1: Spacer
A Spacer hull is designed for use only in hard vacuum. It may be any shape, and does not require streamlining. Once mechanical necessity is dealt with, design becomes an art form; folk with money are drawn to the latest styles, even if their baroque design provides no practical enhancement. Folk without money can cobble together whatever scraps they find; SEE Tramps (below.)
Pure functionality is a sign of a low-tech boor.
2: Skimmer
Skimmers are streamlined for flight in the upper reaches of gas giants where they take on unrefined hydrogen, which must then be filtered and refined for use as fuel.
Hull price increases for Skimmer streamlining are minimal, as there is little reinforcement needed for upper atmosphere skimming. For deeper diving, the streamlining plus reinforcements to withstand increased atmospheric pressure and friction burning actually create a Splasher (see below) while for the deepest, you're actually building a Submersible.
3: Lander
Streamlined and capable of landing on a hard surface, whether a full-facility groundport or an emergency barren area of (hopefully stable) rock, a Lander may be an independent ship or it may be a shuttle or "ship's boat" for a larger vessel.
The cost of streamlining and reinforcement is minimal for smaller ships, but increases as dramatically as hull tonnage increases, due to the need to absorb greater shock and mass when landing, and withstanding many G's of force during liftoff; few large craft are Landers. A "floating city" would require gravity neutralizing technology to land, at which technology level it might use a force-field rather than physical streamling.
4: Splasher
Streamlined and re-enforced to be capable of landing by splashing down in open seas, Splashers are less expensive to build than Landers, especially at larger sizes; they let the water do the shock-absorbing and cradle their mass.
Splashers are generally fitted with "boating engines" to allow it to move to a waterside dock. If not, they will rely on boats, either surface traveling "water boats" or flying boats (which may actually be regular trans-atmospheric shuttles.). It will be weighted, and have a (possibly retractable) keel to keep it upright and stable. The hull reinforcements also allow "Splashers" to dive deeper into gas giants when skimming for raw hydrogen, if equipped with skimmers. Many planetside spaceports are located along waterfronts so as to accommodate both Landers and Splashers.
5: Submersible
The most streamlines of vessels are Submersibles. Depending on ones point of view, there are either submarines capable of spaceflight or spaceships capable of flying underwater. With their pressure hulls, submersibles can also dive quite deeply into gas giants.
Slow Boats Slow-Boats, traveling at sub-light speed, are the most basic of propulsion systems.
Given a network of Gates and Links, Slow-Boats may be quite sufficient for local travel needs. Beyond the Gates or Links, neighbors in the same mundane universe may be reached by slow-boats traveling at sub-light speeds. Depending on what propulsion systems a particular universe's physics permit, and the physical distance to the next star system, such interstellar travel within a mundane universe may take only a few months or it may take centuries.
Millennia of long-range Seeding and Colonization programs have lead to the majority of worlds Empire has brought into the Harmony being are months, decades, or even centuries apart. These latter are generally classed as "unincorporated territories."
"Slow-boat universes" are those in which other means of travel are rarely possible. Theory requires that there be a large number of universes where other means of travel are, in fact, utterly impossible. It is, of course, impossible to empirically verify this, as, by definition, we cannot visit any such universe.
Slow-Boat propulsion systems
Solar Sails
Solar power or the solar winds may be gathered by great gossamer light-sails. This is an elegant method of travel within a dense star cluster, and to Gates or Links nearby. The depths of space, however, are too dark and still for solar sails to gather energy. They must coast across such "deserts."
Ramjets
Ramjets use the loose hydrogen of space. This requires a huge scoop (rarely physical; more often an electromagnetic field is deployed). This doesn't operate well in "thin space;" maps will show "clouded space" where ramjets will be efficient. Such areas of interstellar hydrogen can be graded by richness. This is only for normal space travel; pseudo-FTL Drives do not interact with normal space, which is a good thing, as otherwise the thinnest gas would rip through the ship like a sandstorm through soap bubbles.
