The U. S. Coast Guard Thursday renewed its search a fourth day for Jim Gray, 63, one of the original relational database System R researchers, combing more than 40,000 square miles of ocean. So far, no sign of the frequently honored computer scientist can be found.

The Coast Guard launched its C-130 search plane Thursday due to favorable search conditions -- unusual clear days with high visibility -- to search 200 miles west of San Francisco, or 173 miles beyond the Farallon Islands that were Gray's original destination Jan. 28. He set sail Sunday morning on his 40-foot Tenacious to scatter his mother's ashes at the islands. He was expected back before nightfall and never showed up.

The Coast Guard also is searching 200 miles south of the islands. It's already gone over quadrants north of the islands toward Humboldt Bay. Gray, 63, on previous trips, had shown a predilection for sailing to the north after reaching the islands.

The added flight was a reversal of the Coast Guard's plan to call off the search at 1 a.m. Thursday. The C-130 took off at 8 a.m. to renew the effort.

So far, the best search methods available can't find a boat, a human being floating in the water, or a slick of debris that might indicate the fate of the 40-foot Tenacious and its skipper. Gray worked with IBM, Digital Equipment, and in his latest job at Microsoft's eScience Group in San Francisco to extend the power of computing and database technology to more people. He was a guiding force of the TerraServer project, a predecessor to the kind of database system that runs Google Maps, and the Sloan Digital SkyServer, a database of astronomy information that correlates different types of telescopic data to the same plots of sky.

His many friends and admirers in the Silicon Valley are resistant to the possibility that he can't be found. They've just watched as official searches in the Oregon mountains bungled the hunt for James Kim, an editor at San Francisco's CNet, who died trying to walk out on his own to save his family.

They have pointed out in messages to InformationWeek that Jim used to regularly sail the Farallons route. He is a former volunteer sailor carrying supplies to the naturalists who reside on the islands. He knew the route so well that he could have found his way back in dense fog, violent storms, or heavy seas. Instead, conditions were unusually favorable for the trip, with light winds and clear visibility.

His sailing friends are mystified about what might have happened. The Coast Guard's Capt. David Swatland told me last night that Gray's boat, along with the usual emergency gear, carried a radio signal transmitting beacon that could be manually activated or automatically triggered when it hit the water. Searchers don't need to find the beacon. When activated, it transmits to a satellite receiver, which automatically relays the signal to the Coast Guard and fixes its transmitting position. The Coast Guard has a registry of such beacons and can immediately identify the owner, the boat on which it's used, and how to contact the owner's family.

Let's say the worst has happened, Tenacious accidentally jibed and Jim was knocked overboard by the swinging boom of the sail. Where's the boat? It would sail on, or drift to some point presumably within range of the Coast Guard search.

Or let's say Jim sailed into a mostly submerged cargo container accidentally dropped by the procession of container ships coming into the Bay, which is known to happen in storms occasionally. Such a collision would break open a sailboat's fiberglass hull. In trying to fight the onrush of sea water that would ensue, let's say Jim lost his VHF radio as electrical systems shorted out, so he couldn't issue a Mayday call. Still, where's the signal from that automatic distress beacon? Where's the boat lying on its side in the relatively short distance, by Pacific standards, between the city and the islands?

It doesn't make any sense, this disappearance. Jim's friends in the Bay area are committed to continuing the search. Before the Coast Guard made its decision to renew, friends said they would hire aircraft to go aloft and hunt some more.

We are all hoping that Jim will somehow, once again, beat the odds, as he did so often in his scientific and research work. He is smart and resourceful, and he won't quit.

But we are stunned at that potential loss looming before us.