Tides and tidal calculations
Tides and tidal calculations test whether you can turn a tide table into a height of tide and a safe decision. You need standard and secondary ports, the rule of twelfths, chart datum and tidal stream information.
Tide tables and heights
Tide tables list high and low water times and heights for a standard port. For a secondary port, apply the time and height differences in the almanac, paying attention to signs. Times are usually UTC; in summer, convert to local clock time (BST = UTC + 1). Heights are above chart datum. MHWS, MHWN, MLWS and MLWN describe typical ranges and help you judge spring or neap conditions. Interpolate between HW and LW to find the height at a required time.
Rule of twelfths
Over the six hours from low water to high water, the rise is approximately 1, 2, 3, 3, 2, 1 twelfths of the range. Use it for a quick height estimate, not precise navigation. The range is HW height minus LW height, not the height of tide itself.
Worked example: LW is 0.8 m, HW is 4.4 m, so range = 3.6 m. You need the height two hours after LW. In the first two hours the rise is 1 + 2 = 3 twelfths of the range: 3/12 × 3.6 = 0.9 m. Height of tide = 0.8 + 0.9 = 1.7 m above chart datum.
Tidal streams
Tidal stream atlases give set (direction) and rate (speed) for springs and neaps. Interpolate for the day, then use the stream in a course-to-steer or EP calculation. Near headlands and harbour entrances the stream can run much harder than the atlas suggests; allow for wind against tide.
Traps: using the range as the height of tide; applying the rule of twelfths from HW; forgetting the clock change; adding secondary-port differences with the wrong sign; using spring rates on a neap tide; confusing drying height with charted depth; and ignoring the need for under-keel clearance.