It happens at all speeds over the normal power range. It does not get worse at any particular speed that would indicate a "speed matching a harmonic frequency" situation. When I changed the cutlass bearing I did a realignment on principle and it didn't change anything. The shaft is hard-coupled to the transmission.

Some pulsation is normal since the water flow past the prop is not parallel to the axis to the shaft and prop. The down-coming side of the blade will have more effective pitch than the up-coming side because of the skewed water flow. I think that is what we are feeling.