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| Magnetic Bearing |
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The bearing of an object after magnetic variation has been considered, but without compensation for magnetic deviation.
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| Magnetic Course |
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The course of a vessel after magnetic variation has been considered, but without compensation for magnetic deviation.
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| Magnetic Deviation |
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Compass error. The difference between the reading of a compass and the actual magnetic course or bearing due to errors in the compass reading. These errors can be caused by metals, magnetic fields and electrical fields near the compass. Prior to using a compass, magnetic deviation should be recorded for many different points on the compass as the error can be different at different points. The act of checking for magnetic deviation is called swinging.
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| Magnetic North |
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The direction to which a compass points. Magnetic north differs from true north because the magnetic fields of the planet are not exactly in line with the north and south poles. Observed differences between magnetic and true north is known as magnetic variation.
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| Magnetic Variation |
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The difference between magnetic north and true north, measured as an angle. Magnetic variation is different in different locations, so the nearest compass rose to each location on a chart must be used.
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| Main Mast |
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The tallest (or only) mast on a boat.
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| Mainsail |
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The main sail that is suspended from the main mast.
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| Mainsheet |
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The line used to control the mainsail.
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| Make Fast |
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To attach a line to something so that it will not move.
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| Marina |
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A place where boats can find fuel, water and other services. Marinas also contain slips where boats can stay for a period of time.
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| Mark |
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(1) Marks used on a lead line or anchor rode indicating the length of the line at that point. (2) A buoy or other object used to mark a location.
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| Marl |
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To wrap a small line around another.
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| Marline |
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A small line used for whipping, seizing, and lashing.
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| Marlinespike |
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A pointed tool used to separate the strands of a rope or wire.
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| Mast |
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Any vertical pole on the boat that sails are attached to. If a boat has more than one mast, they can be identified by name.
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| Mast Boot |
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A protective cover wrapped around the mast at the deck on a keel stepped boat to prevent water from entering the boat.
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| Mast Box |
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A box where a deck stepped mast is stepped.
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| Mast Partners |
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Supporting structures to take the load of the mast at the deck.
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| Mast Step |
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The place that supports the bottom of the mast. The mast step usually has a built in pattern fitting a matching pattern on the bottom of the mast, enabling the mast to be accurately positioned.
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| Mast Track |
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A track or groove in the back of the mast to which the sail is attached by means of lugs or the bolt rope.
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| Master |
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The person in charge of a vessel. The captain.
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| Masthead |
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The top of a mast. Wind direction indicators and radio antennas usually collect on the masthead.
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| Masthead Light |
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Also known as a steaming light. The masthead light is a white light that is visible for an arc extending across the forward 225° of the boat. When lit the masthead light indicates that a vessel under power, including sailboats with engines running. Masthead lights are usually located halfway up the mast rather than at the top.
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| Mate |
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An assistant to the captain.
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| Mayday |
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An internationally recognized distress signal used on a radio to indicate a life threatening situation. Mayday calls have priority over any other radio transmission and should only be used if there is an immediate threat to life or vessel. Mayday comes from the French "M'aidez" which means "help me." For urgent situations that are not immediately life threatening there is the PAN PAN identifier. Less urgent messages such as navigational hazards should send a SECURITE message.
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| Mean Low Water |
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A figure representing the average low tide of a region.
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| Mean Lower Low Water |
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In an area with two tides, this figure represents the average of the lowest of the low tides.
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| Measured Mile |
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A course marked by buoys or ranges measuring one nautical mile. Measured miles are used to calibrate logs.
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| Mercator |
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A type of projection of the globe used when making charts. Since the world is a sphere, it is impossible to draw accurate charts on flat paper. A Mercator projection shows all of the meridians as straight vertical lines rather than lines that would intersect. This is the type of projection used on a typical world map, but the distances become very distorted near the poles.
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| Meridian |
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A longitude line. Meridians are imaginary circles that run through both poles.
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| Messenger |
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A small line used to pull a heavier line or cable. The messenger line is usually easier to throw, lead through holes or otherwise manipulate than the line that it will be used to pull.
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| Midchannel Buoy |
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A red and white vertically striped buoy used in the United States to mark the middle of a channel. Midchannel buoys may be passed by on either side. Also see nun and can buoys.
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| Midships |
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A place on a boat where its beam is the widest.
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| Mile |
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(1) Distance at sea is measured in nautical miles, which are about 6067.12 feet, 1.15 statute miles or exactly 1852 meters. Nautical miles have the unique property that a minute of latitude is equal to one nautical mile (there is a slight error because the earth is not perfectly round.) Measurement of speed is done in knots where one knot equals one nautical mile per hour. (2) A statute mile is used to measure distances on land in the United states and is 5280 feet.
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| Millibar |
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A unit of pressure used to measure the pressure of the atmosphere. 1 millibar equals 0.03 inches of mercury.
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| Minute |
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(1) When used to measure location a minute is one sixtieth of one degree. One minute of latitude is equal to one nautical mile. Each minute is divided into sixty seconds. (2) When measuring time a minute is one sixtieth of one hour.
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| Mizzen Mast |
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A smaller aft mast on a ketch or yawl rigged boat.
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| Mizzen Sail |
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The sail on the aft mast of a ketch or yawl rigged sailboat.
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| Mizzen Staysail |
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A small sail that is sometimes placed forward of the mizzen mast.
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| Monkey Fist |
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A large heavy knot usually made in the end of a heaving line to aid in accurate throwing.
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| Monohull |
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A boat that has only one hull, as opposed to multihull boats such as catamarans or trimarans.
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| Moor |
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To attach a boat to a mooring, dock, post, anchor, etc.
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| Mooring |
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A place where a boat can be moored. Usually a buoy marks the location of a firmly set anchor.
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| Mooring Buoy |
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A buoy marking the location of a mooring. Usually attached to an anchor by a small pendant.
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| Mooring Line |
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A line used to secure a boat to an anchor, dock, or mooring.
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| Morse Code |
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A code that uses dots and dashes to communicate by radio or signal lights.
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| Motor |
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(1) An engine. (2) The act of using an engine to move a boat.
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| Motor Sailer |
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A boat designed to use its motor for significant amounts of time and use the sails less often than a normal sailboat.
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| Mount |
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(1) An attachment point for another object. (2) The act of putting an object on its mount.
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| Mouse |
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Also mousing. Tying a line so that it will not come undone, such as when attaching a line to a hook.
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| Multihull |
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Any boat with more than one hull, such as a catamaran or trimaran.
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| Mushroom Anchor |
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A type of anchor with a heavy inverted mushroom shaped head. Mushroom anchors are used to anchor in mud and other soft ground.
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