In his reconstruction of the campaign of Marathon, Prof. N. G. L. Hammond postulates that the Persian fleet accomplished its hurried voyage from Marathon to Phaleron after the battle in a time of 9 hours, and in theory could perhaps have done it in 8 (JHS 1968, p. 43). This very fast time (9 hours for 58 sea miles = 6½ knots; 8 hours = 7), necessary if the fleet is to arrive in Phaleron in time to confront the Athenians on the same day as the battle (sic Plut. Aristeides, v, 5; but cf. Mor. 350 E), is justified by two arguments: (1) the wind blowing at the time was a north-easter, providing ‘the fastest conditions for sailing’; and (2), the Phoenician galleys in the Persian fleet were faster than Greeks, making figures based on Greek performance irrelevant.
(1) A strong north-easter is indeed very probable. During the summer and until mid-September (i.e., there is a strong probability that Marathon is covered, whichever date one prefers for it) the etesian winds (nowadays known as the meltemi) are blowing in the Aegean. These winds are of great strength and regularity, blowing only by daytime, and more or less from the North (Dem. iv 31; viii 14; Arist. Meteo, ii 361–2; A. R. Burn, Persia and the Greeks, p. 388). But the conditions they offer are not favourable for fast sailing from Marathon to Phaleron. Off the east coast of Attica a very choppy sea builds up. The seas come rolling down from the North, and in the funnel-shaped Thorikos Channel, between Makronissi and the mainland, build up to some really heavy weather between Lavrion and Sounion, particularly in the afternoon. This would delay the war galleys. Little is known about Phoenician war vessels, but they appear to have been triremes of some sort—light craft that can make good speed only in calm water. Far from a ‘following sea’ being favourable, a trireme would not give of its best in a sea of any kind, coming from any direction.