You may have heard that there have been a lot of home runs in the MLB in 2019. I’d link some articles, but there are so many that I’ll just recommend Google. There is also evidence that the seams on the balls currently being used in MLB are smaller than in the past. Many are calling the ball “juiced,” which is a term I hate because it sounds like the elasticity of the ball has changed. It has not, but the seam height has. How would a lower seam affect drag and carry of the baseball?
It is intuitive that a lower seam would result in lower drag, and that is indeed the case over a range of seam heights. In their 2015 paper (Kensrud, J. R., Smith, L. V., Nathan, A., & Nevins, D. (2015). Relating baseball seam height to carry distance. Procedia Engineering, 112, 406-411) Kensrud et al. showed that carry distance increases with lower drag and that drag decreases with decreasing seam height (Fig 7). So lower seams mean more carry and more home runs. They reported a drag coefficient as low as 0.33 and as high as 0.45. Some of the baseballs they studied were far outside the range of seam height one would see in an MLB ball.
I would like to point out that there must be a limit to this behavior. A ball with no seam (in other words, a smooth ball) has a higher drag than a baseball. The plot below is repeated from my earlier post on Drag Crisis and is based mostly on data from Jeff Kensrud’s thesis. I’ve added the grey box that indicates the range of Reynolds numbers (i.e. velocities) that are typically encountered in baseball. Baseball drag depends on the orientation of the ball, and other ball types are also presented, but it is clear that over the range of velocities in baseball, smooth balls always have a larger drag than any kind of baseball.
Also, if the seams are sufficiently large their role will change. Seams on the front of the ball will cease causing transition to turbulence and delayed boundary layer separation. Instead, they will cause separation on the front of the ball. This will be accompanied by a very large increase in drag. I’ve sketched this hypothesis below.
Drag is the vertical axis and seam height is horizontal. The range studied in Kensrud’s 2015 paper are labeled and a linear increase of drag with seam height was reported in that range. We don’t know how far that range extends as yet, but I have indicated it with a red line. My point in this post is that the drag is higher for a smooth ball (seam height 0) and for sufficiently large seam height (to the right of “baseballs”) the drag will increase more rapidly. Currently, we do not know where the “corner” to the left of the “baseballs” region lies.
So, if some nefarious person wanted to increase home runs by making the seam smaller, they would risk exactly the opposite.