Perhaps something similar to Paradox's system when you under pay for your fleet? The ships are at 5% health, and morale and organization are near zero.
In Aurora terms, training is now 0, and *all* ship systems are damaged except crew quarters or some such (to prevent boomies). Shipyards must be used to bring the ship back online, and then the crew must be trained. Ships would also need to be refueled and supplied. Essentially, the ship must almost be rebuilt.
Could be useful if you are the one starting the war and you have time to gear up, but not so much for a surprise attack.
Aurora doesn't really have an equivalent to morale/organization for ship crews which inflict crippling penalties when low. We do have crew training but that effect is not very strong, certainly I'd rather have 2x as many ships with minimum crew grade rather than half as many with 100% training in almost any circumstance.
Repairing damaged systems is more promising, and usually comes up in these discussions. However, it will either be so expensive that there's no reason to mothball at all (which I think will certainly be the case if shipyard repairs are necessary), or if mothballing is viable then what prevents players from building new ships into mothballs anyways? This latter part is really the big problem that no mothballing mechanic yet proposed has solved, frankly it is not a big exploit to keep a bunch of ancient ships around that will probably be horribly outmatched in the modern space combat arena, the problem is when you build a dozen shiny new battleships and immediately shuffle them into reserve for that sweet -80% maintenance discount or what have you.
What is the difference between mothballing and pausing ship construction at 99%? The mechanism exists at the cost of bulding more slipway and shipyards.
Slipways and shipyards are very expensive and usually once built are needed 100% of the time unless there is a resource shortage (and even then, a good player will often find clever ways to leverage limited resources as well as refit mechanics to keep the shipyards working).
The best idea I've ever seen for a mothball mechanic has been to require a slipway for each mothballed ship. This makes the mechanic properly too costly to just build ships into reserve, but by doing that it makes mothballing arguably just too costly, period - basically, why would you ever mothball an old ship if you could mothball a brand-new ship instead?
In the Real World
TM, the reason that new-build ships don't usually get built directly to mothballs is because governments only authorize that construction if the ship will immediately perform some essential mission for that country. In Aurora, we don't really have that same impetus unless there is a war on; PPV and the new Raider spoilers help but don't come close to replicating the wide range of "peacetime" missions a real-world navy has to perform. This means that not only is any mechanic we devise to make mothballing non-optimal also going to be artificial, but there isn't a clearly good way to make mothballing only a reasonable option for old ships and not new ones.
