Hi, you may be interested in a few earlier threads on this subject
There are huge gaps between islands in the Pacific, what methods did the Polynesians/Islanders use to discover them and to stay on course in a trip between two islands they already knew existed? Do we know how many/what percentage were lost at sea? - /u/thefairyguineapig lays down 9 pages of explanation and sources. Further down are links to a few previous posts
How could Polynesians navigate so well without compasses or sextants and how could they build ships capable of traveling deep ocean? a recent thread featuring /u/b1uepenguin