Defending America's Cup champion Dennis Conner bounced back with two victories in protest-riddled trial racing today, while sure-fire challenger Australia II took a thrashing at the hands of the British, then faced allegations of sandbagging.

Much developed before the eyes of few on Rhode Island Sound, including the closest trial race in six years--Liberty's one-second triumph over rival Defender--two protests by Defender skipper Tom Blackaller and the general sense that the competitive fires are starting to build among the Americans in these final days before the Cup.

Dull gray skies and spitting rain kept all but the hardiest observers from the course eight miles out to sea, but those who braved it saw a classic finish in the second American trial race.

Not since Lowell North and Ted Turner finished one second apart in a 1977 trial had one been this close. Spectators watched, astonished, as Conner's Liberty and Blackaller's Defender, which had started their second race of the day exactly even, turned up 20 miles later side-by-side.

Only one observer was unmoved--a lonely lobster fisherman who inexplicably sat smack in the middle of the finish as the sleek 12 meters bore home. The lobsterman never flinched as the boats finished 10 yards on either side of him, dropping their sails to lower wind resistance and shooting into the wind just a heartbeat apart.

The victory-by-an-eyelash climaxed a race in which the lead changed hands on every leg but one and it buoyed spirits in the Liberty camp after back-to-back defeats to the third U.S. boat, Courageous, on Wednesday. Conner was particularly encouraged that his boat overtook Defender three times on upwind legs.

But who gets the final credit for the win was not known because Blackaller protested one of Conner's overtaking tactics. A hearing will be held within a few days.

Blackaller also protested the first race, in which Conner stumped him at the start and romped away to a 3-minute-plus victory. Blackaller contends Conner's starting tactics in that race were illegal. Conner and his crewmen maintain both the protests are "chicken feathers."

"He protests us every race," said Liberty tactician Tom Whidden, "and we beat him every race. We're getting pretty good at defending (against the protests)."

Elsewhere, Australia II, undefeated since July and running away with the semifinal trials to pick a Cup challenger, suddenly turned up hopelessly slow against principal rival Victory '83, losing their race by 2 minutes 50 seconds. The Aussies said a bad choice of mainsail slowed them down, but Conner blasted the turnaround as "sandbagging."

"Obviously they threw it," said Conner. "They're trying to take the heat off."

The heat he meant is the heat his organization is applying to have the Australians' radically designed keel ruled illegal by the International Yacht Racing Union.

Indeed, British syndicate chief Peter de Savary offered the advice to the Americans to "drop all the moves to stop the Australian boat, get out on the water and, if Australia is the challenger, beat her. Today we demonstrated that can be done."

Said Conner, "Tell de Savary thanks for the advice. We're going to take it."

Victory '83's win gave her and Australia II identical records of 5-1 in the nine-race challenger semifinal round to eliminate two boats. Azzurra, which beat Canada 1 today, is 2-6 and needs just one loss to be eliminated. Canada is 0-6.