HAZMAT Classes 1–9 — International Maritime Dangerous Goods
The IMDG Code divides dangerous goods into nine classes. Know the class number, what it covers, and the segregation requirements — all are tested on the USCG exam.
Examples: Ammunition, fireworks, detonators
Requires segregation from all other classes. Master's special permission required.
Examples: LPG (flammable), CO₂ (non-flammable), chlorine (toxic)
Subdivisions: 2.1 flammable, 2.2 non-flammable, 2.3 toxic gas.
Examples: Gasoline, diesel, acetone, paints
Most common USCG exam HAZMAT class. Flash point below 60°C (140°F).
Examples: Matches, metal powders, wet cotton
Includes spontaneously combustible materials and dangerous-when-wet substances.
Examples: Hydrogen peroxide, potassium permanganate
Must be separated from flammables. Can cause spontaneous combustion in other cargo.
Examples: Pesticides, medical waste, biological samples
Includes 6.1 toxic substances and 6.2 infectious substances.
Examples: Medical isotopes, industrial radiography sources
Requires special permits and notification. Segregated from all personnel spaces.
Examples: Battery acid, hydrochloric acid, caustic soda
Frequently tested in context of segregation from foodstuffs and from other chemicals.
Examples: Dry ice, lithium batteries, magnetized materials
Catch-all class for goods presenting a hazard not covered by classes 1–8.
Dangerous Goods Documentation
Shipper's Declaration (DGD)
The Dangerous Goods Declaration (DGD) is prepared and signed by the shipper — not the carrier. It must accompany every shipment of dangerous goods.
- ▸Proper Shipping Name — full IMDG name, no trade names alone
- ▸UN Number — 4-digit ID prefixed UN (e.g., UN 1203 for gasoline)
- ▸IMDG Class — primary hazard class and any subsidiary risk
- ▸Packing Group — I (great danger), II (medium), III (minor danger)
- ▸Quantity and type of packaging
- ▸Emergency contact and 24-hour telephone number
- ▸Shipper's certification — signed declaration of accuracy
Cargo Manifest
The cargo manifest lists all cargo aboard. For dangerous goods, the manifest must identify each item, its location on the vessel, and the applicable IMDG class.
- ▸Must be available to master at all times
- ▸Copy provided to port authority on arrival
- ▸Under SOLAS, 72-hour pre-arrival notice required for dangerous goods
- ▸Copy kept ashore or available to shore personnel in emergencies
IMDG Segregation Requirements
Segregation prevents dangerous interactions between incompatible cargo. There are four segregation categories:
Away from
Keep the goods as far apart as practicable — minimum 3 meters horizontal or one deck vertical separation.
Separated from
Store in different compartments or hold spaces, or on different decks. No shared access way.
Separated by a complete compartment or hold from
Intervening space between the two goods must be a complete, enclosed, structural compartment.
Separated longitudinally by an intervening complete compartment or hold from
The most restrictive. The separating compartment must be fore-aft of each item, not athwartships.
HAZMAT Placarding
Packages carrying dangerous goods must display IMDG labels corresponding to the hazard class. Large freight containers must display placards (minimum 250mm × 250mm) on all four sides showing the class diamond and UN number. When dangerous goods are loaded in a hold or container, the vessel's cargo documents must reflect the placard information and stowage position.
Cargo Stowage & Plans
Stowage Plan (Bay Plan)
A stowage plan shows where every unit of cargo is positioned aboard the vessel. For container vessels, it uses a bay/row/tier numbering system. Required elements:
- ▸Container position: bay number (fore/aft), row (port/starboard), tier (height)
- ▸Weight of each unit and verified gross mass (VGM)
- ▸Dangerous goods — class, UN number, flash point if applicable
- ▸Reefer (refrigerated) units and temperature set points
- ▸Out-of-gauge cargo and oversized flat-racks
- ▸Stack weight limits and longitudinal stress limits must not be exceeded
General Stowage Principles
- ▸Heavy cargo low — lowers the center of gravity and improves metacentric height (GM)
- ▸Dense cargo inboard — prevents excessive list and free surface effect in wing tanks
- ▸Fragile cargo above — prevents crushing by stacking weight
- ▸Liquid cargo separated — from dry cargo to prevent contamination in case of leakage
- ▸Odorous cargo isolated — provisions and food cargo must not be adjacent to chemicals, paints, or animal products
- ▸Deck loading limits — never exceed the Allowable Deck Load (tons/ft² or t/m²) stamped on hatch covers and deck plates
- ▸Ventilation — cargo requiring ventilation (grain, green timber, coal) must be placed near ventilator openings; sealed cargo away from openings
Container Operations
Verified Gross Mass (VGM)
SOLAS amendment (effective 2016) requires shippers to provide verified gross mass of each container before loading. Two approved methods: (1) weigh the packed container; (2) weigh all contents and add tare weight. VGM discrepancies can cause dangerous weight distribution and stack collapses.
