Why are there small ships on the trade routes?
In the early days of EU4 development, they were created to make the ingame trade-system more complex, but it turned out that they made it too complex to handle and made the game very laggy. So Paradox had to trash the system, but kept the ships.
What is the best type of ship for Exploration?
Exploring with a fast ship in EU4 is typically the best idea, as they will have the least impact with naval attrition as they will clearly arrive fastest. Use 3 ships for exploring, as 1 ship will likely sink fast, when getting further away from land
How long is the period of a Curia Controller period?
At anygiven time no more than 49 cardinals can serve, with a new one appointed every year at the beginning of a year, if there are openings. The death of the Pope gives way to the election of another Pope in The Papal State, who assigns control of the Curia to a particular country until the next Papal election.