Sid Meier's Pirates!
Get the Ship of The Line (Frigate class):
Relentlessly attack shipping of the English, French, or Dutch. Keep watch for a "New Warship" Ship of The Line sailing about.
Get the Brig of War (Brig Class):
Found as Military Payroll carriers, or Troop Transports, attack as normal, but beware of large amounts of enemy crew.
Get the Mail Runner (Pinnace class):
Offer to escort a war declaration or peace offering from a colony. Attack the courier once out of port.
Get the Royal Sloop Ship (Sloop class):
Found as a Pirate Hunter out of Spanish ports. Attack shipping around a Spanish port and one may come out to attack you.
Thanks to Revolution reader cipherz!