Bioship Propulsion

(TO WRITE: stl via enzymatic creation and release of ionised gases)
(TO WRITE: implanted fusion engines giving bettter mobility and cleaner operation at the cost of requiring invasive surgery)
However due to small reaction mass reserves and an impractically low thrust/weight ratio, bioships have had to develop alternative means of using the small changes in velocity they can generate on their own.
A bioship's wing membranes catch and interdict an otherwise very weakly-interacting omnidirectional radiation pressure, originally theorised to be the expansion of the universe itself, and partially block it in a cross-sectional direction based on a function of the bioship's changing velocity. In practical terms this means they can get an alcubierre-like warp boost in normal flight by using or inducing a change in heading or velocity. Speeds comparable to the galactic standard for manufactured starships can be achieved by swooping, angling, or flapping the wings in the right way, but need to be provided with that initial acceleration in some way for the wings to gain purchase.
More remarkable is the use of the same wings to gain access to FTL travel. By spreading the wings fully and pushing against that pressure in all directions, carefully neutralising all forces to prevent any loss of potential energy to alcubierre movement, a bioship can develop enough dimensionally transcendental pressure to squeeze itself out of realspace and into very low-energy subspace bands. Much like Rupeean methods of ballistic subspace travel that rely on the initial energy of the drive to set the speed and course, the bioship relies on its entry velocity relative to the nearest massive object (and to a lesser extent massive objects in its intended path) to set its pace.
A particularly experienced bioship with the right dimension-sensitive equipment or adaptations can go one further, traversing relative depths of subspace by carefully gaining or bleeding that potential energy. In this way a bioship can find bands of contracted apparent space or small relatively stable energy bands that any manufactured ship hasn't the sensitivity or need to take notice of, allowing for improved results on what is essentially a very efficient if slow FTL method.
Traversal of subspace occurs essentially by feel, and only the most experienced pathfinders of ossific bioships are able to perform the task. No arboreal bioship of the Everjoy or Storm lineages have yet managed to do this - even KSF Storm of Leaves needs to keep a Kitsune-made FTL drive running to keep herself dimensionally stable despite her plethora of survey data, astrogation equipment and huge wing area.
To reenter realspace a bioship can in theory just close or unbalance its wings and fall out of subspace entirely, but in practice this causes transient spatial warping and immense physical pain as a vessel moving in 4 dimensions tries to fold itself back into 3. Navigating to a very low-energy subspace level with more realspace-like conditions before performing the dimensional transition is far easier than navigating subspace on the wing and is apparently instinctual, with essentially every bioship on record able to perform it. This easy drop into realspace also accounts for the unusually quiet FTL wakes that precede a bioship's arrival.
The technology been replicated for manufactured starships but is not particularly common, due to the extremely fine tolerances required in even the simplest of operational tasks and the limited suitability for high-stress or dangerous environments. However some civilian craft make use of full rigs of wing-imitating metallic sails for economic sublight transit, and competitive races involving small sailing starships has become a small but prestigious international sporting event, exported as far afield as the Karqwati.