A Fire Upon The Deep
System Defense Boat
A non-starship specifically intended for defensive operations inside a star system. Developed on the principle that a non-starship (because of the increased armament made possible by its lack of jump drives) can normally defeat a starship of equal tonnage. System Defence Boats are typically stationed at the vital points of a system (the gas giants, the asteroid belt, the major world, and so on), and they attack invading vessels according to one or more predetermined plans.
System defence boats range in size from displacements of 100 to 5000 and are constructed at all tech levels. There are hundreds of different types, tailored to their specific mission.