Dead Reckoning

Dead reckoning is the calculation of your position from a known starting point using only your heading, your speed and the time elapsed.

No landmarks. No stars. No satellites. You know where you were, you know how fast you have been going and in what direction and for how long, and from that you compute where you are.

It is arithmetic and it is entirely reliable and it is always slightly wrong.

The instructor

Captain Aled Morrisey is seventy-one and has taught the module at the college for nine years, having spent thirty-four at sea before that.

The module is two days. It is not examined. It sits in the syllabus under a heading called Traditional Methods and it survives because a review committee has never quite got round to removing it.

Everyone in the room has GPS. Everyone in the room will have GPS for their entire careers. Aled says this on the first morning, before anything else, so that nobody has to spend the two days privately wondering.

The eleven

Cadets, aged nineteen to twenty-six. Bright. Polite. Bored on the first morning in the specific way of people who have been told the next two days do not count.

Aled has run this eleven times and knows exactly what happens and when.

Hour one

He gives them the theory. It takes forty minutes and there is no difficulty in it; the mathematics is trivial and every one of them can do it.

Hour two

He gives them a problem. Departure position, a heading, a speed, six hours elapsed. Where are you?

All eleven get it right in under four minutes.

Hour three

He gives them the same problem with a current.

Nine get it right. Two make the same error, which is the error everybody makes, and which is that they apply the current for the wrong duration.

Hour four

This is where it starts.

He gives them a problem with a current, a wind, and a helmsman who has been steering two degrees high for the whole watch because he is nineteen and tired and does not know he is doing it.

They cannot do it. Not because it is hard — because there is no correct answer.

Two degrees over six hours at twelve knots is about two and a half miles of error, and you do not know it is there, and there is no way to find out except by seeing something.

One of them, every year, says: "So it's wrong?"

And Aled says: "Yes."

Day two

Day two is the part the college does not really understand it is buying.

He gives them a chart, a departure, and eleven hours of a passage in poor visibility, and he makes them work it as a running plot — position every hour, on the chart, in pencil, with the error growing.

By hour four the plotted position is a point. By hour eight it is a point with a circle round it. By hour eleven the circle is four miles across and there is a rock in it.

The exercise is to decide what to do.

There is no right answer to that either. There is a set of defensible answers and a much larger set of indefensible ones, and the whole of the second day is spent in the space between those.

What he is actually teaching

He has never written this down and has never said it in these words to the college.

He is not teaching navigation. They will never navigate this way.

He is teaching them what it is like to be responsible for a decision when your information is definitely imperfect, definitely degrading, and the only alternative to acting on it is not acting.

That is every day of their working lives and there is no other module in the syllabus that touches it.

This year

Eleven cadets. The usual shape: bored, then interested, then, on the afternoon of day two, quiet.

A cadet called Ines asked the question that about one in three cohorts produces.

She said: "If the plot's always wrong, why plot at all? Why not just wait until you see something?"

Aled said, "Because you might see it at four hundred metres."

"So the plot's wrong but you keep it."

"You keep it and you keep it honest," he said. "You write down what you actually did, not what you meant to do. Then when you see something, you know how wrong you were, and by how much, and in which direction."

He tapped the chart.

"That's the only thing that lets the next hour be better than the last one. Not being right. Knowing your error."

The last hour

He ends the module the same way every year, with a story he tells badly on purpose.

In 1981 he was second officer on a ship in the Bay of Biscay in fog with a radar that had failed at 0200. They ran on dead reckoning for nine hours. His plot at 1100 put them four miles east of where they turned out to be.

He got that number because at 1100 the fog lifted and there was a headland.

"Four miles," he says. "Over nine hours. That's very good."

Then he says the last bit, which is the reason he has taught this module for nine years for a fee that does not cover his fuel.

"The master had his own plot. He'd kept it independently the whole watch, and he never once looked at mine, and I never looked at his. That's the rule and it's a good rule."

"His was out by eleven miles. West."

"He was a better navigator than I will ever be and he had thirty years on me and he was wrong by eleven miles, and I was right by four, and neither of us knew until the fog lifted."

He lets that sit.

"So when it lifts," he says, "go and look at your plot. Not his. Yours. And write down the number."