In Life of Pi, in Part Two, the author / narrator's voice disappears; all of the chapters are told in Pi's voice, recollecting the shipwreck and what followed it. In terms of the structure of the novel, why does Martel do this?

The novel's structure consists of three parts organised around a mise en abîme, which means that there is a story within the story.

The first and third parts, which frame the main story, are reflections about spirituality and the relativity of truth rather than a narration of events. They enable the narrator to address his readers more directly. However, during the second and biggest part, the narrator relinquishes his hindsight and lets the character speak as the story unfolds. This is a way to make the events more impactful, more vivid, because it gives the impression that they are being told in the moment by the very person who lived them.