Symbolism, Imagery, Allegory
Have you noticed yet how there are a lot of mysteriously-guided rudderless boats in the "Tale of the Sankgreall"? Galahad, Percyvale, and Bors all board some strange ship that carries, them, captain-less, to yet another mysterious ship where they find an ancient sword, then to a castle whose occupants they defeat. Later, Galahad and Launcelot spend quality father-son bonding time on that same rudderless boat. It's all a wee bit creepy, huh? Where are these boats headed, and why are all these knights so willing to jump aboard?
Wondering what that's all about? Well, it turns out that in Christian-influenced medieval literature, the rudderless boat represents the good Christian's sacrifice of his will to God. You see, by boarding the rudderless boat, the Christian signals that he's putting himself completely in God's hands. In that sense, these boats aren't rudderless at all – they're steered by God.
But still, that knight has no idea where he's going, so it's definitely a leap of faith. And did you notice that the knights' adventures in the rudderless boat don't happen until near the end of the Grail Quest? Could that mean that it's only at this point in their journey, after they've been tested and have passed with flying colors, that they're ready to fully yield themselves to God?