M – Code flag; My vessel is stopped and making no way. Sound signal; My vessel is stopped and making no way, in restricted visibility. Morse Code;  ▬ ▬.

M.T. – Motor Tanker.

M.V. – Motor Vessel.

Mac Ships – Merchant vessels converted into aircraft carriers during World War II. Name derived from initials of ‘Merchant Aircraft Carrier’.

Mackerel Sky – Cloud form in which cirrocumulus and alto-cumulus are arranged in bands in a blue sky.

Mackerel – Brightly coloured and oily schooling fish.

Madden-Julian Oscillation (MJO) – is a 30-50 day wave of periodic rainfall enhancement over the Australian tropics signalling a burst in monsoon activity during the tropical wet season.

Made Bill – Bill of exchange drawn and endorsed in United Kingdom and payable abroad. Specifies name of person entitled to payment.

Made Block – Built block. Shell is made up of parts.

Made Eye – ‘Flemish Eye.’

Made Fast – Secured.

Made Mast – Built mast, as differentiated from mast made from a single tree.

Madrepore – A genus of common perforated coral.

Maestro – NW wind in Adriatic and Ionian seas and off coasts of Sardinia and Corsica.

Magazine – Compartment or structure in which explosives are kept. A munitions storage compartment. Name is given also to a warehouse.

Magellanic Clouds – Three masses of whitish nebulae near south pole of celestial concave.

Magnet – Body or substance having the property of attracting iron and steel. Usually iron or steel, but may be cobalt or nickel. Can have natural magnetism or electrically-induced magnetism.

Magnet Steels – Steels capable of being permanently magnetised. Usually alloyed with cobalt or tungsten.

Magnetic Amplitude – Angle of bearing measured from east or west point of a compass whose needle lies in magnetic meridian.

Magnetic Anomaly – Localised unusual magnetic error.

Magnetic Azimuth – Angle east or west of magnetic meridian and expressed in degrees.

Magnetic Bearing – Bearing of an object when referred to a compass whose needle lies in magnetic meridian.

Magnetic Compass – Compass whose directional property is due to its needles seeking the magnetic meridian.

Magnetic Compensation – Introduction of soft iron or magnets in the vicinity of a compass so that magnetic effects of iron or steel in ship or cargo can be neutralised or reduced.

Magnetic Dip – The tendency for a compass needle to dip vertically as it points directly at the distant pole rather than horizontally at a tangent to the earth’s circumference. See angle of dip.

Magnetic Equator – Line, on surface of Earth, passing through all positions at which there is no dip of a freely-suspended magnetic needle. It forms a dividing line between Earth’s red and blue magnetisms.

Magnetic Field – Area, around a magnet, in which its magnetism is manifest. Magnetic Meridian. Line of magnetic force, between North and South magnetic poles, in which a freely-pivoted magnetic needle would lie if all disturbing forces were removed.

magnetic force is at a maximum.

Magnetic Needle – Thin strip, needle, ring or cylinder of magnetised steel so pivoted that it is free to respond to Earth’s magnetic field and seek and lie in the magnetic meridian.

Magnetic Poles – Of Earth: areas in high latitudes North and South at which a freely-suspended magnetic needle would remain vertical. One area is near Hudson Bay, the other in South Victoria Land. Each area is about 50 square miles in extent. 2. Of magnet: are those points, near its ends, at which its

Magnetic Storm – Disturbance of Earth’s magnetic field lasting for a period that may be measured in hours or days. Is world wide in effect and possibly due to sunspots.

Magnetic Variation – Horizontal angle, at a given place, between the magnetic and geographic meridians. Is the angle that a freely suspended and undisturbed magnetic needle would make with the geographic meridian at the place.

Magnetism – Property which can be imparted to certain substances, so causing them to attract or repel other magnetised substances and to exert a force on conductors of electrical current in their vicinity. Ferrous metals, cobalt and nickel can receive magnetism. 2. The attractive or repulsive property of a magnet.

Magneto – An electro-mechanical device that produces high tension from revolving coils in the field of a powerful magnet, used to power the spark plugs in a simple petrol internal combustion engine.

Magnetometer – Instrument for measuring intensity of Earth’s magnetic force at any place.

Magneto-Striction – Contraction of a magnetic metal when subjected to magnetism. Particularly noticeable in nickel; this property being utilised in supersonic sounding apparatus.

Magnetron – In a radar it acts as an electronic valve, using a powerful permanent magnet to produce pulses of microwaves. The transmitter of a radar set.

Magnitude – The brightness of a celestial body- lowest numbers are brightest.

Magnitude of Star – Its comparative brightness, as compared with other stars. May be ‘relative Magnitude’ or ‘Absolute Magnitude’.

Mahogany – Generic term for many varieties of South American decorative timbers with good workability and marine durability. See Brazilian mahogany.

Maiden Voyage – First voyage made by a new ship after all trials have been carried out and she has been taken into service.

Maierform, Maier Form – Hull design that allows displaced water to flow aft by shortest route, reduces wave making to a minimum and keeps wetted area to small proportions. Forward and after deadwoods are largely removed. Pitching is reduced and directional stability is good.

Mail – Letters, posted papers and parcels shipped for conveyance. 2. Interwoven metal rings fastened to a stout backing material and used for smoothing the surface of newly-made ropes.

Mail Boat – Vessel carrying mails.

Mail Room – Compartment in which mail is carried in a ship.

Mail Steamer – Steamer carrying mail.

Main – Ocean, or open sea. 2. Principal. 3. Main mast. 4. Mainsail.

Main and Foresail Rig – Has a fore staysail and a fore and aft mainsail that is loose-footed or stretched along a boom.

Main and Mizen Rig – Small boat rig having two masts, each carrying a foursided fore and aft sail, main being larger than mizen.

Main Beam – The main longitudinal deck beam on a ship..

Main Body – The bare hull of a vessel.

Main Boom – Spar to which foot of a fore and aft mainsail is extended.

Main Breadth – Greatest distance between any two opposite frames.

Main Course – Sail attached to main yard of a square-rigged vessel.

Main Deck – Principal deck of a vessel. Next below upper deck in five-deck ships.

Main Halyard – Rope by which a mainsail is hoisted.

Main Hatch – Principal hatch, usually the largest.

Main Hold – Space entered through main hatch. Principal hold.

Main Mast – The principal mast of a vessel.

Main Pendants – Two short pieces of strong rope having thimbles in lower ends. Secured under upper ends of shrouds of mainmast to take hooks or shackles of main tackles.

Main Piece – Of rudder, is the vertical piece to which the steering-gear is attached. Of a wooden ship, is a piece stepped into stem head and notched for heel of bobstay piece.

Main Post – ‘Stern Post.’

Main Rigging – Shrouds and ratlines on main lower mast.

Main Sail – In all vessels is applied to the principal sail. The lowest square sail set on the mainmast of a sailing ship.

