Err, 30 amps @ 12 volts makes 360 Watts. I seem to recall from my schooldays that one horsepower was 746 Watts, so we're talking about half a horsepower!. The problem with engine driven fans is how to provide adequate airflow at idle revs without wasting power at high revs. The viscous coupling is a compromise, electric fans are best as they run at a constant speed.
Check out http://www.kenlowe.co.uk