WACE Marine Maritime Studies ATAR key terms

The vertical distance from the waterline to the upper deck edge at the side of a vessel. It indicates reserve buoyancy and helps prevent waves from boarding.

The volume of enclosed watertight space above the waterline that can be flooded before the vessel sinks. It provides safety margin when loading or in rough seas.

The distance between the centre of gravity and the metacenter. A positive GM indicates initial stability; too large a GM causes rapid, uncomfortable rolling.

The torque that tends to return a heeled vessel upright, equal to the vessel's displacement times the horizontal distance between the centre of gravity and the centre of buoyancy.

The geometric centre of the underwater volume of a hull. It moves as the vessel heels or changes trim, affecting stability calculations.

A mark on the hull showing the maximum permitted draught under different conditions. It ensures minimum freeboard and reserve buoyancy are maintained.

The weight of water displaced by a floating vessel, equal to the vessel's total weight. It is usually expressed in tonnes and determines draft and stability.

The total weight a vessel can carry, including cargo, fuel, fresh water, stores and crew. It is the difference between loaded and light displacement.

The ability of a vessel's hull, decks and closures to prevent water ingress. It is maintained through watertight doors, hatches and proper sealing.

The maximum efficient speed of a displacement hull, approximately 1.34 times the square root of the waterline length in feet. Beyond this, wave-making resistance rises sharply.

A hull shape designed to rise and skim over the water surface at speed, reducing wetted area and drag. It requires sufficient power to reach planing speed.

The difference between the theoretical distance a propeller would advance in one revolution and the actual distance travelled. It is expressed as a percentage.

The theoretical forward distance a propeller would move in one revolution if it turned in a solid medium. It is a key specification for matching propeller to engine.

The formation and collapse of vapour bubbles on a propeller blade due to low pressure. It causes noise, vibration and erosion of blade surfaces.

An engine cycle with four piston strokes: intake, compression, power and exhaust. It requires two crankshaft revolutions to complete one cycle.

An engine cycle completing a power stroke every crankshaft revolution, using ports rather than valves. It is simpler but less fuel-efficient and more polluting.

The ratio of cylinder volume before compression to volume after compression. Higher ratios improve efficiency but may cause knocking in petrol engines.

A device that converts mechanical energy from the engine into alternating current for battery charging and electrical loads. It is belt-driven and regulated.

A manually operated switch that disconnects the battery from the electrical system. It is used for safety, maintenance and to prevent parasitic drain.

Electrical supply taken from a marina or dock through a cable and inlet. It requires correct polarity, circuit protection and a galvanic isolator.

The reference level from which depths on a nautical chart are measured, usually lowest astronomical tide. Charted depths are therefore conservative.

The vertical distance of the sea surface above chart datum at a given time and place. It is predicted using tide tables and adjusted for atmospheric effects.

The vertical distance between the deepest part of a vessel and the seabed. It is calculated from charted depth, tidal height and vessel draught.

A navigation method estimating position by advancing a known position using course and speed through the water, without external position fixes.

A confirmed position obtained from external references such as GPS, visual bearings or celestial sights. It is used to update the dead reckoning position.

The actual direction of a vessel's movement over the seabed, combining heading and the effect of current and leeway. It differs from heading.

A numerical scale from 0 to 12 describing wind strength based on observed sea conditions. It is used in weather reports and forecasts.

A line on a weather chart joining points of equal atmospheric pressure. Closely spaced isobars indicate strong winds.

The leading edge of a cooler air mass advancing under warmer air. It often brings squally winds, heavy rain and a sharp temperature drop.

A documented plan for a voyage including route, waypoints, hazards, contingency anchorages and emergency procedures. It is prepared before departure and updated en route.

A geographical position on a route, defined by latitude and longitude, that a vessel intends to pass through or turn at during a passage.

The international distress signal used when a vessel or person is in grave and imminent danger and requires immediate assistance. It has priority over all other communications.

The international urgency signal used when a vessel or person is in difficulty but not in immediate danger. It requests assistance without distress priority.

Emergency Position Indicating Radio Beacon. It transmits a distress alert via satellite to rescue authorities, providing identity and position when activated.

A personal flotation device worn to keep a person's head above water and support them in a survival situation. It must be correctly fitted and maintained.

A protective garment that reduces heat loss and keeps a person dry in cold water. It is worn during abandonment or rescue operations in cold conditions.

An emergency situation in which a person falls from a vessel into the water. It requires immediate action including marking the position and manoeuvring for recovery.

The International Regulations for Preventing Collisions at Sea. They set rules for conduct, lights, shapes and sound signals to prevent collisions.

In a crossing or overtaking situation, the vessel required to take early and substantial action to keep clear of the other vessel under the collision regulations.

The vessel that must maintain its course and speed in a crossing situation, while remaining ready to take action if the give-way vessel does not comply.