If you haven’t studied for a Yachtmaster theory course for a while, you’ve probably forgotten about the ‘computation of rates’ table which sits quietly minding its own business in the front of an Admiralty Tidal Stream Atlas. You won’t find one in your almanac. The table’s sole reason for existence is to interpolate between the spring and neap figures for any tidal arrow inside the book, and it is a lot easier to use than it looks. All you do is make a pencil mark on the ‘neap’ dotted line for the neap tidal speed, and another one on the spring line for the spring velocity. On this example, they are 1.0 knot and 2.1 knots. Draw a straight line joining the dots. Now take a peep in the tide tables to discover the range of tide for the day (the difference between high and low water). Enter the table on the left-hand side with this value and trundle across to the line you’ve drawn. Now go up to the rate axis and read it off. If the range here were 4.8m, the rate would be 1.6 knots.