Lashing Plan
The Cargo Securing Manual (CSM) specifies the lashing arrangement for each position and tier. The lashing plan is calculated by the ship's officer or specialist software accounting for sea state, route, and stack weight. Lashings must be checked at sea after the vessel encounters its first heavy weather.
ISO Container Standards
Standard ISO containers: 20-foot (1 TEU) and 40-foot (2 TEU). Standard height 8'6"; high-cube 9'6". Corner castings must conform to ISO 1161. Max gross weight 30,480 kg (20-ft) and 32,500 kg (40-ft). Tare weight must be marked on the door.
Cargo Securing — Methods & Equipment
Inadequate cargo securing is a leading cause of cargo loss and vessel casualties. The Cargo Securing Manual (required under SOLAS Chapter VI) specifies approved methods for each vessel and cargo type.
Twist Locks & Cone Locks
Exam-TestedISO container corner castingsStandard interlocking fittings connecting containers to each other and to the ship's deck. Must be fully engaged and locked before departure.
Lashing Rods & Turnbuckles
Exam-TestedCross-lashing container stacksSteel rods with turnbuckles tensioned to prevent fore-aft and athwartship movement of upper tiers. Typically used on the 3rd and 4th tiers and above.
Wire & Chain Lashings
Heavy vehicles, project cargoSteel wire or chain secured to deck pad eyes or D-rings. Tightened with chain binders or lever hoists. Minimum 2 lashings per unit, typically 4.
Dunnage Bags (Airbags)
Break-bulk, palletized cargoInflated bags placed between cargo units to fill void spaces and prevent movement. Must be inflated to manufacturer's specification after loading.
Timber Wedges & Chocking
Cylindrical cargo, vehiclesWooden wedges prevent rolling of cylindrical tanks, pipes, and round objects. Must be nailed or otherwise fixed — not just placed.
Shore Bars & Stanchions
Bulk and break-bulk holdsTimber shores braced between cargo and bulkheads. Used to prevent large items from shifting under extreme sea conditions.
Dunnage — Uses and Materials
Purposes of Dunnage
- ▸Elevate cargo above bilge water and condensation
- ▸Prevent cargo from shifting under roll and pitch
- ▸Distribute point loads across deck structure
- ▸Prevent contact damage between adjacent units
- ▸Allow air circulation around cargo to reduce sweat damage
Common Dunnage Materials
- ▸Softwood boards and planks (most traditional)
- ▸Matting and burlap (moisture-absorbing layer)
- ▸Inflatable dunnage bags (fills voids in containers)
- ▸Polystyrene sheets (moisture barrier)
- ▸Rubber mats (non-slip under vehicles)
Bulk Cargo Considerations
Dry Bulk Cargo Hazards
- ▸Liquefaction — certain bulk cargoes (nickel ore, iron ore fines, coal) can liquefy when moisture content exceeds the Transportable Moisture Limit (TML), causing catastrophic stability loss
- ▸Self-heating / spontaneous combustion — coal, woodchips, and some ores can heat and ignite. Monitor hold temperature and CO levels.
- ▸Toxic gas emission — some cargoes (DRI, ammonium nitrate) emit toxic or asphyxiant gases. Confined space entry rules apply before accessing holds.
- ▸Shifting — high-angle-of-repose materials (grain, fine ore) can shift in rough weather, creating a dangerous list
Grain Cargo (SOLAS Chapter VI)
- ▸Grain has a tendency to shift — surface must be leveled or bagged
- ▸Document of Authorization required to carry grain in bulk
- ▸Grain loading booklet specifies maximum heeling moment allowed
- ▸Saucering or strapping required in partially-filled holds
- ▸GM must not be less than 0.30 m throughout the voyage after grain shift
IMSBC Code
The International Maritime Solid Bulk Cargoes (IMSBC) Code is mandatory under SOLAS for vessels carrying solid bulk cargo. It classifies bulk cargoes into three groups:
Group A
Cargoes that may liquefy — require moisture content testing before loading; Transportable Moisture Limit (TML) must not be exceeded.
