Verse 17 notes that is the building of the ship which starts the murmuring. It was not the making of the tools, which again is at least suggestive that the process of smelting the ore and making tools was not unknown to them. Ship building was, however, and they call their brother a fool for believing that he can do it. Laman and Lemuel are not shipbuilders either, but they know enough about ships to understand that the requirements of a vessel to cross the oceans are high. For this reason (and their lack of faith) they deride Nephi's efforts.