Home Crafts & Hobbies How marine clutches and gearboxes work

How marine clutches and gearboxes work


Are you familiar with how marine gearboxes work?

The marine diesel engine gearbox has three functions: reduce the maximum rotation speed of the propeller, allow both forward and backward rotation of the propeller and keep the propeller stationary while the engine is running. In contrast to a car’s gearbox, a marine gearbox usually has only one forward speed and the clutch is located within the gearbox.

Due to the fact that modern engines operate at too high speeds for propellers to function normally, gear reduction is needed. The reduction ratio is between 2:1 to 3:1. Different manufacturers use different ratios.

Gear selector

The output shaft is not connected to the input shaft of the gearbox.

Two clutches can be used to connect the output shaft to the gearing that is required to provide forward or rearward propulsion. These clutches are controlled by the gear selector, not the gears. And so, there is one clutch to be used for forward and one for behind. You should never use both clutches at the same time.

Two-Shaft Gearbox

The simplest and smallest gearbox has only an input shaft and output shaft. The input shaft is used to drive the forward gear and reverse gear through an idler. From the neutral, centre position, a cone clutch or a plate clutch is moved along the freely rotating shaft to connect the forward or reverse gears with the output shaft. As the clutch engages more force can be applied automatically.

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. The input shaft is equipped with one clutch, while the layshaft has the second. In forward gear, the output shaft rotates the same way as the engine. The clutches are normally operated by hydraulics in these gearboxes.


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.

The post How marine gearboxes work appeared first on Yachting Monthly.

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