Tides and Course to Steer

Prerequisites

You have worked through the lesson Charts and Position Fixing: you can measure direction and distance on a chart, convert between True and Magnetic, and you know that charted depths are measured from chart datum.
You know the basics of the ICC coastal test from the lesson Passing the ICC Practical Assessment.
You have a plotter (Portland or Breton style), dividers, a pencil and a rubber, and ideally an almanac or a tide table for a port you know.

Learning Objectives

By the end of this lesson you will be able to:

Explain what causes tides, and the difference between spring and neap tides and why it matters for heights and streams.
Use a tide table to give the times and heights of today's high and low waters, allowing for time zones and summer time (test guidance, Navigation 4).
Find the direction and rate of the tidal stream from a tidal diamond or a tidal stream atlas, for example "two hours after high water springs" (test guidance, Navigation 5).
Construct a simple course-to-steer triangle and give the magnetic course to steer to allow for a tidal stream (test guidance, Navigation 3).
Plan for tidal and non-tidal waters, including the Mediterranean, as part of a simple passage plan (ICC coastal syllabus, Planning 2).

What the Examiner Will Ask

The RYA guidance tells ICC examiners to ask three tidal questions, and to keep them simple:

"Give the candidate a tide table and ask for the times and heights of today's high and low waters."
"What is the direction and rate of the tidal stream at (a tidal diamond), two hours after high water springs at the reference port?" The guidance adds: "Keep the question as simple as this, do not ask for interpolations."
"You are at (a) buoy. Assuming the tidal stream is 180(T) at 1 knot, what is the magnetic course to steer for (a) harbour, with a boat speed of 6 knots and no leeway?" Again: "A high level of precision is not necessary, the candidate should show that the principles of allowing for tidal streams is understood."

So the bar is clear: read a table correctly, read a stream correctly, and draw one clean triangle. The rest of this lesson builds the understanding behind those three answers, so you can also use them on a real passage.

What Tides Are

The cause

Tides are the regular rise and fall of the sea caused mainly by the gravitational pull of the Moon, and to a lesser extent the Sun, as the Earth turns. Around most of the UK and the Atlantic coast of Europe the tide is semi-diurnal: two high waters and two low waters a day. Successive high waters are about 12 hours 25 minutes apart, so each day's tides arrive roughly 50 minutes later than the day before.

Key words:

High water (HW) and low water (LW): the highest and lowest levels in each cycle.
Height of tide: the height of the sea surface above chart datum at a given moment. Tide tables give heights at HW and LW.
Range: the difference between the heights of a HW and the next LW. A big range means a lot of water moving, and strong streams.
Tidal stream: the horizontal flow of water caused by the tide. The flood stream runs while the tide rises, the ebb while it falls. The turn of the stream is not always exactly at HW or LW, which is why you use tidal diamonds and atlases rather than guessing.

Springs and neaps

When the Sun and Moon are in line (at new moon and full moon), their pulls add together. The result is spring tides: higher high waters, lower low waters, the largest range and the strongest streams. Springs usually arrive a day or two after the new or full moon.

When the Sun and Moon are at right angles (at the quarter moons), they partly cancel each other. The result is neap tides: lower high waters, higher low waters, the smallest range and the weakest streams.

The cycle from springs to neaps and back to springs takes about two weeks, so springs and neaps are each roughly seven days apart. Spring streams can be twice as strong as neap streams in the same place.

Spring tides at new and full moon, neap tides at the quarter moons

Why it matters to you:

At springs, the low waters are lowest. That is when a shallow anchorage or a drying harbour entrance is most likely to catch you out.
At springs, the streams are strongest. A 3-knot foul tide (a stream against you) halves the speed of a 6-knot boat. Wind against a spring stream raises steep, dangerous seas off headlands.
At neaps, the high waters are lowest, which matters if you need water over a bar or a sill.

To tell which you have, look at the range in the tide table (a big range means springs), or at the tide table's moon symbols, or simply look at the moon.

