Croatia Master Mariner key terms

The actual direction of a vessel's movement over the seabed, measured from true north, derived from successive position fixes and affected by current and leeway rather than by heading alone.

The rate at which a vessel moves relative to the seabed, obtained from GPS or from plotting successive fixes, as distinct from speed through the water shown by the log.

The calculation of an approximate position by advancing a known starting point using courses steered and distances run through the water, without reference to external position fixes.

The most probable position of a vessel, found by applying estimated current and leeway to a dead reckoning position, and conventionally marked with a triangle on the chart.

The lateral drift of a vessel downwind of her heading, caused by wind acting on hull and superstructure, which must be allowed for when plotting a course to be made good.

The vertical distance between the lowest point of a vessel's hull and the seabed, monitored through echo sounder and charted depth plus predicted tide and squat.

The reduction in under keel clearance experienced by a vessel moving through water, caused by lowered pressure around the hull; it increases with speed and is greater in shallow or confined water.

The horizontal reference level from which depths and drying heights on a nautical chart are measured, normally set near the lowest astronomical tide so that actual depths are usually greater than charted.

The predicted or observed vertical rise of the sea surface above chart datum at a given place and time, obtained from tide tables, tide gauges or tidal prediction software.

An abnormal rise of sea level above the predicted astronomical tide, generated by wind stress and low atmospheric pressure in a storm, which can flood harbours and reduce charted depths.

A descriptive scale from 0 to 12 relating wind force to observable sea and wave conditions, used in weather reporting and in judging wind effect on vessel handling and cargo operations.

A description of the roughness of the sea surface, usually by significant wave height and the WMO sea state code, used to assess safety of cargo work, launching of boats and passage planning.

The average height of the highest one third of waves in a sea area, used in forecasts and ship design because it approximates the height an experienced observer would report.

The ratio of a vessel's underwater volume to the volume of a rectangular block of her length, breadth and draught; a measure of hull fullness affecting resistance, squat and stability.

The vertical distance between the centre of gravity and the transverse metacentre; positive GM indicates initial stability, and its value governs the period of roll and the comfort of the ship.

The horizontal distance between the centre of gravity and the vertical line through the centre of buoyancy at a given angle of heel, representing the vessel's ability to return upright.

The angle to which a vessel with negative initial stability lists and then rests, where the righting lever is zero; dangerous because the ship may flop violently from one side to the other.

The apparent rise of the centre of gravity caused by liquid moving freely in a partially filled tank, reducing metacentric height and stability; minimised by pressing tanks up or keeping them empty.

The mark on a ship's side showing the maximum permitted draught in different seasons and water densities, assigned under the International Convention on Load Lines and verified by survey.

The vertical distance from the waterline to the freeboard deck at side; it provides reserve buoyancy and is controlled by the assigned load lines.

The volume of watertight hull above the waterline that can be flooded before the vessel sinks, giving the ship capacity to survive damage and heavy weather.

The study of a vessel's residual stability and floatation after flooding of one or more compartments, assessed against SOLAS subdivision and survivability criteria for the ship type.

The condition of hull, bulkheads, closures and penetrations being capable of preventing water ingress, maintained through inspection, testing and correct operation of doors and hatches.

The total mass a ship can carry when loaded to her permitted draught, comprising cargo, fuel, fresh water, ballast, stores and crew; the difference between loaded and light displacement.

The volume occupied by unit mass of a cargo, usually expressed in cubic metres per tonne, used to decide whether a cargo will fill a hold before the ship reaches her deadweight limit.

The point at which the total mass of a stow can be considered concentrated; its longitudinal and vertical position determines trim and stability after loading.

The difference between forward and aft draughts, expressing the vessel's longitudinal attitude; positive by the stern, and controlled by distributing masses along the ship's length.

The separation of incompatible hazardous cargoes by distance, deck or compartment according to the IMDG Code segregation table, to prevent dangerous reactions in stow.

The shipper's document giving the correct technical name, UN number, class, packing group and emergency information for a consignment of dangerous goods, required before loading.

A document describing the hazards, handling, first aid and emergency measures for a chemical substance, required under MARPOL Annex II procedures for noxious liquid cargoes.

The mandatory washing of a cargo tank with water before the residue is discharged, required under MARPOL Annex II for certain categories of noxious liquid substances.

A statutory record, kept in the prescribed format under MARPOL Annex I, of all oil transfer, ballasting, cleaning and disposal operations carried out on board, subject to inspection.

The inspection of foreign ships in national ports to verify compliance with international conventions, with detention possible where deficiencies show the ship is not fit to proceed to sea.

The structured system of procedures, responsibilities and records required by the ISM Code to ensure safe operation of ships and pollution prevention.

A formal written authorisation for a high-risk task such as hot work or entry into a confined space, defining the precautions, isolations and supervision required before work starts.

Entry into a tank, hold or void that is not designed for continuous occupancy, permitted only after atmosphere testing, ventilation, a standby person and an authorised entry permit.

The posted schedule assigning each crew member emergency duties and an assembly station, required under SOLAS and exercised through regular abandon ship and fire drills.

A float-free satellite transmitter that activates automatically on immersion or manually, broadcasting distress identification on 406 MHz to the Cospas-Sarsat system.

The SOLAS-mandated framework of satellite and terrestrial communications, alarms and procedures ensuring that distress alerts can be transmitted and received anywhere at sea.

A VHF, MF and HF radiotelephone technique using coded digital signals to alert specific stations or all stations, and to relay distress alerts with identification data.