Weird that nobody has mentioned using a bicycle computer. Much easier than messing about with scopes etc.

There are some available that can read both RPM and speed which could be calibrated to show wind speed on the speed readout. You can extend the wire with no problems.

I use one which only reads speed, so I have calibrated it to read the speed as RPM. It also remembers top speed (i.e. top RPM). I'm at work now, but when I get home to the boat, I can check what wheel circumference I entered to make kph correspond to RPM. It's a constant, not related to prop diameter. (You want to know how many times it turns, not how far the tip of the prop travels.) If you put your thinking cap on, you can calculate it, but I'm at work now, so I left my thinking cap at home... :)

Readout is with one decimal point moved. In other words. 30.5 kPh means 305 RPM. Mine has been a bit flaky lately. Probably moisture got into where I extended the cable, or maybe it needs a new battery.

Don't try a wireless bicycle computer. You probably won't get enough range and even if you did, it would be a pain to have to climb up the tower to change the battery in the sender.