Average is a word that doesn't work too well here, cuz the winds rarely blow steady at any speed.
Too bad the old forum is Toast. E-bone made a good list of winds ranges and about how many days per season they occur.
If you could ride every single time it blew more then ten knots, my guess would be:

... a lot of days are 11 to 16 (80 or more)

Then, we have days that are from 14 to 25 sometimes 30 (up and down all day.. mainly East beach days). About 30 days a year.

Then we have the 20 to 35 days (typically the NW'rs and the following morning NE). About 12 days a year.

Hurricane wind days... a couple a year.

That's my guess... might be off since every single season is different.

So, our winds tend be more like within a range, no true mode, not even on one day.

More like days with ranges of 11 to 16, days from 15 to 25, and days from 20 to 35 plus. I am on SLE kites (and thank God for them) because of the fact that St. Pete rarely has truly consistent wind.

If we averaged all the ride-able time on the water over the course of the year, I think the average speed would be about 18, like you said.
Hopefully though, you get a feel for what I am trying to say.

29? I am forty.. c'mon, don't make me feel more like a pterydactyl than I already do!