"I regard this as a serious piece of thuggery," the judge said.
Pratt told the court that he had been disturbed in his sleepout by an intruder tampering with his car. He said he chased the intruder up the road and acted in self-defence when the man "jumped" him.
He said that he punched the man and stomped on him once while wearing only socks.
But he said the man he hit was not Navy marine technician Brent Salisbury, who was assaulted in McGrath St that night and who was off work for a month and a half after suffering significant head injuries and concussion.
He said Salisbury, who was in the court, "didn't look like the guy I chased".
Salisbury said he and a number of shipmates had come ashore to have dinner at a restaurant before moving on to a party in McGrath St to celebrate nearing the end of a sea-going assignment.
He said he had drunk five drinks at each location. When defence counsel Scott Jefferson asked if he was intoxicated, he agreed that he was.
Salisbury said he stepped out of the party to phone his girlfriend and call the driver of a Navy van which had been arranged to take the sailors to and from their ship.
Salisbury told the court that what happened next was "all fuzzy to me" but his statement to police said that he remembered being hit and kicked in the head by someone wearing boots.
Salisbury suffered abrasions to his mouth and lips, severe swelling to the left side of his face and was unable to open his left eye for several days.
McGrath St resident Sarah Cameron said she saw from her window two men in the street wrestling each other to the ground. One then punched the other from a kneeling position seven times before stomping on him three times "very hard, with the heel".
Judge Collins found Pratt guilty and remanded him on bail for sentencing on March 16.
The patrol vessel HMNZS Taupo was in the Port of Napier last July 16.
