I've had this happen also with an original V-8 four-blade fan on my Model A 35 years ago when rust weakened the rivets holding the two blades together let loose - with not quite as dramatic results as "mercman from oz" shows above - but bad enough to require hood and radiator core repair. $$$$$$$$ Lesson learned the hard way. I have since refused to ever use another four-blade fan and have removed probably 30-35 of these disasters-in-waiting from Model A's I have repaired over the years. I have a box of the things in the basement that I can't GIVE away! Guys think that changing to a four-blade fan will magically correct their poor cooling situation in their Model A's, when 90% of the time a new radiator core is needed. Even in hot, old Phoenix, Arizona, and while driving across deserts on the way to Iowa a two-blade fan kept my Model A's cool. The secret is no secret: clean engine cooling passageways, proper ignition timing and a good radiator. The number of fan blades is only incidental and is no answer in itself.
Marshall