I knew from the outset this was to be no ordinary cruise, of the floating-hotel-and-glitzy-ports variety. After all, how many cruise ships desert the calm waters of the Caribbean or the Med for the east coast of Africa, and some of the most impoverished parts of the world?

But there can be few better reminders than being almost hurled out of your bed at 3am, with the contents of the bedside table clattering across the room, as a storm hits the ship. A storm that will last all day and see you steam past two destinations in order to outrun the weather. This doesn't happen when you're gliding around Barbados with a gin and tonic in your hand.

And another reminder comes when we dock in Beira, the second city of Mozambique. The entire port comes to halt as everyone turns out to watch us - we are only the second cruise ship to call here since the 1980s (and the other one was a small, 40-passenger vessel). As we get off the ship, everyone seems agog at our arrival.

And it's easy to understand why. In Beira, poverty is endemic and the average life expectancy is 40; virtually nobody gets fat or reaches middle-age. A horde of white, plump, old people must have looked like a landing by aliens.

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But that is what a trip on our ship, the Spirit of Adventure, sets out to do: to take its passengers to places few other cruise ships would touch with a mooring rope.

Its capacity of 300 passengers means it can go into small, unusual ports. Its itineraries take it around Africa and to Madagascar; up to the far north of Norway and deep into the Baltic Sea; and over to South America, where it will sail 1,700 km up the Amazon into the rainforest. And at the end of the year it embarks on a 23-day trip to Antarctica.

What is perhaps surprising is that the firm behind this rather daring venture is Saga – the company for the over-50s, whose holidays are more linked, in the public imagination, to elasticated-waist trousers and sandals-with-socks rather than exploration of the globe's far-flung parts.

But with the Spirit of Adventure, Saga is aiming at the more intrepid holiday-makers. And not just those in the traditional Saga age bracket. In a bold expansion, the trips have been opened up to anyone over 21.

In fact, Saga were so keen to stop this cruising programme being labelled "oldies only" that when the Spirit of Adventure launched last year, the company kept its name quiet – preferring to sell this as a fresh brand aimed at all holidaymakers.

That tactic has since changed slightly: to show the new venture has a reliable parent group, Saga's name now appears in the literature – but only in the small print. (For the benefit of customers who might, understandably, hesitate to set sail for the frozen wastes of the Antarctic with what looks like an unknown outfit.)

But will younger holidaymakers be attracted to the Spirit of Adventure? And once they get onboard, what's life like among the Saga crowd?

Arriving at the ship in Cape Town, for the start of a two-week trip round the Cape of Good Hope and up the east coast of Africa, it doesn't look the most impressive vessel on the high seas.

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