Nautical Terms Glossary for Boat Buyers

Buying a boat means encountering a vocabulary that can feel like a foreign language. This glossary explains the most common nautical terms in plain English so you can read listings with confidence, ask the right questions during a viewing, and understand what a surveyor is telling you. For broader context, see our guides on how to read a boat listing, understanding boat dimensions, boat types explained, hull types explained, understanding keel types, rig types for sailboats, and engine types explained.

A

Aft: Towards the rear (stern) of the boat. When a listing describes a cabin or cockpit as "aft", it is located at the back of the vessel. The opposite of forward.
Aground: When the hull or keel makes contact with the seabed in water that is too shallow. Going aground can damage the hull, keel, or rudder, so a boat's history of groundings is worth investigating during a survey.
AIS: Automatic Identification System — a transponder-based tracking system that broadcasts a vessel's position, speed, and identity to nearby ships and shore stations. A Class B AIS unit is standard equipment on most cruising boats and enhances collision avoidance significantly.
Anchor: A heavy device lowered to the seabed on a chain or rope (rode) to hold the boat in position. Common types include the plough (CQR), Delta, and Rocna. Check that any listed anchor gear is appropriately sized for the boat.
Antifouling: A specialised paint applied to the underwater hull to prevent the growth of weed, barnacles, and other marine organisms. Antifouling must be reapplied regularly (typically annually) and is one of the key recurring costs of boat ownership. See our guide on antifouling.
Autopilot: An electronic system that steers the boat to a set compass heading or GPS waypoint without constant input from the helm. A reliable autopilot is essential for short-handed offshore sailing and is a significant value-add in any listing.

B

Backstay: A wire or rod running from the masthead to the stern that supports the mast from behind and counters the forward pull of the forestay. Some backstays are adjustable, allowing the skipper to tune the sail shape underway.
Ballast: Heavy material — usually lead or iron — placed low in the hull or keel to lower the boat's centre of gravity and improve stability. The ballast ratio (ballast weight divided by total displacement) gives a useful indication of a sailboat's stiffness and righting ability.
Beam: The maximum width of the boat measured at its widest point. A wider beam generally means more interior space and greater initial stability, but can affect performance in some conditions. See our guide on understanding boat dimensions.
Berth: Either a sleeping space inside the boat (also called a bunk) or a mooring space in a marina. When a listing says "six berths", it means six sleeping places. A marina berth is what you pay for to keep the boat permanently moored.
Bilge: The lowest internal part of the hull where water collects. Some water in the bilge is normal, but excessive or oily water can indicate a leak or engine problem. Always inspect the bilge during a viewing.
Bilge pump: A manual or electric pump used to remove water from the bilge. Boats should carry at least one electric and one manual bilge pump as a safety requirement. Check that both are functional and that the electric pump has an automatic float switch.
Binnacle: A pedestal or housing mounted in the cockpit that holds the steering compass, and often also the helm, engine controls, and instrument displays. A well-organised binnacle keeps critical navigation information directly in the helmsman's line of sight.
Boom: The horizontal spar attached to the base of the mast that holds the foot (bottom edge) of the mainsail. The boom swings from side to side as the boat changes direction. Many modern booms incorporate a furling or reefing system.
Bow: The front of the boat. A "bow thruster" is a small propeller in a tunnel near the bow that aids manoeuvring in marinas at low speed.
Bulkhead: A vertical partition wall inside the hull that divides the interior into separate compartments. Bulkheads are structural components — delaminating or cracked bulkheads are a serious defect that a surveyor will flag.

