The gearbox on a marine diesel engine has three purposes: to reduce the maximum speed at which the propeller rotates; to enable both forward and astern rotation of the propeller; and to enable the propeller to remain stationary when the engine is running. Unlike a car gearbox, there is usually only one forward gear and the clutch is contained within the gearbox.

Because modern engines run at too high a speed for normal operation of the propeller, gearing reduction is required. The reduction gear ratio is usually between 2:1 and 3:1, with different manufacturers often using differing ratios.

Gear selector

The input shaft to the gearbox is separate from the output shaft.

It can drive the output shaft by either of two sets of gears to give forward or astern propulsion, with the output shaft connected to the appropriate set of gears using one of two separate clutches. It is these clutches that are operated by the gear selector rather than the gears themselves, as is the case in a car. In other words, one clutch for ahead, one for astern. And never both at the same time.

Two-Shaft Gearbox

The simplest and most compact gearbox just has an input shaft and an output shaft. The input shaft drives the forward gear directly and the reverse gear via an idler gear. A cone clutch or plate clutches are moved along the freely rotating output shaft from the centre, neutral position, to link either the forward or reverse gears to the output shaft. The clutch mechanism can be designed so that, as the clutch engages, more force is provided automatically to ensure full engagement.

Layshaft Gearbox

The principle of operation of the other most common gearbox, the layshaft, is similar to the two-shaft box, but a separate shaft – the layshaft – is used. One clutch is mounted on the input shaft and the other on the layshaft. The output shaft rotates in the same direction as the engine in forward gear. These gearboxes normally use hydraulic actuation of the clutches.

Maintenance

Other than checking the gearbox oil level weekly, there’s usually no other routine maintenance required. Gearboxes may use the same grade of oil as the engine, a different grade, or automatic transmission oil (ATF), but it’s vital to use the oil specified in the handbook. Gear oil is thicker than engine oil.