Complex sentence: Krusenstern proposed to appoint second class captain Vasily Golovnin as expedition leader, however, Golovnin was on his own round-the-world voyage at that time.
Split into simpler sentences: Krusenstern proposed to appoint second class captain Vasily Golovnin as expedition leader. But Golovnin was making his own round-the-world voyage at that time.