Main Saloon – The principal living area of a vessel.

Main Sheet – The rope controlling the main boom.

Main Tack – Rope or purchase by which weather clew of mainsail is hauled out and down.

Main Tackle – Purchase attached to main pendant when setting up main stays.

Main Top – Platform at head of mainmast. 2. Division of the watch in R. N.

Main Yard – Lower yard of a mainmast.

Mainmast – Principal mast. Second mast in vessels having two or more masts, except when second mast is smaller—in which case forward mast is mainmast, after mast is mizen.

Mainsail Haul’ – Order given when tacking a square-rigged vessel. As ship’s head comes to wind the main yard and all after yards are braced for new tack— yards on foremast are then aback to assist ship’s head to pay off.

Mainsail, Main Sail – Principal sail. In fore- and aft-rigged ships is sail on after side of mainmast; in square-rigged vessel is sail bent to main yard. ‘

Mainsheet Horse – An arched rod that carries the lower mainsheet block clear of the deck and steering gear, allowing it to slide from windward to leeward when tacking.

Make and Mend – Afternoon watch allotted to R. N. seaman for making and mending clothes. Afternoon watch in which no work is done.

Make Colours – Hoisting the ensign at sunrise.

Make Fast – To secure a line.

Make For – To head in the direction of.

Make Land – To reach a landfall.

Make Sail – To set sail, or to increase the sail already set.

Make the course good – To steer accurately. Related Article: 7 Tips on How to Steer a Ship and Become a Better Helmsman.

Make the Land – Steer towards land so as to bring it into sight.

Make Water – To take in water through a leak.

Making – Said of tide when it is increasing from neap to spring tide.

Making Her Number – Said of a vessel when indicating her name by signal.

Making Iron – Rather large caulking iron used for final hardening up of oakum when caulking a seam.

Making Way – Moving ahead or astern through the water. See ‘Under Way’.

Malacca Max – Maximum draught of cargo vessel that can transit the Malacca Straits.

Malinger – To feign illness to avoid work.

Mallet – Small  wooden hammer.

Mallock’s Rolling Indicator – Instrument for indicating amplitude of roll of a vessel. Consists of a circular box in which a paddle-wheel is mounted in jewelled bearings and immersed in liquid, on a vertical bulkhead. As ship rolls, paddle-wheel maintains an attached and graduated scale in true horizontal. A vertical line on glass face of box indicates amount of roll.

Malone CO2 Indicator – Instrument that measures CO2 content of air, as a percentage, by automatically weighing unit volume of air against unit volume of air mixed with CO2.

Mammal – A warm-blooded vertebrates producing milk to feed their young.

Mammatocumulus – Cumulus cloud with udder-shaped projections downward from lower edge.

Mammatus – Udder-shaped cloud form.

Man – To provide with men, or manpower.

‘Man Overboard’  Call and report given when a person has been seen to fall into the sea from a vessel.

Man Ship – Naval ceremony indicating compliment and, formerly, peaceful intentions. Crew are spaced at regular intervals along sides of upper decks. 2. To provide a crew for a vessel.

Man Yards – Ceremonial disposition of crew of sailing ship. Men are placed at intervals on all yards, standing on the yards and being steadied by jackstays stretched from mast to lift of yard. Discontinued by R. N. in 1902, by order of King Edward VII.

Managing Owner – Shipowner who actively controls the commercial affairs of his ship or ships.

Manavalins – Seaman’s word for ‘odds and ends’.

Manganese – Black oxide of manganese sometimes found on seabed when sounding.

Manganese Bronze – Copper and zinc alloy with traces of aluminium and manganese.

Manger – Space, on cable deck, between hawse pipes and thwart-ship breakwater in vessel where cable deck is below forecastle deck. The perforated bottom of the chain locker allowing water to flow to the drains.

Mangrove – Tropical tree, or shrub, that covers large areas of coast in tropics. Seed germinates on trees and send roots down to the water. Wood is straightgrained, elastic and hard; often used for boat and shipbuilding.

Man-Harness Hitch – Put in a rope when dragging or towing by manpower. Large ‘half crown’ is made in rope and laid across it; bight on one side of rope is taken under the rope and through the other bight.

Manhelper – Paint brush fastened to a long wooden pole.

Manhole – An opening for a man to gain entry into a tank.

Manifest – Document given to Master when cargo is shipped. Contains particulars of cargo, shipper’s name, marks and numbers, quantities, where loaded, Master’s name, ship’s name, tonnage and port of registry. A document listing the ship’s cargo using the bills of lading.

Manifold – Group of valves for pump suctions and deliveries. Small compartment in which such valves are placed.

Manifold – Manifold (bilge). A valve chest connecting suction or discharge pipes to/from spaces and pumps to enable fluid transfers.

Manifold (engine) – A multi branched pipe connecting several cylinder’s discharge ports to the main exhaust pipe.

Manila – The fibres of the abaca plant used to make rope.

Manilla Hemp – Product of a species of banana, principally from the Philippine Islands. Use for making rope.

Manilla Rope – Made from manilla hemp. Contains natural oil, so does not need tarring. Is about three-quarter weight of hemp rope of same size, and has a higher breaking point.

Manning – The operational crew of a vessel.

Manning Scale – Statutory scale (1936) specifying minimum number of efficient deck hands to be carried in steamships of stated tonnages. Now subject to SOLAS Chapter V 13. Related Article: SOLAS Convention of 1974 Chapters and Regulations in Summary.

Manoeuvre – Regulated change of direction, position or speed to attain a desired end. 2. To change direction, position or speed for a specific purpose.

Man-of war – A warship. A type of jelly fish.

Man-of-War – A warship. Related Article: Types of Warships in a Navy Fleet.

Manometer – Instrument that measures pressure and elastic properties of gases. Barometer and steam gauges are examples.

Manrope Knot – Made in end of manrope to form standing part at eye of stanchion on platform of accommodation ladder, or at a hatchway. Is a wall knot with crown above and all parts followed round once.

Manropes – Protective ropes at side of a ladder or inclined wooden steps. Short ropes, used when embarking or disembarking from, or into, boats from lower platform of accommodation ladder; ropes being attached to stanchions on the platform.

Marconi Rig – Nickname for ‘Bermuda Rig’.

Marcq St. Hilaire Method – Procedure for finding ship’s position line from an observation of a heavenly body and the calculation of the altitude and azimuth it would have if ship were at estimated position. The difference between the calculated altitude (or zenith distance) and observed altitude (or zenith distance) is the error in locus of assumed position line. The position line is moved towards or away from the geographical position of the observed body according to the amount the two altitudes (or zenith distances) differ.

Mareel – Shetland name for phosphorescence of the sea.

Maregraph Plongeur – Instrument for measuring changes in sea level by automatically recording changes in pressure at sea bottom. Two Bourdon tubes are open to pressure and mechanism records the resultant movements of their free ends.

