So, pulling together ideas from this thread and other things we know, and throwing in a whole bunch of conjecture and assumption:
Using Southern technology, a field of dilated time could be generated around a ship. Because the ship is sufficiently large, the bubble will move with the ship, effectively multiplying the ship's velocity. I wasn't able to find that 1:8 statistic the OP is referring to, but it seems to me that Bendalloy dilates time a lot more than that. People become blurs, and bullets hang in the air. Here's a description courtesy of Wax in AoL:
It's pretty safe to ballpark the bullet at the speed of sound (340 m/s). From Wax's perspective, the bullet was almost certainly going slower than 1 m/s, so we'll use that to get a low end estimate. That means that time was flowing at least 340x faster inside of Wayne's ordinary speed bubble.
That's pretty cool, but this is even better:
So, stack enough speed bubbles and we could get things going at pretty ridiculous speeds. There are still a couple of problems, however. According to Wikipedia:
Granted, sci-fi Scadrians will almost certainly be able to launch ships faster than that, but let's take this as a realistic minimum. Continuing our assumption of 340x speed bubbles, the Voyager would need 53 layered speed bubbles to crack the speed of light, which is probably an unrealistic number. Even then the nearest star to Earth, for example is more than 4 light-years away. If they don't want these journeys to take years, they'll need something more. The other problem is that a standard Bendalloy bubble is not going to surround the entirety of the ship.
Fortunately, we've already seen a solution to these problem - Allomantic Compounding! In general, there are two facets of Allomantic powers: range and potency. A more powerful Misting will increase both the range and the potency of their Allomancy. For example, the Lord Ruler's soothing was both able to affect more people, and affect those people more profoundly than any other Soother. It's fairly safe to assume that Bendalloy works similarly. With that in mind, it's a simple matter to create an unkeyed metalmind that would alloy a Bendalloy misting to compound his or her Allomancy. They could temporarily boost their Allomancy to incredible levels (100x normal, for example) and then use it to prime an ettmetal device. Presumably, the device would be similarly amped. If not, then perhaps they could burn through ettmetal faster to achieve a similar result, though this would force them to bring and use up a ton of the stuff.
Boom! We have a speed bubble large enough to cover the entire ship and which dilates time enough to get us moving at least 2x the speed of light. Improved propulsion technology, more layered speed bubbles and compounding Allomancy further would get this ship to go even faster. And as SafestPear mentioned, it might be possible to increase that even further by somehow using iron feruchemy to store the ships mass.
There's still one major problem left in this scenario - to the outside world, the ship is now moving faster than the speed of light, but time is still passing at an accelerated pace for those inside. If the ship travels just one day, assuming we have a 34,000x speed bubble, the passengers will have aged 93 years. Interestingly, no matter how much time is dilated, the journey will take the exact same amount of time for those inside. Here are two possible solutions:
Create localized Cadmium bubbles within the ship to cancel out the Bendalloy. I'm not very keen on this because it seems like it would be really hard to keep the Cadmium bubbles compact while simultaneously leaving them potent enough to exactly cancel the overlaying Bendalloy bubble. Or, the ship's crew just wear aluminum-lined uniforms. Better yet, the ship itself would have a layer of aluminum in the exterior except for a small part which would contain the time dilation device.
Relevant WoBs:
Thanks to Kurkistan for posting the links!
So there we have it! Using what we know and some extrapolation, we have viable Allomantic FTL.
Knowing Brandon, I'm sure there's a lot more to it and my mind will be blown when he finally reveals his reasoning. And there's still so much we don't know, like what Harmonium does...