Reading a Tide Table

Standard ports

Tide tables give the predicted times and heights of HW and LW at standard ports (reference ports), usually for each day of the year. You find them in a nautical almanac (Reeds in the UK), in Admiralty Tide Tables, on the websites of national hydrographic offices (the UK Hydrographic Office's EasyTide, the French SHOM, and others) and in tide apps. Smaller secondary ports are worked out from a standard port with time and height differences; that is beyond the ICC test but well within Day Skipper.

Reading one day

Each day shows up to four lines: two HWs and two LWs, each with a time (24-hour clock) and a height in metres above chart datum. The higher heights are the high waters. A typical day looks like this:

Time	Height
0451	0.9 m
1057	4.8 m
1717	0.8 m
2320	5.0 m

That reads: LW 0451 at 0.9 m, HW 1057 at 4.8 m, LW 1717 at 0.8 m, HW 2320 at 5.0 m. The range from the morning HW to the evening LW is 4.8 − 0.8 = 4.0 m.

The time-zone trap

The heading of the table tells you which time zone it uses. This is where candidates lose marks.

UK tables are in UT (GMT). During British Summer Time (late March to late October) you must add one hour. Some almanacs shade the summer period and remind you in the page heading.
Continental European tables are often printed in the country's standard time, which is UT+1 for France, Spain, Italy and Croatia (and UT+2 for Greece). Reeds marks these pages with a time zone of −0100, meaning "subtract one hour to get UT". In summer, add a further hour for local summer time.
Apps and websites may already show local time. Check the setting.

When you convert, the date can change. A HW at 2320 UT becomes 0020 BST on the next day, so it is no longer "today". The diagram below shows a whole question worked through.

A tide table page with today's times converted from UT to BST

Worked example: today's tides

The examiner hands you the table above and says: "It is Wednesday 15 July. Give me today's high and low waters." The heading says UT, and July is in BST.

Find 15 July (WE).
Add one hour to each time: 0551, 1157, 1817, and the 2320 HW becomes 0020 on Thursday.
Answer: "Low water 0551 BST, 0.9 metres. High water 1157 BST, 4.8 metres. Low water 1817 BST, 0.8 metres. The next high water is just after midnight, 0020 BST Thursday, 5.0 metres."

Say the units. Say BST. Say which is high and which is low. That is the whole question.

Heights between HW and LW (useful, not tested)

For a quick estimate of the height of tide between HW and LW, the rule of twelfths assumes a six-hour rise or fall in which the tide changes by 1, 2, 3, 3, 2 and 1 twelfths of the range in successive hours. So halfway between LW and HW, the tide has risen half the range; one hour after LW, only a twelfth. It is a rough guide, and wrong in places with odd tidal curves (the Solent, for example), where the almanac's tidal curve diagrams are needed. Combine the height of tide with the charted depth as shown in the lesson Charts and Position Fixing.

Tidal Streams

Direction and rate

A tidal stream is described by:

Direction (set): the direction the water is flowing towards, in degrees True. This is the opposite convention to wind: a 090° stream flows towards the east, while an easterly wind blows from the east.
Rate (drift): its speed in knots.

Streams change through the tidal cycle, so each hour is referred to high water at a standard port: "HW Dover −3" means three hours before high water at Dover. Each "hour" covers the half hour either side, so HW+2 applies from 1.5 to 2.5 hours after HW.

Tidal diamonds

On Admiralty charts and many others, a magenta diamond with a letter marks a point where the stream has been measured. A table on the chart (usually near the title) gives, for each diamond:

Its latitude and longitude.
The standard port the times are referred to.
For each hour from 6 hours before HW to 6 hours after: the direction in degrees True, the spring rate and the neap rate in knots.

How to answer the examiner's question:

Find the diamond on the chart and its column in the table.
Find the row for the hour asked: "two hours after high water" is the +2 row.
Read the direction, and the rate from the springs column if the question says springs (neaps column for neaps).
Give the answer as direction and rate: "235 degrees True at 2.4 knots."

The diagram below shows that exact question answered.

Reading a tidal diamond table for two hours after HW springs

On a real passage the tide is rarely exactly springs or neaps. You then interpolate between the two rates using the day's range against the mean spring and neap ranges printed for the standard port. A quick approach: if today's range is about halfway between the neap and spring ranges, use a rate about halfway between the neap and spring rates. The examiner is told not to ask you this, but you will use it when you skipper.

