Crews of more than 100 yachts set sail 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.

- 'Boat-breaking' -

Six men died, five boats sank and 55 sailors were rescued in 1998, when a deep depression exploded over the fleet in the Bass Strait.

"We are seeing strong wind warnings developing through the afternoon today and getting up to gale, so about 35 knots," meteorologist Gabrielle Woodhouse said in the last briefing hours before the start.

A strong south-westerly change would move across Bass Strait early on Friday morning, potentially bringing showers, lightning, thunder, and waves of up to 4 metres (13 feet).

"It's going to be a fast and furious first night out there on our way down to Hobart," said Matt Allen, co-skipper of Comanche.

"It's probably going to be boat-breaking sort of conditions," Allen said ahead of the race. "The really small boats are really going to cop it a few times in this race."

Fellow skipper James Mayo said strategy would be key for Comanche, which surged over the finish line in one day, 9 hours, 15 minutes and 24 seconds to set a new mark in 2017.

"It's about keeping the boat in one piece, you know there's going to be some good rides," Mayo said.

- 'Worst forecast' -

LawConnect, too, expects a bumpy trip.

"These conditions are probably the worst forecast I've ever had to go through. The odds of boat damage are obviously very high," LawConnect skipper Christian Beck said ahead of the race.

Two other supermaxis are entered -- Wild Thing and Maritimo.

Other previous overall handicap winners in the fleet include Centennial 7 (formerly Celestial) and Love and War.

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