The Essential Guide to Understanding Your Gearbox

September 30, 2016 - 9:00 AM by Samantha Pudney

Photo by PHGCOM, June 2008. CC-BY-SA 3.0

Photo by PHGCOM, June 2008. CC-BY-SA 3.0

So, think you know everything there is to know about your gearbox? Don’t be mislead by this surprisingly simple term, as this component is one of the most complex parts of an engine. In most installations, the gearbox serves three fundamental functions.

Firstly, to connect the engine to the propeller shaft. Secondly, to provide reverse gear and lastly, to reduce the drive speed from the engine. When considering the efficiency of each of these functions, it is important to recognise the three main types of gearboxes.

Helical Gears

A helical gearbox incorporates two multi-disc clutches and two gear trains, terminating in the output shaft. While one clutch is engaged for forward, the other will be engaged for reverse. Because of the rotating action of the gears in the lubricating oil, helical gearboxes tend to run the hottest. Larger gearboxes and those working in unusual circumstances, therefore, need oil coolers.

No adjustment is needed as the clutches are self-compensating for wear. However, it’s vital to ensure that full movement is obtained when your control arm is engaged. This will help you to avoid the chance of a clutch slip.

The lubricating oil especially used for helical gearboxes is usually Automotive Transmission Fluid, known also as ATF.

Epicyclic Gearboxes

Epicyclic gearboxes are a sun and planet gear train which reverses when the outer gear is restrained. Put simply, they are placed so that the centre of one gear will revolve around the centre of the other.

The engagement adjustment will need to be positive in order to avoid slippage. Again, follow closely the manufacturer’s recommendations in regard to lubrication as this can be different for each individual make.

Cone Clutch Gearboxes

Cone clutch-type gearboxes incorporate two gear trains and in this way, are similar to the helical gearbox. The cones work as clutches when the two parts are forced together, providing a fixed ratio of propeller shaft speed to engine speed.

In most cases, lubrication is with engine oil. But in some, Gear oil will be specified. Always use the oil recommended by the engine or gearbox manufacturer to lubricate your gearbox.

Best Practice for Your Gearbox

Although there are gearboxes able to survive being put in reverse at full engine speed, the best practice is to reduce engine speed to tick over when changing the gear either way. This is far safer for the quality of your gearbox.

Helical gearboxes will withstand more punishment than metal-to-metal cone clutch types, but sympathetic use will extend the life of the clutch, as well as your skipper.


Saildrive Units

Saildrives have integral gearboxes and drive the propeller via a bevel gear train and integral propeller shaft, in a manner similar to an outboard engine. They need to be regularly lubricated and maintained to your manufacturer’s recommendations.

Hydraulic Drives

Hydraulic drives consist of a constant displacement pump driven by the engine. This forces hydraulic oil down high-pressure tubes to turn a constant displacement motor and drive the propeller shaft. They have a number of advantages over conventional drives.

Hydraulic drives can be reserved at full engine speed, without any damage as a control valve limits the system pressure to a safe level. Comparably, the biggest advantage is that the engine can be almost anywhere in the boat due to the connections to the motor being through flexible pipes.

Perhaps the main disadvantage of hydraulic drives is that the system needs to have an adequate-sized oil reservoir. By this, we mean a heavy one in order to obtain reasonable life from the hydraulic oil. Furthermore, the efficiency of the drive is lower than that of a conventional gearbox. In order to keep the system running at optimum temperature, a raw water or oil cooler must be fitted.

General Maintenance


Now you know a little more about the functions of your gearbox, but what about maintenance? Debris in the oil will reduce the efficiency and lifespan of your drive. Components are manufactured to an extremely low tolerance and any foreign matter, especially of a metallic nature, is likely to cause gearbox damage.


A filter of minimum standard must be fitted and the element changed in line with a credible maintenance program. Water in the oil reservoir will cause similar problems as it does in a fuel tank, and the tank breather should also be fitted with a filter.

Finally, low oil levels can lead to pump failure, where any air bubbles that get into the system can implode inside the pump when compressed. A level gauge should be fitted to avoid this and enhance the longevity of your gearbox.

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