I cross the border riding pillion on a rusty Colombian motorbike, buy a ticket at the ramshackle docks. "Welcome to Brazil," the woman says. "We leave at 12 – don't be late."

I board the MV Dom Manoel, her orange hull quietly decaying above the shallow, muddy waters. Watching the others, I sling my hammock, and wait. We all wait. Hours pass. The docks are a cacophony of noise and a cocktail of odours: fish, diesel, rotting fruit.

Eventually the loading of cartons and sacks slows, the thumping of heavy objects hitting the deck falters, and for a moment all is calm.

I close my eyes. The thumping starts again. Louder now, and different. Music. An incongruous bell. "The bar is open," announces an orange-shirted crewman, much to general delight.

A couple of beers and I'm making friends. Antonio – a toothless grinning pensioner, his every word spraying sticky drops of saliva – wants me to teach him English. Felix, a giant genial hulk of a man, proudly wears a camouflage uniform and an inexhaustible smile. He swigs neat cachaça from the bottle.

With a couple of blasts of the ship's horn and a cheer from the assembled throng, we finally set sail around dusk. Antonio has forgotten the English I taught him an hour ago. Felix has nearly finished his cachaça.

Everyone had asked the same thing: "Four days to Manaus by boat? "Why don't you fly?" Standing in the steamy Amazon dusk sipping cold beer, watching pairs of scarlet macaws flit above the endless jungle, I know. The sun sinks behind an ocean of trees and the muddy, brown water turns a violent shade of red. "Salud," slurs Antonio.

As night falls, the monotonous hum of the engines dwindles and we moor against a long wooden jetty poking out of the mud in front of some meagre dwellings. The crew drops a gangplank. A dozen people disembark and vanish into the sticky darkness of some nowhere village at the edge of the world. Where have they come from? Where are they going? I ask Antonio the name of this place. He just shrugs.

We drift down the mighty river, nights and days blurring. Nowhere to go, nothing to do. The journey takes on a pleasing sense of routine; never monotonous or repetitive, but rhythmically predictable. Hammock, beer, river, music. Felix drinks cachaça from dawn till dusk, a bottle a day, and tells tall stories of adventures and girls and jungles, his youthful, chubby face at odds with the things he's seen and done. Antonio swigs beers and grins and forgets the English I teach him.

We stop four or five times a day; people get on, people get off. Sometimes tiny villages, sometimes decent-sized towns. All have names, unknown to me: people, places.

But on board, the boat is our world. The beer, the hammocks, the music. And always the river, the great brown muddy river, carrying us inexorably toward the sea.

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