Battle of Cape St . Vincent ( 1337 ) The Battle of Cape St Vincent of 1337 took place on 21 July 1337 between a Castilian fleet commanded by Alfonso Jofre Tenorio and a Portuguese fleet led by the Luso - Genoese admiral Emanuele Pessagno ( Manuel Pessanha ) . The fledgling Portuguese fleet was defeated , bringing a quick end to the brief Luso - Castilian war that begun in 1336 . Background . In 1328 , the Alfonso XI of Castile married his cousin , Maria of Portugal , a daughter of his maternal uncle King Afonso IV of Portugal . But the marriage soon turned to scandal , when , shortly after , Alfonso XI dispatched Mary to a convent and took up openly with his mistress , Leonor Núnez de Guzmán . Hoping to avail himself of disaffection among the Castilian nobility , Afonso IV of Portugal declared war on Afonso XI Castile in 1336 , citing the mistreatment of his daughter . The Genoese captain Emanuele Pessagno ( Manuel Pessanha ) had been hired by King Denis of Portugal in 1314 to develop a permanent navy . Pessanha , who was made the first admiral of Portugal in 1317 , was eager to try out his new fleet . Soon after the declaration of war , the fledgling Portuguese fleet of 20 galleys , commanded by Pessanha , was dispatched to the coasts of Andalusia , intending to disembark a Portuguese amphibian force to seize Seville . However , the fleet was fended off and the landing prevented . Pessanha ' s fleet spent much of the remainder of 1336 off the coasts of northern Castile , supporting Afonso IV ' s invasion of Galicia . The Castilians responded with counter - invasions in the regions of Minho ( led by Fernando de Castro ) and the Alentejo ( led by Alfonso XI himself ) . In the Spring of 1337 , Pessanha set out from Lisbon with a fleet of 30 galleys , to once again attempt a landing on the Andalusian coast . But simultaneously , a Castilian fleet of 40 galleys sailed out of Seville , under the Galician captain Alfonso Jofre Tenório ( Señor de Mogue ) , probably intending a similar landing on the Portuguese coast . Violent tempests prevented an encounter , and both fleets , damaged by the storms , were forced to return to port for repairs . Both fleets set out once again in early July , Pessanha ' s Portuguese fleet now reduced to 20 galleys and Tenorio ' s Castilian fleet reduced to 30 . The Battle . The Portuguese and Castilian fleets finally met each other near Cape St . Vincent ( the southwestern tip of Portugal ) . With the wind in their favor , the Portuguese gained the initial advantage and managed to seize as many as 9 Castilian galleys early in the encounter . But the winds soon changed and the luck turned . Tenorio ' s Castilian fleet fell on the Portuguese and soon overwhelmed them with their larger numbers . Jofre Tenorio managed to capture the captain ' s galley and took Pessanha prisoner . With their admiral gone , the Portuguese fleet broke up and took flight , with the Castilian galleys in pursuit nearly all the way back to Lisbon . In all , the Portuguese lost 14 galleys - 8 captured , 6 sunk . Aftermath . Alfonso Jofre Tenorio did not attempt a landing , but hauled his prizes back to Seville . The victorious admiral was greeted there by a delighted Alfonso XI . The Portuguese prisoners , including the admiral Pessanha , were paraded in triumph through the streets of Seville . The defeat of the Portuguese fleet brought the Luso - Castilian war to a sudden halt . Afonso IV of Portugal reluctantly agreed to a truce mediated by the Bernard de Alby , Bishop of Rhodes ( legate of Pope Benedict XII ) and the Bishop of Rheims ( envoy of Philip VI of France ) . The final peace and reconciliation between Afonso IV and his son - in - law Alfonso XI was effected only two years later , with the Treaty of Seville in July 1339 , mediated by Maria of Portugal . Among the treaty ' s provisions was a commitment by Afonso IV of Portugal to assist Alfonso XI of Castile against an impending Marinid invasion force , then assembling in Morocco . This would culminate in the Battle of Salado in October 1340 .