Tidal Streams

 

TIDAL STREAMS are the horizontal flow of water caused by the rise and fall of the tide.  Spring Tides produce strong tidal streams and Neap Tides produce relatively weak tidal streams.

Tidal Streams repeat on a regular cycle and for convenience are related to the time of High Water at a Standard Port.  This port may be Dover, Plymouth etc. and will be noted on the Chart or Tidal Stream Atlas.

Tidal Stream information can be found on Charts via Tidal Diamonds or in booklets called Tidal Stream Atlases.  Tidal Diamonds tabulate the Tidal Stream for a particular spot whereas a Tidal Stream Atlas provides a complete picture for the area covered.

 

TIDAL DIAMONDS

Tidal Diamonds on Admiralty Charts and their circular equivalent on Imray Charts show the position of Tidal Stream information tabulated on the chart.

Tidal diamonds on Admiralty Charts 

The Tidal Stream table shows the reference port ie. 'Tidal Sreams are referred to HW at Plymouth (Devonport)'.

So at the location of Tidal Diamond J, two hours before High Water Plymouth, the Tidal Stream will be setting in a direction of 040 degrees, at a rate of 3.1 knots on Springs and 1.6 knots on Neaps.

Tidal Diamonds show the Tidal Streams at that location only, a nearby Diamond may have very different data, sensible interpretation and comparison may be used to estimate the Streams at intermediate positions.

Tidal Stream Atlases provide a better overview for Tidal Streams as a picture for a whole area is provided.

 

 

TIDAL STREAM ATLASES

Admiralty Tidal Stream Atlases have 13 pages giving a picture of the tidal stream hour by hour.  The centre of the atlas shows the High Water page, working back from High Water the previous pages show from 1 Hour Before HW to 6 Hours Before HW.  The Pages after HW work from HW + 1 hour to HW + 6 Hours.


The image below shows a typical page from a Tidal Stream Atlas, this is One Hour before HW at Devonport (Plymouth).

 


Admiralty Tidal Stream Atlas How to use

 

Tida; Stream around Start Point

 Crown Copyright and/or database rights.  Reproduced by permission of the controller of Her Majesty's Stationery Office and the UK Hydrograhic Office (www.ukho.gov.uk)

The arrows show the Set of the Tidal Streams and help to indicate Rate by their weight.  The Rates in knots are also shown numerically.  Close to Start Point you will see 15,30 this means that on Neaps the Rate will be 1.5 knots and on Springs it will be 3.0 knots, the commma indicating the position of the reading. 

More often than not, you will not be exactly on Springs or Neaps but somewhere in between and will need to estimate the actual rate.  A mental estimate will be good enough most of the time, but if greater accuracy is required use the Computation of Rates table found at the front of the Tidal Stream Atlas.


COMPUTATION OF RATES

Computation of rates on tidal stream atlas

 Tidal Stream Atlases provide a great overview for planning a passage, they don't go out of date - although they may be revised occasionally as more accurate data is included.  Crossing the English Channel to Guernsey would require reference to several atlases, you would want NP250 English Channel, you would need NP264 The Channel Islands and Adjacent Coasts of France and would probably also refere to the atlas covering your departure point, say NP337 The Solent and Adjacent Waters.

 

TAKE ADVANTAGE OF THE TIDAL STREAMS

 

Planning your passage around the tidal streams is simply good seamanship.  If you sail at 4 knots and have 2 knots of tide with you will cover 12 miles in two hours, if the 2 knots are against you will cover only 4 miles.  Using the tidal streams to your advantage makes the difference between a quick pleasant passage and a relentless slog.

 

PUBLISHERS OF TIDAL STREAM DATA

The UKHO (United Kingdom Hydrographic Office), the publishers of the Admiralty Tidal Stream Atlases do have some competition.  Various other publishers produce Tidal Stream Atlases often based on local knowledge and research.  Peter Bruce produces the popular 'Solent Tides', Mike Fennessy publishes a pair for South Devon and South Cornwall and Adlard Coles Nautical publishes the popular 'Yachtsman's Manual of Tides' and the various areas by Michael Reeve-Fowkes.  This later series has a different presentation to the Admiralty Atlases but they are easy to master and of great value.  They also have a unique method of calculating tidal stream rates and ready reckoner for tidal heights.