The first thing you notice about Fernando Tatis Jr. is the name. Yes, he's the son of 11-season Major Leaguer Fernando Tatis. Then, maybe his size. He stands 6-foot-3, weighs 185 pounds -- big for a shortstop, big for anyone. After that, probably the age. As an 18-year-old, he was young for the Midwest League, where he started 2017, and that was especially the case for the Texas League, where he ended the campaign.

Go all the way down the list, and eventually, you'll get to his speed. That should be an eye-opener too.

Just as with isolated slugging percentage last week, Toolshed again turns to the more advanced statistic of Speed Score to see which prospects outperformed what we believe their speed capabilities to be via MLB.com' speed-tool grades.

A couple quick notes on Speed Score or Spd. The metric was designed by legendary baseball statistician Bill James in the 1980s in order to boil down a player's speed into a single metric on a 0-10 scale. There are different varieties of Speed Score out there, but for the purposes of this story, we'll be using the four-component model laid out by FanGraphs. They basically measure:



Stolen-base percentage: How good is the player at stealing a base once he decides to go?

Frequency of attempts: It's one thing to be perfect in stolen bases, but if perfect means 1-for-1, that doesn't tell us much. Speedy players are more likely to attempt to steal more often than their slower counterparts.

Percentage of triples: Players with speed can turn singles into doubles, doubles into triples, triples into inside-the-park homers. Triples are most closely associated with speed and more frequent than inside-the-parkers, so this measures how often a player turns balls in play into triples. (This component typically results in a number between 0 and .03 but is put on a 0-10 scale

Runs-scored percentage: By the same token, speedy players are more likely to wheel around the bases and score than slower runners, who could be thrown out or stopped prematurely and stranded at third.

Dee Gordon

Byron Buxton

Spd leaders for top-100 Prospects

You take the average of those four components, and in theory, the fastest players in the game (or at least those who take the most advantage of their speed) should rise to the top. For example, it shouldn't be a surprise that the three Major League Spd leaders were Billy Hamilton (9.0),(8.5) and(8.2). But did MLB.com's Top-100 prospects fit this bill as well?

In short, yes. All five Spd leaders among the Top 100 are considered at least plus runners with three (Lewis, Robles, Trammell) earning a 70 grade on the 20-80 scale for speed and one (Mateo) earning a top-of-the-line 80.

Loyal Toolshed readers will recall that Mateo was featured as the biggest power overperformer last week, but perhaps only because his speed allows him to take extra bases, thus inflating his ISO. Spd shows his burner prowess off more purely. He led the Minors with 18 triples, attempted 67 stolen bases and was successful for 52 of them (tied for second in the Minors) and added 90 runs. He basically ticked off every Spd box.

Lewis showed why his speed earned him the top overall pick in June by going 18-for-21 on the basepaths in the Gulf Coast and Midwest Leagues. Kingery, for all that was written about his power jump, remains "Scotty Jetpax" for a reason as he continued to show off plus speed in the Phillies system, and Robles' ability to grab an extra base earned him a September callup and a Major League postseason roster spot.

So we know Spd can do a fairly good job of measuring just how effective players' speed can be over the course of a season. It's far from perfect, of course, but it paints a solid picture.

As we did with ISO and power tools, let's see which Top-100 prospects beat their Spd expectations, given their speed tool grades. Again, we use average and standard deviation to get both Spd and grades on the same planes. In this case, average Spd was 5.2 with a standard deviation of 1.66 while an average speed tool grade was 51.4 with a standard deviation of 11. Again, these are elite prospects; their averages will be better than those of other groups. Put all of that together and these are the biggest overperformers. (Note: Some numbers may appear off but are results of rounding up or down.)



Biggest spd overperformers among top-100 Prospects (min. 100 at-bats)

This brings us back to Tatis.

The shortstop was a breakout star in the Padres system during his first campaign with a full-season affiliate, and speed had a lot to do with it. Despite having only turned 18 on Jan. 2, Tatis was aggressively moved to Class A Fort Wayne to begin the season and responded in ways that make for an impressive Spd. Combined with his time at Double-A San Antonio, the teenager stole 32 bases in 47 attempts, raced for seven triples and scored 84 times in 131 games during the 2017 season.

