BATH — Ever since U.S. Coast Guard Lt. Jerry Smith Jr. became captain of the Thunder Bay, he’s been preparing to break ice on the Kennebec River.

He’s done it on the Great Lakes, on the Hudson River in New York and New Jersey and in Maine’s Penobscot River; but Smith said the often narrow, often shallow Kennebec is the most taxing mission.

“It’s the most dangerous river that we operate in,” he said.

On Thursday, Coast Guard ice-cutters set out from Bath Iron Works to blaze a 12-mile trail north to Richmond and back to clear ice and fend off flooding in an annual rite of spring in communities in Sagadahoc, Lincoln and Kennebec counties coordinated with other federal and state agencies.

The Thunder Bay, stationed in Rockland with a 17-person crew, is one of nine 140-foot Coast Guard cutters active in the nation and the only one based in New England. Three 65-foot cutters work with it, breaking up ice annually along the Penobscot and Kennebec rivers.

The biggest cutter can’t pass the Richmond-Dresden Bridge because of shallow water on the other side; but the three smaller cutters, Bridle, Tackle and Shackle, will make trips as far as Gardiner through Saturday. A bridge there prevents those boats from going any farther.

But as one would imagine, Thunder Bay is the most effective cutter of that bunch, both for its size and other capabilities.

It uses a “bubbling” system on the ship’s side, which pumps air between the hull and the water to cut down on resistance against the ship to cut through ice at good speeds — usually between 10 and 15 knots, or up to 17 mph.

The ship creates large wakes. When the hull itself fractures the ice, the waves ripple through ice off to the sides of the boat.

On the bridge, it sounds somewhat like a car going through slush. On the deck, it’s a crashing and crunching sound. Fractures create more fractures, often going most of the way to shore.

But cutting isn’t a brute-force operation. It’s more of a carefully crafted ballet, with navigating, turning and docking the ship taking the attention of many of the crew members.

They need to pay close attention to the tides, their speed and their course. Certain narrow spots are the most treacherous, including The Chops, where the steady ice began and converging water flows mark Merrymeeting Bay’s connection to the lower Kennebec.

In many spots, 10 yards can mean tens of feet in depth, and the Thunder Bay needs to be in 13 feet or more water to keep going.

“If we miss anything by 15 minutes even, it potentially increases the risk of us running aground,” said Lt. j.g. Dan Miller of the Thunder Bay crew.

This year, cutting may be more important than usual. Last week, Maine’s spring flooding risk was upgraded to above normal by the National Weather Service after unseasonably cold temperatures allowed ice to form and snow to stay into the spring.

Temperatures were around the freezing mark in Bath. On the water, it was cold and windy. But crews, expecting to find a foot or more of ice, found only 8 inches at most. Rain last week might have melted some of it.

Also, a long stretch of above-freezing weather was expected to come to the area starting Saturday. That could start a long melt of Maine’s deep snow and ice packs.

For that reason, “the timing did appear to work out well” on the cutter missions, said Lynette Miller, spokeswoman for the Maine Emergency Management Agency. The mission went well, too.

On the way back from Richmond, after the cutter headed back through The Chops, free-flowing chunks of ice had made their way well downriver, just an hour after the Thunder Bay had pushed through.

Those floes would be down by Bath Iron Works in four or five more hours, crewmen said. Then they would pass out to sea — the point of the mission, freeing up the waterway.

“We made this,” Miller said, looking at the water. “We’re kind of proud of it.”

Michael Shepherd — 370-7652 [email protected] Twitter: @mikeshepherdme@mikeshepherdme