It is a 4 cyl with a firing period (at 2000 rpm cruising speed) of about 66 Hz, and a prop blade frequency of about 60 Hz (the transmission ratio is 2:19). That gives a beat frequency of 6 Hz and a fairly lengthy period in which the engine and prop are near sync. Rotating the prop on the hub will do no good since the transmission ratio is not an integer. The theory seems to be that the lower the beat frequency the more likely it will excite vibrations throughout the boat. And there is a little vibration everywhere, even the bow pulpit vibrates a bit under power. I bought at vibration transducer, tried it to see if I would be able to measure the frequencies, but it seems finicky: when I try it on a little Briggs & Stratton I get pretty broad band frequencies, most of the energy is way below the engine crank speed and its pretty hard to isolate anything related to crank speed. I assume that all the little mechanical bits get excited at their natural frequencies at some complex subharmonic of engine speed. Anyway it doesn't look like I will be successful at that, may take trained professionals to do it, haven't looked into the expense. At this point I don't think a confident conclusion could be reached except by trying a different prop. A three blade would raise the beat frequency to 20 Hz.

Of course Variprop says only a 4 blade will work, while other manufacturers are confident a three blade will work (of course, that's what they have for sale....). But not so confident that they will offer a guarantee. The one I am looking at is the Autostream, it has a very different blade planform concentrating more area into the tip. Also the hub design places the blades about 2.75 inches further aft of the drive leg, which might be significant it terms of turbulance. When I plot the blade shape, the 4th moment of area of the recommended 3 blade 20" Autostream is actually greater than the 19" 4 blade Variprop. However it is about a $3K gamble.