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Tidal Streams Tidal Stream Interpolation Handout
Tidal stream is the horizontal movement of water as the tide rises and falls. When plotting a course across the tide you have to allow for the sideways movement of the tidal stream; to do this you use a tidal stream atlas or the tide diamond for the area you are sailing in. The direction and speed of the tidal stream varies with height of tide, and can be found for each hour before or after high water at the Standard Port, to coincide with the time you are making the passage. A spring tide has a speed about double that of a neap tide. If in between springs and neaps, for maximum accuracy you have to interpolate between the spring rate and the neap rate. (Using a Computation of Rates chart).
See Tide ladder for a chart to use. See tiderate.pdf for a Computation of Rates Table chart. You will have to draw your own Spring and Neap lines for your selected Standard Port. Here is a WORD version tide_comp_rate.doc
The Yachtsman's Manual of Tides
|The best book on tidal streams planning, with 4 Tidal Atlases for the
English Channel, use it with Cherbourg tide tables here:
The Reeve Fowkes book has an easy method of interpolation of tidal streams with a simple look up table, see here for details: Tide planning course to steer
Calculation of Tidal Streams and Computation of Rates Table - Interpolation between Springs and Neaps
Key notes (SEE Time_Zones )
Always use the Standard Port for Tide Diamonds as defined on the chart or Tidal Atlas - for the RYA exercises it is VICTORIA.
Always adjust your boat time to correspond with the time zone of High Water in the Tide Tables.
The tide tables tell you add one hour in Summer in unshaded areas if you are
in the same time zone - 0000
ie for DST (BST) you add 1 hour to HW Victoria.
For Southern Peninsular Daylight Saving Time, which is UT + 2, (Time Zone -0100) you add 2 hours to HW Victoria to correspond with boat time.
Prolonged strong winds cause surface currents which can modify tidal streams by up to a knot, even in the Mediterranean.
Tidal streams, calculation of computation of rates, tidal stream interpolation, cross channel tides, DST, BST, SPDST
posted 4th May 2011