YouSail

Loading this page…

Australian sailing — learn, improve, connectManage a listing Join YouSail

Marine Services Retailers Marine Hardware Sailing Clubs

Map Directory Marinas Charters Passages Guides

FAQs Glossary Sailing Schools Courses Videos Boats Boats for sale Academy

Brokers Boat Insurance Online Retailers Brands

Brands Meet the Sailors

Search YouSail

Search YouSail

Explore YouSailClose menu⌄

Search YouSail

Directory⌄

All Directory Marine Services Retailers Marine Hardware Sailing Clubs

Destinations⌄

All Destinations Map Directory Marinas Charters Passages Guides

Learn⌄

All Learn FAQs Glossary Sailing Schools Courses Videos Boats Boats for sale Academy

Marketplace⌄

All Marketplace Brokers Boat Insurance Online Retailers Brands

Explore⌄

All Explore Brands Meet the Sailors

Australian sailing intelligence for learning, planning and finding the local people who can help.

Explore

Directory Charters Sailing clubs Marinas Marine Services

Guides & tips FAQs Glossary Videos

Marine Insurance

Sailing YouTubers

Meet the Sailors

More from YouSail

Sailing Passages

A safer starting point

YouSail supports decisions; it does not replace current charts, official warnings, local notices or the skipper’s judgement.

Join YouSail Manage a listing

Australian marine directory

Browse every marine service

Search the directory →

Supplies & chandlery

Chandlery
Marine hardware
Safety equipment
Ropes, canvas & upholstery
Online sailing store
Boat trailers

Marinas & boatyards

Marina service
Berthing & moorings

© 2026 YouSail · Learn, improve, connect · About · Sitemap · Contact

Haul-out / slipway

Boat cleaning & detailing

Mooring service

Sails & rigging

Sailmaker
Sail repair
Rigger
Rig inspection

Mechanical & electrical

Engine service
Marine diesel service
Outboard service
Marine electrical
Marine electronics
Navigation electronics
Marine communications
Marine refrigeration
Watermakers
Marine batteries

Hull & deck

Boatbuilding & repairs
Fibreglass & composites
Antifouling & painting
Boat builder

Professional services

Yacht brokerage
Marine surveyor
Marine insurance
Boat transport
Sailing charter company
Boat dealer / brand sales

Training

Sailing academy / training
Skipper & safety training
Sailing club

views

Sailing reference

The YouSail sailing glossary

Clear explanations of the language used in navigation, pilotage, weather, safety and life under sail.

All A B C D E F G H I J K L

642 terms

Page 13 of 22

Nautical Publications
Navigational Warnings (NAVAREA, Coastal and Local)
Urgent broadcast messages about new hazards or changed marks
The World-Wide Navigational Warning Service divides the oceans into NAVAREAs, each with a coordinator issuing long range warnings, supplemented by coastal warnings for national waters and local warnings for harbours. Subjects include failed lights, drifting objects, new wrecks, buoys off station and military exercises. Delivery is by NAVTEX on 518 kHz in English, by SafetyNET over satellite, and by VHF broadcast. Warnings supersede printed publications until formally cancelled or incorporated.
Read definition →
Emergency Navigation & Distress
NAVTEX
An automatic printer or screen receiving coastal safety broadcasts
NAVTEX delivers maritime safety information by narrow-band direct printing on 518 kHz for international English language broadcasts, with 490 kHz used for national language services and 4209.5 kHz in some tropical areas. Messages carry a station letter and a subject character, so a receiver can be set to reject categories such as pilot or Loran messages, though navigational, meteorological warnings and search and rescue information cannot be rejected. Coverage is typically several hundred miles.
Read definition →

Tidal Theory

Neap Tides

The smallest tidal ranges, near first and last quarter Moon

At quadrature the solar and lunar contributions oppose, so M2 and S2 partly cancel and range falls to a minimum, often around a third to a half of the spring range in European ports. High water is lower and low water higher than at springs, and the times of high water shift relative to springs. Neaps present the greatest risk of a vessel that grounds remaining stranded through subsequent tides.

Read definition →

Tidal Calculations

Negative Surge Allowance

Reducing your expected depth when weather is pushing the sea down

High barometric pressure, typically above 1013 hPa, and persistent offshore winds can lower observed level below prediction, occasionally by more than a metre in shallow shelf seas such as the southern North Sea. Apply roughly 0.01 m per hectopascal above 1013 as a first approximation, then add a further allowance for wind set-down. National authorities issue negative surge warnings; deep-draught vessels and shoal-draught passages should treat these as operational limits.

