USCG Merchant Mariner Credential (MMC) Exam — Questions and Answers

Question 1: Which of the following fittings has to be used when installing pipework so that the pump can be removed for maintenance?

Union (Correct answer)
Coupling
Nipple
Quick disconnect

Correct answer: Union

A union is a pipe fitting specifically designed to allow two pipes to be connected or disconnected without disturbing the rest of the piping system. This makes it ideal for installing components like pumps, valves, or filters that may need to be removed for maintenance or replacement. Unions consist of three parts that can be easily separated and reconnected, facilitating quick and efficient servicing.

Question 2: A vessel is considered to be 'not under command' when she is?

Anchored in a fairway
Operating in restricted visibility
Unable to maneuver as required by the rules due to exceptional circumstances (Correct answer)
Towing another vessel

Correct answer: Unable to maneuver as required by the rules due to exceptional circumstances

COLREGS Rule 3 defines a vessel not under command as one that through exceptional circumstances is unable to maneuver as required and cannot keep out of the way of another vessel.

Question 3: Which type of breathing apparatus is required for fighting fires in enclosed spaces?

A self-contained breathing apparatus (SCBA) (Correct answer)
An emergency escape breathing device (EEBD)
A smoke hood
A simple dust mask

Correct answer: A self-contained breathing apparatus (SCBA)

Self-contained breathing apparatus (SCBA) provides firefighters with their own air supply, enabling them to work in smoke-filled and oxygen-deficient spaces for extended periods.

Question 4: What is a 'Vessel Response Plan' (VRP) required to address?

Only fuel transfer procedures
Crew training schedules
Equipment maintenance logs
Procedures for responding to a worst-case oil spill scenario (Correct answer)

Correct answer: Procedures for responding to a worst-case oil spill scenario

A Vessel Response Plan must include procedures for responding to a worst-case oil discharge scenario, identifying response resources, contacts, and cleanup procedures required by OPA 90.

Question 5: Under the Inland Rules, a white light visible all around the horizon is required for which vessel?

A vessel less than 7 meters at anchor (Correct answer)
A sailing vessel underway
A power-driven vessel underway
A vessel under oars

Correct answer: A vessel less than 7 meters at anchor

Under Inland Rule 30, a vessel less than 7 meters at anchor may show an all-round white light instead of anchor lights.

Question 6: What lights must a vessel restricted in her ability to maneuver display?

Two red lights over one white light
Three red lights in a vertical line
Red, white, red lights in a vertical line (Correct answer)
Two red lights in a vertical line

Correct answer: Red, white, red lights in a vertical line

COLREGS Rule 27 requires a vessel restricted in her ability to maneuver to show three all-round lights — red over white over red — in a vertical line.

Question 7: In a refrigeration system, what is the role of the expansion valve?

Circulates refrigerant throughout the system
Reduces refrigerant pressure to cause it to cool and absorb heat (Correct answer)
Compresses the refrigerant to raise its pressure
Condenses refrigerant vapor into liquid

Correct answer: Reduces refrigerant pressure to cause it to cool and absorb heat

The expansion valve reduces refrigerant pressure, causing it to expand and evaporate in the evaporator, absorbing heat from the refrigerated space.

Question 8: Which light is required on the stern of a power-driven vessel underway?

A white sternlight (Correct answer)
A yellow light
A red light
An all-round white light

Correct answer: A white sternlight

COLREGS Rule 23 requires a power-driven vessel underway to exhibit a white sternlight showing an arc of 135 degrees.

Question 9: What is a 'fire party' on a vessel?

A designated team of trained crew members responsible for fighting fires (Correct answer)
A method of fire suppression
A group from the Coast Guard called in to fight fires
Crew members celebrating after a fire drill

Correct answer: A designated team of trained crew members responsible for fighting fires

A fire party is a designated group of trained crew members with assigned roles — hose team, boundary cooling, ventilation control — to respond to and combat fires aboard the vessel.

Question 10: Which signal word announces a safety message regarding navigational hazards or important meteorological warnings?

MAYDAY
DANGER
SECURITE (Correct answer)
PAN-PAN

Correct answer: SECURITE

SECURITE (pronounced 'say-cure-ee-tay,' spoken three times) is the safety signal used to precede announcements about navigational hazards, ice reports, or meteorological warnings.

Question 11: What is the purpose of a fixed fire suppression system in the engine room?

To flood the engine room with seawater
To sound the fire alarm
To ventilate the engine room during a fire
To automatically deliver suppression agent to control or extinguish a fire without crew entry (Correct answer)

Correct answer: To automatically deliver suppression agent to control or extinguish a fire without crew entry

Fixed engine room fire suppression systems (CO2 or clean agent) automatically discharge suppression agent into the protected space, controlling fires without requiring crew entry into a dangerous environment.

Question 12: When shall a vessel in a narrow channel keep to the starboard side?

At all times when safe and practicable (Correct answer)
Only at night
Only when another vessel is approaching
Only during restricted visibility

Correct answer: At all times when safe and practicable

COLREGS Rule 9 requires every vessel proceeding along a narrow channel to keep as near to the outer limit on its starboard side as is safe and practicable.