Tidal stream atlases

A tidal stream atlas (or the tidal stream pages of an almanac) shows the whole area on one small chart for each hour, with arrows showing the direction and numbers showing the rates. On Admiralty atlases the numbers are written as two figures separated by a comma, such as "09,18": the neap rate is 0.9 knots and the spring rate 1.8 knots. Thick, long arrows mean strong streams.

To use one:

Look up the time of HW at the atlas's standard port for today.
Turn to the page for the hour you need (for example "2 hours after HW").
Find your position, read the arrow direction with your plotter against the page's meridians, and read the spring or neap rate.

A tidal stream atlas page, with arrows for direction and neap/spring rates

Diamonds give exact figures at a point; the atlas shows the overall pattern, including back-eddies close inshore and races off headlands. Use both.

Why streams matter

Speed over the ground. A 6-knot boat with 2 knots of fair tide covers 8 miles in an hour; with 2 knots of foul tide, only 4.
Cross-set. A stream across your track pushes you sideways, onto rocks if you do not allow for it. That is what a course to steer fixes.
Sea state. Wind against tide makes short, steep, breaking seas, worst at springs and off headlands. Plan to round headlands with the stream running the same way as the wind, or at slack water.
Tidal gates. Some passages (a headland race, a narrow sound) are only sensible at certain states of tide. Your passage plan should time them.

Course to Steer

The idea

If you point the boat straight at your destination in a cross-tide, the stream carries you sideways and you arrive downstream of where you meant to go, or on the rocks beside it. Instead, you aim upstream by just enough that the stream's push cancels out, so the boat's track over the ground runs straight to the destination. That heading is the course to steer (CTS).

Three lines make up the triangle:

Ground track: where you want to go over the ground, from start to destination. Marked with two arrowheads.
Tidal stream: the direction and rate of the stream for the time it applies. Marked with three arrowheads.
Water track: the course and distance through the water, which is your course to steer. Marked with one arrowhead.

Building the triangle (one hour)

Draw the ground track from your start position (A) to the destination, and beyond it if the destination is less than one hour away.
Draw the tide from A for one hour: in the stream's direction, for a length equal to its rate in miles. Mark its end C.
Open the dividers to your boat speed (in miles, for one hour) using the latitude scale.
Put one point of the dividers on C and swing an arc to cut the ground track. Mark the cut D.
Join C to D. The direction of C to D is your course to steer (True).
Convert to Magnetic with variation (and to Compass with deviation, if you have a card).
Optional: speed over ground is the distance A to D, because that is how far you move along your track in the hour.

The examiner's question, worked

"You are at A buoy. The harbour is due east (090°T). Assuming the tidal stream is 180°T at 1 knot, what is the magnetic course to steer for the harbour, with a boat speed of 6 knots and no leeway? Variation is 3°W."

Draw the ground track from A to the harbour: 090°T.
From A, draw the tide 180°T for 1 mile: point C is 1 mile due south of A.
Open the dividers to 6 miles. From C, cut the ground track at D.
Measure C to D: about 080°T. The stream sets you south, so you aim about 10° north of the track.
True to Magnetic, add West: 080 + 3 = 083°M.
A to D measures about 5.9 miles, so your speed over the ground is about 5.9 knots.

The diagram below shows the finished triangle.

The examiner's course-to-steer triangle: tide 180°T 1 kn, boat 6 kn, track 090°T

Checking it in your head

The 1-in-60 rule gives a quick check for small angles: the offset in degrees is about 60 × cross-tide ÷ boat speed. Here: 60 × 1 ÷ 6 = 10°. So the answer must be about 10° upstream of 090°, which it is. Use this as a sanity check on the plot, or as a quick answer when you are on deck without a chart.

If the stream is at an angle to the track, only its cross-track part pushes you sideways. A stream at 45° to your track has a cross-track part of about 0.7 of its rate; at 30°, half its rate.

A second example, with the stream at an angle

You want to go 045°T. The stream is setting 120°T at 1.5 knots. Boat speed 5 knots, variation 1°W.

