Tidal streams are most accurately found using an Admiralty Tidal Stream Atlas or equivalent.
Arrows on each hourly chart indicate the direction and rate of the stream. Figures indicate the rate at neaps and springs in 1/10ths of a knot i.e. 21,38 indicates 2.1kn at neaps and 3.8kn at springs.
To determine the rate between springs and neaps it is often satisfactory to interpolate to give an approximate value. Using the above values for instance to find a tidal stream rate half way between springs and neaps you could simply halve the difference between the two values 2.1 and 3.8 i.e. 2.9kn approx.
In order to find a tidal rate accurately however you need to use the Computation of Rates diagram normally found on the inside front cover of the Tidal Stream Atlas. To use this diagram I recommend reading the instructions provided in the Tidal Stream Atlas. Below is an example that you can follow however:
Find the tidal stream rate off Lizard Point at 1600 BST Sat 27th October 2012.
- Find the Mean Range of the tide at Dover for that day. This is the height of HW minus the height of LW. (Note: always use the Dover value not your nearest standard port).
|Dover tides||Sat 27th Oct 2012|
In this case mean range is 6.3 – 1.4 = 4.9m.
- Find the neap and spring rates from the appropriate page and location in the Tidal Stream Atlas. HW Dover is 1111 BST. Appropriate tide page for 1600 BST is 5 hours After HW Dover. The values in the atlas are: 07,13 i.e. 0.7kn, 1.3kn.
- Draw a slanted line joining the dots on the Neaps dotted representing the neaps tidal rate (i.e. 0.7) to the dot on the Springs dotted line representing the spring tidal rate (i.e.1.3). (Note: your line should connect values on the dotted line – not the values at the top and bottom of the diagram. )
- Draw a horizontal line from your mean range value on the left axis (i.e. 4.9) and read the value on the top or bottom axis where this line meets your slanted line i.e. 1.1kn. That is your accurate tidal stream rate!
Using computation of rates may seem laborious if you have many tide rates to calculate but there is a neat trick. Once you have worked out one rate and noted its difference in relation to the neap or spring rate, you can apply the same proportional difference for all other rates that day.
So following from the example above, we see that the calculated rate of 1.1kn is about 80% of the given spring rate value of 1.3kn. Lets say that the next rate we need to check appears in the tidal atlas as 13,23 then we take the value of 80% of the spring value 2.3 which is about 1.9kn.
Note: Computation of rates may sometimes give us values that are lower than the neap value or higher than the spring value.