Read definition →

GNSS & Satellite Navigation

NMEA Position Sentence (GGA and RMC)

The standard data messages that carry your fix to other instruments

NMEA 0183 sends comma-separated sentences over a serial link, commonly at 4800 baud. GGA carries time, latitude, longitude, fix quality, satellites used, HDOP and antenna height, while RMC gives time, date, position, speed and course over ground, and magnetic variation. NMEA 2000 replaces this with a 250 kbit/s CAN bus carrying binary parameter groups, allowing multiple talkers and listeners on a single backbone with far greater throughput.

Read definition →

Passage Planning

No-Go Area

Shaded chart area the vessel must never enter

An area highlighted on the chart during planning that the vessel cannot enter at any state of tide, either because charted depth plus the least expected height of tide is less than draught plus the required under-keel clearance, or because of wrecks, rocks, obstructions or prohibited zones. Shading them makes the safe water obvious at a glance. Areas passable only above a certain tidal height are marked separately as conditional rather than no-go.

Read definition →

Points of Sail & Sailing Manoeuvres

No-Go Zone

The arc either side of the wind where sails will not drive

Within roughly 40 to 45 degrees either side of the true wind direction for a typical cruising yacht, the sails cannot generate lift and simply flap, so the boat stops. The total blocked sector is therefore about 80 to 90 degrees wide, narrower for a performance keelboat and wider for a heavy long-keeled vessel. Ground to windward is gained only by tacking around the edges of this sector.

Read definition →

Nodal Cycle (18.6-Year Cycle)

A slow 18.6-year wobble in the Moon's orbit that alters tidal range

The regression of the lunar nodes causes the Moon's maximum declination to vary between roughly 18.3 and 28.6 degrees over 18.61 years. This modulates constituent amplitudes by a few per cent, increasing M2 slightly when declination range is small and strengthening the diurnal constituents when it is large. Prediction software applies nodal factors f and u to each constituent for the year concerned, which is why very old tide tables degrade.

Read definition →

Lights & Light Characteristics

Nominal Range

How far a light can be seen in average clear weather

Nominal range is the luminous range of a light in a homogeneous atmosphere of meteorological visibility 10 nautical miles, and is the figure printed on British Admiralty charts, for example 18M. It depends only on intensity, not on height or on the observer's eye. In visibility better than 10 miles the actual luminous range is greater; in haze or rain it can be far less, and the luminous range diagram in the light list converts between them.

Read definition →

North Cardinal Mark

Pass to the north of it to stay in safe water

Topmark is two black cones, both points upward. The body is black above yellow, matching the upward-pointing cones. The light is white continuous quick flashing, Q, or very quick flashing, VQ, with no group and no interruption; typical rates are around 50 to 60 flashes per minute for Q and 100 to 120 for VQ. Safe water lies to the north, so a vessel should pass with the mark to the south of it.

Read definition →

COLREGs - Lights, Shapes & Sound Signals

Not Under Command (NUC)

Vessel unable to manoeuvre because of some exceptional breakdown

A vessel which through exceptional circumstance, such as steering gear or main engine failure, is unable to manoeuvre as the rules require and so cannot keep out of the way. She exhibits two all-round red lights in a vertical line, or two black balls by day, adding sidelights and a sternlight only when making way through the water. In restricted visibility she sounds one prolonged followed by two short blasts every two minutes.

Read definition →

Nautical Publications

Notices to Mariners (NtM)

Regular official updates that keep charts and books current

The UKHO issues a weekly edition containing chart corrections, amendments to Sailing Directions and Lists of Lights, temporary and preliminary notices, and navigational warnings. Corrections are applied in ink to paper charts and recorded in the bottom left margin. An annual summary edition reissues standing notices. National authorities issue their own series, and the US Coast Guard publishes Local Notices to Mariners for inshore waters not covered by the national edition.

Read definition →

Meteorology for Navigators

Occluded Front

A front formed when the cold front catches the warm front

Depicted with alternating purple triangles and semicircles. The warm sector is lifted clear of the surface, so surface temperature contrast is small but cloud and precipitation remain extensive. Weather resembles a warm front followed quickly by a cold front, with rain, a poorly defined pressure minimum and a wind veer that is usually less sharp. Occlusion marks the mature or decaying stage of a depression's life cycle.

