I have had a suspicion that my Yanmar's tachometer was reading high. How to test it? There are purpose built tools for the job, but I happen to have a bicycle aboard with a wireless speedometer. A guy on the dock gave me a small magnet which I taped to the inside of the crank end alternator pulley. Then I simply held the bike odo pickup near the magnet and recorded the MPH reading for each tachometer test value. From there, I took the circumference setting on the bike computer and worked backward to get the RPM value it was seeing. My tests were limited to about 1500 rpms since the bike computer wasn't prepared to deal with 80+ MPH. Anyway, the result showed that the tachometer and the bike computer agreed within a percent or so. So, when boat speed is low for the tach reading, it's still time to clean the bottom and propellor. Running the engine faster still isn't the answer.

At the suggestion of a techie friend, I also tried using a spectrum analyzer app for the iPhone, but didn't have any success getting a stable reading. Perhaps I chose the wrong app. One workable solution is enough.