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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?

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What about a spring engine instead?

There is a novel called The Windup Girl which is set in a post-peak-oil world where the world no longer has access to petrochemical products, no alternative power sources have been developed, and biological warfare between competing agricultural firms has left the vast majority of the planet unable to grow safe food crops. In this book, several alternatives to a steady electrical grid or fuel supply have been developed.

Things like treadle computers, lifts that use people as counterweights who can climb back up manually, biogenetically engineered animals that are quite mammoth like, and springs.

There is a scene in the book where an industrial spring is being wound. Basically, it's a large, long band of metal that is being wound into it's casing, like winding a watch except that it's being done by a team of mammoths. Once the spring is fully wound and the casing secured, it is shipped off to a factory where it will power the machines in that factory by turning an axle, much the way steam engines did at the start of the Industrial Revolution.

Springs have a lot of advantages over rubber bands for this kind of work, not the least of which is that by comparison to rubber bands, they are reasonably efficient at storing and releasing kinetic energy in a controlled way. In the long term the spring itself will eventually wear, either becoming fatigued or actually break within the casing unless it is released and rewound on a periodic basis (and even then fatigue is just delayed) and arguably, it might not scale as neatly as one might hope from a wristwatch. But, it's a similar idea to your rubber band and has the added benefit of being contained in a casing, meaning that you could literally swap them out on ships between journeys, sending the old one to be re-wound.

In terms of energy storage, springs were well understood early on meaning that equations are available to determine how much energy a spring can store for you and whether or not it would power your ship, but ultimately the limiting factor would be how fast you want it to go through the water. A spring could almost certainly power a 2000 ton vessel through water if there is no current, but whether it could do so at a given speed, or against a current of a given flow, would be a matter of how big you could make the spring, and what kind of animals or other energy sources you could use to wind it up to store your potential energy.

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  • $\begingroup$ Similar spring engines are used to power airships in Billy O'Shea's Kingdom of Clockwork. goodreads.com/book/show/22836452-kingdom-of-clockwork $\endgroup$ – Klaus Æ. Mogensen Oct 22 '19 at 7:37
  • $\begingroup$ This does not answer the question. I did not ask about springs, I asked about rubber. Yes, it is a concievable alternative... but it is not viable considering the state of the world's metallurgy. $\endgroup$ – Monty Wild Oct 22 '19 at 23:08
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First of all, you would have large losses in the form of heat, resulting in the rubber warming up. That energy is lost, so your "rollercoaster" maneuver wouldn't take you very far.

Then, as opposed to toy aircrafts where the propeller is largely oversized with respect to the plane and you don't care about any specific flight plan, real airplanes need to control their propeller rotation regime. How are you going to give gas when the rubber cannot rotate the propeller faster?

Last but not least, if you ever plan to recharge your rubber motor in flight (you know, crashed airplanes tend to not attract many prospective passengers), you need to carry, together with the weight of the rubber, the weight of thing you will use to recharge it, be it slaves/workers/beasts of burden or fuel for an engine. But if you are using an engine you better strap it to the propeller, and if you are using slaves/workers/beasts you are using cargo space. On top of this, you will need a redundant engine: while one is being recharged, the other needs to supply energy. This turns again into a reduction of available space for whatever you plan to carry.

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    $\begingroup$ Actually, an expending rubber band (I assume untwisting is similar to expending) tend to cool down. I'm sure there would be great losses of Energy, but it may not heat up while running (it would heat massivelly while "recharging") hackaday.com/2016/08/25/a-refrigerator-cooled-by-rubber-bands $\endgroup$ – Madlozoz Oct 22 '19 at 13:27
  • $\begingroup$ While the ship is flying under sail, the variable-pitch keels attached to the are set to a shallow angle so that the rubber can be wound while not slowing the ship too much. In the event of a dive, they can be set to a steeper pitch so as to both slow the dive and quickly wind the rubber. When wound sufficiently, a brake is applied and the keels feathered to eliminate drag. When thrust is needed, the keels would be set to a steep angle and the brake disengaged. The feathering of the keel-propeller (and therefore the thrust) could be controlled by a governor/throttle setup. $\endgroup$ – Monty Wild Oct 22 '19 at 23:22
  • $\begingroup$ So, the ship's motion winds its own rubber - it is wound either by wind power or gravitational kinetic energy. The keels are unlike a propeller in that they don't 'bite' into air, they 'bite' into the Ether with much greater efficiency - a cubic metre of keel could hold a 2000 ton ship aloft so that if it was not moving, it would descend at a minimum of perhaps a metre per minute. $\endgroup$ – Monty Wild Oct 22 '19 at 23:28

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