Ontario-class Corvette

The Danube-class runabout is an extremely successful ship design. Its modularity allows for various mission profiles and its sturdy construction made it suitable even for more dangerous assignments. However, it is still lacking in some areas. Its range, while larger than a shuttlecraft's, is still limited and it is rare for a runabout to go on missions longer than a few days. Also, while runabouts may occasionally be used to defend their home base, they are not designed for combat.

The Ontario-class corvette is supposed to remedy this. At 100 meters long and with 3 decks, the ship is larger than a runabout but smaller than a frigate. It was designed for a crew of 5 to 10 and could go on missions of several weeks. It is intended as larger auxiliary craft for frontier space stations and starbases.
However, like the Danube-class, the Ontario-class is a very flexible design. It's saucer and stardrive can both be swapped for different module variants and the saucer has a large room of 10x19x10 meters that can slotted in from the bottom. This area can serve as a cargo bay with a total volume of almost 1900m³ or contain laboratories, additional living area or mission specific equipment.
Armed with 6 phaser strips, 2 pulse phaser cannons and a forward microtorpedo launcher, the Ontario-class is also a rather competent fighter which can take on ships significantly larger than itself.
Two shuttlecrafts, a Type 6A ( https://www.artstation.com/artwork/R395xA ) and a Type 8, serve as auxiliary craft and could be used as escape vessels in case of an emergency.
As such, this ship class not only serves as auxiliary ship for starbases, but also as patrol ship, as combat ship or to transport supplies, valuable cargo or even to aid in evacuations.
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The idea for this ship came out of nowhere. I kind of wanted to create an "upside down" Starfleet ship. Thus the Ontario-class has the stardrive section above the saucer. Also, main engineering is located on deck 1 while the bridge and crew quarters are on deck 3.