10 is OK....
I have (had) 10 on my H23. Seems OK, but 15 would be easier to see... One thing I just learned this weekend, REMOVE it before trailering.... Apparently, its not designed to take 60 mph wind in the wrong direction! It broke in half, although the little tabs stayed in place... BTW, I've tried the Davis Spar Fly and never even mounted it on the mast. I thought it'd be great because its bigger and cheaper, but it did not work too well in changing wind direction. It has too much moment of inertia and keeps going back and forth.... Windex is much better...ChrisS/V Doghouse