At the beginning of the war (22 June 1941), the Marina Regala Romana (Romanian Royal Navy) had only one submarine: the NMS Delfinul. It had been built in Italian shipyards, at Fiume, and had been launched on 5 May 1936.

On the other side, the Soviet ChF (Chernomorsky Flot = Black Sea Fleet) had 47 submarines. But the existence of this one Romanian submarine forced the Soviet fleet divert a lot of resources to anti-submarine duties in the first two years of war. For this reason it was used very carefully, since its loss was almost irreplaceable in that period. It was also obsolete from a technical point of view in some respects and this limited its operational capabilities. The diving time was relatively long and it wasn't able to recharge its batteries while moving.

Cpt. Constantin Costachescu led the NMS Delfinul in its first war patrol between 22-27 June 1941, in the area 60 miles East of Constanta (the main Romanian port and naval base). It was only a reconnaissance mission. On 26 June, at 0030 hours, it reported the presence of a Soviet task force, which was closing in on the city, allowing the coastal defense to prepare and achieve good results in the following engagement with the enemy destroyers and cruisers. The flotilla leader Moskva was sunk and the Kharkov and the Voroshilov were damaged.

The next mission took place between 10 – 20 July, under the command of cpt. Corneliu Lungu. Delfinul had to patrol in the southern part of Crimea and to asses the AA and AS defense of the Soviet naval base at Norovosiysk. On 13 July it arrived Southwest of Idokopas Cape and on 15 it was South of Feodosiya. In that area the submarine spotted an enemy ship, which disappeared after a while. During the following night, it was surprised by a Soviet patrol vessel and made an emergency dive, but it escaped the attack. On 16 July, the gyrocompass broke down and it had to return home. Some 95 miles East of Cape Shabla, on 19 July, Delfinul discovered a small Soviet submarine, probably a Maliutka class ship, which it tried to attack with its 102 mm gun, but the heavy sea and the sudden appearance of two unidentified seaplanes forced it to disengage.

On 30 July, it had to leave port in another mission at 1600 hours, but the engine malfunctioned and it had to return to Constanta three hours later.

On the third war patrol (12 – 20 August) Delfinul was again under the command of cpt. Constantin Costachescu, who had to disrupt traffic in the in the Odessa area. However, it had only two small opportunities during the night of 13/14 August and on 19, but it couldn't maintain contact. The sector was intensely patrolled by Soviet aircraft and MTBs. It also scouted the Crimean coast and, on 20 August, on the way home, while surfaced it was attacked with one torpedo by the submarine M-33 some 4 miles from Constanta. The torpedo passed by the ship's stern. The M-33 surfaced partially because and the Romanian sailors fired the machine-gun on it.

The fourth mission (3-19 September) took the NMS Delfinul deep in Soviet controlled waters. This time it finally the opportunity to attack Soviet surface ships. After it had sighted two convoys, which it couldn't pursue, on 9 September, 0935 hours, it was surprised on the surface 5 miles off Cape Otrishenok by a Soviet cruiser, which cpt. Corneliu Lungu identified as the Komintern. The submarine maneuvered with care, because the area was filled with underwater rocks, but lost contact. The following days it had several other opportunities, but the bad weather prevented the attack. On 10 September it closed to firing range on a convoy made up of two large vessels 20 miles West of Otryshenok Cape, but a MTB in the escort saw the sub and headed towards it, forcing it to retreat. Another favorable situation was on 16 September West of Novorosiysk, when Delfinul sighted at 0505 hours an oil tanker and an MTB. The latter noticed the periscope and headed towards it. Again the submarine gave up the attack and lowered it. After a while, it raised the periscope, but this time a torpedo boat had joined the escort and cpt. Lungu decided that he should respect the orders he received and not take too many risks. Thus, he abandoned the pursuit. On 17 September it started the return trip, during which it was discovered several times by VVS-ChF patrol airplanes and possibly even attacked by submarine hunters.

