Conquered by Afonso de Albuquerque in 1510, Goa stood on an island surrounded by crocodile infested rivers that could, however, be waded across in some areas during the dry season. The closest and most important fording point to the city of Goa was the Passo Seco , defended by the fortress of Benastarim. As no more than 650 soldiers were left to defend Goa, every able-bodied man from among the casados  and 1500 Christian lascarins were mobilized. 1000 slaves were armed and divided into four squadrons; even  further, 300 clergymen and 200 retired soldiers volunteered to participate in the defence of the city. Dom Luís de Ataíde decided to distribute his forces in 19 critical points along the eastern river banks, where artillery batteries were established, garrisoned with 20 to 80 men and a contingent of lascarins, to keep the colossal army of Bijapur from crossing. Every battery was to have visual contact with the next and their garrisons were not to leave their posts unless ordered to. The deeper waters of the Mandovi and Zuari rivers, to the north and south respectively, were patrolled by four galleys, a half-galley and twenty small galleys called foists. On the opposite shore northwest of Goa, the Portuguese fortress of Reis Magos was supported by an anchored galleon. As the Viceroy had information that the Turks might join the "league", he armed a further 125 crafts of many different sizes to secure the control of the waters around Goa, although by then there weren't enough men to crew all the ships and defend the city simultaneously.
Question: How many more galleys than foists were patrolling the area?
Answer: 16