Understanding tachyons at all is something that we mortals can't yet grasp. Achieving inertial FTL by "riding" anything is known to be impossible, because your local velocity would exceed lightspeed (violating relativity). The convenient part about converting your ship into tachyons is that you can get around relativity by just saying that since you're a tachyon you have to travel faster than light; ordinary bradyons like the ship in your riding scenario would still be incapable of FTL.

So, if the crew are converted into tachyons, does that mean they technically die in the process and that reassembling just produces a clone who has their personality and memories?

May i recommend this: youtube.com/watch?v=6akmv1bsz1M - its a very visual introduction to anti-universe and inter-universe travel and lightcones. I think you can also visualize in those graphs how a infinite big lightcone would look like.. as you could interact with the whole universe at a split second..

Might I suggest that you perhaps phrased the question in such a way as to prevent answers that would be useful to you. "Does any of this seem plausible from a theoretical standpoint?" was the error. You've people answering that rather than giving you encouragement with your tachyon-conversion idea (which seems a splendid science-fiction excuse for FTL to me).

Cont.: Take a look at the search results for "tachyon FTL", then this post's first answer (the last paragraph's advice). You can create your own brand of FTL, as long as you don't technobabble too much - by all means inject some real science and your own flavour of invented words.

Cont.: To try and explain something that's impossible with a fully realistic set of convincing-sounding hypotheses/working theories is one hell of a writing task - and technobabble creation is off-topic here. Don't let the technical impossibilities stop your writing a wonderful story, the "science" is just a backdrop after all, a vehicle for the excitement/drama/comedy/bathos/pathos and character interactions.

How would it most likely work? if tachyons did what they are theorised to do? it most likely wouldn't .. IF they existed, and IF they acted as theorised, then they could "perhaps" form the basis of faster than light communication, but if you harness them in any way to something with mass to carry it with them then that all collapses, you can't add something with mass to them and expect them to still be able to do what they're theorised to do because they can only be theorised to do what they are theorised to do if they and anything they are part of has no mass.

You could (with the employment of considerable quantities of handwavium, my rough back of the envelope calculations suggest around 107.4% of all universally available quantities should suffice) perhaps use it as a long range Trek transporter, but you'll need a receiving dish at the other end, and what you build from the signal received at the other end isn't the ship and crew scanned at the sending end, they're either still there, or dead if you're using the traditional style (destroyed by the scanning process) Trek transporters.

Or you can just use magic, and call the magic tachyons (or anything else that strikes your fancy), it's an approach that worked for both Star Trek and Star Wars in various guises.

Please set your expectations. You say in your post, but the technology in it is at least theoretically possible. Tachyons are fictional, so it's a hard sell to ask for help developing an engine that's theoretically possible that uses an entirely fictional particle.

Oh, and if you're going to use them to 'beam' stuff ST transporter style you'll have to travel to the destination by STL first, then build the tachyon receiver, and every time you use it the thing you sent will arrive before it was sent because the tachyons are traveling back in time, the further you're sending them the further back in time.

@controlgroup, Regarding, "...we mortals can't yet grasp" and "... you can get around relativity..." Actually, it was a mortal physicist who thought up the idea of tachyons, and all he did was describe how a particle with imaginary mass—a tachyon—would behave according to the theory of relativity. en.wikipedia.org/wiki/Tachyon

I hope for the sake of your story that the tachyon drive is working very well ;-).

@JBH, tachyons aren't fictional, they're theoretical. Fictional particles can behave however the author wants; theoretical particles are restricted by the known laws of physics.

@Mark They're not theoretical, at best they're hypothetical. The fact that fictional particles have been defined by someone else doesn't make them any less fictional. Science demands repeatable tests with measurements - show me one that adheres to the known laws of physics. Until then, they're fictional.

Ship and crews are not made up of one type of particle so how the hell are they even people during transit? The massive loss of information during people to tachyons transformation means that there is no way for you to put them back together afterward. Bad science all round!

Tachyons are more like photons, aka, you could use tachyons for stuff like FTL telecommunications and stuff. But alas, general relativity is a bitch😭

@stackoverblown, "Tachyon" is not the name of any specific species of particle (e.g., like "electron," "gluon," etc.) Any object whose mass could be appropriately described in the theory of relativity by a pure imaginary number would be a tachyon.

@JBH, So, would you say that the Higgs Boson was "fictional" up until the moment when researchers claimed to find its "signal" in data from experiments at the LHC? OK, I admit, I'm making a somewhat lame analogy because failure to observe the Higgs would have been a problem for the Standard Model, whereas failure to observe a tachyon in no way threatens the theory of relativity. Still, the fact remains that the existence of tachyons was suggested by a valid physical theory, and if a tachyon ever is observed, that will only strengthen that theory.