Maritime Dictionary – Everything that Starts with the Letter “O”

by | Last updated Nov 24, 2023 | Maritime Dictionary | 0 comments

O – Code flag; Man overboard. Morse Code; ▬ ▬ ▬.

O.B.O. Ship – A multipurpose ship that can carry Ore, heavy dry Bulk goods or Oil. They can make return journeys with cargo rather than empty as single-purpose ships often must. Related Article: Types of Merchant Ships – How Many Do You Know?

Oak – European timber, very durable for marine use due to high tannin and knurled grained, used for frames, chocks and knees. American variety is straighter grained and more easily split and not as durable.

Oakum – Rope that has been unlaid and the yarns teased out. Used for caulking seams, filling fenders, and other purposes.

Oar – Rowing implement made of wood and consisting of a blade, a long shaft and a loom that forms a hand-grip. Blade may be flat or curved.

Oar Swivel – Name once given to a ‘crutch’.

Oarlocks – Rowlock; a device for holding an oar in place when rowing or steering. Most oarlocks consist of a “socket” and a U-shaped or round “horn”. Traditionally other systems have been used. See Thole.

Oarweed – A large marine alga that grows along rocky shores.

Oasis – Fertile place in a desert.

Object Glass – That lens, in a telescope or binocular, at which light rays from an object enter the instrument.

Oblate Spheroid – Solid produced by semi-rotation of an ellipse about its minor axis. Sometimes applied to a spheroid that rotates on its minor axis—as do Earth and planets.

Oblique Sailing – Sailing obliquely to the meridian, so that latitude and longitude are changing.

Oblique Sphere – Terrestrial sphere as it appears to an observer who is not at equator or a pole. Apparent movement of all heavenly bodies are along circles oblique to horizon.

Obliquity of Ecliptic – Angle between plane of Equinoctial and plane of Ecliptic.

Observation – Measurement of altitude and bearing of a heavenly body. 2. Measurement of tidal phenomena.

Observed Altitude – Sextant altitude of a heavenly body when corrected for index error.

Observed Distance – Angular distance between two heavenly bodies when corrected for index error of sextant.

Observed Position – Position found by observing heavenly bodies.

Obstruction – In a chart, signifies something below chart datum that is a possible menace to navigation. An unspecified danger to navigation.

Occlude – To enclose; to shut up.

Occluded Front – When a cold front moving faster than a warm front overtakes it to form a combined front.

Occlusion – Line, on a meteorological chart, indicating the meeting of cold and warm fronts. Also the interposition of the cold front between the surface and the warm air mass under which it has intruded.

Occultation – Obscuration of a heavenly body by a nearer body passing between it and an observer. 2. The hiding of a light by the interposition of a screen.

Occulting – Said of light when its period of visibility is not less than its period of occultation.

Ocean – Originally a great river that was supposed to encircle the habitable world. Now applied to the great seas that extend between the continents. A large expanse of sea called the North Atlantic, South Atlantic, North Pacific, South Pacific, Indian, Arctic, or Southern Ocean.

Ocean Greyhound – A fast passenger liner.

Ocean Plait – An under and over plaiting of a single rope that is often used for making mats.

Oceania – The islands of the Pacific.

Oceanic – Of the ocean.

Oceanography – Study of oceans.

Oceanus – In ancient mythology was the god of the river Oceanus. He and his wife, Tethys, were parents of all rivers, and of 3000 nymphs (Oceanides).

Octant – Reflecting instrument with arc of 45°, and graduated to 90°.

Oertz Rudder – Streamlined rudder designed to reduce eddy currents from propeller streams and to eliminate dead water. Related Article: Types of Ship Rudders, Their Parts and Profiles.

Off and Fair – Order to take off a damaged member of a vessel, to restore it to its proper shape and condition, and to replace it in position.

Off and On – Said of a sailing along a coast when progress is made by alternately standing in towards the land and standing off it.

Off the Wind – Sailing with wind abaft the beam. Sometimes used to mean being further from wind than justifiable while sailing close-hauled. Said of cowl ventilator when back to wind.

Officer of the Watch – Officer in charge of a watch for the time being. Related Article: Want to Become an Officer? Bridge Standing Orders You Must Familiarize.

Official Log – an important legal document required by Merchant Shipping Laws and is a record of activities carried out on board. It is  issued to the Master of a ship when engaging crew. It entered with information required by Merchant Shipping Acts concerning such matters as draught of water on sailing, any casualty affecting the vessel, marriages, births, or deaths occurring on board, disciplinary action taken against offenders, particulars regarding boat and other drills, deck cargo carried, etc.

