What was the most effective way for a medieval country to increase naval capacity?
Short answerExpand the merchant fleet, because most medieval naval forces were mobilized by converting commercial vessels for war.
NotesExpanding merchant shipping gave most medieval countries more hulls to convert for war. Cogs could receive fighting castles at the bow and stern. Venice, other Italian maritime states, and the Ottomans provide examples of powers with purpose-built warships. China also had purpose-built vessels in the period, although converted merchant ships remained common in the encounters described.