Mare’s Tails – Tufted cirrus clouds.

Margin line – An imaginary line drawn at least 76 mm below the bulkhead deck. It provides a margin of safety to the designer’s calculations.

Margin of Safety Line – Line drawn parallel to bulkhead deck at side line, and three inches below the upper surface of that deck at sides. SOLAS Chapter II-1, Regulation 2.

Margin Plank – Edging plank around deck openings

Margin Plate – Plating forming side of double-bottom ballast tank.

Margin Plate – Strengthening member in steel construction at the turn of bilge.

Marina – A yacht harbour which usually provides fuel, fresh water and other facilities besides moorings, for yachts. Facility to berth many small vessels.

Marine – Pertaining to the sea. 2. The corps of soldiers who serve chiefly on board naval ships.

Marine Barometer – Mercurial, Kew-pattem barometer mounted in gimbals.

Marine Borers – Salt water organisms such as the teredo worm, which attack unprotected planking by boring into it causing deterioration and structural collapse.

Marine Engine – In general, is any engine designed for propulsion of ships. More especially applied to compound, triple-expansion and a quadruple-expansion engines that are fitted with a condenser.

Marine Glue – Usually a compound of crude rubber, oil and shellac. Used for paying deck seams, closing small leaks, and other purposes.

Marine Growth – Marine vegetation and fauna that attach to the underwater body of floating craft.

Marine Incident – Where there is a risk or potential risk to life and property from some event that has caused concern for the safety of the Master and those on board, (with due regard to the operational status of the incident vessel)

Marine Insurance – Insurance against losses occurring at sea, or in sea-going ships, or in relation to a marine adventure.

Marine Insurance Act, 1906 – First codification of rules governing marine insurance. Before this date marine insurance was governed, largely by precedent.

Marine Insurance Corporations – Companies who insure against marine risks and are corporately liable for payment. Insurance made by Lloyd’s is paid individually by all underwriters who subscribe to the contract.

Marine Notices – Advice on matters of navigation and safety issued by AMSA.

Marine Society – Instituted 1756; incorporated 1772. Principal objects are to train and fit out poor boys of good character for the sea services, and to ensure a steady stream of lads of good character and physique into the Royal and Merchant Navies.

Marine Store – Warehouse or shop in which rope, canvas and other ships’ stores are bought and sold.

Marine Store Dealer – Person who buys and sells marine stores. His business is controlled, in part, by Merchant Shipping Act, 1894, Sections 538-542. See also Anchor and Chain Cables Act 1967.

Marine Surveyor – One who surveys ships to ascertain if all statutory and appropriate measures have been taken for the good order and safety of ships, cargoes and personnel. 2. One who surveys seas, and waters connected with them.

Mariner – In general, a person employed in a sea-going vessel. In some cases, applied to a seaman who works on deck.

Mariner’s Compass – Compass used for directing course of a ship. The card, or ‘fly’, is mounted in a ballasted bowl fitted with a lubber line or pointer that coincides with ship’s fore and aft line; the compass graduation coinciding with this indicates direction of ship’s head. Usually mounted in a binnacle containing lighting arrangements, corrector magnets and soft-iron compensations.

Mariner’s Splice – Long splice put in cable-laid rope. Made in same manner as long splice with the addition that the three small ropes forming the strand are unlaid into ‘readies’, and are also spliced.

Maritime – Pertaining to the sea, to marine navigation, to shipping and to shipping commerce. 2. Bordering on the sea.

Maritime Air – Air mass that has travelled over the sea for a considerable distance.

Maritime Courts – Those courts of law that deal with shipping matters

Maritime Interest – Interest chargeable on a bottomry bond.

Maritime Law – Law as relating to shipping, seamen, navigation and harbours.

Maritime Lien – Legal right of Master and seamen to have a ship held as security for wages unpaid. Takes precedence over any other lien on ship.

Maritime Polar Air – Air stream, from high latitudes, that has passed over an appreciable expanse of sea before reaching observer.

Mark – One of the marked fathoms in a lead line. 2. Beacon or erection, marking a navigational danger or a position of navigational interest. Distinctive object used for navigation.

Marl (ing) – To bind or frap with small line in such a manner that each turn is an overhand knot. To twist a small rope around another as a protective coating. Crumbly mix of sand and clay.

Marline – Superior kind of spunyard laid up left-handed. Small two-stranded string, used for seizing and whipping. A finer kind of spun yarn.

Marline Hitch – Half hitch made by passing end over and under, so that an overhand knot is formed.

Marline Spike – A tool for opening the strands of a rope while splicing. Tapered and pointed iron pin used for opening up a rope when splicing. Has a perforation at larger end to take a lanyard.

Maroon – To put a man ashore, forcibly, on desolate land. Formerly done as a punishment, or as a criminal act. 2. Pyrotechnic signal exploding with a loud report.

MARPOL – International Convention for the Prevention of Pollution from Ships. Convention of the IMO to limit pollution at sea.

Marry – To interlace the strands of two ropes preparatory to splicing. 2. To put two ropes together, side by side, so that they can be hauled upon simultaneously. To To join.

Marry the Gunner’s Daughter – Old Navy nickname for a flogging, particularly when across a gun.

Marryat’s Code – ‘Code of signals for the Merchant Service’, compiled by Captain Frederick Marryat, R. N., 1817.

Mars – Fourth planet from Sun and, therefore, next outside Earth. Diameter is about half that of Earth. Has two satellites, Deimos and Phobos. Named after Roman god of war, on account of its ruddy colour.

Marsden Square – Area of Earth’s surface bounded by meridians 10° apart and parallels of latitude 10° apart, so forming 100 squares. Used for identifying position of a meteorological report, first figure denoting latitude, second figure denoting longitude.

Marsh – Wet area of country.

Mart – To traffic. ‘Mart’ is often corrupted to ‘Marque’.

Martelli’s Tables – ‘Short, Easy and Improved Method of Finding Apparent Time at Ship.’ G. F. Martelli has not been identified. The tables give quite trustworthy results, but the tabulated quantities have been so disguised that the underlying principle has been obscured.

Martinet – ‘Martnet.’

Martingale – Stay leading downward to prevent upward movement of a jib boom or flying jib boom. An iron spar below the bow sprit, sometimes shaped as a spear or harpoon, that acts as a tensioning spreader for the bow sprit to stem supporting chain.

Martnet – Leechline for tricing up a square sail when furling.

Mary Celeste – 19th century ship found mysteriously abandoned but sailing onward near the Azores.

Mascaret – Local name for tidal bore in Seine and Garonne rivers.

Mast – Vertical wooden pole—made of single tree or lengths of wood or a tube of steel or metal, erected more or less vertically on centre-line of a ship or boat. Its purpose may be to carry sail and necessary spars, to carry derricks, to give necessary height to a navigation light, look-out position, signal yard, control position, radio aerial or signal lamp. Tall masts are usually made up of three or four masts, one above the other. From the deck, they are named lower mast, topmast, topgallant mast, royal mast —the last two usually being one piece.

