A 1934 study showed as only a half of the demand could had been satisfied trough the Italian oilers fleet, while the second half, to import via third parts oiler would raise transport prices beyond control.
A specific board (“Studio sul rifornimento della nazione in guerra”) proposed in 1934 to build a fleet od submersible oilers, whose specs were:
6000 tons of payload;
11000 tons displacement;
106 m L, 15 m W, 8,5 m FLD;
2000 CV diesel engine, 10k; Diesel propulsion in immersion via “Ferretti” snorchel;
1400 CV electron engine, 4K for 10 hours;
Four minutes to submerge;
100m max depth;
These oiler would had been able to cover the routes to Liverpool, Lisbon, Valencia with seven to thirteen trips/year.
Ten of these oilers would had been needed to transport 10% of one year’s demand, a a price of 45 million lire each against the 20 million of a normal oiler.
The project was abandoned in 1937; nonetheless, in 1943 two of twelve submarines specifically designed for strategic good transportation were launched (RS Romolo and RS Remo), demonstrating at least the operational need for such a project.
(Source: Una petroliera sommergibile degli anni trenta, G. Colliva, Storia Militare, March, 1995)