A while ago we were discussing engine cooling fans; direct drive fans, and vicous clutch fans. Chrysler started using a non-thermal viscous clutch fan drive in 1957, called the "Silent Flite". The thermal version came in around 1961.
I came across the chart below and thought you might be interested in actual numbers, power required versus RPM. The "disengaged fan drive" and "engaged fan drive" lines are for a thermal fan drive -- they demonstrate quite a savings in horsepower over the fixed drive fan.