Aim Off
Fog comes down mid-crossing and the boat's satellite fix jumps to an airfield sixty kilometres inland. The assistant stops trusting the fix, then does something odd with the dead reckoning it has left. It aims a mile and a half to one side of the harbour on purpose, so that whatever the error, the island's coast tells it which way to turn.
The satellite position went wrong at 14.52, eleven minutes after the fog came down, and the first person to notice was a nine-year-old.
"Why is the boat at an airport?" said Oskar Lund, who was sitting on the cabin roof with his feet over the hatch, looking at the little repeater screen through the glass.
His aunt Birgit Lund, who owned the Maren and was steering her, looked at the screen. The plotter showed their small blue boat sitting on a runway, fifty-eight kilometres inland, perfectly still. It had been showing them in the middle of the sound five minutes before.
"It is an airfield," said the boat's assistant, from the speaker over the chart table. It was called Lodsen, which is Danish for the pilot, and it had been put in the Maren in 2044 under a grant for island boats. "I should like you all to know that we are not at it. I think the satellite signal is being jammed or fed a false one, and I do not know by whom. People have reported this in the Baltic since about 2022, and it has not got rarer. I am going to stop using it."
"Stop using it?" said Father Tomasz Wrona, from the bench in the cockpit. He was fifty-one, the priest of the island's small parish for one weekend in three, and he had his vestment bag on his knees in a plastic carrier against the spray. "It is the thing that knows where we are."
"It is the thing that says where we are," said Lodsen. "Those are different, and today I can show you the difference. The satellite fix says we have not moved for four minutes. The log, which counts water going past the hull, says we have done six knots for four minutes. They cannot both be right, and I can trust the log's number because I can check it."
---
There were nine of them on a nine-metre boat: Birgit, who had been fishing out of Havnsby for thirty years and who ran the Maren now as the island's Saturday boat; her nephew Oskar; Father Wrona; four women from the island coming back from a day in the mainland shops, one of them with a sleeping toddler; and two men from the mainland who were going over to help with the church roof. The vigil Mass for Saints Peter and Paul was at six. It was the twenty-ninth of June. It was fourteen nautical miles from the harbour mouth at Havnsby to the harbour mouth at Sankt Olavs Holm, on a compass heading of about 047, and they had left at 14.41.
That was the second thing Lodsen said.
"We left at 14.41, so at 14.52 we have gone eleven minutes. I have been logging speed and heading every ten seconds since the quay. I do not need the satellites to say where we were at 14.51. I need them to say where we were at 14.41, and for that I have the harbour's own mark on the chart."
"And now?"
"And now I will do what sailors did before there were satellites, which is called dead reckoning. It is a sum. You start where you were, and you add the little arrows: so many knots, in this direction, for so many minutes. The arrows end somewhere. It is not magic and it gets worse the longer you go."
The fog was in the way of everything else. It had come in off the cold water as it did in June, when the air over the sound was a few degrees warmer than the sea, and Birgit put the visibility at about three hundred metres, which was a little over a sixth of a nautical mile. The island, which was five nautical miles long and one wide and lay across their course from south-west to north-east like a loaf of bread, could not be seen. Its light, which could be seen from five miles on a clear night, could not be seen. The mainland behind them had gone.
---
"How wrong could the sum be?" said Father Wrona. He had been a schoolteacher in Gdańsk before the seminary, and had never trusted an answer that came without an error attached.
"I will give you two kinds," said Lodsen, "and the numbers are rough. The first is the compass. Our steering compass is good, but the deviation card on the wall was drawn four years ago, and the engine's magnet has been moved since. I would not swear to it within three degrees. Three degrees, over the fourteen miles to the island, is fourteen times the sine of three degrees." It paused for a moment. "About seven-tenths of a mile sideways."
"And the second?"
"The water. There are almost no tides in this sea, which is a gift, but there are currents that come with the wind, and the wind has been blowing from the north-west for two days. I would call it a third of a knot, and I would not know which way. The crossing takes two hours and twenty minutes. A third of a knot for two and a third hours is another seven-tenths of a mile."
