Back in October, the Seattle Mariners announced that they would be moving the fences in at Safeco Field—a notorious (justified or not) pitcher’s park. As a result, fantasy analysts and players are sure to be bumping Mariner hitters up their draft boards and Mariner pitchers down. And, in all likelihood, they’ll be overestimating how much to move those players.

The day after the change was announced, Colin Wyers ran a study to estimate how many additional home runs the change might create. He found that the Mariners and their opponents would likely combine for an additional 22 home runs. Seattle’s internal studies reportedly found that number to be somewhere between 30 and 40. That leaves us with a range of an additional 11 to 20 home runs that the Mariners, as a team, will hit in 2013. Over at the The Book Blog, MGL translated Colin’s numbers to estimate that Safeco’s home run park factor would change from 0.90 to 0.93 from the fence shift.

If you haven’t surmised as much already, these are very small changes. As MGL goes on to say, Safeco, despite its reputation as a deadly destination for power, still only cost “the average full time player for the M’s… less than one home run per year” even prior to the fences coming in. For a Mariner hitter that averages 20 home runs per 650 plate appearances, a ballpark factor change like this would add just 0.3 home runs to his total. Even if you bump it up the high range of Seattle’s cited internal study, we’re still only look at 0.6 round-trippers—maybe a jump from 20 home runs to 21.

The effects, for the average hitter, are negligible. If a free-agent hitter moved from Angel Stadium to Tropicana Field, would you bump up his value, or would you say, “This guy is moving from one moderate pitcher’s park to another, let’s keep him the same”? That’s essentially what’s happening here, except many are going to treat this as if it will add a few extra dollars of value to Mariners sluggers.

Of course, this is a bit simplistic, since we know that parks affect different hitters differently. The Mariners aren’t moving the fences in symmetrically; the right field fences aren’t being changed at all, while the left-center fences are being moved in the most. At U.S.S. Mariner, Dave Cameron notes that “it seems likely that the most significant changes are going to come in the form of helping right-handed pull power hitters and hurting left-handed fly ball contact pitchers.” The only Seattle pitcher that sort of fits this mold is Charlie Furbush, who is a mere AL-only afterthought and may be traded before the season anyway. On the hitting side, the only hitter who really fits this profile well is Casper Wells, who isn’t even in line for full-time at-bats (but could be a sleeper if he does wind up starting at some point, even without this park shift). It could also help guys like Jesus Montero, Justin Smoak, and Franklin Gutierrez a little since they hit homers to left and left-center, but they’re not strict pull-power hitters.

All in all, you may find yourself having to a pay a premium for Seattle hitters on draft day this year, but don’t be tempted. Stick to your price point and don’t be swayed by the possibility of a huge power boost; it’s not happening. On the flip side, Seattle pitchers may come a little cheaper than they otherwise would; take advantage of this, especially for righty ground-ball pitchers like Felix Hernandez, Hisashi Iwakuma, and Tom Wilhelmsen, for which the park-related damage will be as minimal as possible. This fence shift may well have just built an extra couple dollars of value into the draft day prices of these guys.

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Good article--puts proper perspective on Safeco fence change!
I've long believed the marine air and cool temperatures are more significant factors at Safeco than the fences.
"For a Mariner hitter that averages 20 home runs per 650 plate appearances..."

What kind of impact will the change have, though, on Sasquatch, the Tooth Fairy and Batman?
Do you plan to take a similar look at SD? I'm especially interested in Venable.
Yup, I will. Result will probably wind up about the same.