Offing – Sea area lying between visible horizon and a line midway between horizon and observer on the shore. To keep an offing is to keep a safe distance away from the coast. Distance to shore or offshore feature.

Offline – Distance from shore.

Offsets – Measurements supplied by a designer for the builder in order to lay down the lines of the hull.

Offshore Operations – For the purposes of NSCV Part B, operations within a limit of 200 nautical miles to seaward of the coast; or within such lesser limits as may be specified.

Offshore – The coastal waters seaward of inshore waters. See inshore.

Ogee – A timber moulding of composite concave and convex curves shaped like an S.

Ohm – Unit of electrical resistance. Equivalent to force of one ampere of current at a pressure of one volt. Standardised as resistance set up in a column of mercury, of uniform diameter, with temperature of 0°C, mass of 14.4521 grammes, length of 106.3 centimetres. Ohms = Volts ÷ Current.

Ohm’s Law – Links up relationship between electrical current, voltage, and resistance by formula E=CR when E is voltage, C is current in amperes, and R is resistance in ohms.

Oil Bag – Canvas bag containing oakum, or waste, saturated with oil. Veered, when required, for modifying storm waves.

Oil Fuel – Petroleum residue used in furnaces for raising steam in boilers. Flashpoint must be at least 150°F for marine purposes. Heat value is about 1.3 that of same weight of coal—but about 1.6 bulk for bulk.

Oil Tanker – Vessel specially constructed and fitted for carrying oil in bulk consisting of many tanks. Related Article: Oil Tanker Sizes – Panamax, Aframax,Suezmax, etc.

Oiling – Taking in oil fuel. Lubricating machinery.

Oilskins – Waterproof clothing made of calico and soaked with linseed oil, or covered with a bitumastic preparation.

Oily Record Book – A log where discharge or escape of oil is recorded.

Oily Water Separator – This is a filter unit which separates oil and water.

Okta – Unit equalling in area to 1/8 of sky, used in specifying cloud amount.

Old Horse – Salt beef.

Old Ice – Ice that was not melted during previous summer.

Old Man – Colloquial name for the master of a merchant vessel. The captain.

Oleron – Island at mouth of river Charente, France. Gave its name to the famous maritime ‘Laws of Oleron’.

Ombrometer – A rain gauge.

Omega – A global position-fixing system using Very Low Frequencies and comparing phases of signals (as in Decca) received from earth-based transmitters by reflection from satellites in fixed orbits.

On a Bowline – Said of a vessel when sailing close-hauled with bowlines taut.

On Report – Under a warning after being in trouble.

On the Wind – Close-hauled. Sailing into the wind.

One for Coming Up – Final pull on a rope to obtain rather more than is required, and so allow for slight loss when turning up

On-Scene Coordinator (OSC) – Is a person designated to coordinate search and rescue operations within a specified area.

On-Scene Endurance – The amount of time a facility may spend at the scene engaged in search and rescue activities.

Oomiaks – Sealskin canoes used by Eskimo.

Ooze – Deep-sea deposit composed of very fine grains of foraminifera, shell, diatoms, and other substances.

Open Bearings – Two bearings of the same object, one taken before running a known distance and the other after running it. Used in common pilotage for finding ship’s position at second bearing.

Open Boat – An undecked boat.

Open Circuit – Is an electric circuit that is broken by a broken component, disconnected wire, etc. Electric current will not flow in an open circuit. In the above image, the switch in the off position is an open circuit.

Open Cover – Development of ‘Floating Policy’. Has no limit to value that may be shipped. Premiums not payable until closings are made and policies are issued.

Open Hawse – Said of cables when riding by two cables, one on each bow, and each cable leads directly to its anchor.

Open Hawse to the Gale – Said of a vessel when moored open hawse with the wind right ahead.

Open Link – The unstudded link in each end of a shackle of cable. Not in modern ships.

Open Pack – Pack-ice in which there are some navigable lanes of water.

Open Policy – ‘Floating Policy.’

Open Roadstead – Roadstead that has good anchorage ground but is not sheltered from winds.

Open Sea – The sea when observer is in such a position that there is an uninterrupted sea horizon. 2. Sea in which there is no shoal water. 3. Sea outside territorial limits.

Open Water – Unobstructed water. 2. Navigable waterway through ice.

Open – Two marks that are not quite in transit.

Opening a Transit – Moving to one side of a transit line so that there is an angle between the objects previously in transit.

