America's Cup holders Team New Zealand and Louis Vuitton Cup finalists Alinghi and Oracle unveiled their boats in public for the first time on Tuesday.

Until now the hull shape and underwater features, such as keel and rudder, have been kept under wraps.

But the three teams left in the competition finally removed the skirts that have hidden their well-kept secrets in a special ceremony in Auckland.

The Kiwi clip-on or 'hula'

Team New Zealand, who have yet to decide which of their two boats to race, stole the show with both NZL-81 and NZL-82 sporting the much-talked about "Kiwi clip-on".

The device is a second section of hull attached under the stern, millimetres from the hull proper.

It is designed to increase the boats' overall waterline length and volume and therefore increase their speed.

Team New Zealand boss Tom Schnackenberg insisted the design, dubbed the 'hula' - short for hull appendage - was within Cup rules.

These state that appendages must not touch the hull outside certain areas.

"We had to undergo a very rigorous process with the measurers," Schnackenberg said.

"Their demands were quite stringent. We have made sure the 'hula' does not touch the hull at any time."

But Oracle admitted they were still studying the design from a legal point of view.

"The hula - that's the name now is it? Is that because it's a dance around the rules or something?" said Oracle designer Bruce Farr.

Team New Zealand have until 10 February to decide which boat to use but will not be allowed to make changes from now on.

Alinghi display SUI-64

Meanwhile, Alinghi and Oracle's designs are fairly similar, with both teams having opted to stick with the same boats that have served them well throughout the Louis Vuitton Cup.

Swiss syndicate Alinghi will continue to sail SUI-64 while Larry Ellison's American challenge Oracle will race USA-76.

The two boats appeared to be of a more conventional design with small variations in keel configuration.

"Today people will focus on appendages and hull shapes but you shouldn't underestimate the effect of small detail changes and the tuning of these boats," said Alinghi designer Grant Simmer.

The winning challenger must use the same boat against defending champions Team New Zealand in the America's Cup proper, which begins in Auckland on 15 February.