Take off:
Accelerate slowly to 60kn then apply full throttle

Rotation is around 100Kn

Top speed 180kn

There's very few materials on the internet about this machine.
This plane was created using 3 views drawing from the book "aeroplani SIAI 1915-1935" EDIZIONI AERONAUTICHE ITALIANE

https://en.m.wikipedia.org/wiki/SIAI_S.21

The SIAI S.21 was an Italian racing flying boat built by SIAI for the 1921 Schneider Trophy race. During test flights, the S.21 proved extremely difficult to control, and the only pilot who had success with it was Guido Jannello, who had already made an impression at the 1918 Schneider Trophy, in a Savoia S.13. When Jannello fell ill at the time of the 1921 Schneider Trophy race and was unable to pilot it, the S.21 was withdrawn from the race.