Question 13: What is the correct sequence for initiating a voice MAYDAY distress call?

MAYDAY three times, vessel name three times, MAYDAY once, position, nature of distress, persons aboard, other relevant information (Correct answer)
MAYDAY once, all vessel information, then wait for acknowledgment
MAYDAY twice, vessel name, position, type of assistance required
MAYDAY once, vessel name, position, nature of distress, persons aboard

Correct answer: MAYDAY three times, vessel name three times, MAYDAY once, position, nature of distress, persons aboard, other relevant information

The correct MAYDAY procedure begins with 'MAYDAY MAYDAY MAYDAY,' followed by the vessel's name three times, then 'MAYDAY' once more, then position, nature of distress, number of persons, and any other pertinent information.

Question 14: Small passenger ships must be built with or lined with ____ for paint and oil lockers.

steel or equivalent material (Correct answer)
sheetrock, asbestos, or other material that retards the spread of fire
marine plywood
fiberglass

Correct answer: steel or equivalent material

To enhance fire safety on small passenger ships, spaces containing highly flammable materials like paint and oil lockers must be constructed with fire-retardant materials. Steel or an equivalent fire-resistant material is mandated for bulkheads and linings in these areas. This prevents the rapid spread of fire and contains potential hazards, protecting passengers and crew.

Question 15: What does the acronym PASS stand for when using a portable fire extinguisher?

Push, Aim, Shoot, Spray
Pull, Aim, Squeeze, Sweep (Correct answer)
Pull, Activate, Squeeze, Sweep
Prepare, Aim, Squeeze, Spray

Correct answer: Pull, Aim, Squeeze, Sweep

PASS stands for Pull the pin, Aim at the base of the fire, Squeeze the handle, and Sweep side to side at the base of the flames.

Question 16: What is the watch-keeping requirement for VHF Channel 16 on GMDSS-equipped vessels while at sea?

Continuous watch while at sea (Correct answer)
Only when in port
At scheduled 4-hour intervals
Only during daylight hours

Correct answer: Continuous watch while at sea

GMDSS vessels must maintain continuous watch on VHF Channel 16 while at sea because it is the international distress and calling frequency.

Question 17: Sea fog most commonly forms when?

Air and sea temperatures are nearly equal and humidity is high (Correct answer)
Air temperature is much higher than sea surface temperature
Barometric pressure is falling rapidly
Strong winds mix warm and cold air layers

Correct answer: Air and sea temperatures are nearly equal and humidity is high

Sea fog forms when warm, humid air is chilled to its dew point upon contact with a cooler sea surface, which most often occurs when air and sea temperatures are close.

Question 18: Which vessel has the right of way when a sailing vessel and a power-driven vessel are crossing?

The larger vessel
The vessel on the starboard side
The power-driven vessel
The sailing vessel (Correct answer)

Correct answer: The sailing vessel

Under COLREGS Rule 18, a power-driven vessel underway shall keep out of the way of a sailing vessel, making sailing vessels the stand-on vessel in most crossing situations.

Question 19: What is the primary purpose of a ship's oily water separator (OWS)?

To remove water from fuel oil before combustion
To separate oil from bilge water before discharge overboard (Correct answer)
To remove impurities from lubricating oil
To filter seawater for the freshwater generator

Correct answer: To separate oil from bilge water before discharge overboard

An OWS removes oil from bilge water so the effluent meets the 15 ppm standard required by MARPOL Annex I before discharge.

Question 20: In the case of a flash type evaporator, a high reading at a salinity cell situated in the loop seal between two stages would suggest ____.

faulty operation of the brine overboard pump
leakage at the second-stage condenser
chill shocking is necessary to remove scale
carryover in the first-stage (Correct answer)

Correct answer: carryover in the first-stage

In a flash-type evaporator, the loop seal between stages is designed to prevent vapor from bypassing the intended flow path and to maintain pressure differentials. A high salinity reading in this loop seal suggests that brine (high-salinity water) is being carried over from the first stage into subsequent stages. This "carryover" indicates inefficient separation of vapor from the concentrated brine, leading to higher salinity in the distillate or downstream stages.

Question 21: Which cloud type is associated with thunderstorms and severe weather?

Altostratus
Cumulonimbus (Correct answer)
Stratus
Cirrus

Correct answer: Cumulonimbus

Cumulonimbus clouds are towering convective clouds that produce thunderstorms, heavy rain, hail, lightning, and sometimes tornadoes.

Question 22: What is meant by 'boundary cooling' during a shipboard fire?

Cooling the fire directly with water fog
Using CO2 to cool the fire boundary
Applying water to bulkheads and decks surrounding the fire to prevent spread (Correct answer)
Removing fuel from around the fire

Correct answer: Applying water to bulkheads and decks surrounding the fire to prevent spread

Boundary cooling involves applying water to the metal structures surrounding a fire to prevent heat transfer that could ignite adjacent compartments or spaces.

Question 23: CO2 fire extinguishers are most effective in which environment?

