Eftersom de tog mycket med dem, och det hade varit obehagligt för Saphira att bära allt under flygningen.
Detta har redan nämnts i de andra svaren, men jag tyckte att det var värt att lägga till ett kanonkit från själva boken (betona min):
The vessel looked as Eragon knew it would, for he recognized it from his dreams, and a sense of inexorable fate settled upon him as he gazed at it.
This was always meant to be, he thought.
They spent the night in Silthrim, which was much like Ellesmera, although smaller and more densely built. While they rested, the elves loaded the Eldunari onto the ship, along with food, tools, cloth, and other useful supplies. The ship's crew was composed of twenty elves who wished to help with the raising of the dragons and the training of future Riders, as well as Blodhgarm and all of his remaining spellcasters, save Laufin and Uthinare, who at that point took thir leave.
-- Inheritance, p.842
Det är alldeles för mycket för fattiga Saphira att bära allt på egen hand, även i flera resor. Och det finns inget sätt att veta hur länge resorna skulle vara i alla fall.