Sailing vessels formed the vast majority of the ships in Spanish military service in medieval and early modern times. Nonetheless, Spain also used galley fleets in the Mediterranean in the late Middle Ages, and later expanded their use wherever Spain had a presence in the world, including the coastal waters of the Americas and South-East Asia. Unlike the Republic of Venice in its heyday, Spain used galleys exclusively for military purposes. In the Mediterranean, they formed a crucial element of Spain's defensive and offensive strategy during the confrontation with the Ottoman Empire and its satellite states in North Africa in the sixteenth century.
Because Spanish galleys performed well at the Battle of Lepanto in 1571, King Philip II sought to increase their numbers. However, the government had difficulty finding enough men to pull the oars and enough captains and other wage-earning officers with the necessary skills to organise and command multinational crews. There were a number of reasons for the difficulty, in part because mariners and naval officers had more attractive choices than the galleys. Spain's transatlantic fleets were approaching their peak in the late sixteenth century, with some two hundred vessels involved in the trade each year. Moreover, the military needs of the crown increased in the same period, spurred by English and French incursions into Spanish America in the late 1570s and Philip II's contested claim to the Portuguese throne after 1578. The naval build-up preceding the armada sent against England in 1588 also increased the demand for mariners at all ranks. The recruitment of captains for Spain's Mediterranean galleys required particular care. By the late sixteenth century, most of the galley oarsmen were slaves, prisoners of war or convicted criminals. Keeping order on board, as well as encouraging the best performance from all hands, required more than the usual skills of a competent naval officer. For noblemen, serving the king as a galley captain challenged not only their abilities as leaders, but also their sense of honour.
Considerable documentation regarding galley service by Spanish noblemen exists in the Archivo General de Simancas (AGS), the Archivo del Museo Naval (AMN) and the Biblioteca Nacional de España (BNE), both in Madrid, and in other depositories.