Apologies for not going carefully through the while thread, where this might have been discussed, but I always wondered about the "brigantine sloop" as well, and when I did sing it, substituted "brig and a sloop." However, I have since discovered that "sloop" in RN terms was not descriptive of rigging, but of the vessel's naval function (as a support vessel, I believe, though I am not certain of the exact definition). So I went back to singing it the other way.