system
Given the following post, respond as if you're commenting on Reddit:
human
Why do desktop fans look more like boat propellers than aircraft propellers?
gpt
Slow moving air is much more like a classical fluid than fast moving  so the desktop fan can look a lot like a traditional non-cavitating propeller (efficient in water = efficient in slow air). Look at the fans in large buildings or on windmills, and you can see this difference in shape along the length of the fan blades. They get narrower and more streamlined down the length because the farthest tips of these blades experience higher air speeds.