Open deck fires
Forest fires
Fuel tank fires
Enclosed spaces where the CO2 concentration can build up (Correct answer)

Correct answer: Enclosed spaces where the CO2 concentration can build up

CO2 extinguishers work by displacing oxygen and are most effective in enclosed spaces where the gas concentration can reach levels sufficient to suppress the fire.

Question 24: What is the primary purpose of the Oil Record Book?

To log voyage distances
To document all oil transfer and discharge operations for USCG inspection (Correct answer)
To track fuel purchases
To record engine maintenance

Correct answer: To document all oil transfer and discharge operations for USCG inspection

The Oil Record Book is a legal document required by MARPOL and U.S. law to record all oil transfers, bilge discharges, and oily water separator operations, subject to USCG inspection.

Question 25: In a ship's electrical system, what is the purpose of a bus bar?

To isolate faulty circuits from the system
To step up voltage from generators
To serve as a common conductor connecting multiple circuits (Correct answer)
To supply power to the propulsion shaft

Correct answer: To serve as a common conductor connecting multiple circuits

A bus bar is a metallic conductor that acts as a common connection point distributing electrical power from generators to various shipboard circuits.

Question 26: What does GMDSS weather broadcast format NAVTEX provide to mariners?

Voice weather broadcasts
Printed text weather forecasts and navigational warnings (Correct answer)
Satellite cloud imagery
Real-time radar imagery

Correct answer: Printed text weather forecasts and navigational warnings

NAVTEX broadcasts meteorological warnings, navigational warnings, and search-and-rescue information as printed text via a dedicated 518 kHz frequency.

Question 27: What is a 'small craft advisory' issued for?

Winds above 64 knots
Winds 15–33 knots or seas 5–7 feet (Correct answer)
Winds below 10 knots
Winds 34–47 knots or seas 8–12 feet

Correct answer: Winds 15–33 knots or seas 5–7 feet

The National Weather Service issues a small craft advisory when winds are forecast between 15 and 33 knots or seas are 5 to 7 feet, posing hazards to small vessels.

Question 28: The normal usage of the emergency bilge suction valve is ____.

to connect the rose box to the independent bilge suction
to inject cleaning additives when the bilges are extremely dirty
when the main condenser becomes fouled, in order to provide additional cooling water circulation
if the bilges become flooded and they can not be emptied by any other means (Correct answer)

Correct answer: if the bilges become flooded and they can not be emptied by any other means

The emergency bilge suction valve is a critical safety feature designed to provide a last resort for dewatering the vessel's bilges. It is typically connected to the main circulating pump or another high-capacity pump, allowing rapid removal of large volumes of water in severe flooding situations when the regular bilge pumps are overwhelmed or inoperable. Its use is reserved for emergencies to prevent the vessel from sinking.

Question 29: What is the recommended immediate action when a person falls overboard?

Deploy the life raft first
Contact the Coast Guard before maneuvering
Throw a life ring, shout 'man overboard,' alert the crew, and keep the person in sight (Correct answer)
Sound five short blasts and stop the engines

Correct answer: Throw a life ring, shout 'man overboard,' alert the crew, and keep the person in sight

Immediately throw a life ring toward the person, shout 'man overboard' to alert the crew, assign a lookout to keep continuous visual contact, and maneuver the vessel for recovery.

Question 30: What is the primary purpose of a Traffic Separation Scheme (TSS)?

To designate anchorage areas
To mark submerged hazards
To organize vessel traffic flow and reduce collision risk (Correct answer)
To allocate fishing zones

Correct answer: To organize vessel traffic flow and reduce collision risk

Traffic Separation Schemes, established under COLREGS Rule 10, organize opposing traffic flow into separate lanes to reduce the risk of collision.

Question 31: Wind speed is measured using which instrument?

Barometer
Hygrometer
Psychrometer
Anemometer (Correct answer)

Correct answer: Anemometer

An anemometer measures wind speed; the readings are often combined with a wind vane to provide both speed and direction.

Question 32: When two power-driven vessels are meeting head-on, both vessels are required to do what?

Sound five short blasts and stop
Alter course to port
Maintain course and speed
Alter course to starboard (Correct answer)

Correct answer: Alter course to starboard

Under COLREGS Rule 14, when two power-driven vessels meet head-on, each shall alter course to starboard so they pass port-to-port.

USCG Merchant Mariner Credential (MMC) Exam

The USCG National Maritime Center (NMC) Merchant Mariner Credential exam tests deck officer competencies including navigation, cargo operations, vessel safety, fire prevention, marine environmental protection, and meteorology required for U.S. merchant mariner licensing.

100

Questions

120 min

Time Limit

70%

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These practice questions were written with AI tools. They are not real exam questions. Exam facts are checked against the official sources listed below. Not reviewed by a subject-matter expert. Editorial policy

Sources & references

Exam facts on this page (fees, format, dates, passing scores) are checked against these official sources; the date is our last check. Editorial policy

dco.uscg.mil — Test delivery, registration deadline, passing score, exam information · verified Oct 2026
ecfr.gov — Eligibility, score validity, retake policy, test dates, renewal, fees · verified Oct 2026