Effective Field Capacity

Everything about machinery economics starts here. How much ground a machine covers in an hour decides how many hours the season needs, how much fuel the crop costs, and whether one more machine has to be bought. The relation itself is trivial — a machine sweeps a width, moves at a speed, and the product is area per unit time — but two things about it are worth stating carefully, because both are routinely got wrong.

The first is that the shortcut everyone learns has a unit conversion baked into it. In metric, field capacity in hectares per hour is width in metres × speed in km/h × efficiency ÷ 10. The 10 is not physics: a metre of width travelled at a kilometre per hour sweeps 1,000 m² in that hour, and a hectare is 10,000 m², so the quotient is 10. In imperial the same relation reads width in feet × speed in mph × efficiency ÷ 8.25, and the 8.25 is simply 43,560 ft² per acre divided by 5,280 ft per mile. The two shortcuts look like different formulas and are the same one. Anybody who has wondered why the divisor changes when the units do now has the answer, and anybody who has tried to use 8.25 with metres has the explanation for the result.

The second is that the width must be the WORKING width and the speed the WORKING speed. A 12 m header on a 12.2 m frame covers 12 m of crop, and a sprayer with a 24 m boom running a 23 m pass spacing is effectively 23 m wide. The speed is likewise the speed while cutting, not the average that includes the road trip and the coffee — that erosion is what the efficiency term is for, and counting it twice is a common way to produce a capacity figure that is quietly half of reality.

The practical use of the equation is usually backwards. A grower knows the area, knows how many days the weather will allow, and wants to know what machine is needed. Rearranged for width, it says exactly that, and it usually says something uncomfortable: capacity is linear in both width and speed, so a 20% wider machine and a 20% faster one buy the same 20%. Width is generally the cheaper and safer of the two, because speed degrades the quality of nearly every field operation before it runs out of tractor.