Water borne vessels, big or small, need navigation control to cruise through water and to move through a controlled path. As vessels move forward by pushing the water backward, the guidance of the vessel by turning it right or left can be controlled by directing the pushed water towards respective direction.
A plate of steel, connected to a rod, which is extended to a convenient height of the boat deck, is used to deflect water to both sides, according to the requirement of turning. This plate is called RUDDER PLATE and the shaft extending to the deck, connected to the rudder plate is called RUDDER- STOCK. The boat driver will use mechanical or Hydraulic systems to turn a rod connected to the rudder-stock to do the turning job. This rod, used to turn the rudder plate is called TILLER-ARM. Though control of Tiller-Arm is easy for small boats running slow in backwaters /lakes, it requires high effort and skill to hold tiller-arm in right position in sea, as well as during fast movement in backwaters.
As the fast-moving / heavy modern vessels use an engine to run a propeller, to push water backwards, they use modern mechanisms to guide the vessel. In these vessels, angular movement of rudder-plate is precisely and efficiently controlled by Hydraulic actuators, driven by closed-loop manual/diesel-run hydraulic systems.
Based on mounting position of the engine, vessels are classified into two. In-Board Vessel, with engine mounted inside the vessel, and Out-Board vessel with engine mounted outside the vessel, preferably backside of vessel.