I have surfed at just over 10 knots downwind in a Newport
27 during racing. Conditions were similar to what Bigpuff describes.
I have "part surfed" my own 33 footer, but not to that extent. My hull
speed is around 6.9 knots to judge from the waterline, but I have had in certain conditions 7.3-7.5 sustained for 60 seconds or more. My highest speed on a momentary surf was 8.2 knots on a broad reach in about 25 knots of wind
right in front of the Toronto Islands...I got lucky with a passing front.
I've surfed faster on bigger boats, but I don't count that if I'm not at the helm
The "feel" of surfing a sailboat is quite distinctive. It's reminiscent to me of the feeling you get just before an elevator stops, if that makes sense. There's a perceptible lift
and the bow waves stop splashing and gurgling and start hissing as less hull
Keep your moves very conservative on the helm
. It's very easy to broach in this wondrous state, and the "control surfaces" actually in the water are lessened to a degree.
But it certainly is an exciting part of sailing.