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Sailing reference

The YouSail sailing glossary

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

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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.
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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.
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Tidal Streams & Currents

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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.

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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.

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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.

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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.

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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.

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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.

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Sailing reference

The YouSail sailing glossary

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