YOKOSUKA NAVAL BASE, Japan — Navy gunmen aboard the USS Blue Ridge shot flares, grenades and other explosives Thursday at an approaching boat to test new unambiguous-warning devices that could keep suspicious boats away.

The shooters fired traditional deterrents such as flares and parachuted explosives, and tested a new “flash bang” device the Navy is considering for development.

The drill was designed to measure the success of the new devices in the real world. It also allowed the Navy to see the explosives’ effect on a community like Yokosuka.

The shots are nonlethal explosives that send an unequivocal message to boats that they’re getting too close to Navy ships.

“To deter any type of small-boat attacks,” said Chief Petty Officer Alonzo Tate, a gunner’s mate on the Blue Ridge.

The deterrence might have helped prevent the attack on the USS Cole in Yemen three years ago, Tate said, when a small boat drifted up and exploded, killing 17 sailors.

After the Cole attack, the Navy began exploring ways to improve security for U.S. ships in foreign ports.

The flash-bang devices are designed to be a clear warning — unlike flares that can be mistaken for a distress call, or celebratory fireworks.

The devices emit 175 decibels of sound and a flash equivalent to 6 million candles, said Carl Jarvis, with NAVSEA/ Crane, the Naval Surface Warfare Center in Crane, Ind., where prototypes were created.

Yokosuka is the first real-world location where the device was tested.

“There’s a big difference between shooting it out over a cow pasture [in Indiana] and shooting it out here,” Jarvis said.

If the explosives work well, Jarvis and NAVSEA will put them through additional tests — by shooting, crushing and otherwise disturbing them to see how they react. If the explosives pass, they might be a standard feature aboard Navy ships.

Master-at-arms and gunner’s mates tested the explosives in daylight and at night, using 12-gauge shotguns and M-79 grenade launchers.

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