Why was the world speed record held by a seaplane for 5 years?

When the world speed record was set in an airplane in 1934, it was in
a seaplane - the Macchi M.C.72 at 440 mph. The land plane speed
record was only 352 mph in a Hughes racer. The seaplane has two
cumbersome floats that adds a lot more frontal area and hence drag.

Why was the seaplane faster than the land plane by so much? Why
didn't Macchi drop the floats and set an even faster speed record?

-- Stanford Tran, September 28, 2016

Answers

I think you might find that they did not yet have variable pitch propellers. A coarse pitch prop is required for high speed flight. But, a coarse pitch prop has poor low speed performance (takeoff acceleration and thrust). Hence they needed really long runways to build up speed. The runways were not long enough, so they made them floatplanes where you could use long runs to get airborne.

-- Greg Parnell, November 23, 2020