Lesson 28 of 40 · Module 4 — Tides and tidal streams

Tidal streams: atlases and tidal diamonds

Finding the rate and direction of the tide for your passage.

Day Skipper25 minVideo · Notes · Interactive · Exercises

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0:00 / 4:35Chapter 1 of 9: Tide and tidal stream

Transcript

In the last few lessons you worked out how high the tide will be. But as the tide rises and falls, the water also moves sideways, flowing along the coast and round the headlands. That horizontal movement is the tidal stream. It carries your boat with it, whatever she is doing through the water, so you need its direction and strength for every hour of a passage.
A tidal stream is described by its set and its rate. The set is the direction the water flows towards, in degrees true. That is the opposite of a wind, which is named after the direction it blows from. The rate is the stream’s speed in knots, and the drift is how far it carries you: rate times time. One and a half knots for forty minutes is a drift of one mile.
On the chart, look for magenta diamonds with a letter inside. Around the Isle of Kerran there are four: A in the approach to Kerran Bay, B off Gull Point, C off Kerran Head, and D in Kerran Roads. Each one marks a place where the stream has been measured. A table printed on the chart gives the stream at each diamond for every hour, from six hours before to six hours after high water at Port Kerran.
Say you will be sailing past diamond C at about nine in the morning on Friday the eleventh of June. The tide table gives high water Port Kerran at ten zero eight, in UT. Add one hour for British Summer Time: eleven zero eight. Nine o’clock is about two hours before high water, so use the row for two hours before. Each row covers the hour centred on its time, so this one runs from zero eight thirty-eight to zero nine thirty-eight.
The row gives two rates: one point seven knots at springs and zero point eight at neaps. To choose between them, work out the range. High water is four point three metres and the low water before it one point three, so the range is three point zero metres. The mean ranges at Port Kerran are four point eight at springs and two point four at neaps, so today is a quarter of the way from neaps to springs. The rate is the neap rate plus a quarter of the difference: about one knot, setting zero seven zero degrees true.
Now put it to work. At zero eight thirty-eight you fix your position south-west of diamond C, and steer due north, zero zero zero degrees true. An hour later the log shows four point five miles run, so lay off the water track with one arrow, to the DR. From the DR, draw one hour of stream from diamond C: zero seven zero degrees, one mile, with three arrows. Its end is your zero nine thirty-eight estimated position.
A tidal stream atlas shows the same kind of information as a set of pictures. There is one page for each hour, from six hours before to six hours after high water at the atlas’s standard port. Arrows show the direction of the stream, and the thicker the arrow, the stronger the stream. Beside them, pairs of figures give the rates in tenths of a knot, neaps first. Off Gull Point at high water, eleven, twenty-two means one point one knots at neaps and two point two at springs.
Streams run hardest off headlands, where the water is squeezed round the land. At diamond B, off Gull Point, the spring rate reaches two point four knots, twice the strongest rate at A in the bay approach. Each time the stream turns there is a short period of slack water, when it is weak and changing direction. At A, that happens about four hours before and three hours after high water.
When the wind blows against the stream, the waves become shorter and steeper, and off a headland they can be dangerous for a small boat. With wind and stream going the same way, the sea is much smoother. So time your passage to carry a fair stream, and to avoid wind against tide off the headlands. Heading west past Kerran Head on the eleventh, waiting until the ebb starts in the afternoon turns about zero point nine knots against you into about one point three knots in your favour. Next, in Module 5, you will use the stream to work out the course to steer.

In this lesson you will learn

Explain the difference between the tide and the tidal stream, and describe a stream by its set, rate and drift
Read a tidal diamond table: find HW at the standard port in BST and choose the row centred on your time
Interpolate a stream rate between neaps and springs from the day’s range
Read a tidal stream atlas, including its neap and spring figures
Plot an EP using a stream from a diamond, and use streams to plan a passage: headlands, slack water and wind against tide

Tide and tidal stream

In Lessons 24 to 27 you worked out how high the tide will be. But as the tide rises and falls, the water also moves sideways: it flows along the coast, through channels and round headlands. That horizontal movement is the tidal stream. It carries the boat with it, whatever she is doing through the water.

	Tide (height)	Tidal stream
What moves	The sea level, up and down	The water, sideways
Measured in	Metres above chart datum	Direction (°T) and rate (knots)
Where to find it	Tide tables and tidal curves	Tidal diamonds on the chart, tidal stream atlases
Why it matters	Depth, clearance under the keel	Your EP, the course to steer, passage timing

You met the three words that describe a stream in Lesson 14, where the stream was simply given to you:

Set — the direction the water flows towards, in degrees true. (A wind is named the other way round, by where it blows from.)
Rate — its speed, in knots.
Drift — the distance it carries you in a given time.

drift = rate × time

For example, 1.5 kn for 40 minutes: 1.5 × 40 ÷ 60 = 1.0 M.

