High Quality Content by WIKIPEDIA articles! HMS Boreas was a 28-gun Coventry-class sixth-rate frigate of the Royal Navy. Built in 1757, she was one of five frigates of the class built of fir rather than oak. Fir was cheaper and more abundant than oak and permitted noticeably faster construction, but at a cost of a reduced lifespan; the four fir-built Coventry-class vessels that did not get captured lasted an average of only nine years before being struck off.