A Second Assistant Engineer or Third Engineer is a licensed member

Licensed mariner

A licensed mariner is a person who holds a license issued by one or more countries to hold senior positions aboard ships, boats, and similar vessels. The United States Coast Guard grants licenses to members of the United States Merchant Marine in five categories: deck officers, engineers, staff...

 of the engineering department on a merchant vessel

Merchant vessel

A merchant vessel is a ship that transports cargo or passengers. The closely related term commercial vessel is defined by the United States Coast Guard as any vessel engaged in commercial trade or that carries passengers for hire...

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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.