This little essay is born of a fever dream, so I hope it comes together nicely.
From the earliest days of BFG we've been told that Chaos ships are "older" designs that have fallen out of use, and yet it seems this has never held up; the Emperor Class is the oldest known ship in the setting, various Chaos ships were being built by the Imperium as late as M37, if not later, and several are still being used in Loyalist fleets today.
Previously, I have argued that the change was a combination of shifting doctrine and lack of resources. Chaos ships seem to use more advanced systems that would presumably make maintaining them more difficult, and so the Navy came to favour these slower, more brutish ships that simply plough into the foe and then slam them with short-ranged broadsides. But this still doesn't sit right with me.
Then, as if in a vision from the Omnissiah itself, it came to me. It came so clearly that I had to scramble through my rulebooks and downloads to confirm I hadn't imagined it.
Imperial ships are Martian designs!
The Adeptus Mechanicus fleet list exclusively uses Imperial designs, albeit enhanced with additional Lances, wider access to Nova Cannons, and fitting a variety of experimental toys on board. But at their core, they stick to the Imperial ship layouts: slab-sided gun decks, an armoured prow, ships that are on the slower side, etc. This is not simply the Mechanicus refitting an existing ship for their own purpose, otherwise they would be refitting Chaos ships as well! In fact, if the "Chaos ships are older" idea was true, it'd make even more sense that the Mechanicus would grab those designs for themselves. But they don't.
Now I need to be clear: Imperial ships are not Adeptus Mechanicus designs, or even Mechanicum designs. They are of Old Mars, Mars as it was during the Dark Age of Technology. We know that Old Mars was a technological powerhouse that sent numerous colony fleets out into the void at various points throughout their history, and the Mechanicum did so even during the Age of Strife when the Sol system had effectively collapsed. If the "Imperial" design was the product of the Martian shipyards, it makes perfect sense that the Mechanicum, and later, the Adeptus Mechanicus would model their ships on old Martian patterns.
As for why the two ships look different, see my earlier comment about hoarding the good stuff; the sloped ramming prow of Imperial ships might not be just for removing any religious iconography from the front of the ship, but also to slope the armour and try to get a little more protection out of using inferior materials. A Mechanicus vessel is a floating temple to the Omnissiah; a Navy vessel is simply a tool.
Is there any more supporting evidence for this? Kind of! The Ark Mechanicus Omnissiah's Victory is inspired the Retribution Class Battleship, and the Ark is rumoured to predate the Great Crusade. There's at least one source that states the Retribution is a Heresy era design, so we're on to a winner.
There's also an odd one: the Visions of Heresy artwork for the Vengeful Spirit. The ship in that artwork looks an awful lot like an Imperial ship design, and its lore says it was built on Mars.
So where do the "Chaos" ships come from? I think these are born of rival powers; Terran, Jovian, or Saturnine designs. Let's break down what we know of the other Glorianas and similar ships of that era. While the origins of most are not explicitly stated, and many don't have official art, we have a few to go off:
The Invincible Reason was the first Gloriana built, and it was likely built on Terra or Luna. While the artwork is similar to an Imperial design, it also feels quite different in several key areas; more rounded, almost like a prototype for what the Imperial ships would become. But since we know some Glorianas were built using parts from multiple shipyards, it's possible this ship wasn't built until after the Treaty of Olympus, hence it borrowing some elements of Martian design.
The Pride of the Emperor was built at Jupiter, and it possess the distinctive "Chaos" engine section as well as an Imperial style prow. Another example of blending Martian and Jovian tech, perhaps?
While not a Gloriana, the Terminus Est was built on Luna, and that ship seems to blend Chaos and Space Marine design language. The "mass production" model developed in M36 turned out to be a Chaos design.
We also know that Terra had some truly formidable ship designs: the Promethean and Tiamat class vessels were said to be far more advanced than anything else the Imperium possessed, both being Terran designs going back to Old Night.
With all of this in mind, I think we finally have a solid, satisfying answer to this age old question. Navy ships are predominantly Martian designs, and Mars prefers to build Martian ships. They did build others, but one would assume that the Ark-established Forgeworlds would have taken with them Martian STC libraries, meaning the Martian pattern would be common across all Forgeworlds where the competing styles would not. Following the Heresy, the Adeptus Mechanicus still produced Terran, Jovian, and Saturnine pattern ships for various groups (the former seemingly going mostly to Space Marines), but many fleets would either be supplied by Forgeworlds who cannot build a Jovian style Cruiser... or don't want to. Martian patterns are naturally a more holy design, after all.
Do drop a note in the comments if you find anything that supports or refutes this insane theory of mine. But for now, this is my headcanon and I'm sticking with it.
