According to the latest America's Cup joke, the world's shortest book is called "New York Yacht Club Ethics."

That line got a chuckle around town today after release of telegrams disclosing that the Freedom-Liberty campaign to defend the cup the U.S. has never lost, sought secret plans of foreign contender Australia II's keel last month from a Dutch tank-testing company.

"We are finally convinced of (the keel's) potential and would therefore like to build the same design under one of our boats," wrote Ed du Moulin, a syndicate manager and NYYC stalwart, in a July 21 wire. "Need your maximum input and experience. We can start (work) next week and be ready Aug. 25."

The Netherlands Ship Model Basin refused on grounds "we are contracted to (the Australians) not to test . . . models for any other syndicate."

Two days later NYYC Commodore Robert McCullough formally protested Australia II's measurement and certification as a legal 12-meter yacht on grounds the unconventional keel gave her an unfair advantage.

The inference of the joke-makers is that NYYC officials decided the secret Australia II keel is illegal only after they were told they couldn't have it, too.

Today the speedy Australia II overcame near disaster just before the starting gun to continue her string of victories in trials to select a foreign challenger. She trounced Italy's Azzurra by 3 minutes 20 seconds after hastily replacing the boom, the spar at the foot of the mainsail, after it split in half 21 minutes before the start.

The boom broke with only 11 minutes to go before the first gun, which signals 10 minutes until start. Once the gun sounds, a competing yacht can have no assistance from her tender.

Skipper John Bertrand said Australia II's crew has a boom replacement drill and the operation takes 10 minutes.

A spare boom was rushed aboard from her tender and the crew had it in place in 9 1/2 minutes, leaving them 90 seconds before the first gun. They went on to take the start from Azzurra.

The Freedom/Liberty syndicate, meanwhile, explained du Moulin's request for the keel plans, saying the group did not intend to build a competing yacht using the plans but sought instead to duplicate the keel on a noncompeting boat and use it as a trial horse against Liberty.

Because of the Australia II camp's secrecy about the bulbous, winged keel, said the syndicate in a prepared statement, "It has been difficult for any potential defender to prepare to compete against her . . . It was never our intention to enter an American yacht with the winged keel in competition."

Australia II's syndicate manager, Warren Jones, said his organization had the telegrams to and from the Dutch tank-testing company for three weeks, but didn't intend to release them. New York Yacht Club and U.S. Yacht Racing Union persistence in protesting the legality of Australia II's keel convinced him finally to do so, he said.

Her victory today in 16-knot winds and choppy seas gave Australia II a 38-4 record, far and away the best in trial races to pick a challenger.

She and Britain's Victory '83 are both 2-0 in the semifinal trials that run this week and next. The two undefeated yachts will meet Monday.

The three American yachts, Liberty, Defender and Courageous, will start final trials Tuesday to select a defender for the cup that has never been lost by Americans in the 24 times it has been contested.