C

Catamaran: A twin-hulled boat offering exceptional stability, shallow draft, and large living spaces. Catamarans are popular for cruising and charter. Browse catamarans for sale or read more in our boat types guide.
Centreboard: A retractable fin that pivots down through the hull to provide lateral resistance and prevent the boat sliding sideways. Unlike a daggerboard it swings on a pivot pin, so it deflects upward on grounding rather than jamming. Common on trailerable sailboats.
Chainplate: A metal fitting bolted to the hull or deck that anchors the shrouds and stays. Chainplates are a critical structural connection — any sign of rust staining, cracking, or movement around a chainplate warrants close inspection.
Cleat: A two-horned fitting on deck or on a dock used to secure mooring lines, dock lines, or sheets. Cleats come in several styles including horn, cam, and clam, each suited to different applications.
Cockpit: The open area, usually at the stern, from which the boat is helmed and sailed. A self-draining cockpit is an important safety feature — water that enters should drain overboard quickly rather than filling up.
Coaming: A raised lip or wall around the cockpit, companionway, or hatches that prevents water from running below. Good-sized coamings improve both comfort and safety in rough conditions.
Cutter: A single-masted sailboat with the mast stepped further aft than a sloop, carrying two headsails (a staysail and a jib or genoa). The split headsail arrangement makes sail handling easier offshore. See our rig types guide.

D

Daggerboard: A retractable fin that slides vertically through the hull to provide lateral resistance. Unlike a centreboard it cannot deflect on grounding, but it is typically more efficient to windward. Common on performance cruising catamarans and some dinghy-style monohulls.
Davits: A pair of small cranes or arms mounted at the stern used to lift and carry a dinghy (tender) or liferaft clear of the water. Davits eliminate the need to deflate and stow a rigid inflatable dinghy every time the boat moves.
Dead reckoning: Estimating the boat's current position by calculating from a known past position using speed, heading, and elapsed time. It is a fundamental navigation skill and also serves as a backup if GPS fails.
Deadrise: The angle of the hull bottom measured from horizontal. A high deadrise (sharply V-shaped bottom) cuts through waves more smoothly; a low deadrise (flatter bottom) gives more stability at rest and in shallow water. Important when comparing motorboat hull forms.
Displacement: The weight of water the hull displaces, which equals the total weight of the boat including everything on board. A "displacement hull" travels through the water rather than planing over it, while a "planing hull" lifts out of the water at speed. See our guide on what is boat displacement.
Draft: The vertical distance from the waterline to the lowest point of the hull or keel — in other words, the minimum water depth the boat requires. Shallow draft opens up more anchorages and allows access to rivers and tidal areas. See understanding boat dimensions.
DSC: Digital Selective Calling — a feature built into marine VHF radios that allows a skipper to send an automated distress signal including GPS position at the press of a button. DSC requires the radio to be registered with an MMSI number to function correctly.

E

EPIRB: Emergency Position-Indicating Radio Beacon — a satellite-linked distress beacon that transmits the boat's position to rescue authorities when activated. EPIRBs must be registered and have their batteries replaced on schedule; check the expiry date when viewing any boat.

F

Fairlead: A fitting that guides a line (rope) in a specific direction to prevent chafe and ensure smooth operation. Fairleads are found on anchor rollers, sheet tracks, and mooring bow rollers.
Feathering propeller: A type of propeller whose blades can rotate to align edge-on with the water flow when the engine is off, reducing drag under sail. Feathering propellers are popular on cruising sailboats as they improve sailing performance noticeably compared to a fixed propeller.
Fin keel: A short, deep, torpedo-shaped keel bolted under the hull. Fin keels provide excellent upwind performance and are the most common configuration on modern performance and cruising sailboats. See understanding keel types.
Forestay: The wire running from the bow to the upper mast that supports the mast from the front and provides the attachment point for the headsail (jib or genoa). On boats with a furling headsail the forestay carries the furling foil.
Forward: Towards the bow (front) of the boat. A "forward cabin" is located at the front of the vessel, often in the widest part of the hull and popular for its berth space.
Freeboard: The height of the hull side above the waterline. High freeboard means more interior headroom and better protection from spray, but can make the boat harder to board from the water and more susceptible to windage.
Full keel: A long keel that runs most of the length of the hull, providing directional stability and protecting the rudder. Full-keel boats are favoured for bluewater passage-making for their seakindly motion and robustness. See understanding keel types.
Furling: A system that rolls up a sail around a stay or boom for easy handling and storage. Roller-furling headsails are now near-universal on cruising boats; in-mast and in-boom furling systems are available for the mainsail, though they involve trade-offs in sail shape.

