Tide tables and chart datum

From the chart to the tide

In Lesson 8 you learned that every sounding and drying height on the chart is measured from chart datum, and that the depth you actually have depends on the height of tide: how far the sea surface is above chart datum at that moment. In Lesson 23 you saw why the tide rises and falls, twice a day, with bigger springs and smaller neaps. This lesson joins the two together. You will read a real-style tide table, convert its times to the clock on your wrist, meet the tidal levels that chart datum sits beneath, and use them to answer the question every skipper asks near shallow water: will we clear it?

Reading a tide table

A tide table lists, for each day, the times and heights of high water (HW) and low water (LW) at a main port. Almanacs print tables like this for a set of main ports, called standard ports; Lesson 25 shows how to use them for times in between. Our standard port is Port Kerran, on the fictional Isle of Kerran, and its table covers 1 to 14 June 2027. Here are the first two days:

Each line is one tide: a time (24-hour clock), and a height in metres above chart datum. Many almanacs don’t label which tide is which (our table adds an HW/LW column to help) — you tell them apart by the heights. The big heights are high waters and the small heights are low waters, and they alternate. On Tue 1 Jun high water is at 0122, 5.2 m, then low water at 0732, 0.4 m, and so on: two of each, a little over six hours apart.

Successive high waters are roughly 12 hours 25 minutes apart, so the tides come later each day — on average by about 50 minutes, though the step varies through the fortnight. Every so often the fourth tide of the day is pushed past midnight into the next day, leaving a day with only three tides. In the Port Kerran table that happens on Mon 7 Jun:

Time zones: UT and BST

Look at the heading of the table: the Port Kerran times are in UT (Universal Time — for our purposes the same as Greenwich Mean Time). In summer, from the last Sunday in March to the last Sunday in October, UK clocks run on British Summer Time (BST), which is one hour ahead of UT. June is in summer, so before you use a time from this table, add one hour.

Chart datum and the tidal levels

Chart datum (CD) is the level that charted depths and drying heights are measured from, and the level the heights in the tide table are measured from too — which is what lets you add one to the other. On UK (Admiralty) charts it is set at approximately the lowest astronomical tide (LAT): the lowest level the tide can be predicted to reach under average weather conditions. So the sea is nearly always above chart datum, and a charted depth is close to the least water you should expect.

For a standard port, the almanac also gives the average heights of high and low water at springs and neaps. Together with chart datum and the highest astronomical tide they form a ladder of levels:

The levels also give you the mean ranges from Lesson 23: MHWS − MLWS = 4.8 m at springs, and MHWN − MLWN = 2.4 m at neaps. Compare a day’s range with these to judge where it sits between springs and neaps.

Height of tide is the height of the sea surface above chart datum.

Two things are measured from a different level. Heights of land and the elevations of lights are above MHWS (5.2 m above chart datum at Port Kerran), as you saw in Lesson 8. And vertical clearances under bridges and overhead cables are measured from a level stated in the chart’s notes — recent Admiralty charts use HAT — so read the notes on any chart that shows them. The practice chart has no bridges or cables.

Depth, drying heights and clearance under the keel

With a height of tide from the table, the arithmetic from Lesson 8 gives the depth, and one more step gives the water under your boat. The draught is how far the keel reaches below the waterline: 1.8 m for the course yacht. What is left between the bottom of the keel and the seabed is the clearance under the keel.

Notice that the wreck, with 4.2 m over it at chart datum, leaves room under the keel at every state of tide; the shallow sounding inshore of Mew Rock and Brennan Bank do not leave much at low water springs. Mew Rock shows at low water springs, but it is covered at every high water and, around neaps, all day long: for much of the time it is hidden, and its isolated danger mark is what warns you it is there.

Will we clear it?

Before you cross shallow water, decide on a safety margin — the least clearance under the keel you will accept. It is the skipper’s choice, commonly somewhere between 0.5 m and 1.0 m: more in swell or waves (the boat drops into the troughs), over rock rather than sand or mud, at night, or when you are less sure of the tide. Then work it backwards to find the least height of tide you need:

Low water is the least height of tide between one high water and the next, so if you have enough at the low water height you have enough all through that tide — weather permitting. When you can’t make it work at low water, the next lessons show how to find the time when the tide has risen far enough.