The Coast Guard planned to call off its search early Thursday for an acclaimed computer scientist whose sailboat has disappeared off Northern California.

Despite unusually calm weather, searchers have exhausted any area Jim Gray could have drifted or sailed after leaving for a solo sailing trip Sunday to scatter his mother's ashes at sea, a Coast Guard spokesman said.

"If they were to even find a life raft, they would have located him in their search patterns," said Petty Officer 3rd Class Jonathan Cilley.

The search would be halted at 1 a.m. PST Thursday if no significant development arose, officials said.

A cargo plane, a helicopter and six patrol boats dispatched by the Coast Guard have been scouring the Pacific since Monday for Gray, whose database innovations led to some of the most widespread technological innovations of the past 30 years.

The 63-year-old San Franciscan was last heard from on Sunday morning, shortly after he set out from San Francisco for the shark-infested waters of the Farallon Islands, about 25 miles west of the Golden Gate Bridge.

Searchers have covered 40,000 square miles from Monterey Bay to Oregon and more than 140 miles out to sea but have found no trace of the longtime sailor.

"Our local commanders have pretty much saturated the area with search units," the head of the Coast Guard, Adm. Thad Allen, said during a visit to San Francisco Wednesday.

Two tips to Coast Guard searchers that a sailboat was drifting off the Marin County coast just north of the city failed to turn up any sign of Gray or his 40-foot yacht, Tenacious, Cilley said.

Coast Guard spokeswoman Lt. Amy Marrs called Gray's disappearance a mystery because the weather was good, he was in good health and the boat was equipped with radios, flares and an emergency beacon.