In the Jan 2021 blog I discussed the upgrade from the original 16.5 pitch prop to a 18 inch pitched prop, both 20 inches in diameter. The speed improved by 1-2 knots at all RPM values with a correspondingly increase in fuel burn only above 3000 RPM.
So it was obvious that I should try another increase in the pitch to see if I could further improve the performance. I purchased a 20 inch pitch prop as a spare prop for the business and swapped the 18 pitch for the 20 inch pitch, both being 20 inches in diameter. The chart above shows the previous 16.5 and 18 inch prop data along with the new data (red and magenta lines) obtained with the new 20 inch pitched prop. Notice that the Gallons per Hour (GPH) on the left hand scale has increased significantly above 2750 RPM. This also corresponds with a significant drop in the measured knots scaled on the right hand side. The speed eventually drops to values that are below the 18 inch pitched prop and track the 16.5 pitch prop. One plausible cause put forth is that the flow between the top of the prop and the bottom of the boat has become more turbulent with the higher pitch, reducing the performance. This may explain the fact that I noticed an increase in vibration at these RPM's.
The last point on the graph is at wide open throttle (WOT). As the pitch increased, the WOT RPM decreased. The WOT for the 20 inch pitch is probably close to an unacceptably low value.
So this one goes under the saloon bench for a spare and I'm going back to the 18 inch pitched prop.
I ran all of these tests at slack tide in Puget Sound when the wind speed was minimal. I generally have full fuel and water tanks and an empty waste tank. Not by choice, but because it's in the rental fleet, the renters are required to bring the boat back in the same condition as when they left. The speed was speed over ground (SOG) from GPS and the GPH was from the engine readout gauge for my Yanmar 6BY-220.