Group B
Cargoes possessing chemical hazards — may emit toxic, flammable, or corrosive gases, or deplete oxygen; requires special precautions.
Group C
Cargoes that are neither liable to liquefy nor possess chemical hazards — generally the safest bulk cargo category.
Moisture Declarations
For Group A bulk cargoes, the shipper must provide a Moisture Content Certificate and a certificate that the moisture content does not exceed the TML. The master may refuse to load or may stop loading if the declared moisture content is at or near the TML, especially in rain. USCG exam questions may ask about the master's authority to refuse bulk cargo loading.
MARPOL — Pollution Prevention
MARPOL 73/78 (International Convention for the Prevention of Pollution from Ships) has six annexes. Annexes I, V, and VI are the most frequently tested on the USCG exam.
| Annex | Covers | Key Requirements | Exam |
|---|---|---|---|
| Annex I | Oil Pollution | 15 ppm discharge limit in special areas; Oil Record Book required; Oily Water Separator (OWS) required on 400 GT+ vessels | High |
| Annex II | Noxious Liquid Substances (NLS) | Controls bulk liquid chemicals; Cargo Record Book required; Pre-wash requirements before ballasting certain tanks | Low |
| Annex III | Harmful Packaged Substances | Controls harmful substances in packaged form; Ties into IMDG Code Class 9 and others | Low |
| Annex IV | Sewage | Prohibits discharge of untreated sewage within 3 nm of nearest land; Treated sewage: 12 nm | Low |
| Annex V | Garbage | Garbage Record Book required on 400 GT+ vessels; Plastics: never discharged; Food waste: 3–12 nm depending on special area; Garbage Management Plan required | High |
| Annex VI | Air Pollution | SOx limits: 0.5% global cap, 0.1% in Emission Control Areas (ECAs); NOx Tier standards; No ozone-depleting substances discharge | High |
Oil Record Book (ORB) — Annex I
- ▸Required on vessels 400 GT+ and oil tankers 150 GT+
- ▸Part I — Machinery space operations (bilge pump, sludge disposal, bunkering)
- ▸Part II — Cargo/ballast operations (tankers only)
- ▸Each entry: date, position, operation description, quantity, officer's signature
- ▸Master must countersign each page
- ▸Retained on board for 3 years
- ▸Available to USCG port state control inspection at any time
- !Maximum discharge: 15 parts per million (ppm) oil content in effluent
- !Zero discharge in Special Areas (Antarctic, Baltic, Red Sea, and others)
Garbage Record Book (GRB) — Annex V
- ▸Required on vessels 400 GT+ or certified to carry 15+ persons
- ▸Records every discharge, incineration, or accidental loss of garbage
- ▸Retained on board for 2 years
- !Plastics — NEVER discharged at sea — anywhere, in any amount
- ▸Food waste: >12 nm from land (3 nm in some areas if ground to <25mm)
- ▸Cargo residues (non-hazardous): >12 nm
- ▸Garbage Management Plan required — posted in crew areas
- ▸Placard (in vessel's working language) required in all spaces where garbage is generated
Cargo Calculations — Deadweight, Trim & Load Lines
Displacement
= Volume of hull below waterline × Seawater density (1.025 t/m³)Total weight of vessel and all contents — equal to weight of water displaced.
Deadweight (DWT)
= Load displacement − Lightship displacementMaximum total weight of cargo + fuel + water + stores + crew.
Available Cargo Capacity
= DWT − (Fuel + Water + Stores + Crew effects)Actual cargo space after subtracting all non-cargo weights.
Trim
= Aft Draft − Forward DraftPositive trim = trimmed by stern (normal). Negative trim = trimmed by head (avoid).
Mean Draft
= (Forward Draft + Aft Draft) ÷ 2Average draft used for stability and load line calculations.
Freeboard
= Freeboard Deck Height − Mean DraftDistance from waterline to freeboard deck. Load line marks define minimum freeboard.
