Set and Drift of the Current

You plot a dead reckoning position, you take a fix, and they disagree. The vector from the DR to the fix is everything the reckoning did not know about, and dividing it by the elapsed time turns it into a rate: the DRIFT, in knots, and the SET, the true direction the water was flowing toward. Knowing the current you have been in for the last four hours is how you plan the next four, and it is the reason the DR plot keeps being drawn on a bridge with a working plotter.

The naming convention is a genuine trap. A current is named for where it goes — a current setting 090° flows toward the east. A wind is named for where it comes from — an easterly wind blows from the east, toward the west. Two arrows on the same chart, drawn from descriptions in the same sentence, can point in exactly opposite directions to what you meant. Write the convention down beside each arrow.

Be careful about what this vector actually contains. It is not the tidal stream alone. It is the sum of the tidal stream, the ocean current, the leeway the wind pushed you to, a log that reads a little slow or a little fast, a compass whose deviation card is out of date, and a helmsman who sits half a degree off the ordered course for hours at a time. All of those land in the same number, and treating that number as pure water movement will over- or under-correct the next leg. It is properly called the "set and drift" only by convention; "everything the DR missed" is closer to the truth.

It is also an AVERAGE over the whole interval, which matters most where the current matters most. A tidal stream that ran east for two hours and west for two hours gives a small resultant and a flattering plot, while the boat swung well off track in the middle and came back. That is why fixes are taken frequently in tidal water and the set is worked leg by leg rather than over a whole watch. Where the current is steady — an ocean current, a river — the average is the truth and a single long interval is fine.