Fueling:Hydrogen and Radioactives
Most ships are fueled by hydrogen in fission nuclear reactors. At tech level H or I, the best reactors can digest basically anything, but the simpler - and purer - the fuel the smoother the operation. Most reactors use pure hydrogen, but some use filtered H20 (or can use either.). Often a catalyst (such as dilithium) is used and needs to be restocked (it's not a "perfect" catalyst and degrades with use.)
Lower tech levels require fissionable radioactive materials.
Ships designed to travel within civilized areas expect access to refined fuel. Skimmers and other streamlined ships may have onboard gas hydrogen refining facilities and can scoop up unrefined fuel from gas giant planets. Their refinery will be rated (a %), and the giant will have a % (indicating purity.) These work against each other; a 10% refinery processing a 10 % atmosphere can recharge only 1% of its tanks per day, while a 50 % refinery processing a 50% atmosphere can recharge 25% of its hydrogen tanks per day.
H2O is also a good source of hydrogen, but requires filtration and splitting, an energy-intensive operation. This does yield oxygen as a bonus. Splitting will, of course, need some other power source, so this is likely to be a port facility, not one carried on board.
Cometary ice is the preferred source of H2O, as planetary water has to be lifted from the planet.
Refineries are defined by % (efficiency) and take 10 tonnes of hullspace. A ship can have multiple refineries. Efficiency is determined by tech level; take tech level, subtract 10, then multiply by ten. Thus, tech 10 yeilds a 10% refinery, while tech 15 can build 50% refineries.
In-System Craft are sub-light speed ships, or sub-stellar ships and boats. They may use any hull type.
Slow-Boats are interstellar craft that stay in normal space. These are commonly used where stars are too close together for trans-dimensional Drives to operate. Happily, these shorter distances (often well under a single light year) mean the craft may not have to be generation ships or have a completely frozen crew. Solar sails or ramjets may be more practical in such dense areas than in wide-open space.
Drives allow a ship to translate from the mundane universe into a contiguous para-dimension and then back. The actual traveling is done in the other, "faster" dimension. Because the result allows a ship to travel from one point to another faster than light would (by basically skipping around the distance between), these are sometimes refered to as "pseudo-FTL Drives."
Developing a psuedo-FTL Drive for interstellar travel is the defining feature of a "cosmic" or advanced race.
Even psuedo-FTL ships generally have an in-system slow-boat engine; only well-established routs have "Lollers," which save on tonnage by having only minimal emergency/maneuvering jets, and instead rely on a dedicated tugboat service.
Psuedo-FTL Drives
There is quite a choice of psuedo-FTL drives; Spin-Drive, Jump-Drive, Hyper-Drive, Worm-Drive, each with variations. Many choices! And yes, they are different. A ship may have multiple drive types, although it requires a higher tech level or more than one system may kick in, with disastrous results.
This multiplicity of technology is a result of the fact that every mundane universe and exotic dimension has its own physics, and every inter-dimensional interaction is unique. This means that no Drive will work in every universe.
In common, most Drives do not work near normal matter. Thus, they must enter normal space before reaching a star. Generally, any line of travel will be "captured" by any star it passes near. One result is the "depositing," or dropping back into normal space, on the planet's ecliptic plane outside of its Oort cloud (comets and other debris, or where they would be if absent.)
Another effect of gravity sensitivity that, unless the ship can maneuver and slide around, balancing one star system's influence against another, ships generally travel "line-of-sight" in segmented journeys, even if they have no other reason to stop in between.
Some factors to consider are: Crystal Usage Efficiency, in terms of distance and suppression of quirks. Drive Range: How far can the ship go between recharges in a mundane universe? Repetitions (between refueling and overhaul)?
Time factors: These are always expressed as a range; what will the ship's average will be?
External time: how much time (range) will seem to have passed in the external universe between the time the ship enters Jump and when it returns to the normal universe, per Jump unit.
