Secondary port tidal height calculator

Standard ports and secondary ports

Tide tables give the daily times and heights of high and low water for a few standard ports, such as Dover, Newhaven or Portsmouth. Everywhere else is a secondary port. For a secondary port the almanac prints a line of differences: how much earlier or later its high and low waters come, and how much higher or lower they are, than at its standard port. You start from the standard port's tides for the day, apply the differences, then use the standard port's curve for the height in between.

1. The secondary port's times

The time differences are printed under two column times for high water and two for low water, for example 0000 and 0600. Each also stands for twelve hours later: 0000 covers 1200. Between columns you interpolate: see how far the standard port's time lies between the two columns, and go the same proportion of the way between the two differences. Round the result to the nearest minute.

In the example, Newhaven's high water on 19 September 2024 is at 11:45. That is 5 h 45 min past the 06:00 column (+0001), on the way to the 12:00 column (−0002): 96 % of the 6 hours between them.

+1 + 0.96 × (−2 − 1) = −1.9 → −2 min, so Brighton's high water is at 11:43

Low water works the same way. Newhaven's 05:42 is 95 % of the way from the 00:00 column (−0003) to the 06:00 column (−0008), which gives −8 min and a low water at 05:34. The evening low water, 18:02, gives 17:54.

2. The secondary port's heights

Height differences are printed against the standard port's mean levels: MHWS and MHWN for high water, MLWN and MLWS for low water. Interpolate by height: see how far the day's height lies between the two levels, and go the same proportion of the way between the two differences. If the height is beyond the levels, carry on in the same proportion. Round to 0.1 m.

Newhaven's high water, 7.4 m, is above its MHWS of 6.9 m. Brighton's difference is −0.1 m at both MHWS and MHWN, so it stays −0.1 m and Brighton's high water is 7.3 m. At low water, 0.4 m is below MLWS (0.8 m), 129 % of the way from MLWN (2.2 m, difference 0.0) to MLWS (difference +0.1):

0.0 + 1.29 × (0.1 − 0.0) = +0.13 → +0.1 m, so the low water is 0.5 m

The evening low water, 0.5 m at Newhaven, becomes 0.6 m. Brighton's day is: LW 05:34 0.5 m, HW 11:43 7.3 m, LW 17:54 0.6 m.

3. Springs, neaps or in between

The standard port's curve comes in two lines, springs and neaps, each with its mean range. To choose between them, work out the day's range at the standard port, using the low water on the side of high water you are working on, and compare it with those two mean ranges. At or above the spring range, use the springs curve; at or below the neap range, the neaps curve; in between, read both and go the same proportion of the way from one reading to the other.

For a time before high water at Brighton, the range at Newhaven is 7.4 − 0.4 = 7.0 m. Newhaven's mean ranges are 6.1 m at springs and 3.0 m at neaps, so 7.0 m is at or beyond springs: use the springs curve.

4. Reading the curve

The curve is marked along the bottom in hours from high water. Work out how far your time is from the secondary port's high water, go in along the bottom at that time, up to the curve, and across to the factor: 0 is low water and 1 is high water.

We want the height at 10:40, which is 1 h 03 min before Brighton's high water at 11:43. On Newhaven's springs curve, the factor at that time is 0.918.

5. The height of tide

The height is the low water on that side of high water, plus the factor times the range on that side, both at the secondary port:

Height = LW + factor × range

Before high water the low water is the morning one, 0.5 m, and the range on that side is 7.3 − 0.5 = 6.8 m:

0.5 + 0.918 × 6.8 = 6.74 → 6.7 m above chart datum

That is the answer the calculator gives for the example. The full course works this on real Channel charts, by hand, with every step checked.

When can you get over a bar or a drying patch?

First, the height of tide you need: your draught, plus the margin you want under the keel, minus the charted depth. A drying height, the underlined figure on the chart, is above chart datum, so it is added instead.

Then, for each side of high water, the factor that gives that height: the height needed minus the low water, divided by the range on that side. Find it on the curve and read down to the time; between the two times there is enough water.

In the example, with a draught of 1.8 m, a margin of 0.5 m and a patch that dries 1.2 m, you need 1.8 + 0.5 + 1.2 = 3.5 m. Rising, the factor is (3.5 − 0.5) ÷ 6.8 = 0.44, which the curve reaches 2 h 57 min before high water, at 08:46. Falling, it is (3.5 − 0.6) ÷ 6.7 = 0.43, reached 3 h 23 min after high water, at 15:06. You can get over from 08:46 to 15:06.

To practise the whole passage, from the tides to the plan, start with the course.