This is follow-up to a recent Sound Of Hockey article where we sat down with the Seattle Kraken Ice Breakers to better understand the role they play at Kraken games. There was one question around the science behind scraping the ice that got me wondering how much of an impact ice scrapes have on NHL games, so we are going to dig into the statistics of goals scored following ice scrapes to see if we can uncover anything interesting.
Here was the question:
Blaiz Grubic: Is there any science behind scraping the ice or a benefit to gameplay?
Kevin Ruuhela: If there is a science to it, I’m not aware of it, but from personal experience playing hockey since about the age of six, I know that the puck moves much more smoothly and consistently on ice without snow. When there are patches of snow, the puck can sometimes abruptly change speed on you and throw you off. I also know that goalies typically prefer less snow so there’s less material to get in their eyes from sticks and skates as players move around the crease.
To summarize Kevin’s answer and take it a bit farther, I came up with the following very simple theory:
Playing on smoother ice should increase scoring.
Let’s see how the statistics align with this theory.
Approach
For this data analysis, only even-strength goals (five-on-five) were reviewed. This removes any advantage a team might have and focuses on an even playing surface. Ice time was separated into two buckets:
- Fresh Ice – I’m defining this as the 150 seconds following an ice scrape or period start. The period start is included, as the ice has been freshly resurfaced.
- Rough Ice – Time accrued after the initial 150 seconds until the next fresh ice event.
150 seconds was chosen because, per Ruuhela, there are three planned ice scrapes during each period. Since each period starts with freshly Zambonied ice, there are a total of four fresh ice events. Two minutes and 30 seconds (or 150 seconds) multiplied by the four fresh ice events equals 10 minutes of fresh ice and 10 minutes of rough ice per period. Assuming there are no advantages given to teams during even-strength play, goals should be scored at a linear rate to the time on ice.
Here is a breakdown of how a period splits into the two buckets of ice:
- Fresh Ice bucket = 50% = 12.5% + 12.5% + 12.5% + 12.5%
- Rough Ice bucket = 50% = 17.5% + 7.5% + 7.5% + 17.5%
In reality, ice scrapes are delayed or missed, so the actual percentages of time on fresh versus rough ice are not exactly 50 percent each. Actual percentages came out to 46.86 percent for fresh ice and 53.14 percent for rough ice.
Data was analyzed for the past three seasons for all NHL teams. In total, 3,936 games and 16,250 even-strength goals were reviewed.
Analysis
To prove the theory, there should be more fresh ice goals than rough ice goals. What the data shows is the opposite, and a reduction in goals is observed following a fresh ice event.
The above chart is a bit hard to interpret, so let me try to provide some context. It is showing that 34.36 percent of the game, the ice is fresh following an ice scrape, but only 31.10 percent of goals are scored during this time. On the other hand, the ice is rough 53.14 percent of the time, and 56.52 percent of the goals are scored in these periods. Goal scoring on freshly Zambonied ice is almost linearly equal with 12.50 percent of the time accounting for 12.38 percent of the goals.
The total for fresh ice time is 46.86 percent of games, and 43.48 percent of 5-on-5 goals are scored during these periods.
Overall, there was a 6.76 percent swing between fresh ice scoring and rough ice scoring over the past three seasons. This means offense is up when the ice is rough and down after a fresh ice event.
Here are the raw data:
| 5v5 Goals | Time in Minutes | Delta | |
|---|---|---|---|
| Fresh Ice – Period start | 2,012 (12.38%) | 29,520 (12.50%) | -0.12% |
| Fresh Ice – After Scrape | 5,054 (31.10%) | 81,151.67 (34.36%) | -3.26% |
| Rough Ice | 9,184 (56.52%) | 125,488.33 (53.14%) | +3.38% |
| Totals | 16,250 | 236,160 |
Goaltending
Remember, this is only looking at 5-on-5 goals, so no empty netters or power-play goals are included. Seeing that goal scoring is down following a fresh ice event got me thinking about goalie save percentage. Following similar logic, I analyzed the shot data for the 2023-24 NHL season to calculate league-wide goalie save percentage for even-strength play on rough and fresh ice.
Sure enough, the goaltenders see a boost in save percentage following a fresh ice event. I am speculating here, but for similar reasons we theorized that scoring would be up due to smoother ice. The goaltenders could be even more of a benefactor of smooth ice, as puck movement is more predictable.
Conclusion
This was a fun project, and I was surprised by the results. There is probably more to dig into here, because one thing I thought of is that even after a scrape by the ice crew, the playing surface is certainly smoother at the beginning of a period and following resurfacing than it is at the end. So, perhaps all “fresh” and “rough” ice is not created equal.
Still, in all my years of watching and playing hockey, I have always thought the opposite to be true, that when the ice had received some sort of attention either from the ice crew or from the Zamboni, teams would score more. There was a lot more data collected than presented here, but to quickly summarize it, shots, turnovers, various fresh ice time intervals, and regulation goals in all situations were factored in and showed the same result. There is a reduction in scoring and a boost to goaltending save percentage following an ice scrape.
If you have any questions or thoughts please comment below. You can follow @blaizg on x.com.