NATARAJACurrent, set & drift — the course to steer
Learning outcomes & permitted supports
- Draw the current triangle correctly: ground velocity = water velocity + current, and name every leg
- Solve the course to steer to make good a wanted track, and give the speed made good with it
- Say what it costs to ignore a current: how far off track a passage lands, and what it does to the ETA
- Recover set and drift from a DR and a fix — the direction FROM the DR TO the fix, and drift ÷ interval
- Distinguish DR from EP, and apply leeway on top of the current, into the wind
Supports in this lab: Calculator, or a plotting sheet and dividers. The triangle is solved exactly (a quadratic in the speed made good) and REFUSES tracks a strong current makes impossible rather than returning a plausible number. Golden 961/961.
Glossary: set & drift · course to steer · estimated position · leeway · dead reckoning
The sea moves under the ship
Everything in this lab is one addition: ground velocity = water velocity + current. The ship's engine and rudder buy you the water velocity — the course you steer at the speed you make through the water. The sea adds its own vector regardless. The triangle on the bench is that sum, drawn for one hour.
To make good 285.0° you steer 271.0° — 14.0° to port of it — and the ground slides past at 10.87 kn instead of 12.0.
Ignore it and see what it costs
The reason this is examined so hard: steering the track you WANT, in a current, is not the same as making it good. Hold 285.0° on the compass for 6.0 hours in this stream and you arrive somewhere else entirely.
| Steering the track blindly, you make good | 299.5° at 11.59 kn |
|---|---|
| After 6.0 h that is | 18.0 nm from where you meant to be |
| ETA over 90 nm, current allowed for | 8.28 h at 10.87 kn |
| …and if you had assumed 12.0 kn | 7.50 h — the difference is the pilot boat waiting |
Leeway — the wind's separate gift
Current acts on the hull under water; leeway is the wind pushing the ship bodily downwind of her heading — a light ship in ballast with a big superstructure can crab several degrees. It is applied AFTER the current triangle: the helmsman is given a course that much further into the wind, so the ship's actual path through the water is the one the triangle solved for.
No leeway allowed: the course to steer is the water-track course itself.
Reading the triangle backwards: set & drift
At the next fix, the sea tells you what it did. Take the DR you were keeping — course steered, distance through the water, no current applied — and the fix you have just obtained: the line FROM the DR TO the fix is the current. Its direction is the set, its length the drift, and drift over the interval is the rate. That is where tomorrow's allowance comes from: yesterday's honesty.
This hour's example, worked backwards: a DR at 12.811°N and an EP at 12.781°N give set 030.0°, rate 3.00 kn — the same numbers that went in, recovered from the two positions alone. The golden suite gates that round trip to a millionth of a degree.
| Course to steer | 271.0° |
|---|---|
| Speed made good | 10.87 kn |
| Crab angle (steer − track) | -14.0° |
| Estimated position after 6.0 h | 12.781°N 73.425°E |
| How far the current moved her | 6.83 nm towards 104.9° |