One of the two training boats sailed by the US America's Cup challenge backed by billionaire Larry Ellison snapped its mast in two while sailing off North Head early yesterday afternoon.

Oracle were last night uncertain why the rig broke at its halfway point in a mere six knots of wind, but the boat quickly returned to its base in the Viaduct Harbour with the mast strapped on board so the incident could be investigated.

Remarkably, no one was hurt as the broken half of the mast swung down on to the deck during a sailing duel with its sister yacht at 2.30 pm.

The crew praised the efforts of helmsman Peter Holmberg, who ensured all were safe as the rig fell.

In November, one of the Oracle boats, USA49, lost its keel in the Hauraki Gulf - almost exactly a year to the day since the other training yacht, USA61, suffered the same fate on Auckland waters.

Oracle bought both yachts and their appendages from Paul Cayard's AmericaOne syndicate after the last America's Cup in 2000. The team had resumed their summer training programme late last week after a Christmas break.

Oracle spokeswoman Joanna Ingley said the syndicate was obviously disappointed that it had suffered yet another setback. `But we are all well aware that America's Cup masts can break," she said. "We'll be back on the water two-boat testing later in the week.'

Syndicate owner Ellison returned to the United States at the weekend after spending time with his crew.