Simultaneous with the battle at Thermopylae, an Allied naval force of 271 triremes defended the Straits of Artemisium against the Persians.[116] Directly before Artemisium, the Persian fleet had been caught in a gale off the coast of Magnesia, losing many ships, but could still probably muster over 800 ships at the start of the battle.[117] On the first day (also the first of the Battle of Thermopylae), the Persians detached 200 seaworthy ships, which were sent to sail around the eastern coast of Euboea. These ships were to round Euboea and block the line of retreat for the Allied fleet.[117] Meanwhile, the Allies and the remaining Persians engaged in the late afternoon, the Allies having the better of the engagement and capturing 30 vessels.[117] That evening, another storm occurred, wrecking the majority of the Persian detachment which had been sent around Euboea.[118]