Automate said:
I would think if everything else is equal, the 2 blade mower would be more efficient. Aerodynamic drag goes up with the cube of the speed. So while the inner part of the bade is going as slow as it can and still get a proper cut, the outer part of the blade is going faster than required. This extra speed on the outer parts of the blade are using up energy at a rate raised to a power of 3 of the speed difference.
In a mower, you need the blade tip speed to be above a minimum threshold to make an acceptable quality cut. The outermost portion of the blade is doing the vast majority of the cutting work while the inner portion is doing almost no work. So in order to maintain the cut quality, you have to spin a smaller blade faster than a larger one.
Typical example: 21" mower swings the blade at 3,200 RPM
5.5' circumference means the bale tips are moving at 17,600 FPM
To get a 13" (3.4' circumference) blade tip moving at 17,600 FPM, the spindle speed must be raised to 5,176 rip 'ems.
You'd also be dragging 4 sails through the air vs 2. So how much drag is there swinging 4 sails that are half the size of 2? I dunno but I'd venture a guess that it's more energy to run overall. There are other variables at work here too as the shape of the deck must change with the 2 blade design and this means that the blades are no longer fully shrouded.