km. Once out in the broad Atlantic, the ships were to steer to a point 645 km beyond the Shannon estuary on the west coast of Ireland, giving themselves a clear run to northern Spain. The course taken The Armada's sailing orders were almost impossible to follow. The weather was difficult. Many of the ships and their crew members were in great distress. The navigators' charts were primitive, and their best training and experience in the techniques of dead reckoning and latitude sailing fell far short of what was needed to bring the fleet safely home. The Armada failed to keep its course around the north of Shetland at 61'N. Instead, on 20 August, it passed safely to the south, between Orkney and Fair Isle, and was carried into the Atlantic at about 59'N. From there it was due to sail from North Uist in the Hebrides Islands until it caught sight of the distant islet of Rockall, but failed again. Southerly winds blew from 21 August to 3 September, stirred up by an anticyclone over Scandinavia, which prevented the fleet from running west-south-west as ordered. One report reflects the frustration of the navigators: "We sailed without knowing whither through constant fogs, storms and squalls". The sailing orders were rendered useless by the weather, but the miscalculation of the Armada's position contributed greatly to its destruction. The navigators were unaware of the effect of the eastward flowing Gulf Stream, which must have hindered the fleet's progress – perhaps by as much as 30 km a day. The paymaster of the San Juan Bautista, Marcos de Aramburu, recorded a log of his progress from late August onwards, when the rest of the fleet was within sight. The inference from his observations is that his ship's estimated position as it turned for home was entirely wrong, some 480 km to the west: its real position lay in the east, perilously close to the coasts of Scotland and Ireland. This single deficiency "made the difference between safety and disaster". After seven weeks at sea the opportunity to make landfall and take on supplies and effect repairs must have been welcome, but navigation in these waters demanded intimate knowledge. The experience of Spanish mariners in the intricacies of north Atlantic conditions was largely confined to trading voyages to the south and south-west of Ireland, and it is likely that the fleet's pilots preferred to