abu hasan crossed the gibraltar straits on 14 august 1340, and all through the summer troops and supplies were ferried across to the peninsula. on 22 september the siege of tarifa was formally established, with the help of yusuf i. however the sultan made a serious mistake: believing it would take many months for the castilians to rebuild a fleet, and in the hope of cutting down the enormous cost of maintaining his own fleet, abu hasan prematurely laid up most of his galleys and returned those of his allies, leaving only 12 at algeciras.