Il Principe
Prince
I also agree this should be implemented, with possibly an attrition system set in place for going too far away from your borders. It would make things like circumnavigation a la Magellan possible, if rather harrowing.
I just realized: The movement limit should make a huge jump ( maybe by an auto promotion) for ships after "Refrigeration" which definetly allowed for longer journeys without worryieng about spoiled foods.
Some ways to extend your reach could be either techs or buildings: A cannery or packed meat plant would ease up logistics and extend the reach of a city.
Or new ones, maybe wonders:
Sauerkraut
Gives a range extending promotion to all ships. Obsoletes with modern healthcare.
Pedia: Until the British Captain James cook packed Sauerkraut on his ships for long expeditions as a cheap and easy to sotre alternative to fresh fruits, scorbut took heavy toll among naval crews. Nevertheless about a century passed until his method was adopted for the Royal Navy in general.
Existing techs ( Logistics comes to mind, or for earlier times Catograophy ) could give a nation wide bonus.