A third mate (3/M) or third officer is a licensed member of the deck department of a merchant ship. The third mate is a watchstander and customarily the ship's safety officer and fourth-in-command (fifth on some ocean liners). The position is junior to a second mate. Other duties vary depending on the type of ship, its crewing, and other factors.
Duties related to the role of safety officer focus on responsibility for items such as firefighting equipment, lifeboats, and various other emergency systems.
Watchstanding
International Maritime Organization (IMO) regulations require the officer be fluent in the English language. This is required for a number of reasons. Examples include the ability to read charts and nautical publications, understand weather and safety messages, communicate with other ships and coast stations, and to successfully interact with a multi-lingual crew.
General watchstanding
Emergencies
Emergencies can happen at any time. The officer must be ready at all times to safeguard passengers and crew. After a collision or grounding, the mate must be able to take initial action, perform damage assessment and control, and understand the procedures for rescuing persons from the sea, assisting ships in distress, and responding to any emergency which may arise in port.
The officer must understand distress signals and know the IMO Merchant Ship Search and Rescue Manual.
Controlling ship operations
The officer has special responsibilities to keep the ship, the people on board and the environment safe. This includes keeping the ship seaworthy during fire and loss of stability, and providing aid and maintaining safety during man overboard, abandoning ship, and medical emergencies.
Understanding ship's stability, trim, stress, and the basics of ship's construction is a key to keeping a ship seaworthy. The mate must know what to do in cases of flooding and loss of buoyancy. Fire is also a constant concern. Knowing the classes and chemistry of fire, fire-fighting appliances and systems prepares the officer to act fast in case of fire.
An officer must be expert in the use of survival craft and rescue boats, their launching appliances and arrangements, and their equipment including radio life-saving appliances, satellite EPIRBs, SARTs, immersion suits and thermal protective aids. In case it is necessary to abandon ship, it is important to be expert in the techniques for survival at sea.
Officers are trained to perform medical tasks and to follow instructions given by radio or obtained from guides. This training includes what to do in case of common shipboard accidents and illnesses.
Sea watch
At sea, the mate on watch has three fundamental duties: to navigate the ship, to safely avoid traffic, and to respond to any emergencies that may arise. Mates generally stand watch with able seamen who act as helmsman and lookout. The helmsman executes turns and the lookout reports dangers such as approaching ships. These roles are often combined to a single helmsman/lookout and, under some circumstances, can be eliminated completely. The ability to smartly handle a ship is key to safe watchstanding. A ship's draught, trim, speed and under-keel clearance all affect its turning radius and stopping distance. Other factors include the effects of wind and current, squat, shallow water and similar effects. Ship handling is key when the need arises to rescue a person overboard, to anchor, or to moor the ship.
The officer must also be able to transmit and receive signals by Morse light and to use the International Code of Signals.
Navigation
Celestial, terrestrial, electronic, and coastal navigation techniques are used to fix a ship's position on a navigational chart. Accounting for effects of winds, tides, currents and estimated speed, the officer directs the helmsman to keep to track. The officer uses supplemental information from nautical publications, such as Sailing Directions, tide tables, Notices to Mariners, and radio navigational warnings to keep the ship clear of danger in transit.
Safety demands the mate be able to quickly solve steering control problems and to calibrate the system for optimum performance. Since magnetic and gyrocompasses show the course to steer, the officer must be able to determine and correct for compass errors.
Weather's profound effect on ships requires the officer be able to interpret and apply meteorological information from all available sources. This requires expertise in weather systems, reporting procedures and recording systems.