First responders, families of Sept. 11 victims and the public were invited Monday to a waterfront viewing area, where they could see the crew standing at attention along the ship's deck.
The big ship paused. Then the 21 shots were fired, with a cracking sound, in bursts of three.
The bow of the $1 billion ship, built in Louisiana, contains about 7.5 tons of trade center steel.
After the ground zero stop, the ship sails up the Hudson River to the George Washington Bridge. Then it turns around and heads south to Manhattan's Pier 88. The official commissioning ceremony will be on Saturday.