Complex sentence: No-one knows when the first single-handed voyage was made; it is possible that early Polynesian sailors, who were proficient navigators, may have been first to make a significant single-handed offshore voyage.
Split into simpler sentences: No-one knows when the first single-handed voyage was made. It is possible that early Polynesian sailors, who were proficient navigators, may have been first to make a significant single-handed offshore voyage.