Yankee One Design, I'd usually hear that hiss once she got to near hull speed. In the pre-GPS days, I assumed it to be around 6 knots on my 24'lwl boat. I sailed on a sistership a year ago (new build) and heard that familiar sound. We had a GPS and it was showing an honest 7.1 knots. Ahhh, that was and is an absolutely great sound!