CREATE TABLE table_79838 ("Estimate" FLOAT, "Name" TEXT, "Nat." TEXT, "Ship Type" TEXT, "Principal victims" TEXT, "Date" TEXT)

what is the name of the battleship with the battle listed on May 13, 1915?
SELECT "Name" FROM table_79838 WHERE "Ship Type" = 'battleship' AND "Date" = 'may 13, 1915'
SELECT "Name" FROM "table_79838" WHERE "Date" = 'may 13, 1915' AND "Ship Type" = 'battleship'