Load Line Marks (Plimsoll Marks)
Load line marks define maximum allowable draft for different ocean zones and seasons. The appropriate mark must be at or above the waterline before departure. Overloading a vessel so that the appropriate load line is submerged is a violation of 46 USC and SOLAS.
| Mark | Label | Notes |
|---|---|---|
| TF | Tropical Fresh Water | Deepest allowed mark — warm fresh water is least dense; vessel floats lowest |
| F | Fresh Water | Fresh water has lower density than salt water — deeper draft allowed than 'T' |
| T | Tropical Salt Water | Tropical salt water — warm, slightly less dense than temperate seawater |
| S | Summer Salt Water | Summer salt water — baseline reference mark for all load line calculations |
| W | Winter Salt Water | Winter — vessel must have more freeboard (lighter) due to expected heavy weather |
| WNA | Winter North Atlantic | Shallowest (most conservative) — North Atlantic winter is the most severe ocean environment |
Exam Tips — What USCG Questions Focus On
HAZMAT Class 3 and Class 8
Flammable liquids (Class 3) and corrosives (Class 8) appear most often in USCG exam cargo questions. Know that Class 3 requires flash point below 60°C, and Class 8 must be stowed away from foodstuffs and personnel spaces.
15 ppm Oil Discharge Limit
The maximum oil content allowed in effluent discharged overboard is 15 ppm. This is tested directly and repeatedly. Zero discharge is required in MARPOL Special Areas. The Oily Water Separator (OWS) must be in operation and the Oil Record Book must be maintained.
Plastics: Zero Discharge
Plastics and materials containing plastic may never be discharged at sea regardless of distance from shore. This is the one absolute MARPOL V rule — all other garbage has mileage exceptions. Expect this on the exam.
Shipper Signs the DGD
The Dangerous Goods Declaration is prepared and signed by the shipper, not the carrier or master. However, the master is responsible for ensuring the DGD is on board and accurate before sailing. If the DGD is missing or incorrect, the master may refuse the cargo.
Trim by Stern is Correct
Exam questions on trim: a vessel trimmed by the stern (aft draft greater than forward draft) is normal and desirable. Trim by the head (bow deeper) is dangerous. If a trim question asks which condition is preferable, always choose trim by stern.
Oil Record Book: 3 Years
The ORB must be retained on board for 3 years. Garbage Record Book: 2 years. Both must be available to USCG inspection. Entries must be signed by the responsible officer and countersigned by the master.
Frequently Asked Questions
What are the nine HAZMAT classes and which are most commonly tested on the USCG exam?
The nine DOT/IMDG hazardous materials classes are: Class 1 – Explosives, Class 2 – Gases (flammable, non-flammable, toxic), Class 3 – Flammable Liquids, Class 4 – Flammable Solids / Spontaneously Combustible / Dangerous When Wet, Class 5 – Oxidizers and Organic Peroxides, Class 6 – Toxic and Infectious Substances, Class 7 – Radioactive Materials, Class 8 – Corrosives, Class 9 – Miscellaneous Dangerous Goods. The USCG exam most frequently tests Classes 3 (flammable liquids), 4 (flammable solids), and 8 (corrosives) in the context of cargo segregation and stowage requirements.
What documents are required when carrying hazardous materials by sea?
Hazardous materials carried by sea require: (1) a Dangerous Goods Declaration (shipper's declaration) signed by the shipper identifying the proper shipping name, UN number, IMDG class, packing group, and quantity; (2) a Cargo Manifest or Stowage Plan showing the location of all dangerous goods aboard; (3) an Emergency Response document (MFAG – Medical First Aid Guide for Use in Accidents Involving Dangerous Goods is often cross-referenced). Under 46 CFR 148 and 49 CFR, the master must ensure all required documents are on board before sailing.
What is the IMDG Code and why does it matter for the USCG exam?
The IMDG Code (International Maritime Dangerous Goods Code) is the international standard governing the stowage, handling, and documentation of dangerous goods aboard ships. It is incorporated into U.S. law through 49 CFR. The USCG exam tests knowledge of IMDG segregation requirements — e.g., Class 1 explosives must be separated from flammable liquids and corrosives by a minimum distance or a full compartment. Segregation categories are: 'Away from,' 'Separated from,' 'Separated by a complete compartment or hold from,' and 'Separated longitudinally by an intervening complete compartment or hold from.'
What is required in an Oil Record Book under MARPOL Annex I?
