I was considering this answer I wrote: https://worldbuilding.stackexchange.com/a/21374/75
It got me to wondering... if we had a flying sailing ship with a twisted-rubber energy storage device that was set so that as the ship was sailing through the skies, it had a set of small keels that used the ship's motion and a set of reduction gears to apply torsion to a bundle of rubber cables the length of the ship. Then, once the rubber cables were wound tight, the keels on the impeller could be rotated so that they would become a propeller, and the energy stored in the rubber driving the ship ahead in the absence of any wind, or against the wind.
Keels are a substance that has a differential resistance to movement because of its directionally-dependent permeability to the Ether - the 'medium' which fills the universe through which everything moves. Most things pass through the Ether with no resistance, but keel materials are almost completely impermeable to the Ether in any direction save one, much like a short, wide, straight length of pipe in water. Try to move the pipe sideways through the water, and resistance is high, proportional to the length and width of the pipe, while if you try to move the pipe so that the water flows through the lumen of the pipe, resistance is low, proportional to the thickness of the pipe's walls and its diameter.
Consider a keel to be like an aerofoil… but capable of sustaining vastly higher loading, dependent not on its lifting area, but its volume.
So, the keel-disk that would both wind the rubber and propel the ship is effectively a very much more efficient and compact mechanism equivalent to a variable-pitch propeller. By setting the pitch of the keels to a shallow angle relative to the direction of travel, the disk could wind the rubber without applying much drag. Then, when the rubber was sufficiently wound, a brake could be applied and the keels feathered to minimise drag. When thrust was required, the keels could be set to a steep angle relative to the direction of travel and the brake disengaged. The rubber would then spin the keel rotor, which would push against the Ether, propelling the ship. By attaching a governor to the pitch control, the speed of the keel rotor can be self-regulated by increasing the pitch as the speed of rotation increases. With keel pitch angles less than 45° to the direction of travel, the rotational speed can be reduced while applying less forward thrust - the ether acts as a brake on the keel plate.
I know about rubber-band propelled toy aircraft, so I was wondering just how much energy could be stored in a quantity of rubber, how quickly that energy could be released (and by extension, what propulsive force it might develop), the tensile and torsional forces involved, and whether timber beams would be sufficient to withstand them, how long such an engine might propel a ship, and how long the rubber could be expected to remain twisted before it began to deteriorate.
The sort of thing I'm trying to determine is if this would be practical for something as large as a ship between 2 and 2000 tons mass, and if it is an "It could get us out of the doldrums" thing (long term low-level power), or if it would be more like a regenerative braking sort of thing: the ship dives steeply, the impeller storing some of the energy from the ship's descent, then when it is time to climb again, the system is reversed to add the stored energy so the ship will climb more rapidly, but that exhausts the stored energy - short-term, high power...
Or should I just hold out for steam power?