G

Galley: The boat's kitchen. A well-equipped galley with a gimballed (swinging) cooker, pressure water, and a refrigerator significantly increases liveability. Note whether the galley is "U-shaped" (bracing points for the cook at sea) or open-plan.
Genoa: A large headsail that overlaps the mast, providing more sail area than a standard jib and therefore more power in light winds. Genoas are measured by their "LP percentage" — a 150% genoa overlaps 50% past the mast.
GRP: Glass Reinforced Plastic (also called fibreglass) — the most common hull material used in production boats from the 1960s onwards. GRP is durable, relatively low-maintenance, and easy to repair. Check for osmotic blistering on older GRP hulls. Learn more on Wikipedia.

H

Halyard: A line (rope or wire) used to hoist a sail up the mast. Each sail has its own halyard — main halyard, jib halyard, spinnaker halyard, and so on. Halyards are part of the running rigging and should be inspected for wear.
Heads: The boat's toilet and bathroom compartment. Marine toilets (also called heads) are either manual pump, electric, or vacuum-flush. Check that the holding tank and seacocks are serviceable, as failing marine sanitation systems can be costly to repair.
Heel: The sideways lean of the boat caused by wind pressure on the sails. Some heel is normal and even efficient when sailing; excessive heel is uncomfortable and can indicate the boat is over-canvassed or has insufficient ballast.
HIN: Hull Identification Number — a unique 12-character serial number permanently affixed to the hull (typically on the transom). The HIN is used to verify ownership, check for outstanding finance, and confirm the boat's history. Always verify the HIN matches the registration documents.
Hull: The main body of the boat — the watertight shell that provides buoyancy. The hull form (shape) determines how the boat moves through the water, its stability, and its interior volume. See our guide on hull types explained.

I

Impeller: A small rubber paddle-wheel inside the raw-water pump that draws cooling seawater through the engine. Impellers deteriorate with age and heat and should be replaced annually — a failed impeller can cause rapid engine overheating. Ask when it was last changed.

J

Jib: A headsail that does not overlap the mast, smaller than a genoa. Jibs are easier to handle and tack than large genoas, making them popular on short-handed or family cruising boats. A "self-tacking jib" on a traveller track requires no sheet handling during tacks.

K

Keel: The structural spine running along the bottom of the hull, and/or the ballasted fin attached below it. The keel provides stability (by keeping the centre of gravity low) and lateral resistance (preventing the boat from sliding sideways). See our full guide on understanding keel types.
Ketch: A two-masted sailboat where the smaller mizzen mast is stepped forward of the rudder post. The split rig allows the sail area to be divided into manageable pieces, making a ketch easier to handle offshore. See rig types for sailboats.
Knot: The nautical unit of speed equal to one nautical mile per hour (approximately 1.15 mph or 1.85 km/h). Boat speed, wind speed, and current are all measured in knots. A nautical mile is 1,852 metres, slightly longer than a statute mile.

L

Lazarette: A storage locker at the stern of the boat, typically accessed via a hatch in the cockpit sole or transom. Lazarettes commonly house gas bottles, fenders, dock lines, and other bulky gear.
Lee: The side of the boat sheltered from the wind — the downwind side. The "leeward" side is away from the wind; "leeway" is the sideways drift caused by wind pressure on the hull and rigging. "Lee cloth" refers to a canvas panel on a berth that prevents the occupant rolling out when the boat heels.
LOA: Length Overall — the total length of the boat from the tip of the bow to the furthest point of the stern, including any fixed bowsprits or stern platforms. LOA determines marina berth fees and is the primary length figure quoted in listings. See understanding boat dimensions.
LWL: Waterline Length — the length of the hull at the waterline when the boat is floating at its designed displacement. A longer waterline generally means higher maximum hull speed for displacement boats; LWL is often shorter than LOA because bows and sterns overhang the water.

