Slowly over the years my C30 had been losing speed under power. The engine has always ran steady and consistent. I suspected the transmission might be slipping but several tranny guys talked me out of it. One guy asked about the condition of my prop. I had been using bottom paint on it and had about ten plus years of accumulation. He said it was possible the prop was cavitating. This spring I sanded the prop clean, dressed the edges and used a spray anti fouling paint. The boat was launched this weekend and my cruising speed went from 4.5 knots to close to 6 knots. Needless to say I'm pretty impressed. BTY, I'm measuring speed with a knot meter not GPS. The knot meter gives true speed through water. I use my GPS for reference, I am able to monitor currents using both. For speed over ground I use the GPS.