Abaft

Toward the stern of a boat, or behind it.

Abeam

At right angles to the centreline of a boat.

Adrift

Floating without any means of propulsion, and without mooring.

Aft

At, near, or toward the stern of a boat.

Afterpart

The part of a boat aft of the beam.

Aground

When a boat is stranded on the shore, or on the bottom of the body of water, it is said to have run aground.

Alee

Toward the opposite to the source of the wind side of a boat.

All standing

To have all sails flying when running before the wind.

Aloft

Overhead.

Amidships

In the middle of the boat

Aport

To the port side of the boat.

Apparent wind

The combination of the true wind and the wind caused by the boat's own speed.  This is the wind felt on the boat, as well as the one shown by the telltales.

To be on or to go to the shore.

Aspect ratio

Concerns sails - the ratio of height to the length.  A narrow but tall sail has a high aspect ratio, and a wide but shorter sail has a low aspect ratio.

Astern

Toward the boat's stern.

At right angles to the centerline of the boat.

Autopilot

A device - may be electronic or mechanical - used for keeping the boat on course without having to steer it (the same idea as on airplanes).  It uses a compass, and is attached to the boat's steering mechanism.

Auxiliary/Auxiliary power

An engine that is permanently installed on the boat.  Unfortunately it has to be used sometimes to power the boat...  The engine is also usually used to recharge the batteries - it's in neutral at that time.

To hold a sail in such a way, that the wind will fill it from the opposite to usual side.   This maneuver is used to slow down the boat (as if applying brakes), or to force a boat to tack when in irons.

A change in wind direction running counterclockwise, as in from west to southwest. 

A rigging wire used to keep the mast from moving forward, as well as to vary the amount of bend in the mast.

If your sails are filled with the wind on the opposite side to what you want (for example, if they are trimmed for the starboard tack, but you get the wind from the port side), you are said to be backwinded.

To get rid of water accumulated in the boat.

A very heavy material, such as lead or iron, placed in the keel of the boat, or in the bilge.  It is used to provide stability.  Oftentimes the crew is also a ballast - especially on smaller boats, or in a jocular way.

In a very strong wind it is possible to be propelled by the force of the wind on only the mast and the boom.  To sail in such a way is called "bare poles".

Thin strips of wood or plastic inserted into batten pockets used to stiffen the leech (to preserve the shape of the sail).

The widest part of a boat.

A direction an object is relative to the observer (based on the compass heading).

A pulley - a nautical term.  Often with more than one wheel (sheave being the proper name) to increase its mechanical advantage.

A spar (a wooden or metal pole) attached to the mast at a right angle, used to support the foot of a sail.

A reinforcing rope along the luff or the foot of the sail, it is slid into a slot along the edge of a spar (mast, boom).

The front end of a boat.

Bower

A main anchor of a vessel.  It is carried at the bow.

Bowsprit

A spar that's attached to the bow of a boat, along the centerline of the boat.  The forestay can be attached to it - thus allowing for a greater sail area.

Breakers

Waves that have entered a shallow water, and built up on height.  By doing this they "break" at the crest producing a curled up formation.

A pivoting board that prevents the boat from sliding sideways.

A compartment where the centerboard resides.

The center of the boat: from the stern to the bow.

A nautically specialized map.

A fitting for securing a line.  It can be wooden, metal or nylon.

An aft corner of a triangular sail.

The rear boat area from where the crew operates the boat.

The front part of a boat.

A crew member responsible for cleaning the deck, and an overall boat maintenance.

In sail measurements, the longest reach of the mainsail along the boom.

The longest reach of the mizzen along its boom.

A flag indicating nationality of the vessel.

F

A piece of hardware or equipment (such as a block) used for leading the jib sheets from the deck to the cockpit.  They are located astern of the beam, on each side of the boat.

An attachment on the forestay, comprising a groove into which the luff of the jib can be fed.

The bottom edge of the sail - the one attached to the boom.

To, at or near the front of the boat.

A foresail is the sail (such as a jib) located immediately in front of the main mast.   It is attached to the forestay.

A cable supporting the mast, running from the bow to the top of the mast.

A spar in a gaff rig (four sided sails) to which the top side of the sailed is attached.

A boat that has to stay clear of the right-of-way, or stand-on boat.

The top edge of the side of the hull.

H

A snap - plastic or stainless steel - attached to the luff of the jib, used to attach the jib to the forestay.

A small opening with a "door" on deck, allowing entry under the deck.

