NMS Marsuinul (S2) was launched on 25 May 1941, but like Rechinul it wasn't complete yet and entered officially in the ranks of the Navy in July 1943 and was declared operational in April 1944. It was originally meant to be a mine-laying submarine, but the decision was taken to transform it into an attack sub. So the mine compartments became additional fuel tanks. This is why it has a superior range to the other Romanian submarines.

Marsuinul carried out only one war patrol between 10-27 May 1944. Its task was to continue NMS Rechinul's mission in the Batumi area, under the command of cpt. Grigore Ciolac. The war patrol started very badly when, after exiting the mine barrage, on 11 May, at 0100 hours, a German R-boot confused it with a Soviet submarine and fired its machine-guns on it. After signaling the day's password, the problem was resolved. In the morning, it was bombed by a German seaplane and forced to submerge. The situation became critical because several KFKs with Croat crews responded to the alarm given by the seaplane and launched no less than 420 depth charges from 0830 hours until late in the evening. The submarine descended to 80 – 90 meters below sea level and was rigged for silent running. There was a moment when the captain tried to surface, but, after getting to as much as 20 meters, the explosions got closer and closer and had to return to the safety of greater depths. Fortunately the damages it had suffered were not serious.

After the KFKs ran out of depth charges, they abandoned the pursuit and the submarine headed to Zunguldak and from there to, sailing along the coast. NMS Marsuinul was discovered and attacked for the first time by the Soviet patrol boats on 14 May, at 1110 hours. Then again, around 1500 hours, it was again found and pursued for three hours. Thus the captain decided to retreat for one day near Trabzon, by the Turkish coast. It returned to the area of operations, where it was again located on 17 May, 40 miles North of Cape Yasun and attacked at 0412 hours, having to seek refuge again. But this time it was pursued and on 18 May , around 0400 hours, several submarine hunters launched depth charges in its vicinity. In the evening Marsuinul tried for the third time to approach Batumi, but the following day it was discovered and attacked. The crew counted 43 explosions.

On 20 May it was near Turkish territorial waters, 40 miles North of Cape Yasun. During the night, around 0200 hours, the crew thought to have seen a torpedo and avoided it. There is no record of such an attack on that day from the Soviet side. Two hours later it was depth charged and 31 explosions were counted. The following day, also around 0200 hours, the submarine was again discovered and attacked until 1400 hours. 43 depth charges were launched. From Constanta, cpt. Ciolac received the order to enter the area of operations for the fourth time.On 22 May, the submarine hunters located NMS Marsuinul at 0455 hours, some 45 miles West by Northwest of Batumi. For three and a half hours, 82 depth charges were launched, of which 74 exploded in the vicinity of the ship's hull. But the day wasn't over yet. It was again detected and attacked between 1831 and 1858 hours.

The evacuation of Crimea was over and there was no need to risk a submarine in a screening mission. Thus, on 23 May it received the order to return home, being under constant pursuit. The trip to the area near Zunguldak was made on a very heavy sea, for 37 hours. The weather got better after it left the route along the Turkish coast and headed directly for Constanta on 25 May. The following day, while sailing on the surface, the submarine was surprised by a Soviet seaplane, which dropped 6 bombs and forced it to submerge. Because of this, Marsuinul missed the rendezvous with the escort in the evening and entered the port the next morning.

Specifications

Crew	45
Displacement on surface	508t
Displacement in imersion	630t
Length	68.7m
Width	6.5m
Draft	3.6m
Max speed on surface	16 knots
Max speed in imersion	9 knots
Range	8000 miles
Guns	1 x 88mm
Machineguns	1 x 20mm
Torpedo tubes	6 (4+2) x 533mm