This lesson shows where the set and rate come from, for any hour of any day.

Tidal diamonds and the stream table

On a chart, look for small magenta diamonds with a letter inside. Each one marks a position where the stream has been measured. Around the Isle of Kerran there are four:

The four tidal diamonds on the practice chart. B and C are off the headlands; A and D are in more sheltered water.

A table printed on the chart, usually in a corner, gives the stream at each diamond. Its heading gives each diamond’s position, and it is referred to HW at a standard port — here, Port Kerran. There is a row for each hour from six hours before HW to six hours after, and for each diamond three columns: the direction (°T, the set), the rate at springs and the rate at neaps, both in knots.

Tidal streams referred to HW Port Kerran — direction °T, rates in knots at springs and neaps
Hours	A49°43.1'N 004°08.6'W	B49°48.3'N 004°03.2'W
Hours	DirSpNp	DirSpNp
6 before	2450.80.4	2301.60.8
5 before	2500.50.2	2351.00.5
4 before	3300.20.1	3000.30.1
3 before	0600.60.3	0401.00.5
2 before	0651.00.5	0451.90.9
1 before	0701.20.6	0502.41.2
HW	0701.10.5	0502.21.1
1 after	0700.90.4	0501.80.9
2 after	0750.50.2	0551.10.5
3 after	1500.20.1	1200.30.1
4 after	2400.60.3	2201.10.5
5 after	2451.00.5	2252.01.0
6 after	2501.10.5	2302.31.1

Tidal streams referred to HW Port Kerran — direction °T, rates in knots at springs and neaps
Hours	C49°46.4'N 004°20.4'W	D49°51.4'N 004°09.6'W
Hours	DirSpNp	DirSpNp
6 before	2501.40.7	2700.90.4
5 before	2550.90.4	2750.60.3
4 before	3400.30.1	3500.20.1
3 before	0650.90.4	0850.60.3
2 before	0701.70.8	0901.10.5
1 before	0752.11.0	0901.40.7
HW	0752.01.0	0951.30.6
1 after	0751.60.8	0951.00.5
2 after	0801.00.5	1000.60.3
3 after	1600.30.1	1800.20.1
4 after	2451.00.5	2650.70.3
5 after	2501.80.9	2701.10.5
6 after	2552.01.0	2701.30.6

Read down a column and you can see the tide at work. At A the stream sets roughly east-north-east for the hours around HW (the flood), swings round, then sets west-south-west (the ebb). The rows where the rate falls to almost nothing and the direction jumps are the stream turning.

Step 1: choose the row

Find HW at the standard port from the tide table, and convert it to BST in summer (add one hour, Lesson 24).
Work out how long before or after HW your time is.
Pick the row centred on that time. Each row applies for the hour centred on it: the row for 2 hours after HW is used from HW+1½ to HW+2½. So round to the nearest whole hour.

Worked example

Passing diamond C on Fri 11 June at about 0900 BST

1The tide table gives the morning HW Port Kerran at 1008 UT, 4.3 m. Add an hour: 1108 BST.
20900 is 2 h 08 min before HW, which rounds to 2 hours: use the row “2 hours before HW”.
3That row covers the hour from 0838 to 0938 BST. At C it reads 070°T, springs 1.7 kn, neaps 0.8 kn.

Port Kerran — times UT (add 1 hour for BST), heights in metres above chart datum
Day	Time	m	HW/LW
Fri 11 Jun	0356	1.3	LW
	1008	4.3	HW
	1620	1.2	LW
	2231	4.5	HW

Check your understanding: HW Port Kerran is 1108 BST. Which row do you use at 1250 BST?

Step 2: springs, neaps or in between?

The table gives two rates, for mean springs and mean neaps. Most days are somewhere between. To find out where, work out the day’s range at the standard port — HW height minus the LW next to it, on the same side of HW as your time (the LW before HW if you are before it, the LW after if you are after) — and compare it with the mean ranges: 4.8 m at springs and 2.4 m at neaps at Port Kerran.

rate = neap + (spring − neap) × fraction

Fraction = (range − 2.4) ÷ (4.8 − 2.4): 0 at mean neaps, 1 at mean springs.

Worked example

The rate at C, continued

1Range = HW 4.3 − LW before it 1.3 (0456 BST) = 3.0 m.
2Fraction = (3.0 − 2.4) ÷ 2.4 = 0.25: a quarter of the way from neaps to springs.
3Rate = 0.8 + (1.7 − 0.8) × 0.25 = 1.03, about 1.0 kn.

