On the Mariners drop in bat speed

ARLINGTON, TX - APRIL 07: Randy Arozarena #56 of the Seattle Mariners takes batting practice prior to the game between the Seattle Mariners and the Texas Rangers at Globe Life Field on Tuesday, April 7, 2026 in Arlington, Texas. (Photo by Courtney Kramer/MLB Photos via Getty Images) | MLB Photos via Getty Images

The Mariners are swinging slower and hitting worse, but connecting these dots isn’t as simple as it seems.

Bat speed is down for half the Mariners’ lineup over the first 2 1/2 weeks of the season. These batters are swinging 0.7 mph slower on average than they were last year. It’s become a fixation in the early going, especially as the team entered the weekend with a 79 wRC+ — bottom five in the majors. Some blame the slower swings for the disappointing start at the plate, and others point to Seattle’s frigid temperatures as the culprit for the decline.

These points were brought to Dan Wilson before Saturday’s game. He shot down either premise.

“That’s pretty negligible from what we’ve seen before,” Wilson said of the decline in bat speed. “It’s colder weather- there’s just a lot of different factors that could be involved there. That’s not something that’s concerning us. Obviously, we track, we monitor, but there’s nothing that’s discernible. We’re fourteen games in, there’s so much more season to go. We want to get off to a good start, obviously as a player you want to get off to a good start, but you know, sometimes those things take a little bit of time to get going. It’s not anything different.”

I pretty much agree.

The first thing to point out is only three Mariners have bat speed drops greater than 1 mph. Randy Arozarena has lost 3.5 mph from last year (the largest change in the majors), Leo Rivas has lost 1.6 mph, and Brendan Donovan has lost 1.3 mph. The next thing to point out is Arozarena and Donovan are the top two hitters on the team so far by wRC+. Bat speed is important, but it isn’t everything.

Now, there are several players with <1 mph bat speed drops who have indeed struggled. Josh Naylor, for instance, has lost 0.9 mph, and Cal Raleigh and Julio Rodríguez have each lost about 0.5 mph.

But the further you dig into the decimals of bat speed, the more difficult it becomes to separate noise from signal, especially in just 16 games. Let’s look at Julio’s rolling bat speed, for example:

Bat speed goes up and down naturally over the course of a season. That doesn’t always reflect a change in ability. These ebbs and flows can partially be attributed to the measurement itself. Bat speed is captured at the moment when the sweet spot of the bat crosses with the path of the ball. In other words, it tells us how fast the barrel is traveling at point of contact (or missed contact, in the case of whiffs).

This “point of contact” piece is crucial. A swing isn’t one, constant speed. It starts slow as the batter fires, and gains speed as the batter follows through. The “bat speed” for any given swing not only depends on how fast the batter swings, but how far they progress in their swing by the time the ball reaches the plate.

What does that mean for interpreting bat speed data? Well, that means observed bat speed can change with the pitch type, velocity, location, and the batter’s ability to identify such variables out of the pitcher’s hand. So if Julio took the exact same swing at a 99-mph fastball on the inner-third as he did on an 84-mph sweeper out of the zone away, he’s not likely to record the same “bat speed” on both swings. And that’s before considering how Julio might change his swing to match each pitch. From that perspective, bat speed is not only a matter of physical strength and ability, but a function of timing and circumstance.

For batters like Arozarena, who display very large changes in bat speed, it’s worth considering the underlying changes pushing their swing to new… slows:

But for pretty much every other Mariners’ batter exhibiting a decline (or gain) in bat speed, there simply hasn’t been enough time to say what, if anything, is different. I’m more likely to look at slower swings as a symptom of early struggles, rather than a cause. I kind of think the Mariners’ timing is just off.

Now, one theory that’s cropped up to explain the drop in bat speed is Seattle’s cold weather. Several people have pointed out the relationship between bat speed and temperature, noting swings are slower when it’s cold and faster when it’s hot.

I did some math on this last week for FanGraphs. My conclusion was, yes, bat speed likely depends on the temperature, and colder means slower. As Patrick Dufor points out in an excellent follow-up analysis, some of that could be the drag created by denser air at lower temperatures. But Dufor also notes drag doesn’t quite explain the full change. It’s possible batters might just be less comfortable in the cold. Julio agreed in an interview with the Seattle Times. “We’re not trying to swing slower; it’s just cold as (expletive) in Seattle,” he said, responding to concerns about the team’s bat speed decline.

Regardless, I estimated the change in bat speed at about 0.2 mph per 10 degrees Fahrenheit. That’s not a lot. While Seattle is the coldest city in the majors over the course of the season, it’s typically not the coldest city in the early part of the season. Lots of places are cold right now. In fact, T-Mobile Park is more neutral than you’d expect in April. It’s the late summer, when the rest of the country warms up and Seattle still has some bite after sunset, that we see the park flex its muscles. That’s to say, yes, it’s possible the Mariners are swinging slower because of the cold. But it’s probably not a big issue, and certainly not one unique to them. And it’s not the only thing that could be pushing bat speed down.

Again, my sense is the Mariners were kind of just bad the first two weeks of the season. I think their timing was off, I think they were swinging at bad pitches, and I think they were falling behind in counts. Each of those things could explain their bat speed slump, and I doubt it’s the other way around.

The Mariners entered the weekend with a 79 wRC+. After walking all over Astros’ pitching the last two nights, they enter Sunday with a 93 wRC+. We are not even 10% of the way through the season. It can swing in a jiff.