Two sailors have died in separate incidents in the treacherous Sydney to Hobart bluewater classic, organisers said Friday, while race favourite Comanche was forced to retire.

Both crew members, on Flying Fish Arctos and Bowline, were hit by booms as the yachts were making their way south along the New South Wales coast after setting off from Sydney Harbour on Boxing Day.

Other crew tried to revive them, but were unsuccessful.

"As these incidents are being dealt with by the Water Police and all family members are yet to be contacted, we cannot provide further details at this stage," the race committee said in a brief statement.

"Our thoughts are with the crews, family and friends of the deceased."

Gale-force winds and high seas had been forecast for the 628-nautical mile event, which ends in the Tasmanian state capital Hobart after the fleet tackles the notorious Bass Strait.

Weather is a critical factor in the race, first held in 1945, with winds and conditions often changing rapidly.

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.

So far seven yachts have pulled out from the initial fleet of 104, organisers said.

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

In a major setback, race record holder Comanche is also out with mainsail damage, leaving the door open for its main supermaxi rival LawConnect to win line honours for the second straight year. (AFP)