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For nearly all of the 1930'ies, the fastest aeroplanes were all of the seaplane category (as opposed to landplanes, to use the terminology of FAI).

Presumably, the designers of the two categories had access to more or less the same technology (engines, materials, aerodynamics, etc.) given the large interest in the subject and funding available.

If we for a second rely on Wikipedia for the record listing, we see that the speed record for a seaplane set in 1931 is 655.8 km/h (Supermarine S.6B), while the record for a landplane set in 1935 (4 years later) is "only" 567.1 km/h (Hughes H-1 Racer).

Seeing how fast aviation technology developed in that period, it is remarkable that four years after the Supermarine record, the fastest landplane was still significantly slower than the fastest seaplane.

I would expect the large pontoons on the seaplanes to provide more drag compared to a simple landing gear, and hence by logic the seaplane should be penalised and achieve lower maximum speeds. All things being equal.

However, since the speed differences are so pronounced, all things obviously are not equal. So what are the differences between the two categories (or their designers and manufacturers) that can explain the gap in maximum speed?

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    $\begingroup$ Supermarine S.6B's engine: Rolls-Royce R, 2,350 hp (1,753 kW), H-1 Racer engine: Pratt & Whitney R-1535 radial engine, 700 hp (521 kW). So I guess this question boils down to "Why seaplanes manufacturers chose to stick more powerful engines to their planes" $\endgroup$ – DeepSpace May 28 '17 at 17:45
  • $\begingroup$ In an era of fixed pitch propellers and limited flap technology a seaplane may have had certain operational advantages compared to the airfield needed to support a landplane of comparable speed-optimized fixed configuration in the takeoff/landing regimes where that configuration is not optimal. $\endgroup$ – Chris Stratton May 28 '17 at 19:26
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    $\begingroup$ @DeepSpace Interesting observations. The question though would rather be "why did landplane manufacturers not use the same powerful engines that seaplane manufacturers did?" $\endgroup$ – Monolo May 30 '17 at 8:41
  • $\begingroup$ @ChrisStratton Thanks for the observation. Peter Kämpf's answer goes in the same direction, so this is worth delving deeper into, I guess. $\endgroup$ – Monolo May 30 '17 at 8:49
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The answer: Fixed-pitch propellers.

Until the 1930s propellers were fixed pitch only. From Wikipedia:

The first practical controllable-pitch propeller for aircraft was introduced in 1932. French firm Ratier pioneered variable-pitch propellers of various designs from 1928 onwards, relying on a special ball bearing helicoïdal ramp at the root of the blades for easy operation.

This means all speed records were flown with fixed-pitch propellers, which needed to be optimised to the top design speed to coax as much thrust from the engines as possible. In turn, this means that most of the propeller blade was stalled at low speed, so take-off thrust was low. To achieve high speed, a high wing loading was needed and, in combination with low thrust, the take-off length of such an aircraft was beyond of the length of airfields back then.

On water, however, the maximum field length is close to infinity. Yes, drag with floats is especially high when transitioning from buoyancy to dynamic lift (the "hump"), but that can be overcome when that hump occurs at high enough speed so the propeller delivers enough thrust.

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    $\begingroup$ This makes a lot of sense. Wouldn't it have been conceivable enough though to use some kind of assisted-start system (catapult? tossing line? rocket boosters?) to get a land-based plane quickly to the speed needed to get the propeller out of stall? Or else, to take off in a salt flat or something? $\endgroup$ – leftaroundabout May 28 '17 at 23:28
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    $\begingroup$ @leftaroundabout: There aren't that many salt flats in Europe where the rules were made with respect to record flights back then. I do not know for sure but would suspect that catapults also would had violated the rules. $\endgroup$ – Peter Kämpf May 29 '17 at 7:23
  • $\begingroup$ Very interesting reasoning. I understand the issues of drag while the pontoons are still in the water, but are they not also a significant source of drag at altitude? In light of all the efforts gone into streamlining the fuselage, one would imagine that the presence of pontoons would provide a very relevant contribution to the total drag on plane? $\endgroup$ – Monolo May 30 '17 at 8:28
  • $\begingroup$ @Monolo pictures f the S6B suggest yes, don't they $\endgroup$ – Chris H May 30 '17 at 9:40
  • $\begingroup$ @Monolo: Yes, the floats and especially the sharp edges on their bottom and the steps did contribute a lot to drag. That is why the Italian Piaggio P.7 tried to replace them with hydrofoils. $\endgroup$ – Peter Kämpf Jun 4 '17 at 10:29
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This may only be part of the story, but the Schneider Trophy was the most prestigious air trophy of that era, and it was initially reserved for seaplanes only. In I Could Never Be So Lucky Again, the famous aviator Jimmy Doolittle (writing about events in 1925) said:

The Schneider Cup race, to be held at Baltimore two weeks after the Pulitzer [air race], was reserved for seaplanes only. The race had first been run in 1913 and was sponsored by Jacques P. Schneider, pilot son of a wealthy French industrialist. It was considered the most important international air race at the time and received much press coverage in the flying nations of the world.

So at that time, the way to bring the most attention to your aircraft - or your country's aviation capabilities - was to enter the most famous air race, which happened to be restricted to seaplanes only:

The word came down from Washington that we could attempt to set or break records and make cross-country flights that were considered newsworthy and would project a favorable image of the Army Air Service.

I'm sure there were other factors involved, but publicity and prestige seem to be two big ones.

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    $\begingroup$ According to the book "Race with the Wind: How Air Racing Advanced Aviation" by Birch Matthews, "Schneider thought that seaplanes had a great future since so much of the earth is covered by water, which could be used for takeoff by large, heavy aircraft without the need to build runways." (Wikipedia) I've also read somewhere (but cannot remember the source, sorry) that a further argument was that most settlements at least in the western world are constructed near sources of water, so seaplanes made sense as they could use existing infrastructure (docks) rather than building new airstrips. $\endgroup$ – bjelleklang May 29 '17 at 12:21
  • $\begingroup$ This is the real reason. $\endgroup$ – Vladimir F May 31 '17 at 10:08
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No source on this, but my gut feeling is that it's a funding issue. The Air Force wasn't an independent branch and the navy was quicker to invest in aviation and didn't share with the army, and definitely had a larger budget then private citizens. The army definitely delayed emphasizing their Air Force (based on Rindlesbacher/rickenbacker's experiences). So again, no source other than books like The Aviators, but I would say funding in the Navy was higher and aircraft carriers weren't yet perceived by the navy as a whole as the dominant force they became in the early years of world war 2 (hence the pontoons).

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    $\begingroup$ You might want perhaps elaborate on Eddie Rickenbacher (if he's who “rickenbacher's experiences” refer to). But, more importantly, please correct me if I am wrong: I feel that this answer addresses mainly the American situation, while the years of the apogee of the seaplanes in the 1930s involve also, and perhaps mainly, British and Italian planes, designers and pilots. So perhaps the answer should be widened a bit, as well as sourced. $\endgroup$ – DaG May 28 '17 at 17:56
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    $\begingroup$ Excellent point. Best to delete my answer? I wanted to leave it as a comment, but apparently I can't yet comment on aviation questions, only answer... $\endgroup$ – oeste May 28 '17 at 17:58
  • $\begingroup$ At no point in this answer do you deign to mention what country you're talking about. $\endgroup$ – TRiG May 31 '17 at 9:15

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