Back to the point. With out knowing were to go it is impossible to navigate to a certain place using your way. You need luck to find that place. Because 'obiously' you don't know were you are going...
You are assuming that these people were intentionally sailing to undiscovered islands, and they went directly there. Both of these assumptions are unwarranted. They could have been sailing for other reasons, and they may have taken a circuitous route the first time they went there, gradually developing a more and more direct route.
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