Cambridge School of Navigation

Competent Skipper


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Tides and Tidal Streams


Chart Datum is an arbitrary level from which Heights of Tide and Charted Depths are measured.
Chart Datum represents the level of Lowest Astronomical Tide (LAT):  ie,  the lowest level of tide which can be calculated from the relative positions of the sun and moon and the rotation of the earth.

Height of Tide is measured above Chart Datum
Charted depth is measured below chart datum

It follows that:

Depth of water = Height of tide + charted depth.

On any chart the numbers on the sea represent charted depth (below chart datum).
The numbers on the green parts represent charted depth (below chart datum) but the underscore makes the number negative.

On the left of the diagram 57 means 5.7metres below chart datum. On the right of the diagram 35 means 3.5m above chart datum (negative 3.5m below).
Some American charts show charted depth in fathoms. A fathom is 6 feet (roughly 2m). 54 would then mean 5 fathoms and 4 feet (roughly 11 or 12m). 57 would be meaningless.


In this diagram the height of tide is about 2.3m (above chart datum). The depth of water varies according to the charted depth.
Where the charted depth is 57 the depth of water is 2.3m + 5.7m = 8m.
Where the charted depth is 23 the depth of water is(-2.3m) + 2.3m = 0m
Where the charted depth is 35 the depth is 2.3m + (-3.5m) = -1.2m
or, 1.2m above sea level
In other words:

Depth of Water = Height of Tide + Charted Depth

This diagram shows that Spring tides have a greater range of Height of Tide than do Neap tides.
During Spring tides the high tides are very high and the low tides are very low.
During Neap tides the high tides are lower and the low tides are higher.


Tide Tables

An almanac contains, among other things, tables showing the Heights of Tide at every Low Water and every High Water throughout the year, for Standard (or Primary) ports.
For Secondary ports the almanac will give a table of differences from the relevant Primary (or Standard) port.   Secondary port calculations are described in a separate chapter for CS/YM candidates.

Tide table from RYA training almanac
Tide table from Reeds Almanac

The tide tables and the differences tables can be used along with the tidal curves to find the Height of Tide at any time on any day of the year.

Tidal curves
The Admiralty tidal curve

  • The heading tells you the Standard Port to which the curve refers.   They are all different:  some are very different.
  • A legend box gives mean (average) range of Spring tides and Neap tides and which one is the dotted line.
  • Below the curve is a scale of times either side of HW at the Standard Port.   The boxes should be filled in with times,  as instructed.   Note that the two boxes labelled LW should have the actual times of Low Water.
    The scale of times has marks at 10-minute intervals.
    Note that this is a scale of actual times: it has nothing at all to do with the hour intervals used to calculate tidal streams.
  • The left hand part of the curve represents the rising (flooding) tide. The right hand part represents the falling (ebbing) tide.
  • From the time of High Water, at the bottom, to the time of the curve is an arbitrary scale, labelled 'Factor', from 0 (LW) to 1.0 (HW)
  • To the left of the scale of times is a scale of Low Water Heights of Tide, marked in metres and tenths.
  • To the left of the top of the curve is a scale of High Water Heights of Tide, marked in metres and tenths.

For any calculation of Heights of Tide the procedure starts in the same way:

  1. Transfer the relevant part of the tide table to your notebook.

  2. On the Admiralty curve, enter in the time of HW, and all the other times.
  3. On the top scale,  mark the Height of Tide at High Water .
  4. On the bottom scale mark the two Heights of Tide of the two Low Waters.
  5. Join the High Water mark to each of the Low Water marks with a straight line. Label the two lines to show which is the flood and which is the ebb tide.
    These two lines convert the curved lines of the tidal curve into straight lines. They also convert the arbitrary (0.0 to 1.0) Factor scale into a scale of metres Height of Tide.
    They can now be used to find the Height of Tide at any point in time,  as follows.
  6. What is the Height of Tide at 1819SP on 1 November?
    the times and heights have been entered onto the curve to the right.
  7. Mark the time 1819SP on the time scale.
  8. Draw a vertical line to the appropriate part of the curve (Neaps or Springs).
  9. Draw an horizontal line to the appropriate HW-LW straight line.
  10. Draw a vertical line (up or down) to the Height of Tide scale. Read off the Height of Tide (2.9 metres) at that time.