MARPOL Annex I requires every vessel of 400 GT or more, and every oil tanker of 150 GT or more, to maintain an Oil Record Book (ORB). Part I covers machinery space operations: ballasting and cleaning of fuel tanks, collection and disposal of oil residues (sludge), discharge of bilge water from machinery spaces, and bunkering. Part II (tankers only) covers cargo/ballast operations. Each entry must include the date, position of the vessel, and signature of the officer in charge. The ORB must be retained on board for three years and must be available for inspection by USCG officers at any time.
What MARPOL annexes cover the most common pollution prevention topics on the USCG exam?
MARPOL has six annexes: Annex I – Prevention of pollution by oil (oil record book, 15 ppm discharge limit); Annex II – Control of noxious liquid substances in bulk; Annex III – Prevention of pollution by harmful substances in packaged form; Annex IV – Prevention of pollution by sewage from ships; Annex V – Prevention of pollution by garbage from ships (Garbage Record Book required on vessels 400 GT+, or certified for 15+ persons); Annex VI – Prevention of air pollution from ships (SOx and NOx emission limits). The USCG exam most frequently tests Annex I (oil), Annex V (garbage), and Annex VI (air emissions).
What is dunnage and how is it used in cargo securing?
Dunnage is any material placed between cargo units to prevent movement, damage from sweat (condensation), or contact with the ship's structure. Common dunnage materials include wooden boards, matting, airbags, and plastic sheets. Dunnage serves three purposes: (1) protecting cargo from moisture and bilge water by elevating it off the deck; (2) preventing cargo units from sliding or shifting during rolling and pitching; and (3) distributing load evenly across the deck structure. On the USCG exam, questions on dunnage commonly appear in context of preventing cargo damage during heavy weather and protecting against moisture damage.
How do you calculate deadweight and determine if a vessel can safely load a cargo?
Deadweight Tonnage (DWT) is the total weight a vessel can carry, including cargo, fuel, crew, provisions, and water. Deadweight = Displacement at load draft minus Lightship displacement. To determine available cargo capacity: Available cargo = DWT minus (fuel + water + stores + crew and effects). Load line marks (Plimsoll marks) define the maximum draft for each zone and season. The USCG exam tests the relationship between displacement, freeboard, and load lines. A vessel is overloaded if the appropriate load line mark is submerged.
What is trim and how does improper cargo loading affect vessel stability?
Trim is the difference between a vessel's forward draft and aft draft (trim = aft draft minus forward draft). A vessel trimmed by the stern is preferred for most vessels as it improves propeller efficiency and maneuverability. Improper cargo loading causing excessive trim by the head (forward) reduces maneuverability, can submerge the bow in heavy weather, and raises the aft draft reducing propeller immersion. Improper transverse loading causes a list. On the USCG exam, trim calculation questions typically give you two drafts and ask for the trim value, or ask which spaces to load to correct an existing trim.
What is a cargo stowage plan and what must it include?
A cargo stowage plan (bay plan) is a diagram showing the location of every cargo unit aboard the vessel — required for container ships and vessels carrying dangerous goods. It must include: the position of each container or cargo unit (bay, row, tier), the weight of each unit, any dangerous goods and their IMDG class and UN number, refrigerated unit locations and temperature settings, and out-of-gauge cargo locations. For dangerous goods, the stowage plan must be available to the master at all times and provided to the terminal and port authority. Under SOLAS, vessels must also provide 72-hour advance notification of dangerous goods to port authorities.
What lashing and securing equipment is used for cargo and what do USCG exam questions test?
Common cargo securing equipment includes: twist locks and cone locks (ISO container corner fittings), lashing rods and turnbuckles, wire lashings and chain lashings, D-rings and pad eyes, dunnage bags, and timber wedges. The Cargo Securing Manual (CSM) — required by SOLAS on cargo ships — specifies approved securing methods for each type of cargo. USCG exam questions test whether lashings are appropriate for the sea state and voyage, whether the stowage weight does not exceed the deck's allowable load (deck loading limit, measured in tons per square foot), and whether containers are properly locked before departure.
Related Study Guides
Marine Pollution Guide
MARPOL in depth, OPA 90, spill response, and pollution penalty exposure for mariners.
Deck General Safety
Fire classes, PFDs, EPIRB, stability, and all distress equipment for the USCG exam.
Stability & Loading
Metacentric height, free surface effect, righting moment, and intact stability criteria.
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