Module 05 · The course to steer
Reading notes · Module 05 · Where you're going
The question turned round: not where the water has taken you, but what you steer so that it takes you where you want to go.
Open the interactive chapter →The same material with the chart, the drawings and the exercises.
Turning the question round
The other way up
Part 2 ended on a question it did not answer: “These two parts asked where has the water taken me. The one that matters when you are actually going somewhere is the other way up: what course do I steer so that the wind and the water land me where I want to be?”
This is that part. Until now the chart has answered. You drew what the boat had done — course, log, stream, leeway — and the chart told you where that put you. The triangle is where part 2 left you: the EP for the last leg of that afternoon, taken on its own from the 1543 mark, with both corrections in it.
Now the chart asks. You put a destination on it — here, 5.0 miles away on 075 — and the line from where you are to where you want to be is your desired track: the track you want to make good over the ground. It has two arrowheads, like the ground track you drew in part 1, because it is the same kind of line, a path over the ground. The only difference is that this one has not happened yet.
What does not change: the same three vectors, the same four symbols, the same hour of stream. The only thing that changes is which side of the triangle you do not know. And what you are looking for has a name of its own, the one the examiner uses: the course to steer.
Three vectors, and you know most of each
Before you solve it, count what you already have. Three vectors look like too many unknowns. They are not.
One unknown in two of the three vectors: two in all. The desired track is missing its length, the water track its direction, and the stream nothing. Two unknowns is exactly what a triangle lets you solve — so this can be drawn, not guessed.
And that is where the next topic starts. You do not know which way to point, but you know exactly how far you go through the water in an hour. Open your dividers to your speed and they sweep every place you could be at the end of that hour — and only one of those places is on the track you want.
The one-hour triangle
In this order, and no other
Four moves, always in this order. This is the only part of the course to steer you have to learn by heart.
1 · From where you are, lay the desired track as a line — no length yet. It is only a direction for something to land on.
2 · From the same point, one hour of stream: three arrowheads.
3 · From the end of the stream, open your dividers to your speed through the water for one hour, and cut the line.
4 · From the end of the stream to the cut is the water track, one arrowhead, and its direction is the course to steer. From where you are to the cut is the ground track for the hour.
And the difference from part 1, which is the one that catches people. There the stream went on the end of the reckoning, because it corrected something already sailed. Here it goes on the start, because it corrects nothing: it asks. Same vector, same drawing, a different place and a different job.
The construction, drawn
From here, you want to go there. That EP, and the desired track on 075, running on towards a destination 5.0 miles away. For now it is only a line.
One hour of stream, from the same point. The set is 287 and the drift 1.4 knots — the tidal stream atlas at the EP for the hour from 1607, worked below — so the end of the vector is where the water alone would carry you in the hour.
Now the dividers. Open them to 5.0 miles — your speed for one hour — and swing them from the end of the stream. Every place you could be at the end of that hour is on this arc, and it crosses the desired track once ahead of you.
Join the end of the stream to the cut. That is the water track, one arrowhead, and it measures 084: your course to steer, 9° into the stream. And from the EP to the cut is the ground track for the hour: 3.76 miles — so the 5.0 miles take 80 minutes.
Down to her compass
The leeway goes the other way now
Topic 2 called 084 your course to steer — and with no wind in it, that is exactly what it is. Put a wind in it and 084 turns out to be what it was all along: the track you need through the water. What you steer is the heading that makes it good, and when the examiner asks for the course to steer, that is the one meant: the one with the leeway already in it.
In part 2 you moved the mark to leeward. The wind had already pushed, and the drawing caught up with it: the bow pointed one way and the water track fell downwind of it.
Steering, it goes the other way round. The bow goes to windward of the water track: the same number of degrees, on the other side.
The wind is from 340 and she makes 10° of leeway. It pushes her towards 160, to starboard, so you point 10° to port: 074. Point 074, crab 10° downwind, make good 084.