Remembering that

Depth of Water = Height of Tide + Charted Depth
Depth at High Water = Depth now + Rise of Tide to high water
Depth at Low Water = Depth now - Fall of Tide to next low water

the Admiralty tidal curve can be used to answer a number of questions.

  • Having anchored here,  what will be the clearance under the boat at the next LW?
  • Having anchored here,  how much scope of rode do I need to veer for the next HW?
  • Having moored to the scrubbing posts,  at what time will my keel touch the grid?
    At what time will the grid become uncovered so that I can start scrubbing?
    At what time will the tide cover the grid again?
    At what time will my boat float again so that I can leave the posts?
  • If I moor alongside this harbour wall, will my keel touch the bottom before LW?
  • Having run aground on this sandbank,  when will we float off again?

  • Having anchored here,  what will be the clearance under the boat at the next LW?
    Let's anchor in Hamilton Sound,  at 1619SP on 1 November in 5m of water. Our draught is 2m.

    We know that the depth of water is 5m because the sounder tells us so.
    We have already calculated that the height of tide at 1619SP is 2.9m. The tide will rise another (5.3 - 2.9) = 2.4m to HW at 2139SP.
    It will then fall (5.3 - 1.2) = 4.1m to the next LW at 0349SP. The overall fall of tide between now (1619SP) and the next low water (0349SP) will be (2.9 - 1.2) = 1.7m.
    If the depth now is 5m and the tide falls 1.7m, the depth at 0349SP will be 3.3m.
    Since we draw 2m, we shall still have (3.3 - 2) = 1.3m clearance below the keel.

  • Having anchored here,  how much scope of rode do I need to veer for the next HW?

    The depth now (1619SP) is 5m,  with a height of tide of 2.9m.

    At high water (2139SP) the height of tide will be 5.3m: ie, the tide will rise 2.4m.
    So the depth will increase by 2.4m to (5 + 2.4) = 7.4m.
    We'll need to veer (7.4 x 4) = 25.6m (say 30m) of chain, or 44.4 (say 45m) of rope.

  • Having moored to the scrubbing post,  at what time will my keel touch the grid?

    We're using a grid in Hamilton Sound on 1 November.
    We tie to the posts at 1015SP in a depth of water of 3m.
    It's the same boat, so our draught is still 2m, and we have 1.0m of clearance below the keel as we tie up.
    On the tidal curve,
    we draw the tide falling from 5.1m HoT at 0855Sp to 1.0m HoT t 1518SP, and then rising to 5.3m HoT at 2139SP.
    At 1015SP the height of tide is 4.4m. It will fall 1.0m, to 3.4m HoT at 1115SP. So the keel will touch the grid at 1115SP, which is when we must heel the boat to prevent it falling away from the posts.
    The tide needs to fall another 2m, to 1.4m HoT, to dry the bottom of the keel. This will happen at 1345SP.
    If we wear our waders we can probably start scrubbing at about 1330SP. Luncheon at noon, then.
    The tide will rise again to the bottom of the keel at 1649SP, and we'll need to stop scrubbing at about 1700SP.
    We'll have about 3.5 hours: from 1330SP to 1700SP. You, me and Fred, with decent brushes and waders, will get the job done.
    The boat will float off the grid at 1849SP, and we'll be back onto our mooring for supper.