Stream at C, 0838–0938 BST: 070°T at 1.0 kn

A computation of rates graph, drawn for Port Kerran’s mean ranges. Join the neap rate (on the neaps line) to the spring rate (on the springs line), then read across from the day’s range and down to the rate.

Tidal stream atlases print a graph like this, called computation of rates, so you can do the interpolation without arithmetic. It also works outside the two mean ranges: just extend the line. In this table neaps fall on days 7 and 8, when the range is only about 2.0 m — less than the mean neap range — so the rate comes out a little below the neap figure. At B at HW, for example: 1.1 + (2.2 − 1.1) × (−0.17) = 0.9 kn.

InteractiveTidal diamond reader

Choose a day, a time (BST) and a diamond. The reader finds HW Port Kerran, converts it to BST, picks the row, works out the range and the rate, and draws one hour of stream at the diamond. Try the same time at B and at A, and a spring day against a neap day.

Day (June 2027)

Time (BST)

Diamond

HW Port Kerran. The nearest is 1008 UT on Fri 11 Jun (4.3 m). Add an hour for BST: 1108 BST.
The row. 0900 BST is 2 h 08 min before HW, so use the row “2 hours before HW” (HW−2). It covers 0838–0938 BST.
Diamond C, off Kerran Head. That row gives 070°T, springs 1.7 kn, neaps 0.8 kn.
The range. HW 4.3 − LW 1.3 (0456 BST) = 3.0 m. Fraction of the way from neaps to springs: (3.0 − 2.4) ÷ 2.4 = 0.25.
The rate. 0.8 + (1.7 − 0.8) × 0.25 = 1.03, about 1.0 kn.

Stream at C: sets 070°T at 1.0 kn — 1.0 M of drift in an hour

Port Kerran — times UT (add 1 hour for BST), heights in metres above chart datum
Day	Time	m	HW/LW
Fri 11 Jun	0356	1.3	LW
	1008	4.3	HW
	1620	1.2	LW
	2231	4.5	HW

Tidal streams referred to HW Port Kerran — direction °T, rates in knots at springs and neaps
Hours	C49°46.4'N 004°20.4'W
Hours	DirSpNp
6 before	2501.40.7
5 before	2550.90.4
4 before	3400.30.1
3 before	0650.90.4
2 before	0701.70.8
1 before	0752.11.0
HW	0752.01.0
1 after	0751.60.8
2 after	0801.00.5
3 after	1600.30.1
4 after	2451.00.5
5 after	2501.80.9
6 after	2552.01.0

Tidal stream atlases

A tidal stream atlas gives the same kind of information as a set of pictures. It has thirteen pages, one for each hour from six hours before to six hours after HW at its standard port (for many UK atlases, Dover; each atlas says which). Each page is a small chart of the area covered with arrows:

The arrows show the direction of the stream; in most atlases, the heavier the arrow, the stronger the stream.
The figures beside them give the rates in tenths of a knot, neaps first. In Admiralty atlases, 08,15 means 0.8 kn at neaps and 1.5 kn at springs. Check the key in your own atlas.
Each page covers the hour centred on its time, just like a row of the diamond table.

You use it exactly as you use the diamonds: find HW at the atlas’s standard port, turn to the right page, and interpolate between the neap and spring figures by the range — using the atlas’s own computation of rates graph and its standard port’s mean ranges. The page below is drawn from the four Kerran diamonds only; off Gull Point at HW, for example, B’s arrow reads 11,22.

InteractiveAtlas pages round the island

Press Play, or step through the pages from HW−6 to HW+6. Watch the flood build to the east, die away at slack water and turn into the west-going ebb. Pick a day to see the clock times each page covers.

HW on

6 hours before HW Port Kerran

West-going stream: the ebb · with HW at 1108 BST this page covers 0438–0538 BST

A (in the approach to Kerran Bay): 04,08 → 0.4 kn neaps, 0.8 kn springs, setting 245°T
B (off Gull Point): 08,16 → 0.8 kn neaps, 1.6 kn springs, setting 230°T
C (off Kerran Head): 07,14 → 0.7 kn neaps, 1.4 kn springs, setting 250°T
D (in Kerran Roads): 04,09 → 0.4 kn neaps, 0.9 kn springs, setting 270°T

An illustrative atlas-style page drawn from the four diamonds only. A real atlas covers its whole area with arrows; here, the heavier and longer the arrow, the stronger the stream.

