In Irons ~ A sailboat is ‘in irons’ when it’s pointed dead into the wind and can’t get out of it. Usually the result of having insufficient way on when going about.

IRPC ~ An acronym for the ‘International Regulations for the Prevention of Collision at Sea’. See also ‘Colregs'.

Isobars ~ Lines drawn on a weather map connecting places of equal atmospheric pressure.

J...

Jackstays ~ Safety lines that a wise sailor will hook his safety harness to when moving about the deck at sea.

Jacobs Ladder ~ A collapsible ladder usually made of rope, or a combination of rope and wooden rungs that is used to board a boat.

Jam Cleat ~ A fitting equipped with two small spring-loaded blocks that allow a rope to pass through in one direction only, until manually released.

Jib ~ A triangular headsail attached to the forestay that doesn’t overlap the mast. Read more about sails...

Jibe ~ A nautical expression describing the change from one tack to another by turning the stern through the wind. An alternative spelling of this nautical expression is 'gybe'.

Jury Rig ~ An emergency get-you-home rig made up from salvaged spars and sails after a dismasting.

K...

Kedge ~ A secondary anchor.

Keel ~ The main backbone of a boat containing, or to which is bolted, the ballast. Read more about keel types...

The ketch is a two-masted sailboat with the shorter mast (the mizzen) towards the stern.

Ketch ~ A two-masted sailboat with the shorter mast (the mizzen) towards the stern.

A popular rig with cruising sailors as it splits the sail area into more manageable proportions, and provides options when shortening sail.

On the wind though, the mizzen sail adds nothing but drag. Read more about the ketch rig...

Kicking Strap ~ A line or tackle used to hold the forward end of the boom down. See also ‘vang’ or ‘boom vang’.

Knot ~ One nautical mile per hour. The oft-heard phrase 'knots per hour' is nonsensical since it would mean 'one nautical mile per hour per hour'.

L...

Laminate sails aren't woven like conventional sailcloth, but glued together and shaped over a mould.

Laminate Sail ~ A sail made up of layers of film, scrim and tafeta used primarily by racing sailors.

The layers are glued together under high temperature and pressure and moulded into the required shape.

Read more about laminate sails...

Lanyard ~ A short, light length of cord or rope used as a safety line to prevent loss of something overboard, such as a line to raise a bucket of water from the sea or to fasten a knife.

Latitude ~ The distance, measured in degrees, north or south of the Equator.

Lazarette ~ Stowage space located in the stern of a boat.

Lazyjacks ~ Light lines from boom to mast to collect lowered mainsail. Read more about lazyjacks...

Lee ~ The side of a boat that is opposite to the one upon which the wind blows. The protected downwind.

Lee Helm ~ A balance between the hull shape and sailplan that will cause a boat to 'bear away' if the helm is released. Thoroughly dodgy.

Lee Shore ~ Land that lies to 'leeward' of you, onto which the wind is blowing. Not good news in heavy weather.

Leech ~ The aft edge of a sail.

Leech Line ~ An adjustable line built into the leech of a line to control its taughtness and improve the airflow around it.

Leeward ~ The side of the boat facing away from the wind.

Leeway ~ A sailboat’s drift to windward caused by the wind’s pressure on the sails.

Let Fly ~ A nautical expression meaning to let a sheet go instantly, spilling the wind from the sail.

Lifelines ~ Safety lines running around the edge of the boat, supported by stanchions

Limber holes ~ Holes through which bilge water can flow to a point from which it can be pumped out.

List ~ A boat's tendency to lean to one side, owing to an uneven distribution of weight.

LOA ~ An acronym for a boats ‘Length Over All’.

Log ~ (1) An abbreviation for 'log-book', in which all necessary navigational information is recorded. (2) An electronic instrument that measures a boat’s speed through the water.

Longitude ~ The distance, measured in degrees, east or west of the Greenwich Meridian.

Loose-footed Main ~ A mainsail whose foot is not attached to the boom. Read more about mainsail furling systems...

Luff ~ The leading edge of a sail. A sail is luffing when it’s shaking violently due to being headed directly into the wind.

Luff Groove ~ The groove in a foil or spar into which the leading edge of a sail is fed.

Luff Up ~ To head into the wind.

Lugger ~ A small boat fitted with a lugsail.

Lugsail ~ A four-cornered mainsail with no boom, but rigged by attaching it to the mast and hanging it from an upper yard.

Lurch ~ The sudden rolling of a boat.

LWL ~ An acronym for a boat's ‘Length Along the Waterline’.

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M...

Magnetic Compass ~ A navigational instrument that indicates a boat’s heading through a pivoted indicator that reacts to the earths magnetic field.

Mainmast ~ The principal mast on a sailboat. Read more about masts...

Mainsail ~ Or just ‘Main’, the sail attached to the main mast.

Mainsheet ~ The control line - a multi-part tackle - attached to the clew of the mainsail.

Marinized Engine ~ An automobile engine that has been modified for operation as an inboard boat engine.

Marlinespike ~ A steel tool used for separating the strands of a rope to aid splicing.

Marina ~ A commercial facility that offers boaters such services as sheltered dock space, moorings, fuel and water.

Marry ~ A nautical expression meaning to splice together two lines.

Mast Step ~ (1) The socket in the keel into which the base of the mast is located. (2) One of a series of steps fixed to the mast to enable a person to go aloft.

Martingale ~ A short spar supporting a boat’s bowsprit, also known as a ‘Dolphin Striker’.

Masthead ~ The top of the mast.

Mate ~ Next in command to the captain.

Mayday ~ Not only a nautical expression, Mayday - from the French m'aidez - is the international cry for urgent help in any emergency situation on land, sea or in the air.

Measured Mile ~ A distance of one nautical mile, often between transits and marked on the chart.

Meridian ~ A north/south line running through any point on the globe.

Next: Nautical Expressions from Mercator Projections to Pooped...