Tidal curves
The UKHO tidal curve

The UK Hydrographic Office is part of the Admiralty.
Their online tidal predictions are easier to use than the old Admiralty tidal curves. Predictions are available not only for the Standard Ports but for very many of the old Secondary Ports, and more.   On the right is a 7 day prediction for Harwich,  and below that a 1 day prediction from the same week.   These curves have many advantages over the old Admiralty tidal curves.

  • The height-of-tide scale is in metres,  not a factor;  so there is no need to draw the straight lines to convert the factor.
  • The use of a real scale,  not a factor,  shows clearly both the diurnal changes in height of tide and the progress from Neaps to Springs and back.
  • The diagram shows day and night clearly,  and gives an approximate time of sunset and sunrise.
  • It also shows up to seven successive days,  so that calculations may be made from day to day more easily.
  • The tide tables are printed close to the tidal curves,  so that they can be compared one to the other.
  • Calculations with the old Admiralty tidal curves give an impression of precision which may be misleading.   The scales on the UKHO curves are less apparently precise
  • If I moor alongside this harbour wall (in old Harwich), will my keel touch the bottom before LW?

    It's 1100UT on 5 March,  we're in 4m of water and we have a draught of 2m.
    At 1100UT the height of tide (from the curve) is about 2.8m.
    LW is about 0.9m HoT: a fall of 1.9m.
    So the depth will fall by 1.9m, from 4m to 2.1m.
    With a draught of 2m we'll have 10cm of clearance at low water. It's a crystal clear, cold March day, barometric pressure of 1024mB. This high pressure will press the water down, and we may have no clearance at all. Let's lunch, and leave before 1400UT, while we still have enough water.

  • Having run aground on this sandbank,  when will we float off again?

    Actually, we left the harbour wall at 1330UT, crossed Harwich Harbour and touched Shotley Spit at 1400UT.   We ran aground in 2m of water (our draught) at 1.3m HoT.
    The curve shows that the tide will fall to 0.9m HoT and then rise again. It will reach 1.3m HoT (depth of 2m) at about 1645UT.




Comparison of Streams & Heights
Measured in metres (m) above Chart Datum
Set measured in °(T)
Drift measured in knots

Many Standard (Primary) ports

Many Secondary ports

One Reference port for the chart
Tide tables in Almanac

Tidal diamonds on the chart

Tidal stream atlas in the almanac or stream atlas

Tidal graphs and curves in the almanac
Table of diamonds (for set & drift) on the chart
To find depth of water
To find set & drift of tide

Tidal Streams

The Tidal Hour

Tidal Streams are based around the concept of the Tidal Hour.
The High Water Hour is a sixty minute period from 30 minutes before High Water to 30 minutes after High Water.
If High Water (at the Reference Port) is 1239, then the High Water Hour is from 1209 to 1309.
The minus one hour (-1 Hr) is, therefore, from 1109 to 1209, and the +1 Hr is from 1309 to 1409.

In the example on the right, a yacht is in the Farlow Channel (Chart RYA3) at 1730UT.

HW at the Reference Port (Victoria) is at 1424UT: the HW Hour is, therefore, from 1354 to 1454.

The yacht is sailing in the +3 hour,  which is the 60 minute period from 1654UT to 1754UT.


Tidal Stream Atlas

A tidal stream atlas illustrates the way in which tides flow to and fro as the tidal height changes.
In a channel the tide flows in one direction toward high water, and then in the other direction toward low water.
In the open sea the direction (set) of the tidal stream depends upon the shape of the tidal basin.
The North Sea is a complex tidal basin; the tides flow into it, and out of it by way of the Dover Straights in the South and the Pentland Firth to the North. The complexities of the British coast to the West, the Danish, Dutch and Belgian coasts to the East and the French coast to the South-East make the tidal streams very complex.