Mast Bands – Iron bands clamped round a mast to take upper eyes of rigging, belaying pins, etc.

Mast Butt  –The bottom of a mast.

Mast Carlings – Fore and aft timbers—or girders—on underside of deck and on either side of mast. Also called ‘mast partners’.

Mast Clamp – Semi-circular band for securing lower part of a boat’s mast to thwart of a boat.

Mast Coat – Conical-shaped canvas covering over wooden mast wedges. Painted to protect wedges from water; which would cause wedges to set up exceptional stresses, and to rot.

Mast Head – That part of mast between truck and upper eyes of rigging.

Mast Hole – Circular hole in deck through which a mast passes.

Mast Hoop – Circular ring, of metal or wood, that encircles a mast and is free to move up and down it. Often fitted in luffs of gaff sails.

Mast House – Long shed in which masts are built.

Mast Knife – Knife with 9- to 12-inch blade, used for scraping wooden mast.

Mast Lining – Doubling piece of canvas on after side of a topsail. Takes the chafe against topmast and cap.

Mast Partners – Supporting deck beams housing a mast.

Mast Prop – Long spar formerly used to strut a mast when vessel was careened.

Mast Rope – Rope by which an upper mast is hoisted. A ‘Heel Rope.’

Mast Scraper – Triangular scraper used on wooden masts. Edges are concave to fit round of mast. Bevel is away from handle.

Mast Step – Socket into which heel of a mast is stepped. Strengthened fitting to which heel of mast is secured. Securing point at bottom of mast.

Mast Table – A support structure around a mast for the cargo boom pivots.

Mast Tackle – Purchase for hoisting or lowering a mast. 2. Heavy-lift tackle depending from a mast.

Mast Thwart – A thwart through which the mast is secured.

Mast Trunk – Casing into which mast of a small vessel may be stepped.

Master – The title of the captain. Related Article: Ship Master’s Job Description:  Duties, Responsibilities and Salary.

Master Attendant – Officer of Royal Dockyard next in rank below Superintendent.

Master Compass – Principal gyro compass controlling the repeaters.

Master Gunner – Formerly, officer-in-charge of gunnery equipment of a vessel of R. N. Now, superseded by gunnery lieutenant, senior commissioned gunner or commissioned gunner.

Master Mariner – Officer of the Merchant Marine holding a certificate entitling him to command a vessel.  A seaman certified competent to be a captain.

Master-at-Anns – Chief petty officer responsible for discipline.

Masthead – The top of a mast.

Masthead Angle – Angle, at observer’s eye, between truck of a vessel’s mast and the waterline vertically beneath it. By means of tables—or by trigonometry— this can give ship’s distance off when height of eye and height of masthead are known.

Masthead Battens – Vertical strips of wood or metal placed on lower masthead to protect mast from chafe of wire rigging, and to protect eyes of lower rigging from chafe by masthead hoops.

Masthead Light –  A white light visible 112.5º each side of dead ahead carried by a power vessel to indicate that it is underway and making way.

Masthead Rig – Where headsails hoist to the masthead.

Masting – Erecting masts in a vessel.

Masting Sheers – Tall sheer legs used for stepping a mast, or for removing it from a vessel. Erected near edge of a fitting-out berth, with line of splay parallel to edge of berth. Heels are hinged so that sheers can be inclined until head is over centreline of vessel.

Mastless – Having no mast.

Mat – Woven strands of rope, or thrummed yams on canvas, used as protection against chafing, or for controlling a small leak.

Mate – An officer assistant to Master. A ‘Chief Officer‘. From time immemorial he has been responsible for stowage and care of cargo and organisation of work of seamen, in addition to navigating duties. Today, it pertains to licensed officers. Related Article: Shipboard Organization: Seafarers’ Ranks, Duties and Salaries.

Matelot – French for ‘Sailor’. In common use on lower deck of R. N.

Material Safety Data Sheet (MSDS) – Information sheet for the safe use and stowage of a material.

Mate’s Log – Book kept by Mate, and recording work done by crew and, with particular emphasis, all matters concerning stowage, carriage, ventilation and discharge of cargo. A Logbook.

Mate’s Receipt – Document signed by Mate when goods for carriage are received into ship’s charge.

mathematician, in 1614.

Mats – Wooden support framework constructed on deck in order to distribue the weight of heavy loads.

Matthew Walker Knot – Knot put in end of rope. Made by unlaying end of rope, making a bight of one strand, passing second strand over bight of first, passing third over bight of second and through bight of first.

Matting Sword – Length of thin wood used for beating in the wheft of a woven, or ‘sword’, mat.

Maul – Heavy hammer of iron or wood.

Maximum Altitude – Greatest angular distance above horizon.

Maximum Use Concentration (MUC) – The maximum air-borne contaminant concentration that a particular respirator may be used for protection against.

Maxwell’s Rule – ‘Corkscrew Rule.’

May – A legal term requiring voluntary compliance, usually qualified by a stated circumstance or limiting condition – see shall and should.

Mayday – (French ‘m’aidez’, ‘Help me’.) The international radiotelephony distress signal, repeated three times.

Mayflower – An early transport ship carrying puritans from England to a new colony in the Americas.

Mclntyre System – Double-bottom construction involving the cutting of frames and reverse frames and the fitting of a continuous water-tight angle bar to take lower edge of tank margin plate.

Mclntyre Tank – Water-ballast tank seated on top of ordinary floors. Preceded double-bottom tank.

Meade Great Circle Diagram – Graticule for solving great circle sailing problems from projection of course on a Mercator chart.

Mean – The average.

Mean Draught – Half sum of forward and after draughts of a vessel. It differs slightly from draught at half length.

Mean High Water – Mean of all high waters at a place when observed during a lunation of 29 days.

Mean High-Water Springs – Mean height of spring high waters at a place when deduced from an adequate number of observations.

Mean Latitude – That latitude whose numerical value is half the sum of a latitude left and a latitude reached.

Mean Low Water – Mean of all low waters at a place when observed during a lunation of 29 days.

Mean Low-Water Springs – Mean height of spring low waters at a place when deduced from an adequate number of observations.

Mean Noon – Instant in which Mean Sun is on meridian of a place.

Mean Place of Star – That position on the celestial concave at which a star would appear to be if viewed from Sun. It is expressed in declination north or south of Equinoctial and sidereal time angle from First Point of Aries on January 1 of current year.

Mean Sea Level – Level that sea would maintain if no tidal effect were manifested and average meteorological conditions prevailed. For hydrographic and surveying purposes the level is derived from very prolonged and careful observations at an appropriate position—Newlyn, Cornwall, being selected for British standard level.

