In a sea going vessel, navigational officers control the ship’s navigation system from bridge and engineering officers control the propulsion plant from the engine room. Bridge is right on top of the ship, hence the officer on the bridge does not know about the condition of the propelling engine and marine engineer in the engine control room does not knows where the ship is heading as the engine room is situated below the water line of the ship. Hence a fail-safe communication is required in between the navigation and engineer officer to ensure smooth and safe sailing of the ship.
As the word describes, the telegraph on board ship is used as a communicating device to transfer orders of change in speed or direction from the bridge to the engine control room. The engine order telegraph consists of a lever which can be moved over different speed positions for ahead and astern direction.
The telegraph and its bell, also known as telegraph bell, are located both in the engine control room (ECR) and the bridge. A responsible officer from each of the departments handles the telegraph from these locations. One more telegraph is located on the emergency manoeuvring or local manoeuvring station of the main engine. There is a changeover switch located in the ECR for telegraph selection which can be manually or automatically changed between the local control and engine control room telegraph.