Boat Diesel – Hydraulic drives

These consist of a constant displacement pump driven by the engine which forces hydraulic oil down high pressure tubes to turn a constant displacement motor which drives the propeller shaft.

The ratio of the pump’s displacement to the motor gives the drive ratio. A system, for example, with a pump of 50cm3/rev and a motor of 100cm3/rev will give a ratio of 2:1. The propeller speed is therefore 50 per cent of the engine speed.

Hydraulic drives have numerous advantages over conventional drives. They can, for instance, be reversed at full engine speed without any damage. A control valve limits the system pressure to a safe level.

If the propeller jams the pressure control will again allow the oil to bypass the pump, thus limiting stresses on the propeller shaft and the engine.

The main advantage, however, is that the engine can be practically anywhere in the boat as the connections to the motor are through flexible pipes.

Hydraulic drives are manufactured to industrial standards for continuous duty and therefore have an extremely long life. The main disadvantage of hydraulic drives is that the system has to have an adequate-sized oil reservoir, which means a heavy one, to obtain reasonable life from the hydraulic oil. The efficiency of the drive is also lower than that of a conventional gearbox and to keep the system running at the optimum temperature a raw water/oil cooler must be fitted.

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