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I just learned that until shortly after world war 2, flying boats had a longer range than land based aircraft

In the 1930s, a flying boat route was the only practical means of transatlantic air travel, as land-based aircraft lacked sufficient range for the crossing. (link)

Why was that? I'm not aware of any reason why land based aircraft should have a shorter range than seaplanes.

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    $\begingroup$ related: Why were flying boats phased out? $\endgroup$ – JanKanis Jan 18 at 21:43
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    $\begingroup$ @DavidBalažic Seaplanes have a worse aerodynamic shape than land planes because they also need to be boat-like. I'm pretty sure the air resistance of a land plane with retractable landing gear will be less than that of a seaplane, but I'm not sure how seaplanes compare to fixed landing gear aircraft. Do you have sources? $\endgroup$ – JanKanis Jan 19 at 11:00
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    $\begingroup$ Yes. The speed records. Of course the comparison is to non retractable gears. That is why after they started using retractable gears, the tables turned to favor land planes. $\endgroup$ – David Balažic Jan 19 at 11:04
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    $\begingroup$ You're right on the speed record thing, but according to Why were the speed records set by seaplanes in the early thirties? this was due to the runway length needed with the fixed-pitch propellers of the day and not so much aerodynamics. $\endgroup$ – JanKanis Jan 19 at 11:54
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    $\begingroup$ @reirab while the wikipedia picture of a CH-300 is of one on floats, every indication I can find is that the NY-Instanbul flight was a land configuration; news articles clearly state that they rolled down the runway at Floyd Bennett, and the picture ark.digitalcommonwealth.org/ark:/50959/6682zd329 clearly shows the poise of a taildragger. So no, that's 5000 mile landplane flight in 1931, not a seaplane one. $\endgroup$ – Chris Stratton Jan 21 at 19:27
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Since fuel volume in aircraft is a large factor in range, and the volume of an aircraft increases faster than the linear size, larger aircraft have a big advantage for range. In the 1930s, the biggest hurdle for large land-based aircraft was infrastructure. Most airports in the 1930s were not very large, and many parts of the world did not have any at all. The common airliners at the time carried around 20 passengers.

Designing an aircraft that could land on the water allowed aircraft to get larger and have longer ranges without the need for large runways to be built to support them. Aircraft like the Sikorsky S-42 carried almost 40 passengers. It wasn't until at least the late 1930s that land based aircraft were able to reach this size, and after World War II the devolopment of infrastructure and aircraft technology made long-range land-based aircraft much more feasible.

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    $\begingroup$ @JanKanis If the fuselage can't take the weight of the entire plane plus all its cargo, the plane will collapse. So it already has that strength. The skin itself doesn't have to be thick either, because the force of the water is distributed over a wide area. Wooden sailing dinghies are constructed of plywood as thin as 3mm or 4mm. $\endgroup$ – Graham Jan 19 at 9:29
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    $\begingroup$ @Graham I wasn't talking about weight, but about the impact forces of hitting the water and the waves. A landing flying boat is a lot faster than a sailing dinghy. The weight can be carried by a few structural beams, but all of the underside of the hull must be able to withstand hitting a wave at high speed. $\endgroup$ – JanKanis Jan 19 at 9:38
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    $\begingroup$ This is why the "Spruce Goose" got built, isn't it - the last attempt at scaling up seaplane technology before the jet age. $\endgroup$ – pjc50 Jan 19 at 14:54
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    $\begingroup$ Good statement. I had the privilege of growing up in Berlin, which had Hitler's "ORIGINAL" Airport (the first real airport, after the airfields used before) and - it was SMALL. The planes went into Hangars where the terminals where and you walked to the plane IN the hangar. Amazing (at that time only used for mid range turbo prop machines), but it gives a good idea how small the largest aircraft where pre the development of the modern airport and the increase in passenger size and the jet age.Pre this airport basically people used airfields and they were VERY limited. $\endgroup$ – TomTom Jan 19 at 15:41
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    $\begingroup$ @JanKanis For the underside of the hull, you're planing - skimming over the water instead of pushing through it. The force from glancing off wave-crests is very different from planting perpendicularly into the water. For sure it needs to be strong, but only on a similar scale to a boat. If you look at Donald Campbell's various speedboats, they were significantly faster than any flying boat, but they weren't that heavily built. Since seaplanes clearly did work, for all the reasons fooot said, I suggest your intuition may not be a totally accurate guide. ;) $\endgroup$ – Graham Jan 19 at 15:42

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