Mean Solar Day – Interval between successive transits of Mean Sun across a given meridian. Its length is the mean length of all apparent solar days in a year.

Mean Solar Year – Average time taken by Sun to go round Ecliptic or, to be precise, for Earth to go round Sun. Due to slight irregularities in the movement of First Point of Aries the time varies slightly and irregularly. Mean value, when taken over about 100 years, gives 365 days. 5 hrs. 48 mins. 48 secs. (approx.).

Mean Sun – Fictitious body that travels along Equinoctial at a constant speed, and in the same time that true Sun takes to travel round Ecliptic at a variable speed.

Mean Tide Level – Average tidal height above chart datum. Found by evaluating half sum of Mean High Water and Mean Low Water at a given place. Name is sometimes applied to mean level of a tide.

Mean Time – Time regulated by Mean Sun.

Measured Distance – Accurately measured distance used for determining speeds of ships. Most of the so-called ‘measured miles’ are measured distances, and require adjustment when calculating speeds in knots.

Measured Length – Typically it is the distance from the fore part of the hull to the after part of the hull, taken at the upperside of the uppermost, weathertight deck, or, in the case of open vessels, at the height of the gunwale. Used as one of the primary determinants for a vessel’s manning requirements.

Measured Mile – Measured distance of 6080 ft. See ‘Measured Distance’.

Measurement Goods – Cargo in which freight is based on the amount of space goods occupy in the ship carrying them.

Measurement of Ships – Determination of a ship’s displacement. Gross, Net and Under-Deck Tonnages, Draught, Freeboard, Length and Breadth.

Mechanical Advantage – Ratio between force applied and weight lifted when using a purchase or other machine.

Mechanical Integrator – Instrument used for finding a vessel’s displacement and moments of displacement by moving a pointer along line of sections of body plan; this movement actuating registering mechanism.

Mechanical Navigation – Former name for ship handling and manoeuvring.

Mechanical Stoker – Machine that feeds furnaces of a coal-fired vessel.

Median – Middle quantity of a series of values; not necessarily the mean value. 2. Line, from angle of a triangle, bisecting opposite side.

Mediterranean Winds – The eight winds providing traditional sail traders routes across the sea, being; Northerly – Tramontane, North easterly – Gregale, Easterly – Levante, South easterly – Sirocco, Southerly – Ostro, South westerly – Libeccio, Westerly – Ponente, North westerly – Mistral

Medivac – Evacuation of a person from a location for medical reasons.

Medonca, Christophe – Portuguese explorer of Java la Grande in 16th century, claimed by some to be the East and South Coast of Australia.

Medusa – Umbrella-shaped jelly-fish with several long and hairlike tentacles.

Meet Her – Order to helmsman when helm is righted during a turn and he is required to check swing of ship’s head.

Mega – Prefix meaning ‘Great’ or ‘Million’.

Megadyne – Million dynes. Pressure of one megadyne per square centimetre is equivalent to 1000 millibars.

Meltemi – A northerly wind blowing off Macedonia and into the Ionian Sea.

Mend – To put additional service on a rope. 2. To loose a sail and refurl it.

Menhaden – A herring-like fish; Atlantic.

Meniscus – Curved upper surface of liquid in a tube, particularly mercury in a barometer. Curve can be concave or convex. 2. Concave-convex lens.

Menkalin – Star β Aurigae, S. H. A. 271°; Dec. N45°; Mag. 2. 1.

Menkar – Star α Ceti. S. H. A. 315°; Dec. N4°; Mag. 2.8.

Menkent – Star Θ Centauri. S. H. A. 149°; Dec. S36°; Mag. 2.3.

Merak – Star β Ursae Majoris. S. H. A. 195°; Dec. N57°, Mag. 2.4.

Meranti – Asian timber with good workability. Increasingly fast grown plantation stock requires careful selection to find that suitable for marine durability.

Merbau – High quality South American timber with good workability and high marine durability.

Mercantile Marine – Merchant marine; commercial shipping.

Mercator Chart – Projection of Earth’s surface in which all meridians are made parallel and the latitude scale is increased in the same ratio as the expansion of the longitude scale in the area. All straight lines are rhumb lines. Distances are measured by minutes of latitude in area considered. Distortion in high latitudes is very great.

Mercator Sailing – Method by which problems of sailing on spherical surface of Earth can be solved by plane trigonometry. Its fundamental equation is: Difference of Longitude = Meridional Difference of Latitude X Tangent of Course. It requires that difference of latitude shall be converted into the invariable units of the longitude scale; this being done by tables of ‘Meridional Parts’. Disadvantages of this sailing are that: (1) Course is a loxodrome, and not a great circle; (2) Method cannot be used in high latitudes except for very short distances.

Mercatorial Bearing – Bearing of a given position as deduced from a rhumb line passing through the position and position of observer. Differs from great circle bearing by half the con-vergency between meridian of observer and meridian passing through the given point.

Mercator’s Projection – Delineation of Earth’s surface on a chart or map in accordance with the principles used in making a Mercator chart.

Merchant Captain – Captain of a merchant vessel. Captain in the Merchant Navy.

Merchant Navy – A nation’s commercial ships.

Merchant Seaman – Seaman employed in a merchant vessel.

Merchant Service – ‘Merchant Navy.’

Merchantman – Trading vessel.

Mercurial Barometer – Barometer that indicates atmospheric pressure by height of a balancing column of mercury—as distinguished from an aneroid barometer.

Mercury – Innermost of the planets. Greatest angular distance from Sun is about 29°. Very rarely observable, and then only near sunrise or sunset. Nautical almanac then gives a warning that its appearance may cause it to be mistaken for a star or another planet.

Mercury – White metal with melting point of —38.5°C. Specific gravity about 13.55. Used in thermometers and barometers for registration purposes, and in some gyro compasses as a ballistic pendulum.

Mercury Ballistic – Pots of mercury, one on either side of a Sperry Gyro Compass. These have a connecting tube through which mercury flows when compass tilts and tends to wander out of meridian. Extra weight on one side causes gyro to process into meridian and regain horizontal position.

Mercury Trough – Receptacle in which mercury is placed to form an artificial horizon when taking celestial observations ashore. Has a glass roof to ensure unruffled surface of mercury.

Meridian – A great circle that is also a longitude. (passes through the Poles)

Meridian – Semi-circle of terrestrial sphere passing between the poles. All positions on this line have noon at the same instant, and the same longitude.

Meridian Altitude –The greatest altitude of a celestial body when crossing the same meridian as the observer.

Meridian Distance – Distance between any two meridians expressed as the difference in their respective local times.

Meridian Line – Line connecting North and South points of observer’s sensible horizon.

Meridian Passage – Transit of heavenly body across a given meridian.

Meridian Transit – ‘Meridian Passage.’

Meridian Zenith Distance – Arc of meridian intercepted between the zenith and a heavenly body in the meridian.

