Any sailor who has explored the British coast will be well aware of the massive power of our tides. Indeed, Avonmouth has the second highest tidal range in the world with a rise of 15m. With this in mind, it's vital to fully understand and interpret tide tables and, even for the most seasoned mariner a quick pre-season refresher is handy.

*Any sailor who has explored the British coast will be well aware of the massive power of our tides. Indeed, Avonmouth has the second highest tidal range in the world with a rise of 15m. With this in mind, it's vital to fully understand and interpret tide tables and, even for the most seasoned mariner a quick pre-season refresher is handy. *

Fortunately, the RYA have just put together a new handbook which will help keep the basics in place. The RYA Introduction to Navigation has been put together by Tim Bartlett and is an ideal prompt and guide when you are starting to overcomplicate things. This excerpt on tides will help give you a steer:

Now, for some of you this will be child's play, but it's best to start at the beginning and then ease yourself in to the tricky stuff. So here we go:

Suppose that we are intending to visit Namley on September 16, and want to know whether it will be safe to cross the bar across the entrance – shown on the chart as having some parts that are 0.3m above chart datum.

The first step is to turn to the pages that deal with Namley Harbour, and find the column for September.

Each day has its own block of information. For September 16 it says:

0039 1.2

0708 3.1

1322 1.1

1954 3.3

The first column of numbers gives the times of High and Low waters, while the second column gives the corresponding heights.

So we can immediately see that high waters (HW) are at 07:08 and 19:54, with heights of 3.1m and 3.3m respectively, while low waters (LW) are at 00:39 and 13:22, with heights of 1.2m and 1.1m.

It’s important to read the note in the corner of the page which says “For Summer Time add ONE hour in non-shaded areas”. In other words, high water is at 08:08 BST and 20:54 BST, while low water is at 01:39 BST and 14:22 BST.

This doesn’t really answer the question of whether it is safe to cross the 0.3m drying heights on Namley Bar.

At 14:22 BST, at low water (LW), with 1.1m of tide, there will be only 1.1 – 0.3 = 0.8m of water – enough for a dinghy, but not much more.

At 20:54 BST, at high water (HW), the tide will have risen to 3.3m above chart datum, so there should be 3.3 – 0.3 = 3.0m – enough for a small ship.

The tide doesn’t rise and fall at a constant rate. At low water (LW), it starts rising – slowly at first, but increasingly quickly. Then, as it approaches high water (HW), it starts slowing down again.

In most places a graph of the changing height of tide would look something like the bell-shaped curve in the illustration. Yachtsmen’s almanacs and Admiralty Tide Tables include tidal curves alongside the tide tables for each major port, making it reasonably easy to estimate the height of tide at times between high and low water.

The published graph actually includes two curves – one for spring tides, and the other for neaps. Use whichever is most appropriate for the tide on that particular day, or estimate a curve between the two for days that are midway between springs and neaps.

Suppose, for instance, we have narrowed down our expected time of arrival at Namley to “about 18:45” We’ve already found that high water (HW) at Namley on that day is at 20:54, so 18:45 is about 2 hours 10 minutes before HW.

Across the bottom of the graph, there is a scale of time, shown as hours before and after HW, while up the centre of the graph is another scale labelled “Factor”.

From the graph we can see that 2 hours 10 minutes before HW corresponds to a factor of 0.7. This means that at 18:45, the tide should have risen 0.7 of the way from low water to high.

- LW was 1.1m, and HW is 3.3m so the difference between the two (the range of tide) is 2.2m
- 0.7 of 2.2 is 1.5m so at 12:30, the tide should have risen 1.5 from LW
- LW was 1.1m, so the Height of Tide at 1845 should be 1.1m + 1.5m = 2.6m above chart datum
- We are interested in a spot where the sea bed is 0.3m above chart datum so the depth of water over Namley Bar should be 2.6m – 0.3m = 2.3m

It’s often even more useful to be able to do the calculation backwards to find out when there will be enough water. Suppose, we need at least 1.7m of water.