A line used to raise things on a boat, for example "the main halyard" is the line used to raise the mainsail.  It is a part of running rigging.

The top part of a triangular sail.  OR A toilet in a cruiser boat.

Any sail located in front of the main mast.

The body of the boat.

I

In sail measurements, the height of the foretriangle.  It is measured from the deck to the highest useful point on the forward side of the mast.  Can be either the point where the forestay is attached to the mast, or if mounted above the forestay, the top of the spinnaker block.

J

In sail measurements, the base of the foretriangle.  It is measured from the forestay at the stem to the forward side of the mast, horizontally to the waterline.

The front sail.

The line used to pull the jib in or let it out.

K

A weighted extension of a boat running below it that prevents the boat from sliding sideways.

Sometimes used to indicate spinnaker.

A nautical term for speed: one nautical mile per hour.  Also a term indicating a method of tying a line.

L

To tie something using a light rope.

The aft edge of the triangular sail - the one that's not attached to anything.

The direction to which the wind is blowing.

The fore edge of a sail.

To luff up means to bring the boat's bow so close to the wind, that the leech of the sail begins to flap.

M

The largest sail.  (Except for the spinnaker.)

The line used to pull the mainsail in or let it out.

The pole attached to the deck at the right angle, holding up the sails.

The top of the mast.

The sail set on the second (aftermost, or rear) mast - as on a ketch.

Permanent anchorage.   It consists of a heavy weight (or an anchor), a chain of a certain length, and a buoy.  Mooring is also often used for piers, instead of pilings.

N

A book containing all current data: navigational, tidal, astronomical and so on.  It is published annually.

O

A device located on the aft part of the boom, used to secure the clew, so that the foot is kept tense.

P

In sail measurements, the longest reach of the mainsail along the mast.

A wooden structure (although it may be built from other materials) built over the water, used by boats for landing.

A thick post supporting or mooring a dock or pier.  It is deep inside the seabed, and it projects above the water level.

The longest reach of the mizzen along its mast.

The left side of the boat.

Q

R

The distance between two objects (horizontally).

The assembly of the boat.

The underwater, movable plate used for steering, and for providing resistance to sideways motion caused by waves and wind.  It is being controlled by the helmsman (helmsperson?) with a help of a tiller or a steering wheel.

The top part of the rudder - the one being actually attached to the tiller.

S

Part of the indispensable equipment on the boat.  It is a small device used for attaching lines to other things, like sails.

A line used to trim sails.

A phenomenon occurring as the waves enter a shallow water - their movement forward is slowed down because of the bottom friction, and thus their height increases.

The wires holding the mast at the sides.

A general name for all masts, booms, gaffs, and bowsprits.

The wooden or metal struts that are attached horizontally to the upper section of the mast, on both sides.  They widen the angle of the shrouds, and thus provide a better support for the mast.

A boat that has the right-of-way over the give-way vessel.  It must maintain its course and speed.

The right side of the boat.

Wires supporting the mast - fore and aft.

The back of the boat.

A continuous line of breakers at the shore.

T

The fore corner of a triangular sail.

A snap used to hold the tack of the jib at the bow, while the sail is raised.

A pin used to secure the tack of a triangular sail at the mast.

Short pieces of yarn attached to the shrouds, or the sails.  At the shrouds they indicate the direction of the wind (the apparent wind), and at the sails they help to check the air flow over the sail, so that proper trimming is easier.

A spar attached to the rudder by the rudder head, used to control the direction of the boat.  Another possibility for steering mechanism is a steering wheel.

The space on a catamaran, usually made of some kind of mesh, located between the two hulls.  It's a place for the crew (like a cockpit on dinghies and cruisers).

A track (usually metal) with a fixture sliding on it.  The fixture holds the main sheet (usually), and the sliding allows for changing angles of the sail.

The person controlling the shape of the sails on a boat.  It is usually the crew on a dinghy, and a different person for each sail on larger boats.

The strength and direction of the actual wind blowing.  While sailing, the true wind is never felt - it is always a combination of the true wind, and the boat's speed (called the apparent wind), and it is always a little forward to the true wind.

A very small sail, used in a very heavy weather instead of a mainsail.

U

V

W

A mechanical device used to assist in pulling on lines.  It is a reel-like part of the hardware.

The direction from which the wind is blowing.

X

Y

In yachts with square sails - the spar on which the sail is suspended.

A two-masted sailboat.

Z

A gentle, warm breeze.  Also, the wind from the west.

Alternating tacks on approximately equal distances.