Module 05 · What pushes you
Reading notes · Module 05 · Where you're going
The mark the last module left on the chart, and the first of the two things that move it: the water itself.
Open the interactive chapter →The same material with the chart, the drawings and the exercises.
What the water is doing
The mark you left in the fog
This is the afternoon you finished the last module with. A fix at 1449, three legs steered and logged, and three marks: 1519, 1543 and 1607.
Every one of them is drawn correctly. The course was steered, the log was read, the distance came off the scale. There is nothing wrong on this chart.
And that module told you, in as many words, that the last of those marks is not where you are. It named the two things that move it, and said they are taught here.
This is the first of the two, and it is the water itself.
The log cannot feel it
A tidal stream is not a current running past your hull. It is the whole body of water moving, and you are floating in it — the way a leaf on a river goes downstream without swimming.
Which is why the log cannot feel it. The log measures water going past the boat, and the water is going with you. An hour of stream leaves no trace on it at all.
And it is not the tide you have already learned. The tide says how much water there is under you; the stream says where that water is going. They come from the same thing and they are not the same question — and the sum that uses one never uses the other.
One hour of stream, drawn
A stream is given as two numbers, and the first one catches everybody. Set is the direction the water is going TO. Not where it comes from. A wind is named for where it comes from; a stream is named for where it goes. They are opposite conventions, and mixing them puts you exactly twice as far wrong as doing nothing.
Drift is how far it runs in one hour. So a stream of one knot carries the water one mile, which you can take off the latitude scale with the dividers you have been using since module 02.
Here it is drawn from the 1519 mark: 277 at 1.5 knots, one hour of it — what the tidal stream atlas gives at that mark for the hour of the leg, on the day in the box below. Three arrowheads, and that is not decoration — one is the water track, the course through the water that module 03 drew; two is the ground track, where she actually goes over the ground; and three is always the water itself.
From reckoning to estimated position
Where the stream has taken you
You have a reckoning, and you have a stream. The correction is one line. From the far end of the water track you have drawn — the mark the log made — you run the stream's vector, and where it ends is where you think you are. That is the estimated position: the EP.
Two things make it right, and both are easy to get wrong.
From the end of the reckoning, not from the fix. The fix was true at the time it was taken and it has been getting older ever since. The stream is applied to where the reckoning has got to — the far end of the line.
The vector of the time you have run, not of an hour. Drift is miles per hour. Half an hour of stream is half the vector; twenty-four minutes is four tenths of it. This is where one knot, one mile starts to pay.
And here the symbols close. Module 03 drew the water track with one arrowhead. Now: two arrowheads is the ground track, three is the water itself, and an estimated position goes in a triangle — the way an observed fix goes in a circle. One, two, three and a triangle: four marks, and a chart anyone can read at a glance.
Applied to the afternoon, step by step
The first leg of that afternoon. Out of the 1449 fix on 300, 2.5 M through the water, 30 minutes at 5.00 knots, and the mark at the far end is DR 1519. Nothing here gets redrawn. The reckoning is finished; what follows is applied to its far end.
The atlas gives 277 at 1.5 knots for that hour — and the leg took 30 minutes, so what you apply is half of it: 0.75 M off the latitude scale, which is 1389 m on the ground. Three arrowheads, run from the 1519 mark. A whole hour of drift laid on a half-hour leg puts you exactly as far out as doing nothing at all — only on the other side.
The far end of that vector is the EP, and it goes in a triangle. Join the 1449 fix to it and you have the fourth symbol: two arrowheads — the ground track, 295 and 3.20 M, where the water track was 300 and 2.50 M. You steered 300 and you went 295; you sailed 2.50 miles and you made good 3.20. What you have moved is not the boat. It is where you think the boat is.
What it costs to ignore it
The afternoon, both ways
Here is that afternoon twice. The dashed, quieter one is what you brought from module 03 — course steered, log read, nothing applied. The solid one is the same afternoon with the stream put in, leg by leg: each leg run from the estimated position the leg before it ended at, and not from the dry mark.
Each leg has its own stream, read where the leg ends and for the middle of its time: the tidal stream atlas at the first and third marks, and diamond B's table for the second. The second leg ends 0.45 M from diamond B, so that one is read off the diamond's table, below, and not off the atlas. The working for all three is under the table.
The water ran 1.83 miles that afternoon. 78 minutes of stream, at 1.5, 1.3 and 1.4 knots leg by leg, and that is a real distance — 3389 m of water went past the ground under you.
And the two 1607 marks are 3209 m apart. 1.73 of a mile, bearing 290 from the reckoning to the EP.
Nearly all of it, because this afternoon the stream pushed one way. It has a new figure every hour and at every place — 277, 320 and 280 here — but all three set to the west, so laid nose to tail the three vectors add up: only about 5 % of what the water ran cancels out. On another afternoon they turn against each other and more of it cancels; you only find out by drawing them.
Which is not a reason to skip the sum — it is the reason to do it. You cannot know how much cancels until you have drawn all three. And 3209 m is not nothing: in fog it is the difference between the channel and the ground beside it. The reckoning was never wrong. It was just never the whole answer.
What you can do now, and what still pushes you
What you can do now, that you could not at the start of this part. Read a set and a drift and know which way the water is going. Lay off the part of an hour you have actually run, off the same latitude scale you have used since module 02. Run it from the far end of the reckoning and put the answer in a triangle. And read the line from the fix to that triangle for what it is: the ground track, two arrowheads.
One honest thing about all of it: every stream in this part was worked for you. Each one came from this chart's own tidal data for 3 September 2026 — the tidal stream atlas at the first and third marks, and diamond B's table for the second — worked the way Tidal streams, module 04, part 4, taught you: the hour from high water at Aldern Quay, and the rate for the day's range. At sea and in the exam that working is yours, and it comes before any of this.
And two things still move the boat that this part has not taught.
The wind pushes the hull sideways, and the log cannot feel that either. It is called leeway, it is the second of the two, and it is part 2 of this module.
And the whole question turns round. This part asked where has the water taken me. The one that matters when you are going somewhere is the other way up: what course do I steer so that the water lands me there. That is part 3. Both are named here and neither is taught here — the same way module 03 ended by naming this one.
Open the interactive chapter →The same material with the chart, the drawings and the exercises.