The direction of a tidal stream is known as the set, and is measured in degrees true from True North.   You can accurately measure the set in a stream atlas with your Portland Plotter:  treat the margin as True North.
The rate of flow is known as the drift, and is measured in knots (nautical miles per hour). The drift varies from Spring tides to Neap tides. In a typical Tidal Stream Atlas the first number is the drift at Neaps, the second is the drift at Springs. The two drifts are separated by a dot or comma. Each number should be divided by 10. Thus (in the example to the right) a drift of '08.16' is 0.8 knots at Neaps and 1.6 knots at Springs.   The dot (or comma) is not a decimal point:  it represents the place where the tide was measured.   To interpolate drift on days between Springs and Neaps,  use the Interpolation of Rates table in the almanac.

The Stream Atlas is based on the concept of the Tidal Hour.   It is supposed that the tidal set and drift remain constant for the 60 minutes of each Tidal Hour,  and then change suddenly to the values for the next Tidal Hour.   It isn't true,  of course,  but it's a useful approximation.
Each page of the atlas represents one tidal hour.   In the example page,  "2 hours after HW Victoria" does not mean "2 hours after the time of HW Victoria":  it means "during the +2 hour".

The Stream Atlas is referenced to a single Reference Port (nothing at all to do with Standard Ports for Tidal Heights).   Wherever you sail,  calculations of the Tidal Hour must be made for the appropriate Reference Port.

Table of Tidal Diamonds

There are tidal diamonds scattered fairly regularly over the chart.   They represent the places where the set and drift of the tide has been measured or calculated:  the values are presented in a table somewhere on the chart.
The diamonds refer to the High Water Hour (not time) at the Reference Port for the chart. At diamond A, on this part table, during the 60 minutes of the High Water Hour the set of the tide is 271°T and the drift of the tide varies from 0.5 knots at Neaps to 1.1 knots at Springs.
At diamond B, during the 60 minutes of the -2 hour, the set of the tide is 165°T and the drift varies from 1.7 knots at Neaps to 3.2 knots at Springs.
Use the Interpolation of Rates Table to find values for set and drift on days between Neaps and Springs.

The diamonds are referenced to a single Reference Port (nothing at all to do with Standard Ports for Tidal Heights).   Wherever you sail,  calculations of the Tidal Hour must be made for the appropriate Reference Port.


In North America movements of water are called currents.
The flow of water in a river is a current: it always flows in the same direction.
The Gulf Stream is a current: it flows in the same general direction irrespective of the tides.
There are many currents in all the oceans. They are more or less constant in direction and flow, although occasionally they may reverse or otherwise change direction.
Currents are generally constrained by river banks, or by surrounding water of a different temperature or salinity.

Tides are movements of water under the influence of the gravitational forces of the sun and the moon:  currents are not usually influenced by these forces.

It is important to distinguish clearly between tides and ocean currents.

Comparison of Streams & Heights
Measured in metres (m) above Chart Datum
Set measured in °(T)
Drift measured in knots

Many Standard (Primary) ports

Many Secondary ports

One Reference port for the chart
Tide tables in Almanac

Tidal diamonds on the chart

Tidal stream atlas in the almanac or stream atlas

Tidal graphs and curves in the almanac
Table of diamonds (for set & drift) on the chart
To find depth of water
To find set & drift of tide


Tides (from low-German 'tiet' = 'time') are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon and the Sun and the rotation of the Earth.
Most places in the ocean usually experience two high tides and two low tides each day (semi-diurnal tide), but some locations experience only one high and one low tide each day (diurnal tide). The times and amplitude of the tides at the coast are influenced by the alignment of the Sun and Moon, by the pattern of tides in the deep ocean and by the shape of the coastline and near-shore bathymetry.

Tide changes proceed in the following stages:

  • Sea level rises over several hours, covering the intertidal zone; this is the flood tide.
  • The water rises to its highest level, reaching high tide (High Water).
  • Sea level falls over several hours, revealing the intertidal zone; this is the ebb tide.
  • The water stops falling, reaching low tide (Low Water).