Fort Wayne manager Anthony Contreras was impressed by the speed he saw early on, and that sense of wonder only grew over time.

"I think naturally he has the height at 6-foot-3, 6-foot-4, and that leads to some really long strides," Contreras said. "I don't think he'll ever be a little burner type, like Brett Gardner, who can shoot out of the gate really quickly, but those strides really make up for it, once he gets going. Plus, his baseball instincts are the best I've ever seen for someone his age. When it came to stealing bases or really trying to steal third, you could see he would get smarter on the bases the more he was out there and learning what to look for."

Perhaps in that way, Tatis had a bit of an advantage when it comes to Spd. The Class A level is meant to be educational, meaning oftentimes coaches and organizations are willing to put up with some caught-stealings if a player learns something along the way about the art of the stolen base. For a player like Tatis, that meant he had somewhat free reign to swipe the extra bag when he had the chance. When it comes to Spd, that raised his Frequency of Attempts component of the stat, helping his case here.

"At this level, we want to go out and see what they can do," Contreras said. "We want them to make mistakes and learn from them. So for me, he always had a green light. Early on, he'd make some of those mistakes, but because of his baseball knowledge, it didn't take much to see he really used it as a growing moment."

Indeed, Tatis went 12-for-20 in stolen-base attempts over 66 games in the first half with Fort Wayne. Between the TinCaps and the Missions, he was a much-improved 20-for-27 over 65 games in the second half. His triple output also jumped from two to five from one stanza to the other. As Tatis' skills became more of a weapon, his skipper could sense those in opposing dugouts taking note.

"Maybe a little bit," Contreras said. "But he's obviously a prospect, so you're going to keep an eye on him regardless. Once he started to do things as well as he did, teams had to start thinking, 'Oh wow, he's more than just a 6-foot-4 athlete.' They had to note how he moves, how he accelerates. Once they'd adapt to him, he'd adapt right back to them because that's how advanced his instincts were."

But for all this about Tatis' ability and growth when it comes to speed, why would he be given a slightly below-average 45 on the 20-80 scale for the tool? After all, Tatis' 6.9 Spd was what should expect from a 60- or 65-grade speedster, according to the Toolshed method above.

The details can be found in the fine print. MLB.com hands out grades for "future tools," meaning it's not measuring how good a player is right now, but how good they could be if they fulfill their potential in the Majors. When projecting out that way, Tatis' size becomes a bit of an issue.

The Dominican Republic native is listed at 6-foot-3, 185 pounds but as MLB.com notes "has become more physical since signing and still has room to grow." That's promising news for a player who's coming off a 22-homer season as a teenager, but the increase in strength and power could come at the cost of eventually losing a tick or two of speed as he enters his 20's. The Padres seem hopeful Tatis can maintain all facets of his exciting tools package.

"It depends on what the body does," Contreras said. "It's going to mature. Even now, it's still a little gangly, but once he gets a little more coordinated, who knows all the possibilities that could come. He'll at least always be a lengthy guy with some strength. ... A lot of it will be up to him and what he can do with the Padres strength and conditioning guys, but if he can keep his flexibility and strength, I think he can still be a good runner."

For now, the Padres have seen enough to keep pushing their No. 4 prospect because they believe he needs it. Tatis was moved up to San Antonio on Aug. 21, becoming the Texas League's youngest position player this season, and he's currently plying his trade with Estrellas Orientales back home in the Dominican Winter League. (He's yet to attempt a stolen base through five games, but he did enter the week 6-for-15 with a homer and two doubles.)

The day still might come when Tatis loses a step, but as he continues to pass test after test, the Padres seem more than willing to be as aggressive with him as he was on the basepaths in 2017.

"He proved over and over that he was a step ahead of everyone, and he needed a chance to keep evolving," Contreras said. "The better the competition, the better he plays as he tries to stay ahead. I really think that's true of Double-A, and it'll be true of Triple-A and the big leagues when he gets there. He rises up to the competition of wherever he's at. It forces him to keep up, and he's proven that he can."