PROVIDENCE — When a submarine goes under water, it’s not normally cause for alarm. But when the Juliett 484 started sinking Monday afternoon, something was clearly wrong.

The Juliett 484, a Russian ballistic missile submarine and museum exhibit moored in shallow water in Providence Harbor, began to take on water during the storm that drenched the Northeast.

The stern of the boat is now under water, and the bow is arching out of the water at a 30-degree angle. The grounded submarine is listing to its port side in its berth at Collier Point Park.

Yesterday, a salvage expert from New Jersey arrived from the company that helped to refloat the retired American sub Growler, berthed next to the Intrepid in New York Harbor.

Stephen Lappies of Donjon Marine went inside the Juliett and found that the back quarter of the sub, including the aft torpedo room and the aft control room, were largely filled with water.

There also was several feet of water in compartments closer to the center of the submarine.

It’ll be some combination of “pumping and patching” that gets the boat afloat, Lappies said.

It could be days before that begins, and could take some time to accomplish.

Frank Lennon, president of the Russian Sub Museum, said the organization would take its time in refloating Juliett 484, because the boat is secured and already resting on the bottom — there’s no danger of the situation worsening.

“It’s not like the sub is going to float out into the channel. It’s not going anywhere,” he said.

The 300-foot-long submarine is secured by two chains. Additional mooring lines were attached yesterday afternoon, Lennon said.

There was also a small amount of oil — less than 50 gallons — released when the submarine partially sank. The pollution boom surrounding the ship broke during the storm, but the city’s police and fire departments have repaired the boom, and that oil is contained in the area immediately around the submarine.

Museum officials are not clear yet on the details of what happened. A submarine, of course, is meant to take on water. The boat has a pressurized inner hull, containing operations and the crew compartments, and an outer hull, which is exposed to the water. In between are empty areas called ballast tanks. The submarine submerges by pulling water into the ballast tanks. It surfaces by blowing compressed air into the tanks to push the water out.

During Monday’s storm there were abnormally high tides and strong winds coming from the east, an unusual occurrence.

The submarine’s engineering staff believes that the storm pushed the bow up onto a shoal, and the stern came to rest in a small gully carved out by the submarine over the years it has sat in port. Once the stern was under water, water probably began to leak in through the ship’s hatches, which were closed, but not sealed tightly enough to prevent water entry. The water-tight doors were all secured yesterday afternoon, and no additional water is entering the submarine, Lennon said.

At some point, the submarine’s ballast tanks flooded.

To right the submarine, engineers will essentially go through the same process as when a submarine is surfaced from under water. Compressors will be used to blow air into the ballast tanks, shoving the water out.

Bilge pumps will be used to clear the water out of the inner hull. There are no plans yet for exactly how and when the pumping will occur.

Aside from water damage to the inside, the boat is not believed to have suffered any structural damage. Lennon said that the museum has insurance to pay for repairs.

The submarine was one of 16 Juliett-class vessels built to carry nuclear-armed cruise missiles. The diesel-powered sub could carry four nuclear-armed missiles that had a range of about 300 miles at an altitude of 650 to 1,300 feet. It could carry up to 22 torpedoes.

This sub, commissioned in 1965, served in the Soviet Baltic and Northern fleets until its decommissioning in 1994.

The submarine was sold and moved to Helsinki, Finland, to become a restaurant. That venture failed, and the sub was again sold and moved to St. Petersburg, Fla., where it became a museum.

In 2001, the submarine was used in the filming of the Cold War thriller K-19: The Widowmaker, starring Harrison Ford. It opened as a museum in Providence in August 2002.

dbarbari@projo.com