Ground track from A at 045°T.
From A, tide 120°T for 1.5 miles to C.
Dividers at 5 miles; from C, cut the track at D.
C to D measures about 028°T.
True to Magnetic, add West: 029°M.
Check: the stream is 75° off the track, so nearly all of it (about 1.45 knots) is across. 60 × 1.45 ÷ 5 ≈ 17°. 045 − 17 = 028°. It agrees.

Speed over the ground comes out at about 5.2 knots, because part of the stream is helping you along.

Practical notes

Use the right hour of tide. Take the stream from the diamond or atlas for the hour you will actually be crossing. For passages longer than an hour, lay off the tide for each hour end to end from A, then arc off the total boat distance. Do not draw a fresh triangle every hour.
A stream dead ahead or astern does not change the course to steer at all, only your speed over the ground.
Leeway (sideways drift caused by the wind on a sailing boat) is applied after the triangle, by turning the heading into the wind. The ICC question says "no leeway", so leave it out on the test.
Steer the course, do not chase the destination. If you keep pointing the bow at the harbour, you will follow a curved path downstream. Steer the CTS and watch a transit or the GPS cross-track error to confirm.
Recalculate if things change: the stream turns, the wind drops and your speed falls, or you alter for traffic.

Non-Tidal Waters: The Mediterranean

Many ICC holders charter in the Mediterranean, where tides are small. The range in most of the Med is a few tens of centimetres, and tidal streams are generally negligible. That changes your planning but does not remove it:

Heights. Tide tables are rarely needed. But strong winds and changes in air pressure can raise or lower the sea level by a few tenths of a metre, and in places much more. Keep a margin in shallow anchorages and harbours.
Exceptions. Some areas do have noticeable tides or streams: the northern Adriatic around Venice and Trieste (range up to about a metre at springs), the Gulf of Gabès in Tunisia, the Strait of Gibraltar and the Strait of Messina (strong streams that must be planned for).
Currents. General currents and wind-driven surface currents of up to a knot or so can exist, especially in channels between islands. Your course to steer method works for any current: just use its direction and rate in place of the tidal stream.
Wind is king. In the Med, the passage plan is driven far more by the forecast (meltemi, bora, mistral, sirocco) than by the tide. See the lesson on weather and passage planning.

A simple non-tidal passage plan still covers the route, distances and times, hazards, depths at the destination, a bolt-hole and the forecast. A tidal plan adds the times and heights of HW and LW, the streams for each hour, any tidal gates and a course to steer.

Worked Example: A Morning Passage in the Channel

Tom is skippering a charter yacht on the south coast of England in July. Before leaving, he answers the three questions he knows the examiner likes, for real:

Today's tides. The almanac (in UT) gives HW at the standard port 0958, 4.6 m, and LW 1615, 1.1 m. In BST: HW 1058, LW 1715. The range is 3.5 m against a mean spring range of about 3.7 m, so it is nearly springs.
The stream at the headland. He plans to round it at about 1300 BST, which is HW+2. The diamond off the headland, +2 row, springs column: 255°T at 2.2 knots. He is heading west, so it is fair, and the wind is from the west at 15 knots: wind against tide. He decides to round a mile further offshore to avoid the overfalls shown on the chart.
Course to steer. After the headland, the harbour is 7 miles away on a track of 320°T. The +3 stream there is 250°T at 1.5 knots, about 70° off his track, so roughly 1.4 knots of it is across. Boat speed 6 knots: 60 × 1.4 ÷ 6 ≈ 14°. The stream sets him south-west, so he aims north, upstream of the track: about 334°T. With variation 1°W he steers 335°M, and confirms on the chart triangle. Because part of the stream is with him, his speed over the ground is a little above 6 knots, so 7 miles takes just over an hour.

He writes all three answers in the log before slipping lines. If the examiner asks him the same questions on a sample chart, it is a five-minute job.

