Paul Hanson wrote:

Not too cataclysmic. Somebody was smart enough along
the way to realize that 100fpm was almost exactly 1
knot. Plus, since one degree of latitude was a nautical
mile it only made sense to use knots for vertical and
horizontal motion to reduce cockpit workload in the
days before the electronic flight computer (I learned
some of this from a Derek Pigot book). It was actually
a worse situation than that here in the US, before
the fpm/mph days, when most varios here were expressed
in fps and airspeeds in mph which made for even more
math that was very avoidable (there are still a lot
of archaic instruments used, reading in silly mph and
fpm and even fps; old habits die hard)

I don't dispute the abstract benefits of having the vario and airspeed
in the same units. As a practical matter, it isn't much use. Having
the airspeed in knots does help with manual navigation using sectionals.
I have never, however, done any L/D calculations based on vario
readings, and I don't know of anyone else who does. In fact, flight
computers and software do not use instantaneous (or average) vario
readings for navigation or final glide calculations, the vario is only
used to determine speed to fly.

So, the question still remains, why did the US gliding community make a
relatively quick shift from MPH and FPM to knots, when just about
everything else happens so slowly?

Marc