WEST PALM BEACH, FLA. — Besides Manager Dave Martinez, the Washington Nationals’ biggest offseason addition may prove to be touted hitting coach Kevin Long, the guru of launch angle who helped Daniel Murphy transform himself from a good hitter into a star in 2015.

When the New York Mets fired Terry Collins as manager after last season, the 51-year-old Long was on the open market — for an instant. Nationals General Manager Mike Rizzo pounced, offering him a three-year contract. Months of grinning have followed.

“I’m super excited,” Nationals outfielder Bryce Harper said. “When I heard, I was like, ‘Wow, that’s a great asset for us.’ I didn’t even think his name would come up at all.”

Under Long, hitters study angles and planes, spin rates and heat charts, cognitive training and chase rates, and they develop swing mechanics that will improve launch angle. In the end, Murphy claims, “Kevin will make you feel like a killer.”

When managers get fired, the cream of their coaching staff is often poached during the limbo days of a managerial search. The Nats didn’t just subtract manager Dusty Baker; they also lost pitching coach Mike Maddux, one of the best, to the St. Louis Cardinals. But when the Mets fired Collins, the Nats grabbed Long.

In the past 11 years, Long became known as a cutting-edge coach who long ago taught theories that modern analytics now approve. In his first year with the Yankees, New York scored the most runs by a Bronx team since 1937. Two years later, they won the 2009 World Series. In his first year with the Mets, they reached the World Series.

Now Long inherits an offense that already was excellent under former hitting coach Rick Schu, leading the National League in slugging in 2017 while scoring the third-most runs in the league. The Nats hope Long’s insights complement the work Schu already did.

In the past two years, Murphy, the hitting evangelical, has praised Long to his teammates a thousand times. Murphy even lobbied ownership to heist Long from the Mets. At the first hitters’ meeting last week, Murphy said to his mates, “Now you’re in trouble. Now you got two of us.” Then the second baseman asked Long, “Can I talk?”

“No, not today,” answered Long, who chuckled recalling the moment.

“Murph doesn’t want to hit one groundball [ever]. He’ll go crazy,” Long said of his friend and disciple. Murphy will remind lamp posts that grounders produce a .220 batting average and only 7 percent of them go for extra-base hits. For a hitter, salvation (and cash) are found in the sky.

“When Murph’s thinking ‘air,’ he’s thinking ‘to the moon!’ ” Long said. “He’s got to understand that every individual doesn’t think like he thinks.”

After spending the offseason watching “thousands of swings” by every Nationals player, Long hit the ground preaching — but different ideas to different hitters. His first project may be catcher Matt Wieters, who hit .225 last year and .143 in the postseason.

“We are well on our way to get him where he needs to be. Made a lot of improvement . . . some adjustments. He feels pretty sexy about what he’s doing,” said Long, whose idea of joy is a 4½ -hour meeting — all on hitting.

Wieters, who went 2 for 2 with a two-run homer in a 9-3 Grapefruit League win over the Atlanta Braves on Sunday, hit .190 after June 16 last year with bat speed so slow he looked as if he were swinging underwater. Washed up or just bad mechanics? Long, who might justify his entire contract if he got Wieters back to his decent career on-base-plus-slugging percentage of .726, thinks the latter.

“Matt wasn’t using his lower half,” Long said, “and he’s starting to do that. He’s excited.”

Is shortstop Trea Turner the terror of 2016 or the merely pretty good hitter of 2017? Long has some ideas. Turner — and coaches — always have debated whether he should use his speed to beat out groundball hits then steal bases or utilize his power to aim for 60 extra-base hits. Why not do both — but in opposite directions?

In the 2016 playoffs, the Los Angeles Dodgers got a book on Turner that others have copied, including the Chicago Cubs in the 2017 playoffs. Turner tends to hit flyouts to right field when he is pitched hard-and-away. Long hopes Turner will use his natural line drive approach for power to left field but use the ground more to the opposite field. If Adam Eaton bats leadoff with Turner behind him, Turner will get more at-bats with “the hole open” on the right side for groundball hits. “That’s sometimes a pretty easy way to get a hit,” Turner conceded last week.

What about Harper, whose best coach always has been his father? Will Long get in the middle and mess him up after a 1.008-OPS year?

No way, vows Long, who spotted subtle changes the Harpers had made to Bryce’s swing in the offseason. “I told Bryce, ‘Call your dad and tell him how much you love him.’ Because he did a tremendous job,” Long said. The coach would appreciate being in the Harper-Harper loop but knows he will never do more than jot notes in the margins of the Harper thesis.

Even Murphy could improve, in Long’s view, if he reduced his “chase rate” at pitches out of the strike zone and drew more walks, like Joey Votto. Then Murphy also might rise toward the very top of the hallowed OPS chain.

Only one Nationals hitter probably will remain under a bell jar: Anthony Rendon. “He’s got it. He’s a magician. We sat there and marveled at him,” Long said of a Mets dugout that watched Rendon’s 17 RBI in 17 games against the New York in 2017.

“He’s quiet, always on time, looks effortless. His mechanics are flawless, in line, balanced,” Long said. Last year, Rendon even moved closer to the plate. Using his quick hands to smash inside pitches, he now commands the entire zone. “I’m leaving him alone,” Long said. “He’s one guy I’m not going to be able to help too much.”

Spring training is for optimism bordering on delusion. The Nats get that feeling talking hitting with Long. But the sentiment is mutual. “To say I’m excited,” he said, “is an understatement.” So let’s have a 4½ -hour meeting to discuss it.

Send Murphy for the black coffee.

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