America's Cup yachts will be able to use engines to trim sails and shift water ballast between hulls, according to a ruling made by the New York State Supreme Court.
In the latest court fight of a competition that could spark more excitement in courtrooms than on the water, the decision is seen as a victory for two-time defending champion Alinghi of Switzerland over United States challenger Oracle.
Alinghi put an engine on its catamaran to shift ballast and trim sails, prompting objections from Oracle.
The rivals are expected to meet in February for a best-of-three clash in big multi-hull boats.
Alinghi plans to announce the venue next week.
But there will be at least one more court date before the bitter dispute moves from the courtroom to the waves.
Another hearing has been set for 10 August to hear arguments over Alinghi's demand that Oracle submit a certificate of measurement for its trimaran.