BATH — U.S. Coast Guard Lt. Jerry Smith Jr. has had plenty of preparation for the annual spring ritual of breaking up the ice on the Kennebec River.

The captain of the cutter Thunder Bay has conducted ice-breaking missions on the Great Lakes, on the Hudson River in New York and New Jersey and on the Penobscot River in Maine, but Smith said the often narrow and shallow Kennebec is the most challenging.

“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 the built-up ice that can cause flooding in communities in Sagadahoc, Lincoln and Kennebec counties.

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 the Thunder Bay each year to break up ice along the Penobscot and Kennebec rivers.

The Thunder Bay is too large to pass the Richmond-Dresden Bridge because of shallow water on the other side. The three smaller cutters, Bridle, Tackle and Shackle, will make trips as far upstream 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 the bunch, both for its size and capabilities.

It uses a “bubbling” system that pumps air between the ship’s hull and the water to cut down on resistance so the ship can cut through ice at relatively high speeds – usually 10 to 15 knots, or up to 17 mph.

The ship creates a large wake that ripples beyond the ice being broken up by its hull. On the Thunder Bay’s bridge, it sounds like a car driving through slush. On the deck, it’s a crashing and crunching sound. Fractures create more fractures, often extending almost 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 occupying the attention of many crew members.

They need to pay close attention to tides, and their speed and course. Certain narrow spots are treacherous, including The Chops, where the solid ice began Thursday and converging currents mark where Merrymeeting Bay connects to the lower Kennebec. In many spots, 10 yards can mean a difference of tens of feet in water depth, and the Thunder Bay requires at least 13 feet.

“If we miss anything by 15 minutes even, it potentially increases the risk of us running aground,” said Lt. 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 continue building and the snowpack to remain intact later in the season than usual.

Thursday’s 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.

A long stretch of above-freezing weather is expected to begin Saturday. For that reason, “the timing did appear to work out well” for the cutter mission, said Lynette Miller, spokeswoman for the Maine Emergency Management Agency.

On the way back from Richmond, after the cutter had passed 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.

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

Michael Shepherd can be contacted at 370-7652 or at:

mshepherd@centralmaine.com

Twitter: @mikeshepherdme