The record books will show that Defender and Courageous split a pair of America's Cup trial races today, but that won't tell what the radio hams would call, "The rest of the story."

The rest is that Courageous, the rejuvenated, 10-year-old, two-time cup champion, led her stablemate around Rhode Island Sound like a puppy on a leash for five hours in brisk southerly winds.

She gave up her lead only once all day, and then only for about three minutes.

It was a poor three minutes to pick--the finish of the first race, where Defender skipper Tom Blackaller managed to slip by Courageous with a couple of slick maneuvers to salvage a bit of dignity from the day with an official 10-second win.

But Defender's inability to otherwise put away her ageless rival for the right to defend the cup next month, along with two clear-cut victories at the starting line for Courageous skipper John Kolius, dealt a blow to the new blue yacht.

This was the first day of final trials to select a defender from among three U.S. contenders. Defender, whose name matches her mission, has been considered the chief threat to 1980 champion Dennis Conner and his new boat, Liberty, particularly after Courageous all but collapsed in July trials.

In an effort to close in on the favored Conner, Defender underwent major surgery. Her aluminum stern was lopped off, trimmed down and refitted in hopes a slimmer hind-section would improve downwind performance. But today in her first outing since the fanny-lift she gained significant ground on Courageous on only one downwind leg--the one that provided her victory.

Meanwhile, Courageous and Kolius demonstrated that the white yacht is back in form.

She maintained her nearly all-day lead under the watchful and perpetual gaze of the America's Cup Selection Committee, which sometime before Sept. 8 must choose which of the contenders will defend the cup.

The committee insists that it keeps no won-lost records during the trials, but acts rather on its perceptions of which yacht is going best, overall.

Kolius, a 32-year-old upstart from Houston who never raced 12 meters before boarding Courageous, gave them something to ponder, particularly when he buried the veteran Blackaller at the starting line twice.

The first time he forced the newer yacht to tack away from the line and led by five seconds at the gun, a lead he quickly built to 32 seconds by the first turn.

He held that lead through four legs of that race, only to be edged at the last mile when Blackaller lured him further downwind than Kolius should have gone, then tacked quickly and slipped across the finish ahead.

But Kolius got revenge at the start of the second race, when Blackaller led him a merry chase through the spectator fleet, only to be bested once again at the line, this time by four seconds. And this time Kolius improved his advantage at every mark.

Courageous and her crew sailed "better than we ever have," said Kolius. As to how good that is, "We'll find out Wednesday against Liberty," he said.

In the semifinal round of trials to pick a challenger, conducted elsewhere on the sparkling sound today, Australia II continued her domination, defeating Canada 1 by 1 minute 19 seconds, and Britain's Victory '83 beat Italian Azzurra by 28 seconds.

The results give Australia a 4-0 record in the nine-race series to winnow the four challenging boats down to two. Victory is 3-1. Azzurra is 1-3 and Canada 0-4.

Meanwhile, a spokesman for the International Yacht Racing Union said in London that it would decide this week whether to call a special committee meeting to consider the U.S. Yacht Racing Union challenge of Australia II's keel.