Meridional – Pertaining to the meridian. Sometimes has the meaning ‘facing the direction of the noon Sun’.

Meridional Difference of Latitude – Difference of latitude expressed in meridional parts of Mercator projection. Usually given as D. M. P. Difference of Meridional Parts.

Meridional Distance – Old name for ‘departure’ between two places on different meridians.

Meridional Parts – Length of any part of an extended meridian of a Mercator chart when expressed in units that are the length of a minute of longitude.

Meridional Projection – Projection of a sphere to a plane that is parallel to the meridian, or coincident with it.

Mermaid – Fabulous creature having upper part of a woman and lower part of a fish. Dugong is an animal somewhat of this type.

Merman – Masculine counterpart of a ‘Mermaid’.

MERSAR – Merchant Ship Search & Rescue (manual) now superseded by IAMSAR, International Aeronautical and Maritime Search & Rescue (manual).

MERSIGS – Merchant Ships Signal Code, World War II.

Mesh Stick – Flat, wooden slat used when net making. Width of slat forms mesh and regulates its size.

Mess – Group of persons who feed at the same table. The space table, and utensils allocated to a group of persons.

Mess Deck – Deck on which the feeding places and tables of a ship’s company are situated.

Mess Gear – Eating utensils.

Mess Kit – Utensils supplied to a mess for eating, drinking and cooking.

Mess Room – A crew’s dining area.

Mess Traps – R. N. name for ‘Mess Kit’.

Messenger – A small line used to heave in a heavier one.

Messman – A steward who serves meals to the crew.

Metacentre – Theoretical point considered in questions of ship stability. When a floating vessel is upright and afloat in water the upward force of buoyancy is equal to the downward force of gravity, and a vertical line through centre of buoyancy will pass through centre of gravity. When vessel is heeled by extraneous forces, centre of buoyancy will shift to low side. Where the vertical line, passing through the new centre of buoyancy, cuts the former vertical line is termed the ‘metacentre’. This point is more or less static for angles of heel up to about 15°. Similarly, we can consider a metacentre in a fore and aft plane. We thus have a longitudinal and a transverse metacentre, their positions changing with draught and inclination of vessel.

Metacentric Height (GM) – Distance that metacentre is above centre of gravity in a ship. The position of M being a large distance above G (centre of gravity) is an indication of the GZ (righting lever) that a vessel will develop on heel and hence its potential stability.

Metal Mike – Seaman’s name for ‘Automatic Helmsman’.

Metallic Barometer – Name formerly given to an aneroid barometer.

Metalling Clause – Inserted in policy of insurance to exclude loss of metal sheathing due to action of water.

Mete Stick – Horizontal batten extending from a vertical timber. Formerly used for levelling ballast, or for measuring height in a hold.

Meteograms – Weather outlooks presented as a graph.

Meteor Shower – Succession of meteors observable when Earth’s atmosphere passes through a ring of meteors.

Meteorological Visibility – The maximum range at which a large object, such as landmasses or mountains, can be seen. Also referred to as Meteorological Range.

Meteorological Wind Scale – Development of Beaufort Wind Scale, and relating wind-force to wind-speed. Introduced 1926.

Meteorology – Science concerned with study of Earth’s atmosphere, its phenomena and variations. Related Article: The Basics of Marine Meteorology – A Guide for Seafarers.

Meteors – Fragments of solid matter in space that meet Earth’s atmosphere. Friction with atmosphere heats them to incandescent point. Altitude of incandescence is probably less than 120 miles. Size of fragments is usually between that of a pea and that of a grain of sand. Occasionally, fragments of considerable size reach Earth’s surface.

Metonic Cycle – Period of 19 years in which 12 years consisted of 12 lunar months, and 7 years of 13 lunar months. Dates of new Moon were practically invariable, the error being one day in 237 years.

Mexican/Mexico Current – Cold current that sets SE’ly down west coast of Mexico and eventually merging in the NE Trade Drift.

MHz. – Megahertz. Of frequencies; a thousand Kilohertz or a million Hertz.

Miaplacidus – Star β Carinae. S. H. A. 222°; Dec. S70°; Mag. 1.8.

Michell Thrust – Thrust block in which the thrust is taken, primarily, on segments that can cant slightly away from collars on shaft. In the space thus formed is oil, so that frictional resistance and wear are reduced to a minimum.

Micrometer Screw – Screw for making very small adjustments with a large movement. In measuring instruments it permits very accurate measurements to be recorded.

Microsecond – One millionth (0.000, 001) of second of time.

Mid Latitudes – The areas between 30 degrees and 55 degrees latitude.

Mid Stream – Middle line of a stream or current.

Middle Body – The sections around midships.

Middle Deck – Second deck below upper deck in a three-decker (which had five decks).

Middle Ground – Shoal area between two navigational channels.

Middle Latitude – That latitude in which the length of a parallel intercepted between two meridians is equal to the departure made when sailing between the two meridians on a rhumb. Owing to spheroidal shape of Earth, middle latitude differs from mean latitude, but can be derived from mean latitude by Workman’s Table.

Middle Latitude Sailing – A navigation approximation averaging of difference in latitude from one waypoint to another to convert a departure to difference in longitude.

Middle Passage – Area of Atlantic Ocean between West Indies and U. S. A. Is an old ‘Slave Trade’ name.

Middle Staysail – Flying jib – the outer most foresail set on a sailing ship.

Middle Topsail – Deeply-reached topsail formerly set at heel of topmast in some schooners and sloops.

Middle-Latitude Sailing – Method of solving certain navigational problems when sailing between two points in different latitudes and longitudes. The length of intercepted arc of parallel midway between the two latitudes is taken to be equal to the departure made, and is found by formula Dep. = D. Long. x CosMid. Lat. The method is said to have been introduced by Ralph Handson, a

Middy – Colloquial name for a ‘Midshipman’.

Midland Sea – Mediterranean Sea.

Midship – Situated in or near the middle line—transversely or longitudinally—of a vessel.

Midship Beam – Longest beam in midship body of a vessel.

Midship Bend – Largest of the transverse sections of a vessel.

Midship Body – That part of hull of a ship in which there is little change in transverse shape.

Midship Frame – Largest transverse frame of a ship.

Midship Section – The transverse section of the ship amidships. Amidships may be midway between the ends of the LWL.

Midship Section Area Coefficient (CM) – The ratio of the immersed area of the midship section to the area of the circumscribing rectangle having a breadth equal to the breadth of the ship and a depth equal to the draught. CM = AM ÷ (B x T). CM values range from about 0.85 for fast ships to 0.99 for slow ships.

Midshipman – Young subordinate officer in R. N. Intermediate between cadet and sub-lieutenant.

Midshipman’s Hitch – Alternative but little-used name for a ‘Marlinespike Hitch’.

Midshipmen’s Nuts – Broken ship biscuit.

