The Golden Gate Yacht Club has caved in to the rule changes made to next February's 33rd America's Cup regatta and allowed its challenging syndicate BMW Oracle Racing to fit an engine to its giant trimaran.

The yacht, known as BOR 90, emerged from its shed in San Diego this week with a powerful engine fitted in the centre hull and the places for grinders and their winches removed.

The Swiss holder of the trophy, Ernesto Bertarelli, shocked the sailing world earlier this year when his giant catamaran, Alinghi, was unveiled with a Honda engine hanging from its rear cross arm. The engine is used to trim the giant sails and more than halves the number of crew needed to sail.

The BOR 90 team plans to test the engine and the hydraulics that will now be used to trim the sails on land before putting the trimaran back in the water at the end of the month.

The cockpit of BOR 90 has been extensively modified to accommodate the engine.

Should the 33rd challenge regatta go ahead -- something that is still in doubt as various matters are before the New York Supreme Court -- it will be the first time in the 159 year history of the America's Cup that anything other than human power has been used to trim sails and do other work.

BOR 90 design director Mike Drummond says that when the trimaran was originally designed and built, the team assumed they would have to use the crew to provide all the power on board -- as has always been the case in America's Cup regattas.

"With the change to the rules, we've had to adapt, adjust and modify," Drummond says, "Otherwise the engine power that Alinghi designed into their boat would have given them a significant advantage."

Sydney sailor Jimmy Spithill is the helmsman of BOR 90.