Operational Instructions – Involving strategic and operational planning and resource allocation decision-making.

Opposed Cylinder Engine – Internal combustion engine having cylinders on opposite sides of each crank. Impulses are applied at twice the usual rate.

Opposed Piston Engine – Internal combustion engine having two pistons in each cylinder. Charging, compression and ignition take place between the pistons, each of which operates its own connecting rod.

Opposition – Relative positions of two heavenly bodies, particularly solar system bodies, when their celestial longitudes differ by 180°, or their R. A.’s differ by 12 hours.

Optional Pilotage – Pilotage service that is available but is not compulsory for specified vessels.

Orbit – Path of planet around Sun, or of a satellite around its primary.

Ordnance – Unexploded bombs, artillery shells, mines etc.

Ordinary or Regular Lay – In this type of lay of rope or wire the strands of the rope are laid in the opposite direction as the yarns in the strand.

Ordinary Seaman – Seaman aged 18, or more, who has not qualified to be a rated Able Seaman. The initial grade of the deck crew.

Ordnance Datum – Level to which heights and depths are referred in English surveying. Formerly, the level was a rather inaccurate approximation of mean sea-level at Liverpool. Is now mean sea level at Newlyn, Cornwall.

Ore Carrier – A large ship for the carnage of ore. Because of the high density of ore, carriers have a high centre of gravity to prevent them rolling heavily with stress to the hull.

Oregon – American straight grained red softwood timber properly called Douglas Fir. Recently quick grown Oregon is found with large knots, rendering it structurally unreliable in thinner board thicknesses.

Ore-Oil Carrier – A vessel that carries either ore or oil in bulk.

Orient – The Far East. East point. To define a direction by its relation to the east point of compass—a relic of the days when the east point of compass was the prime cardinal point, and was marked by a fleur-de-lys.

Original Bill – The first of a set of bills of exchange to be presented. Prior to presentation, No. 1 is the original bill.

Orion – One of the most notable constellations. Lies between R. A.’s 5 h and 6 h and Dec. N10° to S10°. Name is that of a mythical giant hunter who was supposed to be the son of a peasant. Other myths make him a prince of Tanagra. As Lecky says, ‘Orion moves in the highest stellar society,’ being surrounded by important constellations and stars.

Orionids – Meteor shower through which Earth passes in autumn, and which appears to radiate from constellation Orion.

Orlop Beams – Beams supporting an orlop deck.

Orlop Deck – The lowest deck of a cargo ship.

Ormonde Beacon – Dan buoy used by surveying ships. Buoyancy is supplied by three 25-gallon oil-drums. Is ballasted with a 3-cwt. sinker, its bamboo mast carries a flag, 16 ft. by 10 ft.

Orographic – Pertaining to mountains.

Orographic Rain – Deposit of rain by a saturated air-stream coming into contact with rising land.

Oropesa Sweep – Towed wire used for sweeping sea bottom when surveying for shoals, or locating wrecks. Consists of two kite ‘otters’, one at each end of a 450-ft. sweep wire, and a third otter—close astern—to keep both sweep wires down.

Orrery – Machine that illustrates the relative movements of the solar system bodies.

Orthodrome – A great circle track.

Orthodromics – Science and methods of great circle sailing.

Orthodromy – Art of sailing on a great circle.

Orthographic Projection – Projection of surface of a sphere to a plane perpendicular to line of sight, all lines of sight being considered as parallel.

Osculating – Said of two arcs, or circles, when in contact but not intersecting.

Osmosis – Water penetrating the gel coat of Glass Reinforced Plastic and travelling along the fibres of glass. 2. A natural botanical process. 3. A process producing fresh water from sea water. Progressive degradation of fibreglass.

Ostro – Warm, humid and rain bearing southerly wind – one of the eight traditional traders winds of the Mediterranean Sea.

Otolith – Particles of calcium carbonate in vertebrates inner ears involved in sensing gravity and movement.

Otter – Plane surface towed forward of its middle length so that it will incline and dive. Used in fishing, mine-sweeping or surveying.

Out of Trim – Not on an even keel.

Outboard – Suspended or projecting outside a vessel. At or towards the side of a ship. A freestanding engine assemblage mounted on the transom. A position at the extreme seaward edge of a vessel.

Outboard Motor – Portable motor attached to the stem of a boat with a vertical shaft driving an immersed propeller.

Outer Jib – The fourth foresail set on a sailing ship inside the middle staysail.

Outer Turns – Turns of a seizing, or whipping, that override the inner turns. 2. Those turns of an earing that tend to stretch a sail.