Read definition →

Lights & Light Characteristics

Occulting (Oc)

Light mostly on, briefly blacked out at intervals

Occulting is the inverse of flashing: total duration of light in each period exceeds total darkness, so the eye sees a steady light punctuated by short eclipses. Written Oc 8s, or Oc(2)10s for group occulting where two eclipses occur in each period. Occulting lights are common on harbour entrance structures and sector lights because the long light phase makes the sector colour easy to judge against a bright shore background.

Read definition →

Tidal Streams & Currents

Ocean Current

A large, mostly steady flow of ocean water, unrelated to tides

Driven by wind stress, thermohaline density differences and Coriolis effect, organised into gyres that circulate clockwise in the northern hemisphere and anticlockwise in the southern. Western boundary currents such as the Gulf Stream and Kuroshio are narrow and fast, reaching several knots, while eastern limbs are broad and slow. Mean rates and directions are shown on routeing charts and in Ocean Passages for the World.

Read definition →

Nautical Publications

Ocean Passages for the World (NP 136)

The reference giving recommended long-distance routes and their seasons

NP 136 sets out recommended ocean routes between principal ports for both power and sailing vessels, with distances, notes on prevailing winds, currents and ice limits, and guidance on the best seasons. It is used with the Routeing Charts, which give monthly wind roses, current arrows, ice limits, mean pressure and shipping route distances for each ocean. Jimmy Cornell's World Cruising Routes serves the same purpose for yachts, with more practical detail.

Read definition →

Great Circle & Ocean Routing

Optimum Track Ship Routeing (OTSR)

A shore service that recommends and monitors an ocean track

Originally a United States Navy service and now offered commercially, OTSR issues a recommended departure route based on forecast and climatological data, then monitors the vessel's daily position reports and diverts it as the weather pattern evolves. Objectives may be least time, fuel saving, avoidance of heavy weather damage or arrival at a stated time. Advice is a recommendation only; the master decides.

Read definition →

Position Fixing

Order of Observation for a Simultaneous Fix

Taking the fastest changing sighting last so the fix stays true

A fix assumes all its observations belong to one instant, so objects are observed in a deliberate order: those changing slowly first, typically ahead or astern, and the fastest changing last, typically abeam or close aboard. The time and log reading are noted at the final observation. Taking a whole round of bearings should occupy well under a minute; at 6 knots one minute of delay displaces the vessel 0.1 mile, which can exceed the plotting accuracy the fix is intended to deliver.

Read definition →

Tidal Streams & Currents

Overfalls

Breaking, turbulent water over an underwater ridge or shoal

Formed when a stream flowing over a sudden change of depth or an irregular bottom produces upwelling, boils and short breaking seas. Marked on charts by a distinctive wavy symbol. Unlike a race, overfalls can occur in relatively open water wherever bathymetry is abrupt. They are hazardous to small craft chiefly through steep, close-period waves and loss of steerage, and are considerably reduced at neaps and at slack water.

Read definition →

Electronic Charts & ECDIS

Overscale Indication

The warning that you have zoomed in beyond the chart's real detail

Every ENC cell has a compilation scale reflecting the survey and generalisation behind it. Zooming beyond that scale does not add information, it merely magnifies existing objects, so the ECDIS marks the affected area with a vertical line pattern or an overscale indication. The safe response is to zoom out to the compilation scale, or to load a larger scale cell of the area if one exists in the folio.

Read definition →

Speed, Log & Distance

Paddlewheel or Impeller Log

A small rotating wheel in the hull that counts water flow

A bladed wheel protrudes a few millimetres through the hull, usually forward and below the turn of the bilge where flow is least disturbed, and magnets in the blades trigger a sensor once per revolution. Pulses are counted for speed and totalised for distance. It is cheap and reasonably linear over normal yacht speeds but stalls at very low speed, is sensitive to hull position, heel and aeration, and requires regular cleaning through its seacock valve.

