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Warning: TV Tropes links detected.

In a World In The Sky, floating islands are held up by deposits of a mineral that defies the pull of gravity. Things attached to it are held in a sort of neutral buoyancy in the air. (based on the ratio of the mass of the object to the mass of the deposit)

The people of this world have learned that they can split off small chunks of this mineral and build it into ships to travel from island to island.

My question is this: How would this culture best integrate the material into ships for maximum efficiency and reliability?

Assumptions:

  • The material is handwavium: it makes no physical sense and it is not supposed to
  • Relative technology level is that of earth in the Dark Ages
  • A chunk of the material the size of an average car engine is enough to 'levitate' a medieval galley

About the handwavium:

  • It has the mass of normal stone, but rejects the pull of gravity so strongly that it can suspend normal matter attached to it.
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  • $\begingroup$ I think you do have to explain a little bit about your handwavium. Does it have very low mass, like Helium/Hydrogen gas, does it have zero or negative mass? It obviously interacts with gravity somehow since it always pulls "up", but does the strength of that interaction also follow an inverse square law depending on the distance to the gravity well? $\endgroup$ – Hackworth Aug 11 '15 at 13:19
  • $\begingroup$ @Hackworth I have added some small amount of information. The material is quite mystical and contradictory, and, to be honest, I'm still working out the 'technical' specs myself. The question itself, however, is one of shipbuilding, and where and how in a ship's construction this mystical stuff is best used. $\endgroup$ – Danny Reagan Aug 11 '15 at 13:37
  • $\begingroup$ What has prevented the material from floating off into space? $\endgroup$ – Green Aug 11 '15 at 21:33
  • $\begingroup$ Left alone, it does not float off into space. It is effectively weightless, while still being denser than the surrounding air. $\endgroup$ – Danny Reagan Aug 11 '15 at 22:02
  • $\begingroup$ Now, left to itself, it might drift slowly due to air currenrs $\endgroup$ – Danny Reagan Aug 11 '15 at 22:03
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Given your edit, what I can best explain this material as is that you have a solid rock that works like a lighter-than-air gas. Yes, technically repels gravity, but the result is the same.

Therefore, your shipbuilding designs will be based in part around some lighter-than-air construction principles.

1: The 'lift stone' will need to be placed above the ship's center of mass. When something is in the air, it will naturally try to orient with the heaviest part facing down...so if your lift stone is below the center of mass, your ship will be unstable and likely to flip over, and get stuck that way.

2: For an extremely small, unmanned craft (like a buoy), a single lift stone will be enough to hold it up and keep it stable, however in an occupied craft, it will wobble around a single lift stone. Therefore, you will want multiple, smaller lift stones arranged around the edges of the ship, always equal in distance from the Center of Mass as the stone piece on the opposite side of the COM. So, for clarity, your Bow and Aft stones need to be the same distance from the COM, and the Port and Starboard stones will need to be the same. More stones will provide more stability in the ship.

3: Extra stability can be gained by placing stones on outriggers that extend beyond the body of the ship. But this raises a big problem.

4: Assuming that lift stone truly repulses gravity, you will need to make sure that it is kept very well secured at all times, and is only attached to the ship once construction is complete. This is because a gravity-repulsing stone would naturally eject itself from the planet if not held down. Likewise, a damaged ship that has lost a bunch of mass is going to have problems not simply ascending out of control, and the crew may have to cut loose some lift stones to regain control. For this reason, more small lift stones is better than a few big ones.

5: In combat, the primary target for warships will be the other ship's lift stones. All you have to do is destroy the casing around them, and they will hurl themselves into the sky, and eventually forcing the other skyship to fall.

As an alternative to 'gravity repulsive,' you could instead declare the stones to be extremely buoyant in atmosphere, This would cause them to naturally 'float' up to a certain altitude where their buoyancy is cancelled out by the thinness of the air. The closer they get to that altitude, the less lift they generate. In this case, recovering lost Lift Stones is difficult, but not impossible, and does explain why all the Lift Stones in the world haven't launched into space already.

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  • $\begingroup$ Interesting points, to be sure. I actually didn't say that they repel themselves from gravity, however. Obviously, I need to work on my description and hand waving of this material before I try to use it in a work of fiction. Too many obvious holes at present. $\endgroup$ – Danny Reagan Aug 11 '15 at 14:01
  • $\begingroup$ "And know I dedicate this ship, the S.S. guilsbounty, to sail the seven skies." Breaks bottle on ship. Ship turns over. "Danny Reagan, did you put the lift stones to low in the ship, again?" "Sorry, mayor." $\endgroup$ – PyRulez Aug 11 '15 at 15:22
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Your handwavium provides nothing but constant, upward thrust. It also generates upward thrust to counteract the weight of regular matter attached to it, up to a certain limit. Your description is a little contracdictory there. On the one hand, it "rejects the pull of gravity", but it also tries to stay at a constant distance from the source of gravity and can counteract the weight of other masses pulling it downwards, up to a weight limit. Not totally sure what to make of that.

