Its true that you get more gold for the longer the trip, however, if you are fighting for control of the seas against a good opponent, make yours and your allies dock near each other. You can only have to protect a smaller area, and by keeping the same amount of ships that you would if you were protecting a trade line a cross the map your ships will have more depth and not be stretched out. You wont get as much gold per boat but what good does the gold do if the boat is sunk before it returns with your goods?
The past is useless which is why we move on to the future.