As hopes began to fade for the former Royal Marine, rescuers noticed that an alarm beacon had been activated on the yacht, which is upturned in an inaccessible patch of water, 1,500 miles south-west of Perth.
Mike Calway, senior search and rescue co-ordinator at the Maritime Rescue Centre, Canberra, said: "We have noticed a different mode of transmission from his beacon, which must have been manually done.
"That would indicate that he must be around to have changed the switch from normal to alarm mode. We hope he is safe inside the overturned hull of his boat or in an overturned raft nearby."
Bullimore, 57, who was competing in the Vendee Globe single-handed, round- the-world race, ran into difficulty when his boat Exide Challenger hit severe gales on Sunday.
It capsized in such a remote spot that earlier search planes were unable to investigate the surrounding waters thoroughly.
Mr Calway said the rescue organisation had never attempted a rescue operation so far from land before, adding: "One of our aircraft will be taking off very shortly to proceed to his position, but it will take about seven hours to reach him. We are hopeful. It's just a question of whether help can arrive in time and if he can survive the cold temperatures out there."
A nightclub operator and managing director of a Birmingham-based import- export company, Bradkeyne International, Bullimore is a tough, experienced sailor who has made 27 Atlantic crossings.
Before the start of the race, on 3 November, he said: "This is one of the few things left on this earth that I want to do." His wife, Lalel, has always supported her husband's adventures but had strong reservations about the solo round-the-world voyage. She said last night that not knowing he husband's fate was agonising. "All I can do is sit by the phone and wait."
Fellow ocean-racers and an Australian rescue spokesman said just because Bullimore had not been sighted did not mean they should assume the worst. "Anybody wanting to survive is going to stay inside the hull," said Mark Gatehouse, a Plymouth dock owner who has competed against Bullimore in transatlantic races.
Race competitor Frenchman Thierry Dubois - who got into difficulties in the same area of ocean - was spotted earlier clinging to his upturned vessel and was thrown a life-raft by rescuers. Dubois, 29, clambered into the raft - dropped by an Australian air force search plane - at 11am yesterday, and now awaits the frigate HMAS Adelaide to take him to Perth.Reuse content