Correct answer is... 90 km/h. It is equal to 49 kn or 56 mph. It is higher value then stall speed of other aircrafts of a similar size. Average is about 14 km/h (7 kn or 9 mph).
The stall speed is the speed at which detachment of airstream going over the wing will occur. In practice, it deprives the Savoia-Marchetti S.73 of the lift, the pilot, which leads to the free fall of the airplane. The pilot partly or fully loses the possibility to control the airplane.the stall speed depends on many factors. The pilot can positively influence the stall speed using flaps or slots (if installed). Then, the stall speed decreases. On the other hand, the pilot has to maintain much higher speed when turning. Then, the stall speed increases.
Disclaimer: Our articles and data connected to stall speed of Savoia-Marchetti S.73 are based on public data from various sources. We are very picky to select an reliable data providers, but we cannot guarantee that everything is 100% error-proof. We recommend for any serious purpose - as a rule of thumb - to request offical data about stall speed from manufacturer of Savoia-Marchetti S.73.