But it was very important for some language pedants at that time, and for a couple of centuries after, to show the origins of words in their spelling. So this one was preserved as boatswain.

English spelling is notoriously irregular ... Not only that, but we have complete outliers such as the words colonel and boatswain - pronounced [kɝnl̩] and [boʊsn̩], respectively. However, even though ...