IN Germany they call it Butter Fahrt. Anyone can turn up at the port of Travemunde and demand a free ride to Denmark. The Baltic Sea is the location for one of the strangest phenomena in travel: a fleet of ferries shuttling between Germany and Denmark without charging fares.

This being Germany, passengers are issued with tickets showing a fare of DM0.00. Once on board, most travellers head straight for the duty-free shop. Thousands of Germans and Danes ride these 'butter boats' back and forth across the Baltic purely to buy tax-free food and drink. Some retired people even go contract shopping, buying luxuries such as vodka and wurst to order. The ferry operator makes enough profit on selling provisions to dispense with fares. Needing neither butter nor beer, just a ride to Denmark, I had a cup of tea and enjoyed the spectacle of a three-hour shopping frenzy around the Lurpak counter.

This is an extreme manifestation of the first law of free travel: with the lure of a free or cut-price ride, punters can be persuaded to embark on journeys they might otherwise not make - and part with cash they would otherwise not spend.

Airlines are experts at discount marketing, such as the Sainsbury/British Airways promotion that gives supermarket shoppers 30 per cent off BA flights. On paper this looks a splendid deal but - as has been noted in these pages - it can prove more costly than buying the same flight from a bucket shop.

To fill empty seats, airlines sell tickets through agents at huge discounts. Last month I bought a BA ticket to Entebbe that showed a fare just pounds 10 short of pounds 2,000 - yet it cost only pounds 455. As well as obtaining a discount of 83 per cent, I have avoided filling my larder with industrial quantities of cornflakes.

The BR/Boots free train ticket scheme is more promising. For a start you needed to spend only a fiver at Boots to qualify, rather than a minimum of pounds 200 for the Sainsbury offer. With a voucher, two people can travel by train together for the price of one. The main drawback is an arcane set of restrictions. Condition (j), for example, prohibits the use of vouchers between Harrow-on-the-Hill and Amersham. The scheme ends on Wednesday, so start travelling now (unless you live in Amersham and want to go to Harrow).

Most 'free' deals are hedged with conditions. When Richard Branson launched Virgin Atlantic, he offered each Upper Class passenger a free Economy ticket. It could even be given away to a friend. Shortly after the scheme began, however, the conditions changed subtly. Instead of getting a confirmed transatlantic seat, free-ticket holders now have to travel standby in peak season.

After a well-heeled Upper Class passenger gave me a free ticket, I spent a frustrating summer on a succession of waiting lists at Gatwick trying in vain to fly free to the United States. In the end I converted the voucher into pounds 20-worth of CDs at Virgin Records, but REM proved no substitute for JFK.

The second law of free travel is that public transport fares are in inverse proportion to concern about the environment. The British government's attitude seems to suggest we should all take to the roads. In contrast, cities in the pollution-conscious United States compete to offer the best free public transport. Buses anywhere in downtown Seattle are free, while Portland, Oregon, operates a 'fareless square' covering trams and buses throughout the city centre.

The third law is that nothing sells products like the promise of free travel. Hoover sales soared when the company offered two free flights to America to anyone spending more than pounds 100. So enticing was the prospect of free tickets to New York or Orlando that thousands of people bought vacuum cleaners they did not need; there is still a glut of second-hand Hoovers. The offer baffled the travel industry: how could any company promise flights worth around pounds 400 to customers spending only pounds 100?

Hoover will not say how many people have taken up the offer, how many have flown or how many seats it intends to give away. The company says the offer is 'subject to availability', but will not reveal its definition of availability. Nigel Griffiths, Labour's Consumer Affairs spokesman, accuses Hoover of 'hiding behind the small print' and is calling for a DTI inquiry. He says the company should 'honour its commitment to customers'. Since 20 wide-bodied aircraft fly from Britain to New York every day, it is reasonable to suppose there are sufficient seats on the market if Hoover chooses to buy them. But Mr Griffiths fears some Hoover owners face a summer spent vacuuming at home instead of vacationing in the United States.

One guaranteed way to obtain free travel is to set yourself up as a travel journalist. You can then take advantage of the exotic freebies holiday companies hand out in the hope of receiving favourable reviews. The Independent does not accept free trips, which is why its writers find themselves aboard Baltic butter boats rather than on Caribbean cruises.