BATH – The launching of a Navy destroyer into the Kennebec River looks simple and takes no more than 90 seconds.

But as Saturday’s launch of the Mason at Bath Iron Works illustrated, it takes months of preparation and a combination of calculus, grease and tugboat brawn to make sure the hulk doesn’t fall over or crash into a nearby bridge.

The seemingly straightforward event is actually fraught with peril. A single miscalculation or mechanical problem could spell doom.

“Launching it this way is dangerous and technically complex. Everything has got to go right. Short of combat, it’s the next most dangerous circumstance these ships will find themselves in,” said Tom Wilcox of the Maine Maritime Museum.

Saturday’s launch was the last of its type at Bath Iron Works.

For more than a century, the shipyard has launched ships down an “inclined way” to get ships from soil to sea. In the future the shipyard plans to use a dry dock to lift its ships into the river when they are “launched.”

The work to ensure a smooth launch starts when Bath Iron Works gets a contract to build a ship, said Erik Hansen, manager of hull engineering.

“We have to assess how to get it into the water,” said Hansen, who has witnessed more than 75 launchings in his 33 years at the shipyard. “We look that far ahead, even before we sign on the dotted line.”

The preparation begins in earnest about three months before the launch, when melted wax is poured about a half-inch thick on the concrete incline where the ship slides down into the water.

The hardened wax is topped with a specially formulated grease, then workers build a wooden cradle for the vessel to rest in during the launch.

Behind the scenes, engineers have to calculate how much water to put in the ship as ballast, the relationship between ship’s speed and weight, and the resistance the water in the river will offer.

The process, Hansen says, involves calculus, geometry and physics.

The shipyard even has to calculate how long politicians’ launch-day speeches will last to make sure they don’t miss “slack tide,” the time when the waters are between low and high tides and the river currents are at their stillest. (Hint: politicians’ speeches are usually longer during election years).

On Saturday, teams of workers drove oak wedges – 325 of them on each side of the ship – between the cradle and the ship in a time-honored tradition known as the “driving of the wedges.”

At the sound of a horn at 7:30 a.m., 90 teams of “drivers” took the 80- to 100-pound steel rams and pounded the wedges inch by inch to tighten the cradle to the hull.

Men in hardhats, jeans and work boots drove the wedges over and over in a rhythmic primitive pounding until the horn sounded again. They repeated the process eight times.

Like many other shipyard workers, Jeff Kuchinski wanted to be part of the final driving of the wedges.

Kuchinski had participated twice before. “But we won’t be doing this again, I suppose,” he said.

The actual ship launching occurs hours after the driving of the wedges, when three levers are pulled to release the ship and let it begin its descent down 460 feet of grease-covered concrete into the river.

Engineers estimated that the Mason would reach a top speed of about 18 feet per second, or 12.5 mph, before it splashed into the water, Hansen said.

If the ship’s speed, weight and ballast have not been calculated correctly, the ship’s stern could lift too soon – or not soon enough – as it enters the water. If that happens, the ship could slam hard onto the concrete and cause untold damage.

As the ship enters the water, Earl Walker, the shipyard’s port captain, directs six tugboats from his perch in the ship’s bridge 58 feet above. For Saturday’s launch, the tugs had a combined 10,000 horsepower.

Walker’s biggest concerns are wind, speed and the bridge that connects Bath to Woolwich on the other side of the river.

“Fenway park has its ‘Green Monster,”‘ Walker said, referring to fabled left field wall at the home of the Boston Red Sox. “And Bath has its own Green Monster with the bridge.”

Ship launchings typically go off without a problem. But not always.

Once, on a cold March day, the cradle froze to the hull of a ship when it entered the water, Walker said. It took the better part of a day to separate them.

In the 1930s, a nut fell onto the grease on the concrete and stopped a trawler as it slid down the incline. On another occasion, a cradle holding a ship partially collapsed as the vessel was being launched.

High winds have blown ships dangerously close to the former Carlton Bridge and the shore on the other side of the river.

Future launchings off a dry dock could be more difficult than down the inclines because of strong river currents, Walker said.

“I hope I haven’t jinxed myself,” he said, “but I’m thinking we’re doing the last of the easy launches.”