I had the impression that not loaded and unloaded had different meanings:
But I do not find examples corroborating this difference, as if unloaded could also be used to mean "has never been loaded", e.g. "unloaded ship" for a brand-new cargo ship.
Is there an actual difference between them?
Un- as a prefix on a verb (eg unload) refers to reversing the activity.
Un- as a prefix on an adjective merely says that the adjective does not apply, or that the reverse of the adjective applies, and does not say anything about how the item got that way.
Loaded could be an adjective or a participle, but in practice there is little difference in meaning.
But unloaded, while it could in principle be either, is almost always taken as the participle, so un- has the verbal meaning rather than the adjectival meaning. For the adjectival meaning we have to resort to a paraphrase, such as not loaded.
You're right. They can be confusing, but it's more about emphasis and context than a strict grammatical rule.
Not loaded: This is a neutral statement about the current state. It simply means there's nothing loaded at the moment. This could be because it has never been loaded, or because the load was previously there and has been removed.
Unloaded: This emphasizes the action of removing the load. It implies that something was previously loaded and then emptied. It can be used for things that are typically loaded and unloaded, like ships, trucks, or even computer programs.
However, "unloaded" isn't strictly limited to things previously loaded. It can be used in some contexts to mean "not loaded" as well, especially for things that are designed to be loaded.
Think of a dishwasher. "Not loaded" means there are no dishes in it currently, regardless of whether it's been used before. "Unloaded" means there were dishes in it before, but they've been removed for cleaning.
The key takeaway is that "unloaded" implies a past state of being loaded, while "not loaded" is more neutral about the history.
Both phrases can be used to mean "never was" or "was but it has been undone".
For example, if I say "The order is not boxed", I probably mean that it never was in a box. But I could mean that it was in a box and then someone took it out of the box. (Perhaps because it was the wrong kind of box, or they needed to inspect the item, or whatever. Reason is beside the point.)
Likewise, if I say, "The order is unboxed", the same two possibilities exist.
Same with "not loaded" and "unloaded". In the real world one of the interpretations may be unlikely. Like if I say, "The gun is not loaded/unloaded", maybe it's a brand new gun that was never loaded, but more likely the ammo has all been fired or removed. But if I say, "The order is not shipped", it probably has never been shipped. It's possible that it was shipped and then returned and needs to be reshipped, but probably not.
Note that "unloaded", or "un-" most anything, as a verb, means the process of undoing. If I say, "I am unloading the gun", I mean there was ammo in it and I am removing it. Because it wouldn't make sense to say that I am unloading it if it never was loaded.
If you want to make clear whether you mean "never was" versus "was but now isn't", you have to use additional words. Like, "This order was never boxed", versus "This order was boxed but has now been unboxed."
