CREATE TABLE table_61530 ("Bow number" TEXT, "Ship name" TEXT, "Launched" TEXT, "Commissioned" TEXT, "Service" TEXT)

When did the ship that was commissioned July 1948 with a bow number of ps-31 launch?
SELECT "Launched" FROM table_61530 WHERE "Commissioned" = 'july 1948' AND "Bow number" = 'ps-31'
SELECT "Launched" FROM "table_61530" WHERE "Bow number" = 'ps-31' AND "Commissioned" = 'july 1948'