I discovered what a barco rabelo was while looking through an old book on wine. It seems that the Portuguese used these flat bottom boats to transport their huge barrels of port wine down the Duoro river. "Rabelo" means "little tail" referring to the long rudder at the back of the boat used for steering. Port in it's many forms is a fortified wine. During times of conflict with France, England turned to Portugal to supply it's wine needs. The wine from Portugal was fragile and did not hold up well during shipment. They somehow figured out that fortifying it with brandy solved the problem.
These boats aren't in use anymore but they sure are cool.