Sailor is the next rank, it gives you the ability to control 3 captains and 2 permits. It is the rank where your reputation in all nations will eventually turn to neutral. You can now buy letters of Marque to capture/loot enemy ships as well. If you plant to take this route, the best way to make money is to capture all of the ships and leave them moored in the Governor's town. You don't get rank from selling them and they can be useful if your main ships are in for repair. See this article for more info about battles. Also you may want to sell any buildings you buy if you plan to attack the nation the buildings are in. As they probably will be confiscated. The best plan is probably to do quests as they give a lot of money or ships as a reward, a letter of marque against an enemy is also a good idea to get some ships, you can also loot some towns for a lot of goods.