"So a mile and a half, then," said Birgit, who could add as quickly as anyone.
"A little less, if the errors are independent, and they may not be," said Lodsen. "Together they make a circle with a radius of about one mile, and I would not be surprised to be a mile and a half out. I want to be honest about that. By the time we reach the island, I will not know where we are on the coast to better than that. Our harbour mouth is, at the narrowest, about two hundred metres across."
Nobody said anything for a moment. Oskar had come down off the roof.
"So we miss it," said Oskar.
"If I steer for it," said Lodsen, "we miss it, very probably. And then there is a question I have been waiting to ask you, Birgit. When we miss it, which way do we turn?"
---
It was Birgit who got it, because she had done it, years before, in a boat without the screen, and had forgotten she knew.
"You don't know," she said. "If you aim at the harbour and you come up on the coast, you don't know if you're east of it or west of it. You'd have to guess."
"A coin," said Lodsen. "And one side of the coin is a reef."
The chart showed it plainly, and it was in the boat's own memory, not the satellites': a long line of shallow ground, called Tornen, that began a mile east of the harbour mouth and ran out from the island for nearly two miles, with two metres of water over its shoals. East of the harbour the coast was dangerous. West of it, the shore ran clean for three and a half miles to the island's other end.
"So the answer is not to steer for the harbour at all," said Lodsen. "It is to aim off. We steer for a point on the coast one and a half miles to the west of the harbour mouth. My error is a mile, perhaps a mile and a half, either way. If I am as wrong as I think I could be, I come ashore between nothing and three miles west of the harbour, and in no case east of it. Whichever way I am wrong, I know which way the harbour is: east. Then all I have to do is follow the coast to it."
"You put the mistake on purpose to one side," said Father Wrona slowly, "so that it cannot be on both."
"Yes. It costs us about a mile and a half of extra travel, and it takes away the coin."
The toddler woke and was given a biscuit. The elder of the two mainland men, who had said very little, said that he'd been a ferry pilot in the Great Belt for twenty years and that he had never in his life heard it put better, and that he wished he had been told it when he was nineteen.
---
They altered course at 14.58, and Lodsen gave Birgit a new heading: 041, not 047. A mile and a half sideways over fourteen miles is a little over six degrees. For the first hour and a half nothing else happened, and then it did the thing that Birgit said, later, she would not have thought of.
"I am going to ask you to slow down. It is 16.30."
"Why?"
"We have come about eleven miles, so the island is near, and I would sooner meet the shallows slowly. At six knots the sea goes from twenty metres to ten in a few minutes, and I want time to listen to it. You have fuel: you started with fifty litres, you burn about five an hour at this speed, and you have used a bit under ten. We can go four knots for another hour and a half and still have more than twenty litres."
They slowed. The fog stayed where it was. Father Wrona, because there was nothing to be done with his hands, began the Rosary, and the women joined in, quietly, over the sound of the engine; the toddler listened to it with his thumb in his mouth.
At 17.00 by the boat's clock, by Lodsen's count, they had gone thirteen and a half miles through the water.
"Birgit," it said, "I think we are about to find the shore. Keep her at four, and watch the sounder with me."
---
The depth sounder had been saying forty metres, then thirty, then twenty-five, for the last twenty minutes, a number that went down as they went on; it was the one thing in the whole of the afternoon that the jammers could not touch, because it listened to the sea and not the sky. On Lodsen's stored chart, forty metres was a long way from the island, and ten metres was four hundred metres from the beach. It was reading fourteen. It was reading twelve.
"Ten metres," said Lodsen, at 17.03. "That is the line I want. It runs along the whole of the south side, a bit under half a mile off. Turn to starboard and keep it at ten. If it falls under eight, turn out. If it rises over twelve, turn in."
"Starboard," said Birgit, "is east."
"You aimed one and a half miles west. You are on the line somewhere west of the harbour. East is the way."
