A small fish whose head and the fore part of its body suggest the head and neck of a horse.
The first Seahorse (SS-304) was laid down on 1 August 1942 by the Mare Island Navy Yard, Vallejo, Calif.; launched on 9 January 1943; sponsored by Mrs. Chester C. Smith; and commissioned on 31 March 1943, Comdr. Donald McGregor in command.
Following shakedown along the California coast, Seahorse sailed to Pearl Harbor and, on 3 August 1943, got underway for her maiden war patrol, conducted off the Palau Islands. On the morning of 29 August, while the submarine was gaining attack position on an enemy convoy, she was detected by escorting destroyers and suffered minor damage from a depth charge attack. Seahorse scored three torpedo hits on a transport on 6 September, and then went deep to evade a depth charge attack that caused severe leaks and put her number four torpedo tube out of commission. A week later, she expended eight torpedoes in an unsuccessful attempt to sink a large tanker. The submarine terminated her first patrol at Midway on 27 September.
Following refit, Seahorse sailed on 20 October for her second war patrol. Between 29 and 31 October, the submarine sank three enemy trawlers in surface actions and then commenced a two-day attack on a 17-ship Japanese convoy. Early on the morning of 2 November, following an attack on the convoy by another United States submarine, Seahorse evaded three escort ships and fired three torpedoes into two freighters. Four hours later, she again attacked, sending three torpedoes toward a tanker and another spread at a third freighter. Flames burst from each target as two Japanese destroyers turned toward Seahorse but too late to catch the rapidly departing submarine. Sunk in this action were the 7,089-ton cargo ship Chihaya Maru, and the 5,859-ton cargo ship, Ume Maru. Seahorse later closed again on the convoy but was driven down by depth charges and departed the vicinity.
On 22 November, Seahorse skillfully maneuvered past three enemy escorts, fired four torpedoes from periscope depth, and sank the cargo ship, Daishu Maru. On 26 November, the submarine contacted another enemy convoy and began to close the range. Determined to mount an attack before the targets entered the mined Tsushima Strait, the submarine fired four torpedoes at long range quickly sinking a cargo ship; and then, dodging enemy escorts, let go four stern shots at a second target. The results seemed disappointing until a sudden blast sent flames and debris mushrooming high into the air, completely destroying the 7,309-ton tanker, San Roman Maru.
Seahorse expended the last of her torpedoes on the night of 30 November and 1 December. After maneuvering for several hours, the submarine was finally able to fire her stern tubes at an enemy convoy. However, one torpedo exploded just after it left the tube, and the entire convoy opened fire on the vicinity of the submarine. With so many explosions around her, it was impossible for Seahorse to determine whether any torpedoes had hit. Low on fuel and out of torpedoes, the submarine returned to Pearl Harbor on 12 December from a successful second patrol, with five ships and three trawlers sunk.
Seahorse departed Pearl Harbor on 6 January 1944 for her third war patrol. On 16 January, while en route to the Palaus, she evaded four escorts and destroyed the 784-ton cargo ship, Nikko Maru, with three torpedo hits. She spent 21 January tracking two enemy cargo ships in company with three escorts. In the late evening, she pressed home four consecutive attacks to sink the 3,025-ton cargo ship, Yasukuni Maru, and the 3,156-ton passenger-cargo ship, Ikoma Maru.
On the evening of 28 January, Seahorse began an 80-hour chase of an enemy convoy off the Palaus. After being continually harassed by escorts and aircraft throughout the next day, Seahorse fired three torpedoes at the cargo ship, Toko Maru. After the sinking, the submarine lost contact with the convoy for several hours, but again had it in sight at dawn on the 31st.
Early on the morning of 1 February, Seahorse fired four torpedoes for no hits followed by two more, again without result. With the crew exhausted from the extended chase, the submarine fired her final two torpedoes and headed for deeper water. After evading the escorts, she surfaced in time to see the results of her latest attack as the cargo ship, Toei Maru, slipped beneath the waves. Seahorse terminated her third patrol at Pearl Harbor on 16 February.
Seahorse's fourth war patrol was conducted in the Marianas. She departed Pearl Harbor on 16 March and intercepted a large enemy convoy on 8 April. After nightfall, the submarine fired four torpedoes at overlapping targets, sinking the converted seaplane tender, Aratama Maru. Shortly thereafter, her second spread of torpedoes sank the cargo ship, Kizugawa Maru. Although a counterattack by escorting destroyers drove the submarine from the vicinity, she quickly regained contact and continued the chase into the following day, sinking the cargo ship, Bisaku Maru.
Seahorse took up lifeguard station for the carrier airstrikes on Saipan that commenced on 12 April and, while west of Saipan on 20 April, sighted and sank the Japanese submarine, RO-45. In the same vicinity a week later, Seahorse sank the 5,244-ton cargo ship, Akigawa Maru. The submarine departed her lifeguard station on 3 May to refuel at Milne Bay, New Guinea, and arrived at Brisbane, Australia, on 11 May.
Seahorse put to sea for her fifth war patrol on 11 June 1944, patrolling between Formosa and Luzon. On the morning of 27 June, she sank the tanker, Medan Maru, and damaged two other enemy vessels; and, on 3 July, sank the cargo ship, Nitto Maru, and the passenger-cargo ship, Gyoyu Maru. The following day, the submarine expended the last of her torpedoes sinking the cargo ship, Kyodo Maru No. 28, and returned to Pearl Harbor on 19 July.
Seahorse spent the first part of her sixth war patrol supporting the capture of the Palaus and then headed for the Luzon Strait. Despite intensive efforts, the submarine could locate only one worthwhile target, Coast Defense Vessel No. 21, a frigate of 800 tons, which she sank. Five days later, Seahorse took up lifeguard station for the carrier airstrik on northern Luzon and then returned to Midway on 18 October.
Upon completion of an overhaul at Mare Island Navy Yard, Seahorse put to sea on 9 March 1945 for her seventh war patrol. Following patrol in the Tsushima Strait, she sank a small junk with gunfire on 8 April. On the 18th, an attack by two patrol boats left the submarine's interior a shambles of broken glass, smashed instruments, and spilled hydraulic oil. Seahorse made hasty repairs and headed for Apra Harbor, Guam, and then to Pearl Harbor for overhaul.
Seahorse put to sea for her eighth and final war patrol on 12 July. When hostilities ceased on 15 August, the submarine was on station 40 miles southeast of Hachijo Shima. Following her return to Midway, Seahorse sailed for Mare Island where she was decommissioned on 2 March 1946. She was assigned to the Pacific Reserve Fleet and remained inactive for the remainder of her career. She was reclassified an auxiliary submarine (AGSS) on 6 November 1962, struck from the Navy list on 1 March 1967, and sold on 14 December 1968 to Zidell Explorations Inc., Portland, Oregon, for scrapping.
Seahorse (SS-304) received nine battle stars for World War II service.