- We are interested in a spot where the sea bed is 0.3m above chart datum so we need 1.7m + 0.3m = 2.0m height of tide
- LW was 1.1m, so we want the tide to have risen 2.0 – 1.1 = 0.9m from LW
- LW was 1.1m, and HW is 3.3m so the difference between the two (the range of tide) is 2.2m
- 0.9m is 0.4 of the range so we can cross the bar whenever the factor is bigger than 0.4
- Looking at the curve, this appears to be 3h45m before HW HW is at 20:54, so we should be able to cross the bar any time after 20:54 – 3:45 = 17:09

More basic tide tables may not include the tidal curve. Fortunately, though, the graph for most places is similar to Namley’s bell-shaped curve, so we can use a fairly simple rule of thumb.

It’s called the 'Rule of Twelfths' because it says In the first hour after low water, the tide rises one twelfth of its range In the second hour after low water, the tide rises two twelfths of its range In the third hour after low water, the tide rises three twelfths of its range In the third hour before high water, the tide rises three twelfths of its range In the second hour before high water, the tide rises two twelfths of its range In the last hour before high water, the tide rises one twelfth of its range.

It’s important to be aware, though, that some places, especially where there are islands close to the coast, have tidal curves that are quite severely distorted.

Even such substantial books as Reeds Almanac and the Admiralty Tide Tables cannot include full tables for every port. The solution is provided by difference tables, such as the one, for Port Rampton.

It tells us how the tides at Port Rampton differ from those at the nearest standard port – in this case, Namley. It’s really a very simple table, consisting of one row of information, divided into eight columns. It just looks confusing because the column headings take up more space than the information itself!

The first two columns deal with the time of high water, and tell us that:

- If HW Namley is at midnight or mid-day (00:00 or 12:00), then HW Port Rampton is 35 minutes earlier (-00:35)
- If HW Namley is at 06:00 or 18:00, then HW Port Rampton is 16 minutes earlier (-00:16) The next two columns deal with the time of low water, and tell us that:
- If LW Namley is at midnight or mid-day (00:00 or 12:00), then LW Port Rampton is 6 minutes earlier (-00:06)
- If LW Namley is at 06:00 or 18:00, then HW Port Rampton is 10 minutes earlier (-00:10)

The third pair of columns deal with the height of high water, and tell us that:

- If HW Namley is at 4.0m then HW Port Rampton is 0.5m higher (+0.5)
- If HW Namley is 3.4m then HW Port Rampton is 0.2m higher (+0.2) The final pair of columns treat low water, in exactly the same way, telling us that:
- If LW Namley is at 1.1m then LW Port Rampton is 0.6m higher (+0.6)
- If LW Namley is 0.4m then LW Port Rampton is 0.2m higher (+0.2)

Knowing that HW Nameley is expected to be at 1954 GMT, we can look at the difference tables and see that:

- If HW Namley is at midnight, HW Port Rampton is 35 minutes earlier (-00:35) but if HW Namley is at 18:00, HW Port Rampton is 16 minutes earlier (-00:16)
- HW Namley is at 19:54, which is about one third of the way between 18:00 and midnight.
- So the difference for Port Rampton is about one third of the way between 16 minutes and 35 minutes: say 22minutes earlier than at Namley.
- So HW Port Rampton is at 19:54 – 22 = 19:32
- To complete the job add an hour to convert to BST 19:32 + 01:00 = 20:32 BST

The height calculation is similar. The difference tables tell us that:

- If HW Namley is at 4.0m, HW Port Rampton is 0.5m higher (+0.5) but if HW Namley is 3.4m, HW Port Rampton is only 0.2m higher (+0.2)
- In this case HW Namley is 3.3m, which is as close to 3.4m as makes no difference.
- So the height of HW Port Rampton is 3.3m + 0.2m = 3.5m

Having worked out the time and height of high water, the procedure for finding the corresponding information for low water is exactly the same.

Graphs are rarely provided for secondary ports, but for most practical purposes, the graph for the nearest standard port will do.

*These tips and many more are available in the all new Introduction to Navigation available at the RYA Shop.*