Common Mistakes

Forgetting BST (or the local summer time abroad). Every time on the day is an hour out, and a HW can move into the next day.
Mixing up HW and LW lines in the table. The bigger height is high water.
Reading the wrong column. Springs and neaps are side by side; the question tells you which.
Reading the wrong row. "Two hours after HW" is +2, not −2.
Treating stream direction like wind direction. A stream of 180° flows towards the south.
Drawing the tide at the wrong end. The tide goes from the start position (A), not from the destination.
Arcing the boat speed from A instead of C. The boat speed is measured from the end of the tide line.
Taking A to C to D the wrong way round. The course to steer is C to D, not A to D (that is the ground track).
Forgetting to convert to Magnetic. The question asks for a magnetic course. Apply variation.
Aiming downstream. If the tide pushes you south, the course to steer is north of the track. Check with the 1-in-60 rule.
Assuming no tide means no planning. In the Med, plan around wind, pressure-driven sea levels and local currents.

Summary

Tides are caused mainly by the Moon. In most of north-west Europe there are two HWs and two LWs a day, about 12 h 25 min apart.
Springs (around new and full moon): biggest range, strongest streams. Neaps (quarter moons): smallest range, weakest streams. About a week apart.
Tide tables give HW and LW times and heights above chart datum at standard ports. Check the time zone: UK tables are UT, so add an hour for BST.
Tidal stream direction is where the water flows to, in degrees True; rate is in knots. Diamonds and atlases give both for each hour relative to HW at a standard port.
For "two hours after HW springs", read the +2 row and the springs rate.
Course to steer: draw the ground track, lay off one hour of tide from the start, arc the boat speed from the end of the tide to cut the track, and measure the course from there. Convert to Magnetic.
Check with the 1-in-60 rule: offset ≈ 60 × cross-tide ÷ boat speed.
In non-tidal waters, plan around wind, pressure and currents, using the same triangle for any current.

Check Your Understanding

What are spring tides, when do they happen, and what do they mean for tidal streams?

Answer: Tides with the largest range (highest HWs, lowest LWs), happening a day or two after new and full moon when Sun and Moon are in line. The streams are at their strongest.

A UK tide table in UT shows HW 1340, 4.2 m on a day in August. What time is HW on the boat's clock?

Answer: 1440 BST. UK tables are in UT; add one hour for British Summer Time. The height (4.2 m above chart datum) does not change.

A table in UT shows a HW at 2335 on 3 June. When is that in BST?

Answer: 0035 BST on 4 June. Adding the hour moves it into the next day.

The examiner asks for the stream at diamond C two hours after HW springs. The table's +2 row reads 112°, 2.6 kn springs, 1.3 kn neaps. What is your answer?

Answer: 112° True at 2.6 knots. Use the +2 row and the springs column. The water is flowing towards 112°.

An atlas page shows "07,15" beside an arrow. What do the numbers mean?

Answer: A neap rate of 0.7 knots and a spring rate of 1.5 knots.

You are at a buoy. The harbour lies 000°T. The stream sets 090°T at 1 knot, boat speed 6 knots, no leeway, variation 2°W. Roughly what is the magnetic course to steer?

Answer: The stream pushes you east, so aim about 10° west of north (60 × 1 ÷ 6 = 10): about 350°T. Add 2°W: about 352°M.

In a course-to-steer triangle, which line is the course to steer, and where do you arc the boat speed from?

Answer: The line from the end of the tide vector (C) to the point where the boat-speed arc cuts the ground track (D). You arc the boat speed from C, the end of the tide line, not from the start.

The stream is directly against you at 2 knots. Your boat speed is 6 knots. What is the course to steer, and your speed over the ground?

Answer: The course to steer is the same as the track (no cross-set), and the speed over the ground is 6 − 2 = 4 knots.

Why should you avoid rounding a headland with a strong wind blowing against a spring stream?

Answer: Wind against tide builds short, steep, breaking seas, and races and overfalls off headlands get much worse. Time the rounding for slack water or a fair stream with the wind, or stand well offshore.

You are chartering in Croatia, where the tidal range is small. Name two things you still need to allow for when planning.

Answer: Any two of: wind and air pressure changing the sea level by a few tenths of a metre; local currents in channels (use them in the course-to-steer triangle); the forecast, especially the bora and other strong local winds; depths and margins in shallow harbours and anchorages.