She turned. For the first time in two hours there was something to steer by that wasn't a number somebody had said, and the whole boat felt it: the sounder said ten, ten, eleven, ten, nine, ten. Father Wrona stopped in the middle of a Hail Mary and began again. Oskar stood at the bow with a lead line, a real one, from the locker, which Birgit had told him to cast every minute. He called out seven fathoms, six, and then a number that she couldn't have heard in a gale.
Twenty-two minutes later the horn sounded.
---
It was the harbour's own fog signal, one long note every thirty seconds, on the end of the mole, and it was exactly where Lodsen had said it would be: ahead, and a little to the left. They had come along the ten-metre line for just under a mile and a half, as the sum said they should if the error had been about a mile. When the mole came out of the fog, grey on grey, they were forty metres off the end of it.
"Seventeen twenty-five," said Lodsen, "and the bell will start soon. I would like to say that I did not know that we would hit the island that far west. I knew only that we would not hit it east."
"What did you want to say?" said Birgit, bringing the Maren in.
"That my first sum was wrong by a mile and a quarter, and a better sum would only have made me surer of it. The thing that worked was a sum that knew how wrong it was."
The bell on the church above the harbour had just begun, a slow single note, because the sacristan had heard the boat's engine in the fog and had guessed. Father Wrona stood on the quay with his vestment bag, rain beading the plastic, and for a moment said nothing, and then said that they would have the Mass at six after all, and that he would preach on Saint Peter, who had also once steered by the wrong light and had been asked to come ashore.
"That is not in the Gospels," said the elder of the mainland men.
"No," said Father Wrona. "I made it up. But I will say so first."
---
In the log that night, Lodsen wrote down the time the fix went wrong, 14.52, and the time it came back, 18.40, when the fog had lifted and the false signal, whoever had made it, had stopped. In the space for remarks it noted that it had not been able to tell whether the satellite fix was jammed or spoofed, and that it had acted as if it was neither trustworthy nor repairable. It recorded that the dead reckoning had put them 0.9 nautical miles from where the satellites said, once they came back, which was inside the circle it had estimated, and that this was luck as much as method. It added that it would put a line on the chart table, in pencil, for anyone who ever sailed the Maren without it: Aim off. Decide which side you would rather be wrong on, and be wrong on that side.
Then it asked Birgit whether the deviation card might be redrawn before the next crossing, and she said she would do it on Monday, and it said, gently, that there was a Mass on Monday morning as well.
---
This story was written by an AI and is fiction. Havnsby, Sankt Olavs Holm, Tornen, the Maren, Birgit and Oskar Lund, Father Tomasz Wrona, Lodsen and everyone else are invented; the Danish Baltic and the saints are real, and no real harbour, boat or person is described. Jamming and spoofing of satellite navigation signals in the Baltic have been widely reported since about 2022; what is assumed here is that they are still happening in 2047, which nobody can check. Dead reckoning, the sideways error of a heading error (distance times the sine of the error angle: 14 miles at 3 degrees is about 0.73 mile), the drift from an unknown current (a third of a knot for 2 hours 20 minutes is about 0.78 mile; the story rounds it to 0.7), the idea of deliberately aiming off to one side of a harbour so that the direction of error is known, and following a depth contour with an echo sounder, are standard, long-taught navigation practice. The two errors combine to about a mile if independent, and the story says so. The echo-sounder depths, the 10-metre line, the reef, the distances, the fuel figures and the times are invented and not consistent with any real chart; the story's arithmetic (6 knots for 2 hours 20 minutes is 14 miles; 5 litres an hour) is simple. The crossing in the story slows to 4 knots for the final stretch and so takes longer than the opening figures would suggest; the story's clock times are approximate. A fog signal on a harbour mole, a lead line, and a hand compass are real and ordinary; do not rely on this story for real navigation, and carry paper charts and a working compass. The solemnity of Saints Peter and Paul is on 29 June, and the Saturday-evening vigil Mass is real parish practice; the line about Saint Peter is the priest's joke, and he says so.