The fifth war patrol took place between 2 and 7 November 1941 and was the one during which Delfinul made his first and last torpedo attack. Its mission was to harass Soviet convoys supplying Sevastopol. In the vicinity of Constanta, it spotted an enemy submarine on the surface, but because of the darkness it lost contact. By early morning on5 November, it had reached the area near Yalta. At 0636 it noticed a patrol vessel and continued its stakeout. Around 0805 hours, slt. Constantin Stegaru noticed a large transport vessel on an approximate course of 290 degrees heading towards Yalta. At 0815 hours, the ship turned left, closing in on the submarine and cpt. Costachescu decided to let it pass by and fire on it with one of the aft torpedo tubes. At 0843, the NMS Delfinul fired a torpedo with tube number 6 from a distance of 800 m. First they heard the torpedo's explosion followed soon by another one, which was more powerful. It is possible the ship was unescorted, because the first contact with the Soviet submarine hunters was made after an hour and their attack lasted from 1030 to 1840. There were 23 rounds and the crew counted between 80 and 90 depth charges. While the attacks took place, Costachescu sunk the ship deeper and while the enemy was listening, he shut down the engines. Thus he managed to eventually get rid of the pursuers. Then, they headed for the Turkish coast, where it entered a very powerful storm from which it barely made it to Constanta.

Cpt. Constantin Costachescu was later awarded the prestigious Mihai Viteazul Order 3rd class for this war patrol. Romanian historians identified the ship that was attacked and sunk as the Uralles (1,975 tons), but according to Russian sources, this vessel was sunk in Yevpatoria by Luftwaffe bombers on 29 October 1941. The ship wasn't yet positively identified. The Ostrovsky auxiliary mine laying ship could have been in the area at the time of the attack, but it arrived at Sevastopol without encountering problems.

The next mission was supposed to be carried out against Soviet communications between Batumi and Instanbul. Delfinul left the port on 30 November, but because of the very bad weather it had to return home on 3 December.

The seventh war patrol (6 – 13 December) took the NMS Delfinul back to attack the Batumi-Istanbul line. However, it did not encounter any enemy convoys. As it was returning to base it discovered two Soviet submarines, close to Constanta.

The first mission 1942 took place between 18 – 30 May, under the command of lt. cdor. Lungu, who had been promoted in the meantime. It patrolled north of the Turkish coast, but again it did not encounter any cargo ships. It was attacked by Soviet airplanes on 27 May, but it did not suffer any damage.

The last war patrol of NMS Delfinul (25 June – 3 July) was carried out in the area east of Yalta. On 27 June it arrived in the designated sector, but it was forced to remain submerged, because Soviet aircraft had discovered it. Between 0436 and 1530 hours, the Delfinul was the target of no less than 240 bombs. One fuel tank was lightly damaged. After 1600 hours, several enemy MTBs appeared and cpt. Costachescu gave the order to stop the engines. The following day, it was again surprised on the surface by an enemy airplane and several bullets damaged the tower. On 1 July, the last day during the evacuation of Crimea, the VVS-ChF was extremely busy in the area around the Crimea. Delfinul was discovered and attacked for 13 hours from the air. Between 0725 and 1030 hours, the crew counted the explosion of 107 bombs. Then, around 1300 hours there was an attack with 20 bombs and then another one at 1540 with 24. In the evening, between 1930 and 2000, 82 bombs fell in the submarine's vicinity and 35 further away. In total, 268 explosions owed mostly to aerial bombardment and less likely to depth charges. As it returned home, Delfinul entered general repairs and remained on the dry dock until the end of the war. It was confiscated by the Soviets after 23 August 1944, but was returned to Romania after a while, damaged. It was the most successful Romanian submarine in WWII. In its honor and of those who served aboard it, the only Romanian present-day submarine was also named Delfinul.

Specifications

Crew	40
Displacement on surface	650t
Displacement in imersion	900t
Length	68m
Width	5.9m
Draft	3.6m
Max speed on surface	14 knots
Max speed in imersion	9 knots
Range	2000 miles
Guns	1 x 102mm
Torpedo tubes	6 (4+2) x 533mm