From a post to Baen's Bar BuShips dated December 14, 2002:

Impeller rooms

    Impeller rooms are cylindrical volumes located within the hull and centered fore-and-aft on the impeller ring they serve. The diameters of civilian impeller rooms are approximately 60% of the diameter of the impeller ring and approximately 1.2 times as long as they are across. They normally consist of a single, very large compartment crammed with the required generators and node support hardware. Military impeller rooms are approximately 85% as wide as the diameter of the impeller ring and approximately 1.6 times as long as they are broad. They are also subdivided — normally into quarters in smaller vessels (through CL), eighths (CA-BC), or twelfths (DN-SD) — with the individual compartments heavily bulkheaded and armored to localize and contain damage. Since the outbreak of the war, the RMN and GSN have begun dividing impeller rooms in ships of the wall into 16ths — one for each beta node. These individual compartments are arranged in clusters around the long axis of the ship with each forming a smaller cylinder, bundled together with their fellows within the volume of the overall "impeller room." Thus a modern RMN SD would refer to "Impeller One" or "Impeller Two" to indicate (respectively) the forward or aft impeller rooms, and then to "Impeller Eleven," "Impeller Twelve," or "Impeller Thirteen" to indicate the subcompartments within Impeller One. (That is, "Impeller 1.1" would be referred to as "Impeller Eleven," "Impeller 1.2" would be referred to as "Impeller Twelve," etc.)