The most successful minelaying submarine of the war, Rubis was responsible in her 22 minelaying patrols for the sinking of at least 15 vessels. These included five warships as well as vessels running iron ore in coastal convoys to Germany.
Like the Royal Navy, the French had a six-strong class of minelaying submarines, the 'Saphir' class of 1925-9. These were much smaller than the British boats, being geared to Mediterranean operations. Again, as a mine capable of being launched through a standard torpedo tube had not been developed, the hull design was dominated by the mine stowage. The design for this had been produced by the well-known submarine builder, Normand, but was based on that of the British 'E' class minelayers of 1914-8: 16 vertical chutes were built into the space between the widely-separated double hulls, in four groups of four, and each chute could accommodate two mines, though a weakness of the arrangement was that these were of special manufacture. The British had abandoned the system in favour of laying over the stern with the mines stowed within the upper casing.
Four stretched versions, continuing the 'jewel' names as the 'Emeraude' class, were scheduled to follow in 1937-8. Lengthened by nearly 7m (22.97 ft), they would have carried 25 per cent more mines, but only the nameship was ever laid down and she was destroyed on the slip at the occupation.
Of the 'Saphirs', three (Nautilus, Saphir and Turquoise) were taken by the enemy at Bizerta and one (Le Diamant) was scuttled at Toulon. The Rubis and Perle operated for the duration of the war (the latter was sunk in error by British aircraft in July 1944) under the Free French flag. The Rubis began operating with the British Home Fleet in April 1940, laying mines in Norwegian waters. Between then and the end of 1944 she carried out no less than 22 successful minelaying operations, most to interrupt the enemy's coast-hugging mercantile routes. The total of 15 ships known to have been destroyed on her mines included several Scandinavians carrying German ore cargoes, a minesweeper and four small anti-submarine vessels. She also torpedoed and sank one more, a Finn.
Displacement: 761 tons surfaced and 925 tons submerged
Dimensions: length 65.90 m (216.21 ft); beam 12 m(23.36 ft); draught 4,30 m(14.11ft)
Propulsion: two diesels delivering 969.4 kW (1,300 bhp) and two electric motors delivering 820.3 kW ( 1,100 hp) to two shafts
Speed: 12 kts surfaced and 9 kts submerged
Endurance: 12970 km (8,059 miles) at 7.5 kts surfaced and 148 km (92 miles) at 4 kts submerged
Armament: one 75-mm (2.95-in) gun, three 550-mm (21.65-in) torpedo tubes (two bow and one stern), two 400-mm (15.75-in) torpedo tubes in a tramable mounting, and 32 mines