Tides & Tidal Streams

Tidal Definitions and Datum

Chart Datum is the reference for charted soundings and predicted tidal heights. In appreciably tidal UK waters it is normally Lowest Astronomical Tide or a closely related low-water level...

• Sounding + tidal height = estimated depth
• Tidal height - drying height = estimated water over a drying feature

Using Tide Tables

ADMIRALTY Tide Tables cover about 500 standard and 5,700 secondary ports and provide daily HW/LW times and heights, secondary-port methods, harmonic constants where known and exceptional...

• Confirm source, currency, port and date
• Read the displayed time basis and convert once

The Tidal Curve Method

Start with the bracketing HW and LW and calculate range = HW height - LW height. Express the required time as an offset before or after HW, then use the curve supplied for that port or ex...

• Range = HW - LW
• Use time before or after the stated HW

Secondary Port Calculations

ADMIRALTY Tide Tables provide time and height differences for secondary ports relative to a named standard port. Begin with the correct standard-port HW and LW for the date; the named sta...

• Start from the named standard port and correct date
• Apply HW and LW time corrections separately

Tidal Anomalies

The simple six-hour rise-and-fall model does not describe every port. Basin and coastal response can produce double high or low waters, long stands, asymmetric rise and fall, bores and st...

• Local tides can have double waters, stands, asymmetry or bores
• Stream turns need not coincide with local HW or LW

Tidal Observation Buoys and Beacons

A tide gauge, current meter, authoritative harbour display or published observation station can provide measured height or stream evidence. An ordinary navigation buoy or beacon does not...

• Identify the actual sensor and measured variable
• Check datum, timestamp, quality and location

Tidal Diamonds and Stream Atlases

A charted tidal diamond identifies a position whose table gives set and rate by hour relative to HW at a named reference port. Set is the direction towards which the water flows in degree...

• Set is the direction towards which water flows
• Rate is stream speed in knots

Tidal Streams in Passage Planning and Pilotage

Tidal stream changes course made good, speed over ground and passage time. A fair stream can reduce duration; a foul stream can prevent a yacht making useful progress through a narrows or...

• Stream affects course made good and speed over ground
• Opposing wind and stream can create steep hazardous seas

Worked Secondary-Port and Tidal-Gate Example

Standard-port HW is 0535 at 5.2 m and LW is 1148 at 1.1 m. The applicable secondary differences are HW +0015 and -0.4 m; LW +0025 and +0.2 m. Corrected secondary HW is 0550 at 4.8 m and L...

• Apply separate secondary HW and LW corrections first
• Read whether the supplied factor is rise or fall

Yachtmaster Offshore Standard

All worked values in these advanced sections are fictional, non-navigational teaching data. Use the current publication, applicable observations and vessel-specific margins for a real dec...

Advanced Scenario Check

A yacht is approaching a fictional harbour after a machinery delay. The plan uses a direct digital secondary-port prediction of 2.4 m at 0815. A harbour gauge panel copied into both navig...