M

Mainsail: The primary sail attached to the mast and boom, providing the bulk of the driving force in most conditions. Mainsails on cruising boats are increasingly fitted with slab reefing or in-mast/in-boom furling systems for easy handling.
MMSI: Maritime Mobile Service Identity — a unique nine-digit number assigned to a vessel's VHF radio and AIS transponder. The MMSI is transmitted automatically in DSC distress calls, linking the signal to the registered boat and owner. Verify that the MMSI is properly registered if a boat is sold with its radio.
Monohull: A boat with a single hull — the most traditional and common configuration. Monohulls generally have a lower purchase price than equivalent catamarans, tend to sail more upright in light winds, and berth more easily. Browse sailboats and motorboats.
Multihull: Any boat with more than one hull — including catamarans (two hulls) and trimarans (three hulls). Multihulls offer speed, stability, and spacious living areas but require wider berths. Browse catamarans for sale.

O

Outboard: A self-contained engine unit that clamps or bolts to the transom, combining the motor, gearbox, and propeller in one portable unit. Outboards are common on tenders (dinghies), RIBs, and small powerboats. They are easy to remove for servicing but less efficient than inboard engines at higher speeds. See engine types explained.
Overhang: The part of the bow or stern that extends beyond the waterline. Overhangs were a traditional design feature that increased waterline length (and therefore speed) as the boat heeled. Modern designs tend to have minimal overhang to maximise interior volume.

P

Port: The left side of the boat when facing forward. Port is indicated by a red navigation light. A useful memory aid: "port" and "left" both have four letters. The opposite side is starboard.
Propeller shaft: The rotating steel shaft that connects the inboard engine or gearbox to the propeller. The shaft passes through the hull via a stern gland (or cutlass bearing), which must be properly sealed and lubricated to prevent water ingress. Worn or bent propeller shafts cause vibration.

R

RIB: Rigid Inflatable Boat — a small fast boat with a solid GRP or aluminium hull and inflatable rubber tube sides. RIBs are commonly used as tenders (dinghies) for larger yachts or as standalone patrol and leisure craft. Browse all boats.
Rigging (standing and running): Standing rigging refers to the fixed wires and rods that support the mast — stays and shrouds. Running rigging refers to all the ropes and lines that are used to control the sails — halyards, sheets, and control lines. Standing rigging is typically replaced every 10–15 years or after any significant damage.
Rudder: The underwater fin used to steer the boat by deflecting water flow. Rudders may be hung on the trailing edge of the keel (keel-hung), attached to a skeg (skeg-hung), or mounted on a separate stock at the stern (spade rudder). Inspect the rudder stock bearings for play during any survey.