Overall, your vessels seem to be very similar to zeppelins. Your upward thrust can be regulated by throwing some handwavium overboard, throwing ballast overboard, and/or engines with directional controls, either by changing the direction of engine thrust for vessels with low forward speed or wing-like extrusions with flaps for fast vessels.

The shape of the vessel can be anything, since the volume of the handwavium is not a deciding concern. Therefore, an somewhat aerodynamic shape of the vessel seems most desirable for efficiency, to increase forward speed and maneuverability.

For reliability, the handwavium required to carry an empty vessel (no payload) should be distributed evenly throughout the structural parts of the vessel, to prevent a catastrophic loss of upward thrust if parts of the vessel break off due to accident or combat. Additional handwavium can be taken onboard in accessible, removable containers to counteract the additional weight of crew and cargo, and to have disposable handwavium in case a rapid descent is necessary.

For navigation, the vessel needs some sort of engine and ability to direct thrust. For medieval technology levels, this can be as simple as muscle-powered propellers and directional control like wings and flaps.

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I am assuming more Efficient means smallest amount of mineral, and more Reliable means less likely to break.

Have seen two answers posted that answer some of the technical issues

One of the main difficulties is being able to propel such a craft with Dark Ages technology. Oars would have to be really oversized to work in air.

Sails would work, but interestingly they would need to be all around the boat.

Windmills were invented at the end of the Dark Ages so it is plausible that one could be used as a Medieval Propeller, possibly also used to provide Vertical effort.

As mentioned in another answer information about the Centre Of Mass would be critical. Boats in water act to keep the vessel upright by pressure on the hull, if one side goes down, more of the boat is under water so you get a force that pushes that bit up. if you have a traditional square sail the Centre of Force generated is approximately half way up so you get a push acting to rotate around the centre of Mass, this is resisted by the bow of the ship going into the water. If we assume an airship that is essentially a boat which is supported by the stone to a neutral buoyancy the whole boat would rotate and act like a windsock rather than a boat. Having sails that balanced each others forces would ensure that the vectors of imagined thrust would drive the boat forward. Having the sails simply in one above and one below formation will allow some form of control over the angle that the boat is pointing.

Another Issue to overcome, and I don't have a solution is that the water provides resistance that allows the boat to go sideways rather than straight down the wind direction.

My thoughts on a solution would be a small craft like a coracle made from wicker, Willow and hides, with small stones tied equally around the frame the operator would have a large fan in which they could waft there way along, a pole like gondoliers and punters use.

Alternatively large heavy craft made of oak could be towed by a pack animal.

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NOTE: I am NOT answering the asked question

But I thought I could provide you with highly valuable information for your story so I'm submitting this as an answer but to a question you never asked.

For your handwavium magic material with freaky weird properties, you might check out Negative Matter. Negative matter is NOT anti-matter.

Awesome Engines

 1. Positive mass attracts both other positive masses and
    negative  masses.
 2. Negative mass repels both other negative masses and positive masses.

For two positive masses, nothing changes and there is a gravitational pull on each other causing an attraction. Two negative masses would repel because of their negative inertial masses. For different signs however, there is a push that repels the positive mass from the negative mass, and a pull that attracts the negative mass towards the positive one at the same time.

Hence Bondi pointed out that two objects of equal and opposite mass would produce a constant acceleration of the system towards the positive-mass object, an effect called "runaway motion" by Bonnor

[...]

Such a couple of objects would accelerate without limit (except relativistic one); however, the total mass, momentum and energy of the system would remain 0.

Imagine an engine that requires no energy, requires no propellant, constantly accelerates, and has no "off" button. Could make for a very interesting plot device IMO.

Wormholes

Morris, Thorne and Yurtsever[21] pointed out that the quantum mechanics of the Casimir effect can be used to produce a locally mass-negative region of space–time. In this article, and subsequent work by others, they showed that negative matter could be used to stabilize a wormhole. Cramer et al. argue that such wormholes might have been created in the early universe, stabilized by negative-mass loops of cosmic string.[22] Stephen Hawking has proved that negative energy is a necessary condition for the creation of a closed timelike curve by manipulation of gravitational fields within a finite region of space;[23] this proves, for example, that a finite Tipler cylinder cannot be used as a time machine.

It could exist

Another twist to this is that negative matter and its behaviors isn't forbidden in our current formulations of physical theories.

We have never observed negative matter and there are some ramifications of this material which make scientists skeptical of the existence of negative matter, but it isn't forbidden.

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