The first night heading down the Ucayali river aboard the Henry I, the boat’s stated capacity of 150 people was supplemented by about 100 more. On the passenger deck, hammocks overlapped hammocks, and yet more were strung up at right angles above the others. People slept on top of and under the benches. Luggage was piled wherever it would fit. A trip to the bathrooms at the back of the boat was like a cross between an army assault course and a limbo contest.

It was the start of a journey I'd long wanted to make – four days downriver, into the heart of the Peruvian jungle, from the end of the road at Pucallpa. The destination was Iquitos, an alluring dot on the map in the middle of nowhere, and with a population of 500,000, the world's largest city without a road connection.

On a previous South American trip, my plans for the voyage fell through. But second time around I convinced my friend Hugo from Arequipa to go with me, and there was no backing out.

Like most Peruvians from the coast and the sierra, Hugo knew as little of his country's jungle regions as I did. We both imagined heading through dense, threatening jungle with chattering monkeys and clouds of mosquitos, on a boat crewed by hard-bitten desperadoes: some combination of Heart of Darkness and the movie Anaconda.

It turned out to be different from what we'd pictured. Something like that “real jungle” does exist, but you've got to travel at least a day or two away from the main river to find it. The Ucuyali itself – which eventually becomes the Amazon - is the region's main highway, and the boat the crowded local bus. The riverbanks are covered with secondary vegetation, dotted with patches of cultivation and the odd town or village.

It's still a different world - in some ways even remoter and stranger, yet gentler and more charming, than we had imagined. Time and logic have a slightly different structure, as we discovered when the departure time of the Henry I - “100 percent certain” to be 5:00pm Wednesday – morphed without explanation into Thursday. The other passengers packed onto the boat took it in their stride. “Oh well, I guess I'll come back tomorrow” said one, as he took down his hammock. Hugo couldn't believe how laid back everyone was. “If this happened in the sierra, they'd already have formed a protest march and be about to lynch the captain”, he said.

By the time we eventually left port, we had got to know many of our fellow passengers. Most were heading back to their families or plots of land after trips upriver for work or commerce Out on deck, the air was fresh, and in the middle of the river there wasn't a mosquito in sight.

We listened to some local guys tell a stream of self-deprecating jokes. But they acted a little sensitive when we confessed it was all more pleasant and civilized than we had imagined. “Yeah, people in Lima think we all live in swamps and run around with our faces painted”, they complained. Then they rather undermined their case by telling tall stories about boats that had disappeared without a trace, sucked down by enormous monsters rumoured to live at the bottom of the river.

After that first overcrowded night we arrived early next morning in the little port of Contamana, to a rapturous welcome from the local council, who enthused over the port loudspeaker that “the powerful, the potent motor vessel, Henry I, has arrived!”

Shedding some of its passengers and cargo, our potent motor vessel sailed on, and we started to drift into the rhythm of the river. By day we lazed in the hammocks, trying to stay cool as the boat chugged too slowly downstream. The hour around sundown was delicious, as the heat ebbed away and the sky caught fire behind the palms and banana plantations along the river banks.

After dark the breeze was cool, and a warm sweater or blanket was necessary in the hammocks. We drank cold beers from the ship's bar on the steps of the wheelhouse and chatted to the captain, who peered downstream for signs of any dangerous sandbars. The captain had been navigating the river for 18 years, he told us in a drawling, musical Spanish that was halfway to being Portuguese.

We asked him for stories of the river, but he wanted to talk about women. He had been in four serious relationships, and his first three lovers had all left him during hard times. “Love – it'll make you eat shit”, he assured us.

But fourth time around his fortune had improved. “It's all about luck, primo”, he told me. “You never know when your luck will change”

By both day and night we pulled into the caserios - little frontier farming settlements hacked out of the riverbank vegetation. When the boat came in the entire community gathered by the wharf. The men, bare-chested, casually displayed muscle definition that would make a gym addict weep, earned by constant unglamorous toil with a machete. They sold their bananas to the boat and took delivery of mandarins, flour and beer.

I struggled to imagine what life would be like in such isolation, without electricity or running water. But the people on the shore looked proud and content.

They also provided a steady supply of delicious food. Contrary to warnings we’d received, the meals doled out on the boat, though hardly gourmet, were perfectly edible. But it was the local delicacies which left us dozing in our hammocks, stuffed and content. Wrapped in banana leaves were yellow rice and chicken juanes, or patarashca – river fish with sweet chilis. Fruits included the silky guavilla, orange-like cocona, malty aguaje, and the crisp taperiba, which the locals eat with a sprinkle of salt.

Morning on the third day, when I strolled past the wheelhouse, the captain and his assistant shouted down at me. “Climb up here amigo! Go on, you can climb up on the roof”, they called. “This is where the Marañón joins the Ucayali – it's the start of the Amazon” I clambered up on the roof of the boat, to see the broad mouth of the Marañon, pouring its load of water into the Ucayali. The Amazon. It was the same river, now rather broader and a bit browner. But it felt like I was ticking a box on life's checklist.

It was eight hours more to Iquitos. The last stretch had a joyous feeling, like the end of school. With most of the passengers and cargo unloaded, we swung freely in our hammocks, joking and exchanging phone numbers with our new acquaintances.

Iquitos has a faded beauty, slightly wistful for its past when the 19th-century rubber trade made it a booming frontier, and Gustave Eiffel designed a building in the main plaza. Now its ports are graced mainly by beaten-up barges which take wood down to Pucallpa and gasoline up to the Colombian border. It's a town of lazy hedonism; of buzzing motorbike-taxis, famously beautiful women, and ice cream made from the exotic fruits of the jungle.

On shore, Hugo and I went looking for the open-air discotheque we had been recommended. By the time we found the place, bursting with infectious cumbia beats, we were still swaying with the rhythm of the river.