S

Sailboat: A vessel primarily powered by sails, with an auxiliary engine for manoeuvring in harbour or motoring in calm conditions. Sailboats range from small day-sailors to large offshore passage-makers. Browse sailboats for sale.
Seacock: A valve fitted at every through-hull fitting below the waterline that can be closed to prevent flooding if a hose fails. Every seacock should be operable by hand and exercised regularly to prevent seizing. A boat with seized or corroded seacocks is a safety hazard.
Sheet: A line (rope) used to control the trim of a sail — the mainsheet controls the mainsail, the jib sheets control the headsail. Unlike halyards, sheets are not used to hoist sails; they adjust the angle and tension of the sail relative to the wind.
Shroud: A wire running from the mast to the sides of the boat (chainplates) that supports the mast laterally. Upper shrouds run to the masthead; lower shrouds run to a point partway up. Shrouds are part of the standing rigging and should be checked for broken strands and corrosion at their end fittings.
Sloop: The most common sailboat rig: a single mast with one mainsail and one headsail. Sloops are simple, efficient, and easy to handle, which is why the vast majority of modern production sailboats are sloop-rigged. See rig types for sailboats.
Spinnaker: A large, lightweight balloon-shaped sail used when sailing downwind or at broad angles to the wind. Asymmetric spinnakers (sometimes called cruising chutes or gennakers) are easier to handle for short-handed crews than traditional symmetric spinnakers.
Spreader: A horizontal strut projecting from each side of the mast that pushes the shrouds outward to improve the angle at which they support the mast. Spreaders are usually aluminium or carbon; check that the tips are fitted with boots or end caps to prevent tearing the mainsail.
Starboard: The right side of the boat when facing forward. Starboard is indicated by a green navigation light. Starboard-tack vessels generally have right of way over port-tack vessels under the International Regulations for Preventing Collisions at Sea (COLREGs).
Stay: A wire or rod that supports the mast in a fore-and-aft direction. The forestay runs from the bow to the mast; the backstay runs from the stern to the mast. Stays and shrouds together form the standing rigging. See also: Forestay, Backstay.
Stern: The rear of the boat. Many modern cruising boats feature a "sugar-scoop" stern — a low platform at the waterline that makes it easy to swim, board a dinghy, and use davits.
Stern drive: An inboard engine connected to a steerable outdrive unit (also called an "I/O" or inboard/outboard) that protrudes through the transom. Stern drives offer the power of an inboard with some of the steering flexibility of an outboard, but the bellows seals require regular inspection. See engine types explained.
Stringer: A longitudinal structural member running along the inside of the hull that stiffens the hull shell and distributes loads. Delaminated or cracked stringers are a serious structural defect; surveyors tap the hull and stringers with a hammer to detect voids.

T

Tack: As a noun, the lower forward corner of a sail, or the direction the boat is sailing relative to the wind (port tack or starboard tack). As a verb, to tack means to turn the bow through the wind so that the wind comes from the other side — the fundamental zigzag manoeuvre for sailing upwind.
Through-hull: Any fitting that passes through the hull below the waterline — seacocks, depth transducers, log impellers, and engine cooling intakes are all through-hulls. Each through-hull is a potential source of flooding; verify that all are in good condition and fitted with operable seacocks.
Tiller: A straight bar connected directly to the top of the rudder stock, used to steer smaller boats. Tillers are simple, give direct feedback, and are lighter and cheaper than wheel steering, but can be tiring on longer passages and awkward in confined cockpits.
Transom: The flat (or near-flat) vertical surface forming the stern of the boat. The HIN is stamped on the transom; outboard engines clamp to the transom; and swim platforms or boarding ladders are often mounted on it.
Trim: The fore-and-aft balance of the boat in the water, or the adjustment of sails and engine trim tabs to optimise performance. A boat that floats with its bow lower than designed is said to be "bow-heavy" or "trimmed by the head"; excess weight aft trims the boat "by the stern".

V

VHF: Very High Frequency radio — the standard short-range marine radio used for communication with other vessels, marinas, and the coastguard. Channel 16 is the international distress and calling channel and must be monitored at sea. A fixed-mount VHF with DSC is a legal requirement in many jurisdictions for offshore passages.

W

Waterline: The line on the hull where the water surface meets the boat when it is floating at its designed displacement. The waterline is often marked with a painted bootstripe. The length at the waterline (LWL) is an important performance measurement — see understanding boat dimensions.
Windlass: An electric or manual winch mounted on the foredeck used to raise and lower the anchor and chain. An electric windlass is a significant convenience for a cruising boat, particularly with a heavy ground tackle setup. Check that it is correctly sized for the boat's anchor chain.
Windward: The direction from which the wind is blowing — the upwind side. "Making to windward" means sailing towards the wind. A boat that sails well to windward (close-hauled) is said to point well; this is largely determined by hull form, keel design, and rig efficiency.

Y

Yawl: A two-masted sailboat similar to a ketch but with the small mizzen mast stepped aft of the rudder post. Yawls were popular in offshore racing rules that penalised mainsail area; the mizzen contributes less drive than a ketch's but can carry a mizzen staysail. See rig types for sailboats.

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