Interesting size comparison with the Spruce Goose and others here (from wikipedia) http://upload.wikimedia.org/wikipedia/commons/5/52/Giant_planes_comparison.svg
Yeah, I like those. Kinda drives the point home that the Spruce Goose was before its time... but since it was before its time, it wasn't a plane, it was an Ekranoplan with delusions of grandeur. Hughes H-4 Hercules: 8x3000HP radials = 24,000 HP Boeing 747-8: 4x67,000lb thrust turbofans = 400,000 HP Hughes H-4 Hercules: 250,000 lb Boeing 747-8: 500,000 lb Give that dude 20+ years of engine development and he coulda been a contender. As it is, it was a precursor to the Caspian Sea Monster. It's funny. I live in Playa Del Rey. The Spruce Goose was built a short bike ride away. A lot of the old beach houses around here are built out of leftover cuttings from the assembly line; they aren't that well insulated. I've also shot two television shows in the hangar where they built it. It's a pretty massive space. Not quite as crazy as Boeing's 747 assembly line, but bigger than your average garage, for sure. 
Is that the HP directly equivalent to the amount of thrust it produces? It looks like the Rolls Royce engine on the boeing could produce 50K lb of thrust, cannot seem to find the same for the H-4 but its probably down around 5K lbs?Hughes H-4 Hercules: 8x3000HP radials = 24,000 HP
Boeing 747-8: 4x67,000lb thrust turbofans = 400,000 HP
Good eye. I perused this thread and decided to go with the guy who said There's a good deal more discussion possible, but I figured it wasn't directly salient to the discussion at hand.What I was told many years ago is that 1 lb of thrust equals 1.5 to 3 HP depending on altitude, speed and temperature. But do 3 oranges do the work of one apple?
http://en.wikipedia.org/wiki/Ground_effect_(aerodynamics) The Spruce Goose has a wingspan of 324 feet. It achieved a maximum altitude of 70 feet. To conclusively "fly" it would have had to get up to 700 feet or so; if you read between the lines on the maiden and only flight of the Spruce Goose, Howard Hughes was scared shitless by not only how little power the beastie had but also the disturbing creaks and cracks heard by all aboard. The dude did 130mph for a mile. That means it was not on the water for less than 30 seconds. I'll bet all 28 of them were invigorating.