The Sydney to Hobart yacht race has been left reeling after the deaths of two sailors in wild weather that has forced a fifth of the fleet to retire.

Supermaxi LawConnect was in the box seat on Friday evening to take out line honours for a second year running after rival Master Lock Comanche pulled the pin with mainsail damage.

South Australian Nick Smith, 65, and 55-year-old West Australian Roy Quaden were killed in separate incidents on Thursday's first night of racing off the coast of NSW.

Smith, competing on Bowline in his fifth Sydney to Hobart, hit his head on a winch after being knocked across the yacht when he was struck by the boat's main sheet.

Quaden was hit by the boom - the large horizontal pole at the bottom of the mainsail - while aboard Flying Fish Arctos.

Both crews, who subsequently retired from the event and limped back to shore, performed CPR on the men but couldn't save them.

"It's tragic. You're expecting an adventure ? to do something really special and something like this happens," David Jacobs, vice-commodore of race organiser, the Cruising Yacht Club of Australia, told a media conference.

The deaths were the first in the 628-nautical mile race since the 1998 event in which six sailors lost their lives in storms, prompting mass safety protocol reforms.

As at 8pm on Friday, 25 yachts from the 104-strong starting fleet had retired, citing a range of problems including crew injuries, damage and electrical issues.

Porco Rosso had to recover a crew member who was swept overboard in the early hours of the morning in darkness.

Jacobs said there was no consideration given to calling off the race.

"It is a fundamental principle of yacht racing, that once the race starts, the skipper has the right and the obligation to decide whether it is safe to continue," he said.

"We have a complex structure around the race to help with safety, if we cancel the race that structure falls away."

Jacobs said Bowline and Flying Fish Arctos were dealing with winds of 30-38 knots and seas of 2-3m.

He was adamant forecasters did not warn organisers of fatal weather, but conceded conditions played a part in the incidents, which will be investigated.

"The forecast was strong winds to gale force winds," Jacobs said.

"The weather ? these boats and these crews are used to, they train for, the boats are prepared for, but they were challenging conditions."

In a pre-race interview, LawConnect's skipper Christian Beck said the weather was likely to bring the worst Sydney to Hobart conditions he'd seen.

The boat's veteran captain Ty Oxley said conditions had eased down Tasmania's east coast after Thursday night.

"We held everything together. All of our manoeuvres were done with a lot of planning," he told AAP on Friday evening.

"There was a lot of water down below and coming through hatches."

Oxley expressed his condolences to the family and friends of Smith and Quaden.

"We've still got a job to do at the moment, get our crew safely to Hobart, so we're concentrating on that at the moment," he said.

LawConnect appears likely to cross the line in the early hours of Saturday morning, well short of the race record of one day, nine hours, 15 minutes and 24 seconds.

It was some 18 nautical miles ahead of Celestial V70 at 8pm on Friday.

Flying Fish Arctos, a NSW-based 50-footer designed for around the world sailing, had contested 17 previous Sydney to Hobart events.

It arrived at Jervis Bay on Friday morning, while Bowline was escorted by police to Batemans Bay.

Master Lock Comanche withdrew from the race 63nm off Green Cape in the early hours of Friday while leading the fleet.

It was the first time the quadruple line honours champion, one of the most impressive monohull yachts in the world, had retired in its nine editions of the race.

Comanche had been chasing atonement for last year when LawConnect claimed line honours by 51 seconds in a tit-for-tat battle in the River Derwent.