Engine order telegraph

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Engine room telegraph on board a merchant ship.

An engine order telegraph or E.O.T., often also chadburn, is a communications device used on a ship or submarine for the pilot on the bridge to order engineers in the engine room to power the vessel at a certain desired speed. In early vessels, from the 1800s until about 1950, the device usually consisted of a round dial about nine inches (~20 centimetres) in diameter with a knob at the center attached to one or more handles, and an indicator pointer on the face of the dial. Modern E.O.T.s on vessels which still use them use electronic light and sound signals.

Traditional E.O.T.s required a pilot wanting to change speed to "ring" the telegraph on the bridge, moving the handle to a different position on the dial. This would ring a bell in the engine room and move their pointer to the position on the dial selected by the bridge. The engineers hear the bell and move their handle to the same position to signal their acknowledgment of the order, and adjust the engine speed accordingly. Such an order is called a "bell," for example the order for a ship's maximum speed, flank speed, is called a "flank bell."

For urgent orders requiring rapid acceleration, the handle is moved three times so that the engine room bell is rung three times. This is called a "cavitate bell" because the rapid acceleration of the ship's propeller will cause the water around it to cavitate, causing a lot of noise and wear on the propellers. Such noise is undesirable during conflicts because it can give away a vessel's position.

On most modern vessels the EOT acts as a direct throttle with no intervening engine room personnel.

[edit] Typical dial positions

Many modern ships have the following dial indications:

Full [Ahead]
Half
Slow
Dead Slow
Stop
Dead slow [Back]
Slow
Half
Full

The following dial positions are more common on older ships:

AAIII -- "all-ahead flank," the fastest possible, often unsustainable due to mechanical constraints
AAII -- "all-ahead full," rapid cruising speed, sustainable over long distances
AAI -- "all-ahead standard," normal cruising speed, more energy-efficient than AAII
AA2/3 -- "ahead two-thirds," slow, used for hazards
AA1/3 -- "ahead a third," very slow, used for docking
Z -- "all stop," no motion
B1/3 -- "back a third," very slow reverse
B2/3 -- "back two-thirds," slow in reverse
BF -- "back full," fastest sustainable in reverse
BE -- "back emergency," maximum power in reverse, often unsustainable