> Do you know if and when British seaman wore bellbottoms?

I don't have specific dates immediately at hand, and it might take awhile to track down that detail. However, a very quick glance at what books I could place my hands on state that an official uniform for sailors was not adopted until 1857. (Thus, your 1817 date probably does refer to the American navy.) Regulations as to the decoration used by officers, and some petty officers, came much earlier. Prior to that time it was pretty much up to the Captain how they would be outfitted by the Purser. However, it was becoming much more standardized. The familiar short blue jacket was becoming common, as was the white duck trousers which belled out below the knee. The latter appears to have been a development from baggy breeches worn by sailors in an earlier period.

I remain, etc. Jason E.