Why cubeships? Why not? Efficiency is the primary reason. No extra bits and pieces. It takes more work to build something fancy with superfluous parts sticking out from the hull than it takes to make a cube. Additionally, the cube reflects the Oneness of the Collective as a whole: no front/back, no up/down; axis is irrelevant. And for attackers, there is no cue as to where to fire, no obvious weakness such as weapon ports, power sources, bridge, or engines.
What follows are some brief notes on the types of ship (mostly cubes) encountered in the Star Traks: BorgSpace concept of the universe.
Size: 1.3 kilometers per side
Compliment: 4,000
Extra Alcoves: 250
Max Assimilation: 2,500
*Max Compliment: 6,500
Relatively lightly armed and armored when compared with other classes, this cube can achieve higher conduit speeds and enjoys greater maneuverability than its larger brethren. Power output of reactors to size ratio is top rated, as to be expected in a need push maximum transwarp velocities to the envelope. Basic sensor range as to detecting sentient-made objects or moving ships is greater than other cube classes, although it must still approach within normal operating distances to discern specific variables. The task of Exploratory-class cubes is twofold: (1) to find and make initial assimilation samples of new species and (2) to perform routine reconnaissance of known species and provide advance information of movements of potential/active enemy fleets.
Heavily armed and armoured with the latest in assimilated technologies, Battle-class cubes are trouble looking for a place to happen. While conduit speeds and maneuverability can not match Exploratory-class, agility is still superior to both Assimilation-class and Cargo-class. Large numbers of drones are carried to perform both space (ship/station/asteroid) and planetary mass assaults; numbers of drones lost in such endeavors is expected to be matched or exceeded by those newly "recruited". The task of the Battle-class cube is to (1) defend BorgSpace from invaders, (2) destroy resistance of recalcitrant species, and (3) begin the program of assimilation of colonies and populations.
Although Assimilation-class ships do not have the offensive nor defensive abilities of a Battle-class, they still outstrip the abilities of an Exploratory-Class, which in itself is generally more than most species can handle at odds less than 40:1. Assimilation-class cubes are among the slowest of classes, but then again, their need is generally not urgent, coming into play only after the subduing of the target. Each cube is capable of assimilating and processing 120,000 drones, thanks primarily to power and regeneration systems specially designed to cope with the demand such densities cause. The task of the Assimilation-class cube is to (1) codify the hold on the target and (2) assimilate, process, and transport new drones.
Cargo-class cubes are the slowest and least armed of all Borg ships, but they are also the largest, used to haul immense quantities of material; in a pinch, they can be retrofitted with extra reactor cores and alcoves in order to transport upwards of 500,000 drones in stasis. The active drone compliment is very few in relation to the cube size, and is generally kept in a regenerative stasis, acting as both maintenance as required, and as a back-up system in case the automatic computers of the ship become dysfunctional. In many respects it would seem the Cargo-class cube would be a tempting target for would-be raiders, should any be so stupid, but reality is realized when it is known the the cube has the strongest physical and electromagnetic-based shielding of the Borg fleets. And, there is always the possibility raiders, could they isolate a cube and crack it open, would not find immense salvage riches, but half a million waking drones. The task of the Cargo-class is to (1) convey materials and (2) act as an impromptu mass drone transport.
Structure is nearly half the volume of an Exploratory-class cube. Because the platform is without its own propulsion, except that necessary for station-keeping and similar localized maneuvers, it must be transported to the designated installation site via a Cargo-class cube. Multiple on-board sensor packages, with range extendable using (non-biological) drones. Mission is to observe and record local ambient space and subspace environment. Completely automated, with highly redundant back-up systems and large warehouse space for replacement parts: the Greater Consciousness has calculated efficiency indices, determining it more effective for the Whole to tolerate complete automation than to dedicate units to on-site upkeep.
Following the reemergence of Borg from Hive, vessels developed during the Hive era were retained and continued to be manufactured.
Development of the sphere as a viable vessel platform began prior to the Dark War, but was not fully realized until the conflict commenced. Assault-class spheres build upon the concept of the Battle-class cube, but increases firepower by an order of magnitude. During the Dark War, the most destructive class of weapons - Nova Bomb - was carried by the Assault-class. Power output and maneuverability is increased over the Battle-class despite the gain in vessel volume by use of duel-core design. The Assault-class follows the same task protocols as the Battle-class cube.
Essentially (but not quite) an overgrown Cargo-class cube, the Collective has pushed the limit between "vessel" and "by the Director, that is a ship??" with the Lugger-class. Begin with a cube eight kilometers per edge. Although this vessel class externally resembles other Borg cubes, the internal architecture is best described as a "cube within a cube." The main cargo holds are bound by airtight equatorial bands along X, Y, and Z axes, creating eight vast, open-volume areas; and while it is possible to pressurize each compartment, due to energy requirements such is done only when necessary. Modular struts and movable decking allows creation of many configurations within Bulk Cargo Holds for most efficient cargo storage, with exterior doors large enough to admit an Exploratory-class cube (or asteroid) for transport. Subsection 14, itself nearly the size of a Battle-class cube, includes eight Interior Cargo Holds - fully customizable as to atmosphere, gravity, temperature, and humidity - to transport delicate cargos or those with special needs. Four auxiliary cores are present (not the standard ten), two serving as backups to the primary core and two dedicated to cargo maintenance. Alcove tiers are clustered near Primary Core/Central Engineering, not dispersed throughout cube volume. Defense is the most important aspect of the Lugger-class survival strategy, with the main components a 200 meter thick hull (fifty meters of which is duralloy and ablative armor) and extraordinarily strong shields. The lack of offensive capability is most apparent via an examination of torpedo capability: eight launchers per side (compared to normal sixteen) and magazines which include only moderate yield quantum torpedoes. Similar to the Cargo-class, the Lugger-class can attain only moderate FTL velocities, whatever the warp flavor, and is not very maneuverable. Warp nacelle configuration is modified from other classes: instead of nacelles along the full edge length, 500 meter long nacelle segments radiate outward from each corner (for a total of 24 segments). Retrofitting of the Lugger-class with extra cores and alcoves can achieve a stasis drone carrying capacity of 2 million units.
(*Only achieved after a mass assimilation event)