A Second Assistant Engineer or Third Engineer is a licensed member of the engineering department on a merchant vessel or naval vessel.
The Second Assistant is usually in charge of boilers, fuel, auxiliary engines, condensate and feed systems, and is the third most senior marine engineer on board. Depending on usage, "the Second" or "the Third" is also typically in charge of fueling (a.k.a. bunkering), granted the officer holds a valid Person In Charge (PIC) endorsement for fuel transfer operations.
The exact duties of this position will often depend upon the type of ship and arrangement of the engine department. On ships with steam propulsion plants the Second/Third is in charge of the boilers, combustion control, soot blowers, condensate and feed equipment, feed pumps, fuel, and condensers. On diesel and gas turbine propulsion plants the Second is in charge of auxiliary boilers, auxiliary engines, incinerator, air compressors, fuel, and fuel oil purifiers.