	Tidal diamonds	Tidal stream atlas
Shows	The stream at a few exact points	The pattern over the whole area
Rates	Springs and neaps columns, in knots	Pairs of figures, neaps first, in tenths of a knot
Referred to	HW at the standard port named on the chart	HW at the atlas’s standard port, named on each page
Best for	Plotting an EP or a course to steer close to a diamond	Passage planning: where the stream is fair, and when it turns

Using the stream: an EP from a diamond

Now apply the stream exactly as in Lesson 14, but with figures you have worked out yourself.

Worked example

The 0938 EP west of Kerran Head

1At 0838 BST a fix puts you at 49°44.6'N 004°22.0'W, south-west of diamond C. You steer 000°T; the log reads 2214.6.
2At 0938 the log reads 2219.1: 4.5 M run. Lay off the water track (one arrow) to the 0938 DR.
3This is the tidal hour 0838–0938, so the stream is the one you worked out at C: 070°T at 1.0 kn, a drift of 1.0 M in the hour.
4From the DR, lay off 070°T for 1.0 M (three arrows). Mark the EP and label it 0938.

0938 EP: 49°49.4'N 004°20.5'W; course over the ground 011°T

The 0838 fix, water track to the 0938 DR, one hour of stream from diamond C, and the 0938 EP.

Streams in passage planning

The stream decides how long a passage takes and how comfortable it is. Four things to look for:

Headlands. Water is squeezed round them, so streams run hardest there. At B, off Gull Point, the spring rate reaches 2.4 kn; at A, in the bay approach, never more than 1.2 kn. Charts and pilot books warn of tide races off some headlands.
Slack water. Each time the stream turns there is a short period when it is weak and changing direction. At A that is around HW−4 and HW+3. Slack is often the easiest time to round a headland or pass a narrow channel.
Wind against tide. When the wind blows against the stream, the waves become shorter and steeper; with wind and stream together, the sea is much smoother.
A fair stream. Time your departure so the stream carries you, especially past the headlands, where it is strongest. Points you can only pass comfortably at certain states of the tide are often called tidal gates.

When should we leave? Say you want to head west past Kerran Head (270°T) on Fri 11 June. The stream at C for each tidal hour, and how much of it is with you (+) or against you (−), is:

Tidal hour (BST)	Row	Stream at C	Effect westbound (about)
0738–0838	HW−3	065°T 0.5 kn	−0.5 kn foul
0838–0938	HW−2	070°T 1.0 kn	−0.9 kn foul
0938–1038	HW−1	075°T 1.3 kn	−1.3 kn foul
1038–1138	HW	075°T 1.3 kn	−1.3 kn foul
1138–1238	HW+1	075°T 1.0 kn	−1.0 kn foul
1238–1338	HW+2	080°T 0.6 kn	−0.6 kn foul
1338–1438	HW+3	160°T 0.2 kn	turning: slack
1438–1538	HW+4	245°T 0.6 kn	+0.5 kn fair
1538–1638	HW+5	250°T 1.2 kn	+1.1 kn fair
1638–1738	HW+6	255°T 1.3 kn	+1.3 kn fair

Until early afternoon the east-going flood is against you, at up to 1.3 kn. It is slack from 1338 to 1438, and from 1438 the ebb is with you. At 5 kn through the water, that is the difference between about 3.7 kn and 6.3 kn over the ground — so plan to be off Kerran Head in the afternoon. If the forecast is for a fresh west-south-westerly, though, that fair ebb is also wind against tide: a reason to wait for a quieter day, or to keep well offshore.

Find HW, choose the row, work out the range, interpolate the rate — then let the stream work for you.

So far you have used the stream to work out where it has taken you. In Module 5 you will turn this round and use it to work out the course to steer to arrive where you intend.

Key terms

Tidal stream: The horizontal movement of the water caused by the tide.
Set: The direction a stream flows towards, in °T.
Rate: The speed of a stream, in knots.
Drift: The distance a stream carries you: rate × time.
Tidal diamond: A magenta diamond on the chart marking where the stream table applies.
Stream table: Direction and spring and neap rates at each diamond for each hour from HW−6 to HW+6 at the standard port.
Tidal stream atlas: A booklet of hourly pages showing the stream as arrows, referred to HW at a standard port.
Computation of rates: An atlas graph for interpolating a rate between neaps and springs from the day’s range.
Slack water: The short period when the stream is weak and turning.
Wind against tide: Wind blowing against the stream: shorter, steeper and rougher seas.
Tidal gate: A place best passed at a particular state of the tide, usually a headland or narrow channel.

Exercises

Check your understanding. You get a second try at each question, then the worked answer. Score 70% or more to complete the lesson — your progress is saved in this browser.

What is a tidal stream?

