When David Johnstone went to sea aged 17, slide rules were used to navigate. He now relies on GPS

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I’ve been at sea for nearly 40 years but even I was apprehensive when I first saw the Mary Maersk. She is a Triple E class container ship, officially the largest ocean-going vessel class in the world, and is almost a quarter of a mile long. That’s 1,312ft or four times the length of the Wembley football pitch. The bridge where I work is very high up so I can see over the top of the containers, which are stacked nine high on the deck below. Because the vessel sits 48ft deep in the water when we are fully laden, there are some ports in the world where we are simply too big to tie up.

You need a steady nerve to steer her into port. I have men stationed at the bow and stern with walkie-talkies, updating me on exactly what is happening as we inch in. The ship has twin propellers to make it easier to steer in a tight space but we still need three tugs to nudge us in the right direction. It has to be a gentle operation – we have 18,000 containers on board.

I don’t know where I found my sea legs – I grew up in Wishaw, south of Glasgow, nowhere near the coast. My father was a labourer at the steelworks and my only experience of the sea was at Arbroath, when we went on holiday. After I left school, some of my friends joined the Royal Navy and came back with tales of exotic places that I’d only ever seen on the map. That was when I decided I wanted to sail the world, even though I’d never set foot on a boat.

I joined the Merchant Navy as a 17-year-old. Washing decks and splicing ropes might not seem very interesting but within months I’d visited five continents. I was hooked. I became a third officer by the age of 21 and carried on studying, becoming a foreign-going master mariner.

I’m 56 now but my first captaincy came when I was 31, on a ship called the Pearl of Dubai. It almost ended in disaster when we were hit by a typhoon in the Bay of Bengal and lost all power for three days. Then the second officer had appendicitis and I just missed standing on a cobra that had somehow slithered aboard. I think I lasted about six months before I resigned and went elsewhere.

Nowadays I work three months on, three months off and I rotate my Christmases with another captain. I live in County Wicklow, Ireland, with my partner Rene and, after eight years together, I think she has got used to my routine. I relax by riding a Harley-Davidson – we’ll be going to several rallies later this year.

Although I can sail using celestial navigation, modern vessels such as the Mary Maersk rely on sophisticated GPS systems. I started off my career using a slide rule and logarithm tables but now all I have to do is push a button. I have a crew of 22 and we spend a lot of time poring over meteorological charts to figure out how to avoid the worst of the weather. Any ship feels uncomfortable in big seas and this one is no exception, even though it cost $185m.

People think that because the ship is so big, she must be fast too. In fact, she is designed to sail at an average of only 16 knots, using a system called super slow steaming. The accountants reckon it will save the company an average of around £750,000 on fuel alone on every trip between Rotterdam and Shanghai.

I’ve sailed on dozens of ships but Mary is far and away the most sophisticated. She’s incredibly easy to handle during a passage and at every port people want to come aboard and have a look around. They line up at the harbour to watch us come in. There are often a lot of dignitaries to look after but we haven’t got time for everyone. Sometimes it’s a relief to get back out to sea – we mostly sail between Europe and the Far East.

I have a luxurious cabin below the bridge, with a widescreen television, DVD player, WiFi and air conditioning. It feels like a five-star hotel compared with my first ship but my favourite place is still on deck, looking at the stars and watching the waves below.

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