In the Rider (and other decks, like the Robin Wood) there is usually a ship that looks like a Spanish Galleon, at least one, sometimes two, floating in the background of the Two of Coins. These floating ships usually signify thoughts or events being 'Up in the Air,' or a connotation of being 'ungrounded.'
In the Fool card, being 'up in the air' hints at indecision, inexperience, and probably an innocent sense of ones self-importance. Why be grounded when you can be as light as air? A dirigible being a modern replacement for a floating Spanish Galleon (not too many of those around anymore!) is actually a good choice.