Watchman-class System Defense Boat (Rogue Trader RPG ruleset - modded for the quest, Cobra Destroyer variant)
---Explanation of terminology - "Scarce" means that technologically speaking this is as basic as it possibly gets, at least where Imperial spaceships are concerned. "Rare" means it's still just as basic technologically speaking, but it's either a specialty item that needs a bit of tweaking to fit properly (suggestion: one additional full Techmarine or Techpriest must be committed if one or more of these components is being installed this Turn), it requires unique materials (suggestion: costs more BP), or it simply isn't common on a standard Imperial warship (ie, they don't usually carry Barracks since that's for transports and blockade runners).
---The numbers in parenthesis () in the Ship Stats is what you'd find in a basic Navy Cobra Destroyer.
Ship Stats:
Speed - 10 (10) Manoueverability - 30 (30)
Armor - 17 base, 19 from the front (15) Hull Integrity - 30 (30)
Detection - 15 (10) Shields - 2 (1) Turrets - 2 (1)
Weapons (1 prow, 1 Dorsal):
Disruption Macrocannon Battery - Prow, Damage 1d10+1, Battery Strength - 3, Crit Rating - N/A, Range - 5, "Rare" Availability
Sunsear Laser Battery - Dorsal, Damage 1d10+2, Battery Strength - 4, Crit Rating - 4, Range - 9, "Scarce" availability
For comparison, the "standard" macrobattery of the Imperial Navy escorts and frigates:
Mars Pattern Macrobattery - Dorsal, Damage 1d10+2, Battery Strength - 3, Crit Rating - 5, Range - 6, "Scarce" availability
Critical Components (39/43 Power, 35/35 Space, all of "Scarce" availability and Common quality):
Class 2 Jovian Plasma Drive, Combat Bridge, Emergency Gellar Field, Repulsor Shield Array, Voidsman Quarters, Vitae Pattern Life Sustainer, Mk 201.b Augur Array
"Scarce" Common-Quality Components:
Brig, Cargo Hold and Lighter Bay, Reinforced Prow
"Rare" Common-Quality Components:
Additional Armor Plating, Augmented Retro-Thrusters, Barracks, Fire Suppression System
Suggested Rules, QM is expected to modify as is preferred:
---Watchman-class SDBs have 10-25% more HP than other Destroyer escorts (more armor) and Frigate or Light Cruiser Void Shield integrity (not sure if there's a difference)
---Watchman-class SDBs have +1 Dice on Detection rolls, Boarding and Counter-Boarding rolls, and anti-Fighter/Bomber/Assault Boat rolls (better sensors, Barracks + Brig, and more turrets with smaller target than is usual respectively)
---Watchman-class SDBs may make a Reaction Defense roll to counter Warp-based teleportation or Demonic Incursion (the Emergency Gellar Field)
---Watchmen do not suffer manouver penalties for light terrain hazards such as nebula and sparse debris or asteroid fields (repulsor shields)
---I don't know how you'd rule this in quest mechanics, but I should mention that Disruption Cannons don't actually inflict hull damage. Instead, every 5 points of damage that gets past the armor automatically De-Powers 1 Component which then requires a Tech-Use test with a malus to restart. (randomly if just generally shooting, the targeted part if aiming somewhere specific). Also affects biological "ships" like Void Krakens or Tyranids - it's still an assload of energy.
Fluff:
Most system defense boats are designed to be cheap and brutal, with heavy armor and powerful weapons. Given that they are often looked down on as second or third rate and/or feared by insecure planetary governors as strong points for rebels, emphasis is usually placed on "cheap" and extensive drilling is frequently frowned upon. Such vessels tend to function more as checkpoints and "bunkers" on a defensive line than actual void ships.
The Watchman is a significant departure from this trend, created by an enterprising band of Naval officers and Mechanicus Enginseers who realized that having a single logistics train was far cheaper than skimping on quality, that most space-faring criminals preferred fast ships which the average defense boat cannot hope to catch, and that a faster ship with longer range was better for many system-defense scenarios - especially when outnumbered by invaders. The venerable and time-honored Cobra destroyer was selected as being among the cheapest templates available while still correcting these flaws.
First and foremost, the Warp Drive and torpedos were removed to substantially save on space, cost, and power consumption. This was then redirected to facilities such as Arbites precincts, a small customs office with inspection lighters, a brig for temporary containment, and an extensive turret network for more expeditiously shooting down escapees. So much additional space was left over that it was realized Watchmen could serve as something of a "spare parts" role by filling them with components that were more commonly seen on light and full cruisers - if repairs for a campaign were desperately needed, they could simply be cut out of a Watchman and transferred over. As such, they are typically up-armored and carry doubled void shield arrays.
In terms of weaponry, Watchmen carry a dorsal and a prow macrobattery, the exact type of which is left up to individual dockyards to apply. A common loadout as seen here is two twin turbolaser turrets on the dorsal ridge for harassment and targeted shots at vital components, with two twin ion disruption cannons in the prow for disabling and capturing vessels for boarding. The extensive speed and agility of the Cobra chassis ensures that only the richest or luckiest criminals will escape.