only one at a time can be locked.
Mechanical multi-plate clutch
On small mechanical ZF/Hurth transmissions like the one in our example, the clutch mechanism is a so-called multi-plate clutch. There are two clutches, one for each gear on the output shaft. Each clutch has two sets of plates. One set of plates is effectively locked to the gear and always turns with it, and the other set of plates to the output shaft via an armature plate and always turns with the output shaft. The two sets of plates alternate.
When the clutch is disengaged and the transmission is in neutral, the plates attached to the rotating gears are turning while those attached to the armature plates are not. The output shaft is stationary. When either clutch is engaged (for forwards or reverse), the armature plate on that clutch is pushed towards its gear, jamming all the plates in the clutch together. This locks the plates up as one, transmitting the rotation of the relevant input gear to the output gear and shaft. Depending on which clutch is operated, the output shaft rotates either in the opposite direction to the input shaft (forwards) or the same direction (reverse).
Note that in our animation, we show a simplified version of this mechanism. A real transmission contains more parts, such as bearings and something called a "servo-automatic system" which consists of balls running in tapered groves to force the close stack together.
Make sure to also check out our animation of a planetary reverse gear.