Three weeks ago Dennis Conner said Sept. 16 was a guaranteed victory for his America's Cup yacht Liberty because it was his 41st birthday. Today he won, as promised, but through arguments in a hot conference room and not on the water.

After hearing 4 1/2 hours of testimony, fleshed out by videotape replays from spectator boats and an observation blimp, a five-man international jury deliberated two hours before disallowing a protest of Conner's Thursday triumph over Australia II.

The Australians, who now are 0-2 in the best-of-seven cup final, claimed Liberty maneuvered too close when she took the lead for the first and final time more than halfway through the race. But the jury ruled Australia II had adequate sea way. Racing resumes Saturday.

Australian radio reporters started their broadcasts this afternoon, "It was another bad day for Australia," but on the Liberty docks crew members were crowing that their opponents were suffering from inadequate preparation during the summer of trials for the cup, when Australia II accumulated a 48-6 won-lost record.

"We're following the Ps," said Liberty mast man Bobby Campbell. "Proper planning prevents poor performance. We spent 18 months working 12 hours a day to keep problems like they're having from happening to us."

The Australians worked just as long, but evidently not as carefully. Their losses Wednesday and Thursday both were due at least in part to gear breakdowns.

And the Liberty crew believes that the Australians' lack of close competition during trials all summer has left them bewildered in the kind of close, head-to-head racing that has developed in this cup final.

On Wednesday, Conner held onto a minuscule lead with a daring turn toward Australia's midsection as that yacht bore down on Liberty on the next to last leg. The Liberty crew is convinced that maneuver took their foes by surprise. When Australia turned sharply to avoid Liberty, her steering gear broke and she was left wallowing in the swells.

Again Thursday Conner tacked hard on Australia's bow to steal the lead in the fourth leg of the race, a maneuver that resulted in the protest. Liberty tactician Tom Whidden, who argued his side's case to the jury, said the maneuver, while coming close to a collision course, was more conservative than similar tactics Conner took during trials this summer.

"It looked like a normal situation like we've been in all summer," said Whidden. "It seemed normal to us, but to them, not having been in close quarters all summer, it might have looked different."

The Australians presented testimony from their bowman and tactician that they swerved to avoid collision when Liberty's transom was only 24 inches away from their advancing bow. But Whidden guessed it was more like seven to 10 feet.

The jury settled on a four-foot margin of clearance, which it deemed safe, and ruled Australia II "could have kept clear of Liberty either by maintaining their course or by tacking."

Australian syndicate chief Alan Bond said he would seek further recourse in the protest, but the international jury's decision, by cup rules, is final.

As if to lend credence to Liberty claims of sloppiness in the Australian effort, those crewmen spent six hours today checking and strengthening welds all through the controversial challenger and turned up two cracks in the rudder stern post. "There were weaknesses there we hadn't picked up before," said Bond.

Today's protest hearing was the first in an America's Cup final race since 1970, when another Australia yacht protested that Intrepid had made a violation near the starting line. That protest was disallowed by a panel of members of the host New York Yacht Club and sparked angry recriminations from the Australians, who charged the NYYC with bias.

After that series the club opted for an international jury, and today's hearing was conducted by jurors from Mexico, Sweden, Bermuda and Ireland, under chief hearing officer Livius Sherwood, a criminal court judge in Canada.

The Liberty crowd believes it has an experience edge even in protest hearings, after having spent much of the summer defending themselves against protests from Tom Blackaller, an opponent in the three-boat trials to select a defender for the U.S.

In midsummer Conner appointed Whidden to handle the burgeoning protest load and Whidden said, after today's hearing, that the experience served him well. "It works better procedurally if I argue the case and call Dennis as a witness," he said, though he praised Australia II skipper John Bertrand, who argued for the other side.

Racing Saturday is expected to be wild, with stormy skies and southerly winds of 15 to 25 knots forecast.