Tidal curves: standard ports

Standard ports

In Lesson 24 you read the Port Kerran tide table: the time and height of each high water (HW) and low water (LW), in UT, converted to BST by adding an hour. That is enough if you want to know about the tide at high or low water. But real questions fall in between: how much water will there be over the bank at 1900? or when will there be enough water to cross it? This lesson answers both, using the port’s tidal curve.

A standard port is a port with full daily predictions: the tide tables (in a nautical almanac or the Admiralty Tide Tables) list every HW and LW there, and print a tidal curve showing how the tide rises and falls between them. Port Kerran is our standard port. Smaller harbours and anchorages nearby are secondary ports: they have no daily table of their own, and you work out their tides from a standard port by applying time and height differences. That is Lesson 27; the curve method you learn here carries straight over to it.

Each standard port has its own curve because the shape of the tide varies from place to place. At many ports the rise and the fall take different lengths of time, so the curve is lopsided; a few ports with unusual tides, such as those in the Solent, even have their curves drawn around low water instead. Always use the curve printed for the port you are working.

The table gives you HW and LW. The curve fills in everything between them.

Reading the curve diagram

The Admiralty-style curve diagram looks busy at first, but it has only four parts.

• The time axis along the bottom is in hours from high water, from 6 hours before HW to 6 hours after. On a printed form there is a box under “HW” where you write the time of HW, and you fill in the clock times of the hours either side.
• The factor scale up the middle runs from 0 (low water) to 1 (high water). The factor is simply the fraction of the range that the tide has risen above low water: 0.5 means halfway between LW and HW.
• The curves show the factor at each time. The solid curve is for spring tides (mean range 4.8 m at Port Kerran) and the dashed curve for neaps (mean range 2.4 m).
• The height panel on the left has two height scales: HW heights along the top and LW heights along the bottom. Joining the day’s LW height (bottom) to its HW height (top) gives the range line, which turns a factor into a height of tide.

Port Kerran’s curve is nearly symmetrical, with the rise and the fall each taking a little over six hours, and the spring and neap curves lie close together. At some ports they differ a lot, so judging springs or neaps matters more there.

Before you start: the right HW and LW

Most mistakes with tidal curves happen before the curve is touched. Three decisions come first.

1. Which high water? The one nearest the time you are interested in. The curve is drawn around HW, so every time is measured from it.
2. Which low water? The one on the same side of that HW as your time. Before HW the tide is rising from the previous LW, so use the LW before; after HW the tide is falling towards the next LW, so use the LW after.
3. Springs, neaps or in between? Work out the range (HW height − that LW height) and compare it with the mean ranges printed on the diagram. As in Lesson 23, the day is called springs, neaps or in between by whichever of 4.8 m, 3.6 m (halfway) and 2.4 m its range is nearest. For reading the curve, though, use the actual range, as in the table below.

For example, for the height at 1900 BST on Fri 4 Jun: the nearest HW is 1535 UT = 1635 BST, 4.5 m. 1900 is after it, so use the following LW, 2145 UT = 2245 BST, 1.2 m. Range = 4.5 − 1.2 = 3.3 m.

Our range of 3.3 m is in between (nearest 3.6 m); read about 40% of the way from the neap curve to the spring curve. Never extend beyond the curves: a range bigger than springs still uses the spring curve, and one smaller than neaps the neap curve.

Method A: height of tide at a given time

1. Write the HW time (BST) in the box under HW and work out how long before or after HW your time is.
2. On the height panel, mark the LW height on the bottom scale and the HW height on the top scale, and join them: the range line.
3. From your time on the bottom axis, go straight up to the curve (springs, neaps or in between).
4. From that point go horizontally across to the range line.
5. From the range line go straight up to the height scale and read the height of tide.

The drawing is really doing a sum. Where you met the curve you can read the factor, and the range line just multiplies the factor by the range and adds it to the LW height:

Method B: the time for a given height

Often you know the height you need and want the time: when will there be enough water to cross a bank, enter a harbour or float off a mooring? Work the construction backwards.

1. Pick the HW nearest the time you expect, the LW on the same side (rising tide: LW before; falling tide: LW after), and draw the range line.
2. Find the height you need on the top scale and go straight down to the range line.
3. Go horizontally to the curve, on the rising side (left of HW) or the falling side (right of HW) as required.
4. Go straight down to the time axis and read the time from HW. Convert it to a clock time.

If the height you need is below the LW, there is always at least that much water on that side of HW; if it is above the HW, this tide never gives it.

Putting it to work: crossing a shallow patch

Brennan Bank, south of Gull Point, has a least charted depth of 2.1 m. Crossing it saves a detour round the South Brennan buoy, but our yacht draws 1.8 m. How much water you want under the keel is the skipper’s call: perhaps 0.5 m on a calm day over sand, more in a swell, which makes the boat rise and fall and breaks heavily on shoals. Here the skipper chooses 1.5 m.

Notice that the two answers use different high waters: each time is worked from the HW nearest to it, with its own range line. At neaps the problem can vanish altogether: on Tue 8 Jun no LW falls below 1.8 m, giving at least 3.9 m over the bank, so the yacht has 2.1 m under the keel at any state of the tide.

The same method tells you when a drying hazard covers. Mew Rock dries 1.2 m, so it is uncovered whenever the height of tide is below 1.2 m. On the morning of Tue 1 Jun it uncovers at about 0654 BST and covers again at about 1010 BST, the same times as for the bank, since both depend on the tide reaching 1.2 m. Covered, it is invisible but no less dangerous; its isolated danger beacon marks it at every state of tide, so keep clear.

Port Kerran levels for reference: MHWS 5.2 m, MHWN 4.0 m, MLWN 1.6 m, MLWS 0.4 m above chart datum.