You are not correcting what the wind has done. You are taking off, in advance, what it is going to do.
And this is the classic mistake, so it is said out loud: after the whole of part 2 putting leeway to leeward, the hand wants to do it again. That gives 094 — and steering 094 she makes good 104, 20° off.
All the way down
From the water track to the heading. The triangle gave you 084 through the water. The wind is from 340 and pushes her 10° to starboard, so the bow goes 10° to port, to windward: 074 true.
From true to magnetic. The variation is 6° W, and on the way down a westerly variation is added: 080 magnetic.
From magnetic to her compass. You go into her deviation card by the magnetic, the way module 02 drilled it. 080 falls in the 090 row: the card goes every 30 degrees and 080 is nearest to 090. That row says 2E, and on the way down an easterly deviation comes off: 078 by her compass. That is what goes on the card in front of the helm. Interpolate between the two rows and you get 077: the nearest row is within a degree of it, which is closer than you can steer.
When there is no answer
When the stream is stronger than you
The wind has died and the engine is on. Same Kittiwake, same card, but now she makes 2.5 knots through the water. A different passage, the same day: at 1321 you are in the southern channel, just off diamond F, and you want to make good 203 down it. Read the stream off F's table, below: at 1321 it is the 1 before row, and with the day's range it sets 038 at 3.8 knots.
Build it the way you always do. One hour of stream from the start; from its end, the dividers open to one hour of you, 2.5 miles. The arc does cut the line, and 226 is a real course. But the cut falls 1.37 miles behind where you started: the bow points down the channel, and over the ground she goes astern at 1.37 knots.
That is the answer, not a drawing gone wrong: no course takes you down the channel against that stream. That is a tidal gate — Tidal streams, module 04, part 4, timed one in this same channel. Here it is shut while the stream against you is stronger than your engine. What you do about it is seamanship, not geometry: wait for it to open, go another way, or accept it and plan again.
And the other way it goes wrong — the next screen draws it: if the stream ran across your track faster than you can go, the arc would not even reach the line.
So a cut is thrown out for two different reasons. The one further behind is sailing the wrong way: its course, 000, points away from where you are going. The one ahead can still come out negative, as here: you point ahead and go backwards over the ground. The first throws out a course; the second throws out the trip.
The arc that never reaches
The same channel, the same hour, the same 2.5 knots of engine. This time you want to cross it, straight over, on 308. The stream is F's again, 038 at 3.8 knots: it runs along the channel, right across your track.
Build it. One hour of stream from the start; from its end, the dividers open to one hour of you, 2.5 miles. The arc does not reach the line at all: its centre is 3.8 miles off your track, and the dividers only open to 2.5. No course makes good 308.
Point the bow straight at the far side and you still go down the channel every hour you try. That is the other tidal gate — no course at all — and the answer is the same one: wait until the stream eases below your engine.
What you can do now, and what is still given to you
What you can do now, that you could not at the start of this part. Put a destination on the chart and read the question the other way up. Build the one-hour triangle — desired track, stream from the same point, dividers at your speed — and take off it the water track and the speed over the ground, and from that the time. Take that course all the way down to her compass, leeway to windward first. And recognise when there is no answer.
And the same honest thing parts 1 and 2 ended with. Every stream in this part came from this chart's own tidal data for 3 September 2026 — the tidal stream atlas at the EP, and diamonds G and F — worked for you or read beside the chart, the way Tidal streams, module 04, part 4, taught it. At sea that working is yours. The wind was handed to you in the words of the question: on board you would read it off the anemometer and the wind vane. The leeway is still your own judgement of your own boat: no instrument measures that.
Named here, and not taught here: the passage longer than an hour. It lays each hour's stream nose to tail — each one read for its own hour, the way part 1 read the afternoon leg by leg — and swings the arc at that many hours of you.
Where it goes next. later in the course, Passage planning, is where all of this becomes a plan.
Open the interactive chapter →The same material with the chart, the drawings and the exercises.