The principle of a hydraulic system is very simple. A force applied to one point is transferred to another point by means of a liquid that cannot be compressed. A hydraulic system thus consists in its simplest form of a master cylinder to which a force is applied and a slave cylinder to which the force is transferred.
The master cylinder is connected to the slave cylinder by a hose in which oil, which is not compressible, flows back and forth. That the medium that transmits the power must not be compressible is very important as it would mean large losses in the system. You can find the best hydraulic steering via the web.
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That is why it is so important that there is no air in the system. As you know, the air is very easy to compress. In a manual hydraulic steering system for boats, it is the steering pump that applies the force that causes the cylinder to move back and forth.
For a hydraulic steering system, this means that the force that arises when turning the steering wheel is transferred by means of two hoses from the hydraulic pump, on which the steering wheel is mounted, to a hydraulic cylinder mounted on the engine or rudder.
Two hoses because we want to be able to transfer the power in two directions. The hydraulic pump consists of a number of pistons that pump hydraulic oil back and forth in the two hoses when turning the steering wheel. There are usually 5 or 7 such pistons that pump hydraulic oil when turning the steering wheel.