WHAT ARE 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 you with a complete overview for the area covered. For detail on reading Tide Tables Click here.
TIDAL DIAMONDS EXPLAINED
Tidal Diamonds on Admiralty Charts and their circular equivalent on Imray Charts show the position of Tidal Stream information tabulated on the chart.
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
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 on the right shows a typical page from a Tidal Stream Atlas, this one is for One Hour before HW at Devonport (Plymouth).
SET AND RATE
Arrows show the Set (direction) of the Tidal Streams and help to indicate the Rate (speed) by their weight. The Rates in knots are also shown numerically. Close to Start Point (see the enlarged image) 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 comma indicating the position of the data.
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 They don’t go out of date – although they may be revised occasionally as more accurate data is included.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 for the English Channel, and then NP264 The Channel Islands and Adjacent Coasts of France. You would probably also refer to the atlas covering your departure point, say NP337 The Solent and Adjacent Waters.
COMPUTATION OF RATES
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.
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.