Team New Zealand will sail the newer of their two boats when they meet Swiss challengers Alinghi in the America's Cup, which begins on Saturday.

The Cup defenders have been trialling the two boats against each other in recent weeks and chose NZL-82, though they believe there is little speed difference between the pair.

Both Team New Zealand boats feature the radical new "hula" design, a hull appendage designed to add to the overall waterline length of the boats and increase their speed.

NZL-81 was launched last August, followed two months later by NZL-82.

"We created both boats with some deliberate differences. After a while of sailing them together we decided we preferred 82 to race against Alinghi," said Mike Drummond, Team New Zealand's boat designer and navigator.

Alinghi are sailing SUI-64, the boat they used to beat US software billionaire Larry Ellison's Oracle BMW Racing 5-1 in the Louis Vuitton Cup challenger's final.

Both boats will be officially unveiled on Tuesday.

Meanwhile, Alinghi grinder Christian Karcher will be unavailable for the Cup because of a hip injury.

The Frenchman fell heavily in training in November and aggravated the injury again last week.

"We lose one of the most experienced people in the Cup...but we have another grinder who is ready to occupy this post," said Alinghi director Jochen Schuemann.

Grinders occupy the most physically demanding crew positions on America's Cup yachts and operate winches which raise and lower the boat's massive sails.