Read definition →

Emergency Navigation & Distress

Pan-Pan

The urgency call for a serious problem that is not yet life-threatening

Pan-Pan, spoken three times, precedes an urgency message concerning the safety of a vessel or a person where immediate assistance is not required, such as engine failure in open water, a broken rudder under tow, or a crew member needing medical advice. It ranks below distress but above all other traffic. A medical advice request may be qualified as Pan-Pan Medico, and DSC urgency alerting is available on channel 70.

Read definition →

Radar Navigation

Parallel Index Line (PI)

A line set at your planned passing distance from a fixed object

A parallel index is drawn parallel to the intended ground track at the planned passing distance from a selected charted radar-conspicuous object. Keeping the object's echo sliding exactly along the line confirms the vessel is on track; the echo drifting inside or outside the line reveals set immediately and continuously, without plotting. It requires an accurate heading input and a correctly identified object, and works equally well in relative or true motion.

Read definition →

Passage Planning

Passage Plan

Written plan covering the whole voyage from berth to berth

A documented plan for the entire intended voyage, prepared before departure and built in four stages: appraisal, planning, execution and monitoring. It records courses and distances, waypoints, hazards, tidal windows, contingency ports, communications and reporting points. SOLAS chapter V requires voyage planning by every vessel, including pleasure craft in many states, and the plan should exist in a form that can be checked afterwards, whether a chart plot, a plan sheet or a spreadsheet.

Read definition →

Nautical Publications

Passage Plan Documentation

The written record of how a voyage was planned before departure

SOLAS Chapter V requires every vessel, including pleasure craft in principle, to plan its intended voyage. The documented plan covers the appraisal of charts and publications, the planned tracks with courses and distances, no-go areas, clearing bearings, tidal windows, contingency ports, waypoint lists and abort points. Retaining it in writing demonstrates due diligence and lets any crew member take over navigation. It should be reviewed and amended if conditions change during the passage.

Read definition →

Lights & Light Characteristics

Period

Time for one whole flashing cycle before the pattern repeats

The period is measured in seconds from the start of one flash or group to the start of the next identical event, and is the single most reliable discriminator between similar lights. Time at least three cycles and average, since a stopwatch started on the first flash is prone to reaction error. In Fl(2)10s the 10 seconds includes both flashes, the short eclipse between them and the long eclipse before the next group.

Read definition →

Emergency Navigation & Distress

Personal Locator Beacon (PLB)

A pocket beacon registered to a person rather than a boat

A PLB uses the same 406 MHz Cospas-Sarsat alerting as an EPIRB but is smaller, registered to an individual, manually activated and typically specified for a shorter minimum operating period, commonly twenty-four hours. It is not float-free and many models must be held clear of the water with the antenna upright, so a lifejacket pocket mounting is usual. It is a personal supplement to, not a replacement for, the vessel's EPIRB.

Read definition →

Pilotage & Harbour Approach

Pilot Boarding Ground

Charted position where the pilot boards or leaves ship

A charted area, often shown by a magenta circle labelled Pilots or by a dedicated buoy, where the pilot launch or helicopter meets arriving vessels. Ports publish the boarding position, the notice required, the VHF channel and any alternative bad-weather position. The vessel is expected to arrive at the agreed time, make a lee, reduce to a manageable transfer speed of typically five to eight knots, and hold a steady course while the pilot transfers.

Read definition →

Nautical Publications

Pilot Charts / Routeing Charts

Monthly climate charts of wind, current and weather for ocean planning

One sheet per ocean per month, showing wind roses giving the percentage frequency and force of wind from each of eight or twelve directions, prevailing surface currents with mean rates, percentage frequency of gales and fog, mean air and sea temperatures, ice limits and tropical storm tracks. Admiralty issues them as Routeing Charts, the US NGA as Pilot Charts. They present long term climatology, not a forecast, so they inform departure timing rather than daily decisions.

Read definition →

Pilotage & Harbour Approach

Pilot Ladder

Rope ladder rigged over the ship's side for the pilot

A side ladder rigged, secured and tended under the ship's responsibility, with a responsible officer in attendance and direct communication with the bridge. The climb from the water should not exceed 9 metres; above that a combination arrangement with an accommodation ladder is used. Steps must be horizontal and equally spaced, typically 310 to 350 millimetres apart. A lifebuoy with self-igniting light, a heaving line and adequate lighting are required at the point of access.

Read definition →

Page 13 of 22

Sailing reference

The YouSail sailing glossary

Clear explanations of the language used in navigation, pilotage, weather, safety and life under sail.