A $500,000 Port Authority patrol boat sank this month after a veteran police sergeant took the advice of a clueless civilian safety instructor — and opened a hatch while it was under water, The Post has learned.

“It was like opening a window during a carwash,” one PA insider lamented of the screw-up off Breezy Point, Queens, that left 11 people scrambling for their lives.

Nobody was seriously injured, but eight cops and three civilians had to either swim to safety or be rescued as the 37-foot M-2 model Moose Boats craft sank 40 feet to the ocean floor about 300 yards off shore.

PA sources said the sergeant who ordered the hatch open is expected to face disciplinary action. They refused to identify him. He will likely lose vacation days for failing to maintain supervisory control and basically abdicating his command to a civilian during the Sept. 9 debacle, they said.

The aluminum-hull catamaran was being used for rescue exercises mandated by the Federal Aviation Administration for airport first responders, a source said.

The boat, PAPD No. 7211, was among those on hand to respond to potential terror maritime assaults on JFK Airport and possible airline crashes in Jamaica Bay.

It is equipped with $500,000 in firefighting pumps, hoses, nozzles, marine radios, radar, GPS gear, depth sounders, pursuit lights, sirens, scuba gear and dozens of inflatable rafts designed to carry up to 600 people.

The boat had been carrying three instructors from Ocean Rescue Systems, a Portland, Maine, firm the PA hired to conduct the exercises.

The starboard engine began to display “vibrations” that afternoon, sources said, and some believed a piece of driftwood or rope was clogging the engine.

An Ocean Rescue Systems instructor advised the sergeant to open a hatch near it — below the water line — to see if there was an obstruction, one insider said.

But the hatch should have been opened only if the craft was in dry dock, the source explained.

The sergeant took the instructor’s advice — and the boat sank within a half-hour.

PA officials had an outside contractor raise the boat the next day. It’s unclear how much of its equipment is still usable.

The sidelining of the boat is big blow to the PA’s efforts to keep the waters off JFK safe.

The PAPD is already reeling because its high-tech $100 million airport security system, the Perimeter Intrusion Detection, has been a bust.

In August, a man whose Sea-Doo ran out of gas in Jamaica Bay climbed a JFK fence, crossed two active runways and flagged down a baggage handler for help — all without being detected.