As the tide rises it moves to cover the intertidal zone:  as it falls it moves to uncover the intertidal zone.   These oscillating horizontal movements are known as tidal streams. The moment that the tidal current ceases is called slack water or slack tide. The tide then reverses direction and is said to be turning. Slack water usually occurs near high water and low water,  but there are locations where the moments of slack tide differ significantly from those of high and low water.

Many factors contribute to the tides:  the primary constituents are the Earth's rotation, the positions of the Moon and the Sun relative to Earth, the Moon's altitude (elevation) above the Earth's equator, and the shape of the basin in which the tide is moving.

The semi-diurnal range (the difference in height between high and low waters over about half a day) varies in a two-week cycle. Approximately twice a month, around new moon and full moon when the Sun, Moon and Earth form a line, the tidal force due to the sun reinforces that due to the moon. The tide's range is then at its maximum: this is called the spring tide.
When the Moon is at first quarter or third quarter, the sun and moon are separated by 90° when viewed from the Earth, and the solar tidal force partially cancels the moon's. At these points in the lunar cycle, the tide's range is at its minimum: this is called the neap tide.
Spring tides result in high waters that are higher than average, low waters that are lower than average, 'slack water' time that is shorter than average and stronger tidal currents than average. Neaps result in less extreme tidal conditions. There is about a seven-day interval between springs and neaps.

There is a delay between the phases of the moon and the effect on the tide. Springs and neaps in the North Sea, for example, are two days behind the new/full moon and first/third quarter moon. This is called the tide's age.
The local bathymetry greatly influences the tide's exact time and height at a particular coastal point. There are some extreme cases: the Bay of Fundy, on the east coast of Canada, features the world's largest well-documented tidal ranges, 17 metres (56 ft) because of its shape
Southampton in the United Kingdom has a double high water caused by the interaction between the region's different tidal harmonics, caused primarily by the east/west orientation of the English Channel and the fact that when it is high water at Dover it is low water at Land's End (some 300 nautical miles distant) and vice versa. This is contrary to the popular belief that the flow of water around the Isle of Wight creates two high waters. The Isle of Wight is important, however, since it is responsible for the 'Young Flood Stand', which describes the pause of the incoming tide about three hours after low water.
Because the oscillation modes of the Mediterranean Sea and the Baltic Sea do not coincide with any significant astronomical forcing period, the largest tides are close to their narrow connections with the Atlantic Ocean. Extremely small tides also occur for the same reason in the Gulf of Mexico and Sea of Japan. Elsewhere, as along the southern coast of Australia, low tides can be due to the presence of a nearby amphidrome.

The tidal effects observed along the Menai Strait can also be confusing. A rising tide approaches from the south-west, causing the water in the strait to flow north-eastwards as the level rises. The tide also flows around Anglesey until, after a few hours, it starts to flow into the strait in a south-westerly direction from Beaumaris. By the time this happens the tidal flow from the Caernarfon end is weakening and the tide continues to rise in height but the direction of tidal flow is reversed.
A similar sequence is seen in reverse on a falling tide. This means that slack water between the bridges tends to occur approximately one hour before high tide or low tide.
Theoretically it is possible to ford the strait in the Swellies at low water, spring tides when the depth may fall to less than 0.5 metres (1.6 ft). However, at these times a strong current of around 9 kilometres per hour (5.6 mph) is running, making the passage extremely difficult. Elsewhere in the strait the minimum depth is never less than 2 metres (6.6 ft) until the great sand flats at Lavan sands are reached beyond Bangor.


A few days before Spring tides,  and a few hours before High Water,  the ship would be sailed onto a beach (gently).   As the tide fell and the ship became exposed the sailors would clean it's bottom by scraping away the marine growths.

Modern-day sailors careen their boats by mooring them to scrubbing posts and letting them settle onto the scrubbing grid between tides.   (Actually,  most of them get the boatyard to lift their boats out of the water and use a pressure hose to remove the growth.   They then paint very toxic compounds onto the boat to kill off new growth)