AFAIK the sloop of the time was schooner-rigged (or rather gaff-rigged or Bermuda-rigged) rather than ship-rigged, with anywhere from one to three masts, where as you say a brig was two-masted and ship-rigged. However it is fair to say that the term "sloop" was used sloppily (sorry) to describe the purpose of the vessel as much as the description of the vessel. Also, 18 guns would be a *very* heavily-armed sloop: 4 to 12 guns being more common. An example of a sloop is the famous (at the time) HMS Pickle of 6 guns.
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