Midships – Commonly used form of ‘Amidships’. The middle section of a vessel.

Mile – Unit of geographical distance. Has various values and names, those that concern seamen being Statute, Nautical, Standard Nautical, International Nautical and Geographical miles. Each of these will be found under its prefix. A nautical mile is equivalent to 1 minute of latitude, 6080 feet or 1853.18 mtrs.

Milky Way – Unsymmetrical and irregular belt of luminosity extending round the heavens. Is an immense congregation of stars in shape of a lens—roughly. Diameter is about 150, 000 light years, Earth being about 40, 000 light years from centre.

Millibar – Unit of barometrical pressure. 1/ 1OOOth of a ‘Bar’.

Minelayer – Vessel specially fitted for carrying mines and laying them at sea.

Minesweeper – Vessel specially fitted for sweeping up enemy-laid mines and for destroying them. Modern term is mine clearance. Sophisticated mines cannot be swept.

Minimum Altitude – Altitude of an observed object at the instant its altitude ceases to decrease.

Minion – Small gun that threw a ball less than 4 Ib.

Minor Planet – One of the asteroids.

Minute – Interval of time equivalent to 1/60 of an hour. 2. 1/60 of a degree of arc.

Minute Gun – Firing of a gun, at intervals of about one minute, as a signal of distress.

Mirage – Optical effect, caused by unusual atmospheric refraction, by which distant objects appear to be nearer, or in remarkable positions.

Mirfax – An apparatus for transmitting and receiving weather-charts by radio.

Mirror Dinghy – Small easily managed sailing boat designed for safety, simplicity and cheapness.

Missing – Said of a vessel when no news has been heard of her and it is feared, but not proved, that she has been lost.

Missing Stays – Failing to go from one tack to the other when attempting to go about close-hauled.

Mission Control Centre (MCC) – of the Cospas-Sarsat system that accepts alert messages from the local user terminals (LUTs) and other mission control centres to distribute to the appropriate rescue coordination centres or other search and rescue points of contact.

Miss-stays – Fail to complete a tacking manoeuvre successfully.

Mist – Thin fog that reduces visibility to less than two miles. Meteorologists have a visibility limit of one mile, but the usual practice of seamen is to assume mist to be present when ships and ships’ lights are not visible at two miles.

Mistral – Strong, cold and often persistent north westerly wind, blowing down the Southern France river valleys and into the Mediterranean Sea.One of the eight trading winds of the Mediterranean Sea.

Mitchboard – Wooden stanchion, with curved socket, for taking weight of a boom when not in use.

Mixed Policy – Marine insurance policy covering two or more different kinds of risk, such as ‘sea’ and ‘land’ risks; or a ‘Voyage’ risk with a ‘time’ element incorporated. A ‘mixed’ policy, covering a voyage and a period of time exceeding 30 days, is not uncommon.

Mixed Tide – Tidal undulation in which both semi-diurnal and diurnal constituents are definitely manifested.

Mizen, Mizzen – Fore and aft sail, with gaff and boom, set on after side of mizen mast. Small sail, set on small mast, right aft in a boat having a mainmast.

Mizen, Mizzen Mast – Third mast from forward in a vessel having three or more masts.

Mizzen – A sail set on the mizzen mast, the aftermost mast of a sailing vessel.

Mizzen Staysail – A fore and aft sail hoisted on the mizzen. It is tacked down forward, overlapping the mainsail, to the weather side of the deck and sheeted to the end of the mizzen boom. It must to be lowered when tacking or gybing.

Mizzle – Combination of drizzle and thick mist. ‘Scotch mist’.

Moderate Speed – An expression used in the Collision Regulations. It will vary with circumstances and cannot be defined.

Modulator – A radar component that controls the magnetron, causing it to fire at full power, continue for the pulse duration, and cease instantly.

Mohammedan Calendar – Dates from flight of Mohammed from Mecca to Medina, July 16, 622 A. D. Has 12 lunar months of 29 or 30 days alternately. In every 30-year period there are 11 years of 354 days and 19 years of 355 days.

Mole – A breakwater and landing pier.

Molgogger – A folding or removable fairlead fitted to the bulwarks of a tug.

Mollusc – An invertebrate with a soft body and hard external shell; snails, oysters, mussels, etc.

Moment of Change Trim – Weight and leverage necessary to change trim of a vessel by one inch. May refer to 1 centimetre, care is needed.

Monitor – War vessel in which speed and other considerations have been sacrificed to obtain maximum gun power and armour protection.

Monkey Block – Large iron-bound block bolted to a wooden chock on deck of a sailing vessel. 2. Block fastened on topsail yard as a fairleader for buntlines.

Monkey Boat – Long, narrow canal boat. 2. Small boat used in a dock.

Monkey Chains – Small rigging chains abaft main chains. Royal and topgallant backstays are set up in them.

Monkey Face – Shamrock-shaped connecting piece of a mooring swivel. 2. Triangular plate connecting derrick span bull wire and long link chain.

Monkey Fist – Knot sometimes put in end of heaving line to increase its carrying power. Three round bights are made in end, leaving enough end to cross first turns with three round turns, and then a further three turns going over second turns and under first turns.

Monkey Gaff – Gaff hoisted above a spanker gaff.

Monkey Island – Screened navigating and compass position on top of a wheel house or chart house. A flying bridge over a pilothouse.

Monkey Jacket – Uniform jacket as distinguished from frock coat.

Monkey Lever – Iron lever operating tumblers that release securing chains of an anchor stowed on an anchor bed.

Monkey Poop – Low poop. Sometimes applied to a deck above an after cabin.

Monkey Rail – Light rail on and above a quarter rail.

Monkey Seam – Flat seam put in centre of sail when making it. Made by overlapping selvages of two cloths, and then tabling them.

Monkey Tail. Rope attached to end of a lever so that extra hands can heave on it. A curved bar used as an attachment for the rubber pendants.

Monkey Topsail – Sail that was used in instructional ships for teaching young seamen to hand, reef, furl and loose a topsail; to lay out on a yard; to bend and unbend a sail; to pass an earring. The yard was crossed a small distance above the deck; the names of the various parts of the sail were painted on it.

Monkeys Fist – A rope terminal on the end of a heaving line to make it heavier.

Monocoque – A structure in which the outer covering (planking) carries all or a major part of the stresses.

Monohedron – From the Greek word hedron (a geometrical figure having any number of planes). The theoretical ideal shape for planing over the water surface is one of constant (mono) section. Thus monohedron describes a hull that has a running surface of constant section; in practice the sections may not be exactly the same.

Monsoon – Persistent wind that blows from one direction in summer, and from an approximately opposite direction in winter. Speaking generally, they come from seaward in summer, and from landward in winter. They are well developed in E and S Asia, and may be noticed in China Sea, Indian Ocean and off African coast. The West African monsoon prevails from S to SW throughout the year.

