“A good hitting coach,” the late baseball writer Leonard Koppett once told me, “can add five or six points to a team’s batting average.” But a good pitching coach, he thought, “had about the same effect on pitching staffs as weathermen do on the weather.”

There are a few pitching coaches whose staffs are consistently under the weather, and, considering his team’s natural advantages in wealth and resources, many would argue that Mel Stottlemyre is one of them.

From his 1984 season with the Mets, through April of this year, any positive effect that Stottlemyre has had on his pitchers is difficult to trace. He’s hindered by a penchant for forcing pitchers away from their best power pitch and toward a second pitch that puts more strain on their arms, as well as an inability to correct a troubled pitcher’s mechanics.

That’s the nice way to say it. A not so nice way would be to say that giving your pitching staff to Mel Stottlemyre is like making Dr. Kevorkian your team physician.

Stottlemyre is one of the best-liked men in the Yankees organization, a respected former pitcher with a link to old Yankee royalty and a man who has shown courage in the face of adversity and personal illness. As a pitching coach, though, he is the sacred cow of the Yankees organization, the Teflon man to whom no failure sticks.

Going into last night’s game against the Angels, the Yankees’ staff ranked 11th in the American League with a 4.89 ERA. Much of the damage had been caused by the horrendous performances of Jaret Wright (ERA 9.15), Kevin “Breakdown” Brown (8.25 ERA before last night’s relatively innocuous loss), and reliever Tom Gordon (whose current ERA of 5.00 is more than double that of last year).

Gordon’s recent ineffectiveness could be just an aberration, and it may be fairly asked what Stottlemyre can do with a pitcher with a bad shoulder like Wright’s and a fragile 40-year-old such as Brown. But then, the prevention and repair of breakdowns is an important part of a pitching coach’s job, and Stottlemyre has never been particularly good at it.

The Javier Vazquez horror is still fresh in the minds of Yankee fans. A durable young pitcher with an ERA of 3.52 in his three previous National League seasons, Vazquez came to the Bronx, lost his mechanics, and saw his confidence plummet while his ERA soared to 4.91.

The excuse offered by many for Stottlemyre’s failure to straighten out the 28-year old right hander was that Vazquez was a “head case,” but if so, how did he become one in such a short time? And if Vazquez was a head case, what of Jeff Weaver? Weaver was 26 when he came to the Yankees during the 2002 season. In his previous 351 innings with Detroit, his ERA was 3.77; after pitching 237 innings with the Yankees, his ERA rocketed to 5.36.

Whatever bug Weaver caught was mild compared to what ailed Jose Contreras, who was 7-2 with a 3.30 ERA his first year with the Yankees, 2003. The next year, he quickly fell apart; his ERA went to 5.65 for 18 starts, after which he was shuttled to the White Sox, where his ERA dropped slightly to 5.30. So far this season, he is 3.48 with Chicago. Sterling Hitchcock has never been a very good pitcher, but he’s always been better when not wearing pinstripes. His ERA with Stottlemyre was 5.84; his career ERA without Stottlemyre is 4.68.

A question as intriguing as why Stottlemyre has been with the Yankees so long – his first year was 1996 – is why he was ever given the Yankees job in the first place after his 10 seasons with the Mets. The question of what turned Dwight Gooden from the best 19-and 20-year-old ever to a 30-year-old burnout will never be resolved – was it drugs or Stottlemyre’s pressure to move him away from being a power pitcher?

Jeff Pearlman, author of last year’s exhilarating book about the ’86 Mets, “The Bad Guys Won,” feels it was a little, or possibly a lot, of both.

“Mel had this stubborn insistence that Doc had to develop a third pitch, a breaking ball, to make him more effective,” Pearlman told me.” ‘He’s striking out too many batters’ was his attitude. He didn’t seem to understand that the breaking pitches put a lot of strain on a very young arm. You could see the difference right after the ’85 season. He was a great pitcher in ’86, but he struck out fewer hitters, gave up more hits and more walks, and his ERA climbed sharply.”

In truth, the Mets of the mid-to-late ’80s might have had the best pitching staff in the league without Gooden, and almost none of them lived up to their early promise. Ron Darling, for instance.

“Mel had this thing about strikeouts,” said Ed Hearn, the Mets’ backup catcher in 1986. “He wanted Ron to throw more breaking stuff. He did, and he was never quite as good afterward as he was in ’86.”

That year, Darling’s ERA was 2.81, and he struck out a career-high 184. He never again came close to matching either figure, and in his nine remaining big league seasons, he won just 14 or more twice.

Sid Fernandez wasn’t so much a head case as a stomach case, a poorly conditioned pitcher with spectacular stuff – three times he led the league in lowest opponents’ batting average. But as every Mets fan could see, Fernandez’s awkward motion put too much stress on his arm and shoulder, and he quickly became recognized by opposing teams as a five- or six-inning pitcher.

“Mel just wasn’t very good with mechanics,” said a former Mets reliever who asked not be named. “If you had a problem with your delivery or if you were trying to work things out after being hurt, you were pretty much on your own.”

Many still feel that way. So far, Mariano Rivera has resisted Stottlemyre’s suggestions that he learn the splitter. And in a much-publicized blowout down the stretch in the 2003 season, David Wells rebelled at Stottlemyre’s and Joe Torre’s insistence that he fight a slump by throwing more between starts.

“My problem was a sore back and shoulder,” said Wells. “I needed some rest, not more work.”

Wells, of course, was a whiner, but one you would have thought Stottlemyre could ably handle after two stints with the team in the previous five seasons.

To be fair, there are pitchers who might be chalked up as successes for Stottlemyre, most notably Andy Pettitte and Rivera, though his critics would point out that both had a season to mature before working with Mel. As Baseball Prospectus writer Jay Jaffe noted on his Web site, FutilityInfielder.com, “The only other Yankee product to flourish under Stottlemyre was Ramiro Mendoza. Every other pitcher of lasting note during Stottlemyre’s tenure arrived in the Bronx a finished product.”

Meanwhile, as the Yankees face the prospect of a season with two black holes in the rotation, a bullpen looking at the same kind of overwork that brought it to collapse in 2004, and Mel Stottlemyre as their pitching coach, the forecast seems to be stormy weather.