There are no roads from the Peruvian river port of Iquitos, but the rich take aeroplanes. Those who cannot pay to fly may pay the premium for the 40ft motorised express canoes that take only a day to roar to and from the upriver port of Yurimaguas with its bus station. But losers in the global race cannot afford speed. For them there are only the big, slow, hot, lumbering cargo boats: nearly four days’ journey from Iquitos to Yurimaguas.

So the moment a passenger walks up the gangplank and strings their hammock between the iron rafters of the open–sided deck, we can guess he or she is not one of life’s winners. Anyone who was wouldn’t travel this way.

No less than among the rich there is social stratification among the poor, and the river fast shakes people down. About 24 hours is enough for the passengers on an Amazon cargo boat to settle into a mutual recognition of their relative levels of failure. So it did not take long for everyone on board — all (apart from us) mixed-race Peruvians — to reach an unspoken understanding about who the outcasts were.

I didn’t immediately recognise they were a family. I had noticed first a very black youth among the brown passengers. Black in the African sense, and with entirely African features, he was a fine-looking boy of about 15 and seemed alone and ignored. In the Spanish Andean countries, black people do meet prejudice.

Next, and soon enough, we noticed the crazy guy. He was infuriating. Of medium build and middle age, he stood out first because of his garish tracksuit, secondly because he carried with him at almost all times a portable personal sound-system with loudspeakers and flashing lights, and thirdly because whenever this was not blaring out awful music, you could hear him coming because he never picked up his feet, but just slid his flip-flops noisily along the iron deck. He would slide up to passengers and engage them in loud and intrusive interview, with faintly aggressive questions about their lives, provenance, destination and reasons for travel.

It took a while to realise this was the husband (‘my comedy husband,’ she said) of the crazy woman. Horribly overweight, she walked with a shabby wood crutch: more a disability statement than a necessity. She was friendly but loud and borderline hysterical. She too would corner strangers, to tell us about her life and her health problems. She told me she was diabetic, showing me ‘before’ and ‘after’ photographs of herself to prove that the diet she was on (it did not seem to exclude fizzy drinks) was effective. There was no difference between the photographs.

In the end, strobing lights and mindless hip-hop constantly repeating from a group of three hammocks in one corner of our deck all night led me to understand that these three people, the crazy guy, the self-proclaiming diabetic and the black boy, were a family of a sort. After I had spoken to the boy, who seemed entirely sane, the crazy guy came up and announced (I translate), ‘He belongs to us. Totally black (puro negro).’ This was said as though it were an obvious embarrassment but one that needed to be acknowledged. The boy said nothing.

But he seemed loyal to them, though plainly not their son, and did not look craven or abused. Maybe they were kind to him in their way? Maybe he was homeless and parentless and they were all he had? Maybe he was all they had? I wondered about slipping him a $100 note and begging him to quit the boat at the first opportunity and make his own way in the world — but what did I know?

For three days, as the jungle-clad banks of the great river slipped by, the crazy couple — their adoptive son looking forlornly on — haunted the entire complement of passengers and crew, deafening us, embarrassing us, waylaying us, even invading the captain’s wheelhouse. Everyone tried to avoid them but nobody confronted them about their disruptiveness. In a new way to gain an audience, the crazy guy instructed the now-sheepish boy to rummage in the family’s big, cheap, zipped plastic bags. Out came a huge and disgusting black-painted wooden dildo, which the crazy guy then started waving around, sliding across and confronting groups of his fellow passengers with the object, laughing manically as people looked away.

I suffer from fascist moments. I quickly suppressed the thought that the chances now of this man ever contributing anything to the world were so small, and the likelihood of his continuing to spoil thousands of people’s enjoyment of life were so overwhelming, that it might be better if he were shot.

All his behaviour and most of his wife’s could be explained by a craving for attention: attention on almost any terms, including self-humiliation. It struck me that this is only an extension of what drives many into politics.

Once or twice a day the wife would completely flip and start weeping and shouting at her comedy husband, ostentatiously trying to leave him. On a boat this was difficult. At one jungle halt to take on sawn hardwood, she went wailing on to the bows and attempted, crutch waving, to stagger up the steep, narrow, wobbling gangplank: the crew and the black boy beseeching her to stop. She turned back.

It happened again on our last morning and this time she succeeded: teetering up the plank, still yelling tearful imprecations, comedy husband in pursuit. He and the boy fetched all their belongings and he followed up the steep bank. The couple reached the top, the passengers gawping from our decks, the crew laughing, and the boy left relaying all the heavy cases and bags up the slope. Fellow passengers congratulated each other on finally being rid of them.

We weren’t. Six hours later, from our taxi tuk-tuk in Yurimaguas, we saw the three, still with all their belongings, struggling along the pavement in the intense midday heat. Where were they going? We never saw them again.