SYDNEY (AFP) - Supermaxi favourite Wild Oats XI led the start of the gruelling Sydney to Hobart yacht race Monday, as forecasters predicted heavy weather will hit the fleet within hours.

Cloudy skies were overhead when the cannon fired for the start of the annual 628 nautical mile race to the southern island state of Tasmania and rough conditions are expected for the first night at sea. "As we know from a Sydney summertime thunderstorm, you can get some very, very strong winds and hail out the bottom end of them," the weather bureau's Rob Webb said.

"It might be worth keeping an eye out for severe thunderstorm warnings during today." Eighty-eight boats are competing in this year's Sydney to Hobart -- a race which in 1998 claimed the lives of six sailors and sank five yachts when catastrophic weather hit.

Weather has already played its part in the 2011 race, with crew members from several yachts delayed in reaching their boats in Sydney due to a thunderstorm in Melbourne on Sunday which disrupted their flights. That Melbourne weather is now expected to begin affecting the fleet within hours as it sails south and create winds of 20 to 30 knots.

At the same time, ex-tropical cyclone Fina has whipped up a northerly swell of more than five metres which could also create dangerous sailing conditions. Wild Oats XI, one of the biggest yachts in the race and the favourite for line honours, should manage the weather better than the smaller competitors but conditions will likely mean its race record is safe.

The boat, which is looking for its sixth win line honours in Australia's best known yacht race, posted the race record in 2005 with a time of one day, 18 hours, 40 minutes and 10 seconds to complete the bluewater challenge. In a repeat of last year's start, Wild Oats XI led the pack out of Sydney Harbour Monday as it began its race down the coast, followed by super-maxi Investec LOYAL and Wild Thing.

Coastal currents and the rapidly changing weather make the Sydney to Hobart once of the world's most challenging races. The race attract boats ranging in size from 30-footers (nine metres) to 100-footers.