Month – Period of time based on Moon’s revolution around Earth. It has various values, depending on the epoch used. The Anomalistic, Nodical, Periodic, Sidereal, Synodical and Tropical months are dealt with under their prefixes.

Moon – Secondary planet that is a satellite of Earth. Distance from Earth varies between 225, 463 miles at perigee to 252, 710 miles at apogee. Diameter 2160 miles. Plane of orbit is slightly more than 5° from plane of Ecliptic. Declination varies between 18 1/2° and 28 1/2° when maximum in a lunation. Moon’s gravitational effect on Earth has important effects in tidal phenomena.

Moon Culminating Star – Star sufficiently near Moon, both in R. A. and Declination, that observations could be taken of it to determine difference of longitude.

Moon in Distance – Term denoting that angular distance of Moon from another heavenly body allowed measurement of lunar observations.

Moon Sail – The sail above the highest square sail yard of an extreme clipper, with the point uppermost. See raffee.

Moonraker – Sail that was sometimes carried above skysail in square-rigged ships during fine weather and light airs. Smuggler.

Moonsail – Alternative name for ‘Moonraker’.

Moonsheered – Said of a vessel whose upper deck has exaggerated sheer forward and aft.

Moor – To secure a ship in position by two or more anchors and cables. 2. To attach a vessel to a buoy, or buoys. 3. To secure a vessel by attaching ropes to positions ashore.

Mooring – An arrangement for securing a boat to a mooring buoy or a pier.

Mooring Buoy – Buoy, carrying a large ring or shackle, securely moored so that a vessel can be attached to it and ride in safety.

Mooring Lines – Lines used to secure the vessel to the jetty or another vessel etc.

Mooring Swivel – Strong swivel piece for insertion in cables of a vessel riding by two anchors in a tideway. Each end of swivel is connected to a strong trefoil member that is pierced with three holes, one of which is for its connection to swivel, the other two being for inboard or outboard connections of ship’s cables. Use of swivel necessitates disconnection of each of ship’s cables, and a reconnection to the swivel.

Moraine – The debris of gravel and rock deposited by a glacier.

Moral Hazard – Term used in marine insurance to denote possible risk of dishonesty in an assured person or his representatives.

Morass – Bog or extreme marsh.

Morning Watch – Division of nautical day from 4 a. m. to 8 a. m.

Moro Vinta – Vessel of China Seas. About 40 ft. long. Has tripod mast, lugsail and outrigger.

Morse Code – Alphabet and numbers used in signalling. Each letter or figure is represented by a long sign, a short sign or a combination of them. Signalling system of dots and dashes. May be transmitted by light, sound or graphically. Invented by Professor Morse of Massachusetts.

Mortar – Short ordnance used for firing projectiles at short ranges. Angle of elevation was constant, about 45°, range being increased or decreased by increase or decrease of propelling charge. Now used only for line-throwing purposes.

Mortar Vessel – Small, broad beam vessel in which a mortar was mounted for bombardment purposes. Bomb Ketch.

Mortgage – The granting of a ship as security for money advanced. Any mortgage of a ship must be registered with Registrar at ship’s port of registry. Mortgage can be raised by the owner only.

Mortice Block – Clump block.

Mother Gary’s Chickens – A stormy petrel (sea bird) foretelling bad weather and bad luck.

Motor Sailor – A sailboat, with minimal rig and powerful motor, ideal as a live aboard.

Motor Torpedo Boat – a light, unarmoured, fast attack vessel.

Motor Well – A watertight box in which an outboard motor is fitted.

Motor/Ship Vessel – Vessel propelled by internal combustion engines.

Mould – Pattern or template from which a member of ship’s structure is shaped.

Mould Loft – Large covered space, in shipbuilding yard, where moulds, or templates, are made for the various members to be erected.

Moulded Base Line – The lowest extremity of the moulded surface of the ship. At the point where this line cuts the midship section a horizontal line is drawn, and it is this line which acts as the datum for all hydrostatic calculations. This line may, or may not, be parallel to the LWL depending on the ship type.

Moulded Breadth – The internal extreme breadth of a vessel amidships, from inside the plank, plate of ceiling.

Moulded Depth – The internal extreme height of a vessel from the top of the keel to the underside of the deck.

Moulding – Correctly forming the depth and outline of a vessel’s frames or timbers when building.

Moulds – The patterns taken from a vessel’s lofted plans used as templates to shape its component and structures.

Mount Misery – Colloquial name for an unprotected or unsheltered upper bridge.

Mouse, Mousing – To pass turns of yarn, or small line, across open part of hook to prevent accidental unhooking. Formerly: a knot worked in eye of stay to take chafe against the mast.

Mousing Hook – ‘Self-mousing hook’.

Mozambique Current – Warm ocean current setting southerly along the east African southern coastline east of Mozambique.

Mud – Sediment with grains of less than 0.06 mm diameter.

Mud Scow – A boat used to transport the mud from a dredge.

Mudhole – Small door, in lower part of boiler shell, through which sediment may be withdrawn when not under steam.

Muffle (Oars) – To put soft material around loom of oar, where it rests in crutch or rowlock, to deaden sound when rowing.

Mulct – To fine, or impose a penalty.

Mullion – The vertical divider in the lights of a split window.

Multi-chine – A many chined hull shape. See chine.

Multihull – A vessel which has more than one hull, e. g. a catamaran or trimaran.

Mumbleby – Devonshire name for a fishing boat rather smaller than a trawler.

Mumford Boiler – Water-tube boiler somewhat similar to Thorny-croft type, but having tubes more curved. Practically obsolete.

Munnions. – Decorative work separating windows in sterns of old ships.

Munro Wind-Speed Recorder – Instrument that points a pressure head into the wind and, by means of measuring elements and a clockwork attachment, gives an anemogram.

Mush Ice – Broken ice in pieces up to 6 ft. across.

Mushroom Anchor – Umbrella-shaped anchor. Invented 1809 by Hemman of Chatham. A mushroom shaped anchor without a stock. Related Article: Parts of an Anchor and Its Windlass Arrangement.

Mushroom Head – Domed top of a ventilator.

Mussel Bow – Name given to yachts that were cut away at forefoot, thus having a broad shallow part forward.

Muster – To assemble at a specified place.

Mutatis Mutandis – Latin term used in Lloyd’s Salvage Agreement, and other documents. Means, ‘after making the needful changes.’

Mutiny – Revolt from, or forcible resistance to, duly constituted authority.

Mutual Indemnity Insurance Association – Group of shipowners, and others, who combine to carry those risks excluded from policies of marine insurance. They are conducted on a non-profit-making basis, and mutually subscribe to make good an uninsured loss sustained by a member. ‘Small Damage Club.’

Muzzier – Very strong head-wind.

Mystery Tables – Name given to Martelli’s ‘Tables for Finding Apparent Time at Ship’ on account of the obscuration of their basic formulae.

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