Complex sentence: The course is unusual, as it begins off Cowes, travels to the Fastnet Rock off the southwest coast of Ireland, which they round, and then back to Plymouth via the south side of the Scilly Isles.
Split into simpler sentences: The course is unusual, as it begins off Cowes, and finishes in Plymouth. It goes from Cowes to the Fastnet Rock, which they round, and then back to Plymouth via the south side of the Scilly Isles.