Don’t complain, don’t be lazy, just get busy and obey. Whatever God commands is right and possible. God knows all things, and he is the Master Teacher; of course he can instruct Nephi how to build a ship. There is a precedent: Noah already did it, and his ship must have been bigger.
“If God Had Commanded Me to Do All Things I Could Do Them ”
Righteousness and spirituality generate a palpable, physical power. Light and truth are an actual force in the universe (D&C 84:45–46; 88:6–13). Priesthood power is demonstrable. The brothers didn’t dare touch Nephi, and they couldn’t succeed in being argumentative. There must have been a few days of peace.