The crews of more than 100 yachts set sail on Thursday on a "boat-breaking" Sydney-Hobart yacht race, with gale-force winds and high seas forecast for the gruelling bluewater classic.

Under blue, mid-summer skies, spectators crowded onto boats and lined the shore of a breezy Sydney Harbour as a starting cannon set the fleet on a dash to the open ocean.

In a dramatic opening contest, reigning line honours champion LawConnect lost an early lead when one of her sails failed to unfurl.

Comanche, four-time line honours champion and the event's record holder, took over as frontrunner.

By late evening, Comanche was speeding southwards down Australia's east coast with a lead of more than eight nautical miles over LawConnect.

LawConnect edged Comanche to win by just 51 seconds last year and the two 100-foot supermaxis are again expected to battle for line honours.

Alive, last year's overall winner of the Tattersall Cup, which takes into account boat size and other factors, was the first casualty of this year's race, retiring with reported engine issues.

Of the 104 entrants at the start of the race, 102 remain after the withdrawals of Alive and the two-handed Transcendence Rudy Project, which dismasted.

Race crews face gales, storms and big waves as they sail south and then tackle the notorious Bass Strait towards the Tasmanian state capital Hobart.

Weather is a critical factor in the 628-nautical mile event, first held in 1945. (AFP)