Internal time: how much time will seem to have passed internally (range) per Jump unit.
(The difference between these two is commonly called "slippage.)
Speed of Travel and Internal Time Fluctuation: While one can never be sure how long a trip will take, better technologies narrow the window of how long the trip will seem, and decrease the chances of "beyond normal range" occurrences.) It does little good to arrive in three (external) days while the rest of the Empire experienced three weeks if the internal time was a century more than "ship's average" and you lacked sufficient deep-sleep bunks or food!
Fuel: What kind(s) of fuel(s) can the ship use? How much does it need per Jump unit?
Refining: Can you do any of your own fuel refining from raw materials, or must you rely upon a "filling station?"
Ease of use: How many personnel, and of what skill level (and how specialized) does this ship require, for engines and other features?
Tonnage of the vessel? Of cargo? Berths (of various classes) for crew and/or passengers?
Safety: What are the chances of something breaking down? All systems stress and decay over time, but some manufacturers' equipment has higher rates of decay than others'. Sturdier systems are more expensive - in mass and volume as well as cost.
The Mother-of-Angels are among the most terrifying vessels of the Imperial Ministry of Resolute Harmony, sometimes called the Department of Overt Internal War. They are primarily transport ships for "flights" of "Angels."
Angel-class harmonizer Mother-ships vary in detail because they are incredibly expensive and so are built as special gifts / tribute by various subject and client states granted the elite privilege of upgrading its ship building facilities sufficiently to make such a construction feasible.
The ships are actually shaped like angels. While Seraphim are huge enough to be able to afford the luxury of armor, most Angel mother ships rely upon swarms of tiny Cherub escorts for protection.
Simply coming in to orbit around a planet is usually enough to make a planet petition to reconsider its trespasses and seek harmony with the Empire.
Celestial Palace-class ships
These are better classed as "flying cities" than mere ships. By definition, they are built around their own inter-dimensional drive system, generally an Ancients artifact. Celestial Palaces are built at such a high level of technology that efficiency as a vessel is but a detail of engineering. Functionality as a center of administration and as a display of the might of the Empire through ostentatious luxury is the primary concern.
Celestial Palaces are used as traveling administrative centers by Satraps when they Proceed about their Imperial Provinces, by Imperial Grand Inquitors when inspecting worlds to sniff out Utopias, potentially rebellious AI (Artificial Intelligence), or other treasonous heresy, or by high-ranking members of the Imperial Family when on an official Tour of the Empire, such as when Investing a new local ruler. Basic travel is better done in simpler vehicles.
Celestial Palaces are not military vessels, but they high tech level equipment includes suitable armaments. The Palace Guard of a Celestial City may be relatively small and primarily ceremonial, but at least some contingents will be very well trained. There may even be a flight of Angels (SEE Angels of War.) Celestial Cities are often accompanied by a military escort.
Hosting the visit of a Celestial Palace is very expensive; such a visit can be used as a "white elephant" to empty the coffers of an overly ambitious subject or client state.
Tramps
If it can get from "here" to "there" with its captain and crew alive, preferable with enough cargo and / or passengers to turn a profit, it can be labeled a ship.
At the low end, a tramp vessel is just a hull with life support and a propulsion system or two.
Hollowed out an asteroid, or fuse together a shell of meteors. Cobble together cannibalized parts from a derelict satellite or find an actual abandoned ship. Make an air-tight cabin, even if it's just by spraying a thick layer of insulating foam about the interior and then spraying on a few layers of plastic lacquer the same as spray-on armor) to seal and stabilize it. Bolt in some life support equipment. Bolt in engines for the in-system drive, Jump-drive, and any other propulsion you can afford.
Bribe an official to have your jalopy certified as a ship, and you're all set. It's your ship, and your life, to risk as you will. The only question is, can you get a cargo and / or passengers to pay off the bills?