A great, black cloud hung over town this evening, boosting the spirits of the men who on Tuesday must defend the longest winning streak in modern sport, the 132-year U.S. hold on the America's Cup.

Dennis Conner's red 12-meter yacht Liberty, facing perhaps the stiffest cup challenge ever when she goes against the radically innovative Australia II, likes a breeze. That's what she's been promised once the front moves through tonight, with northeast winds of about 15 knots expected.

There could be other assistance for the besieged Americans. The Coast Guard, charged with keeping the race course clear of spectator yachts, said the controversy surrounding Australia II's winged keel and the prospect of the first real threat to the cup in decades will lure thousands of viewers to the race course eight miles offshore.

Coast Guard Capt. Bill Nolan said 3,000 spectator boats are expected, more than 2 1/2 times the largest spectator fleet in the 1980 cup, and the Guard will have only 38 official vessels on hand to control the crowd.

That could spell rough seas, stirred up by the prop wash of the spectating hordes, and that, too, may work to the Aussies' disadvantage.

"It's going to make a tremendous sea," said Liberty sail trimmer John Marshall. "The Australians could be victimized in the spectator wash," because their winged keel responds poorly to a roiled and confused sea, he said.

There were skirmishes aplenty on this final day before the two 12-meters square off in the best-of-seven series for yachting's most prestigious prize.

Neither boat left the dock for practice as last-minute preparations were undertaken by the crewmen, who have worked two years for the right to compete. "It's funny," said veteran Australian sailor John Longley. "I'm relaxed and I'm excited at the same time."

The Aussies had the worst of the last-minute fixing, because when their boat was certified by official measurers on Friday a slight error in the hull lines was spied by their opponents. Today they had to apply a putty-like filler to the gunwales to come up to specs. This application of plastic "mud," which will have no effect on performance, nonetheless occupied time they could have spent on final tactical planning.

Meantime, Conner was off golfing and his crew had time for a morning rest.

The Aussies reneged on their planned public unveiling of the secret winged keel, with syndicate manager Alan Bond saying he'd simply changed his mind. "You'll see the keel at the same time the cup is presented to us," he said.

He took delivery of a gold wrench with which he was to unbolt the cup from its perch in the New York Yacht Club. "I don't know whether I'll need this to unbolt it or to knock them over the heads and take it," said Bond, who has been embroiled with the NYYC all summer over challenges to the legality of the Australian keel.

The challenges ended in late August when the NYYC decided it was beating a dead horse. But charges that the keel gives the Aussies an unfair advantage and that its design was illegally assisted by Dutch scientists were raised again by Liberty navigator Halsey Herreshoff.

Herreshoff called the keel a "trick" and said, "the truth will come out in the end. It always does."

The Americans, who compiled a 34-17 won-lost record in summer-long trials against two other U.S. contenders, are banking on good overall boat performance and excellent crew work. The Aussies, who were 48-6 against six other foreign contenders, figure they have superior boat speed and maneuverability.

Marshall said the question will be whether the Aussies have "the responses to do the job, after not being tested. They have the heart, but if you play against the Dallas Cowboys, you better be able to put your helmet on straight."

On Saturday, the last day of practice, the crews had a chance to answer the question all observers here are asking: which boat has the speed?

Liberty spied Australia II sail-testing on Rhode Island Sound, slid over near her and took off on the same tack for a quick speed check. Conner shouted over, "Want to drag?" according to Longley.

"But we had our own program going and our own things to do. We told him . . . we'd see him Tuesday."

At last, Tuesday is nigh.

Jimmy the Greek is one of the few bold enough to pick Conner and Liberty as favorites (at 6-5). "He's just so good," the Greek told The Washington Post about Liberty's skipper. "With the boats this close, I like Dennis Conner over John Bertrand. If Australia II makes one bad move, Dennis will pick it up and adjust for it. I like his experience."