The stream table gives 250°T for diamond C at 5 hours after HW. Which way is the water flowing?

On Tue 1 Jun you will pass diamond A at 1700 BST. Which row of the stream table do you use?

Port Kerran — times UT (add 1 hour for BST), heights in metres above chart datum
Day	Time	m	HW/LW
Tue 1 Jun	0122	5.2	HW
	0732	0.4	LW
	1341	5.1	HW
	1950	0.5	LW

On Fri 11 Jun you will round Gull Point, near diamond B, at 1200 BST. What is the rate of the tidal stream there, allowing for the day’s range? (Mean ranges at Port Kerran: springs 4.8 m, neaps 2.4 m.)

Port Kerran — times UT (add 1 hour for BST), heights in metres above chart datum
Day	Time	m	HW/LW
Fri 11 Jun	0356	1.3	LW
	1008	4.3	HW
	1620	1.2	LW
	2231	4.5	HW

Tidal streams referred to HW Port Kerran — direction °T, rates in knots at springs and neaps
Hours	B49°48.3'N 004°03.2'W
Hours	DirSpNp
6 before	2301.60.8
5 before	2351.00.5
4 before	3000.30.1
3 before	0401.00.5
2 before	0451.90.9
1 before	0502.41.2
HW	0502.21.1
1 after	0501.80.9
2 after	0551.10.5
3 after	1200.30.1
4 after	2201.10.5
5 after	2252.01.0
6 after	2302.31.1

On Fri 4 Jun at 1330 BST you are at anchor in Kerran Roads, near diamond D. In which direction is the stream setting?

Port Kerran — times UT (add 1 hour for BST), heights in metres above chart datum
Day	Time	m	HW/LW
Fri 4 Jun	0315	4.6	HW
	0925	1.1	LW
	1535	4.5	HW
	2145	1.2	LW

Tidal streams referred to HW Port Kerran — direction °T, rates in knots at springs and neaps
Hours	D49°51.4'N 004°09.6'W
Hours	DirSpNp
6 before	2700.90.4
5 before	2750.60.3
4 before	3500.20.1
3 before	0850.60.3
2 before	0901.10.5
1 before	0901.40.7
HW	0951.30.6
1 after	0951.00.5
2 after	1000.60.3
3 after	1800.20.1
4 after	2650.70.3
5 after	2701.10.5
6 after	2701.30.6

On an atlas-style page for the Kerran area, “1 hour before HW”, the arrow off Gull Point is labelled 12,24. Today’s range at Port Kerran is 3.6 m (mean ranges: springs 4.8 m, neaps 2.4 m). What rate do you use?

On Fri 4 Jun at 1505 BST you fix your position (plotted below). You steer 330°T; the log reads 3127.4, and at 1605 it reads 3132.4. Use the tide table and the stream table for the diamond nearest your track, B, to find the tidal stream for this hour, then plot your 1605 EP. Lay off the vectors on the plotting sheet, then tap the same spot on the second chart.

Port Kerran — times UT (add 1 hour for BST), heights in metres above chart datum
Day	Time	m	HW/LW
Fri 4 Jun	0315	4.6	HW
	0925	1.1	LW
	1535	4.5	HW
	2145	1.2	LW

Tidal streams referred to HW Port Kerran — direction °T, rates in knots at springs and neaps
Hours	B49°48.3'N 004°03.2'W
Hours	DirSpNp
6 before	2301.60.8
5 before	2351.00.5
4 before	3000.30.1
3 before	0401.00.5
2 before	0451.90.9
1 before	0502.41.2
HW	0502.21.1
1 after	0501.80.9
2 after	0551.10.5
3 after	1200.30.1
4 after	2201.10.5
5 after	2252.01.0
6 after	2302.31.1

Plotting sheet: A starts on the 1505 fix. Move B until the readout shows the direction and distance you need, then pencil in a mark. The dividers then start from your mark for the next vector.

Distance A→B: 0.60 MBearing A→B: 060°TDrag A and B, or Tab to them and use the arrow keysZoom with + / −, drag to pan

At diamond A the rows for 4 hours before HW and 3 hours after HW read 330°T and 150°T, with rates of only 0.2 kn at springs. What is happening then?

You are passing close west of Kerran Head, heading north. Which diamond gives the best idea of the stream you will meet?

Off Kerran Head the ebb sets 250°T at 1.8 kn at springs. Which wind will raise the shortest, steepest sea there?

0 of 10 answered · 0 correct

Tidal stream drill

Five fresh questions each time: choosing the row, reading a rate or direction, an atlas or drift sum, an EP to plot with a stream from a diamond, and a quick check on the ideas.