Outfall – The seaward termination of a sewer pipe.

Outhaul – Any rope or purchase used for hauling outward. Specifically applied to clew of a gaff sail, and to outboard whip of union purchase. Any rope used to pull outwards. A rope or tackle used to stretch the clew of a sail.

Outlying – At a distance from the shore, or from a given position.

Outpoint – To sail closer to the wind than does a rival vessel.

Outport – Customs term for any U. K. port other than London.

Outrigger –. Spreader set up to windward of crosstrees or tops to give more spread to breast backstay. 2. Projecting bracket for carrying rowlock in a small skiff or racing boat. 3. Boat fitted with outriggers for stability.

Outsail – To sail faster than another ship, and leave her astern.

Outside Clinch – Turn of end of rope round its own part and with the end of rope outside the bight thus formed.

Outward Bound – Said of a vessel leaving a port in her own country. Said, relatively, of any vessel going to sea, even if homeward bound.

Outward Clearance – Formalities and procedure for obtaining ‘Clearance Outwards’.

Over Board – Over the side of a vessel and in the water.

Over Carriage – The transportation of cargo beyond its intended port of discharge.

Over Run – To go past or beyond an intended position or distance.

Over Tide – Delayed part of a tidal undulation that arrives after tide has begun to fall. May cause falling tide to rise or stop temporarily.

Over-all – A vessel’s extreme longitudinal measurement, with overhangs.

Overboard – Over and outside the sides of a ship. Overcast. State of sky when it is 7/ 8, or more, covered with cloud.

Overcasting Staff – Measuring scale used by shipwrights to determine the differences in timbers, or frames, from midships towards ends.

Overfalls – Violently disturbed water where a current sets over an irregularity in sea bed. Level tends to sink where bed rises, and to rise where bed sinks.

Overhand Knot – Simple knot made in end of a rope by taking end across rope and round it, and then through bight thus formed. Related Article: Can You Knot? 10 Widely Used Knots on Merchant Ships.

Overhang – Projection of upper part of stem abaft rudder-post. The distance that the bow and stern project beyond a vessel’s waterline.

Overhaul – To examine with a view to repairing or refitting. 2. To overtake another vessel. 3. To extend a tackle so that distance between blocks is increased.

Overhead – Underside of a deck that is above another deck.

Overlap – In yacht racing, exists when two yachts are on parallel courses and forward end of bowsprit of one yacht is forward of stern of another yacht.

Overlapping Rule – When two racing yachts are approaching a mark or obstruction, and an overlap has been established, the yacht further from the obstruction shall allow sufficient room for the nearer yacht to clear the obstruction.

Overload – To take excessive weight into a ship so that the appropriate load line becomes submerged.

Overlop – Old name for ‘Orlop Deck’.

Overset – Capsize.

Overshoot – To go ahead of an intended position or line.

Overtaking –. Coming up to another vessel from any point abaft the other vessel’s beam. In the Rules for Preventing Collisions at Sea an overtaking vessel is one that is coming up with another vessel from any direction more than two points abaft her beam.

Overtide – The duration of a tidal cycle.

Owner – Registered owner of ship.

Owner’s Liability – Statutory limit of liability of shipowner for losses due to the improper navigation of his vessel when occurring without his fault or privity. Limits are £15 per registered ton if there be loss of life, £8 per ton if no loss of life. Now calculated in SDR or Special Drawing Rights of the International Monetary Fund.

Ox Bow – Bend or reach in a river.

Ox Eye – Small cloud, or meteor, immediately preceding a violent storm off Cape of Good Hope. Mentioned by Falconer, but not by meteorologists.

Oxidation – The process in which a material combines with oxygen in the air, releasing heat.

Oxter Plate – Specially-shaped plate where side-plating ends in way of after deadwood. The plate in the curve between the shell plating and the top of the stern post.

Oxygen Deficient Atmosphere – An atmosphere with oxygen content below 19.5% by volume.

Oxygen – The second most abundant gas in air, (21%). Colourless and odourless.

Oya Siwo – Cold counter current setting S’ward along SE coast of Kamchatka and Kurile Islands, and along E coast of Yezo. Floating ice is carried between Kuriles and Yezo in early spring.

Oyashio (Kamchatka) Current – A cool North Pacific current setting southerly along the East Siberian and Northern Japanese coast.

Oyster Bed – On the sea bottom where oysters breed.

Oyster Tray – A man made stage where oysters are bred.

Ozone – Component atmospheric gas that limits solar radiation penetration.



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