Why would there be any correlation between boats and spaceships when it comes to classification. A classification that is completely arbitrary to begin with.
Because of all the parallels we have to what life in the void is like, the Navy comes closest. Consider:
Classical Navy ships (and modern Navy ships for that matter) often had to operate for extended periods without support. They could be weeks or months away from the nearest port, let alone a place they could go ashore. By contrast, the airforce does not deal in such long-term operations; flight times are measured in hours, not weeks.
When we do reach the stars, we will almost certainly be drawing upon the Navy for guidance. Their terminologies are likely to carry over for the simple reason that much of it still makes sense; the difference between a ship and a boat, for example, is just as valid away to distinguish a large, inter-planetary void craft and a smaller void craft designed for short ship-to-ship, ship-to-orbital or ship-to-planet trips. In other words, the Space Shuttle would likely be called a boat in space faring terms.
Finally, let us look at modern usage and see how that compares:
Frigate: A primarily anti-submarine vessel, larger than a Corvette but smaller than a Destroyer.
Okay… so what is a Corvette and what is a Destroyer?
Corvette: A small, maneuverable, lightly armed warship. Modern Corvettes measure about 180-330ft (55-100m) in length and are usually armed with medium and small caliber guns.
Let’s just take that description and run with it a second… excuse me while I boot up Star Conflict and fetch a ruler.
Okay, so, I’ve lined up the camera so that a person loitering under each of my ‘Frigates’ is about 0.5cm high. Assuming a person is 6ft tall, let’s look at the approximate lengths of these ships…
_Harpy: _23.5cm = 282ft.
_Hydra: _27cm = 324ft
Raptor: 24.5cm = 294ft
As you can see, those lengths fit nicely within the Corvette size bracket. But let’s go one better and look at the guns; the weapons on these ‘Frigates’ are little better than Fighter / Interceptor guns. They might be more powerful, but aside from specialist gear like the Disintegrators, Torpedoes or mine layers these ships are essentially packing weaponry off an attack craft.
By contrast, Frigates and Destroyers are armed with weapons intended for full-on warships.
So… the thinking is starting to add up. It’s becoming pretty obvious we aren’t flying Frigates out there.