Radwraith said:

This assumes you fight the marines in conventional warfare. Why aren't people considering things like tenebro mazes to funnel attackers, murder servitors/crew reclamation combinations to go all Borg on their asses, Alien mercenaries, EM weaponry that will jam their power armour, stealth and cloak systems to avoid detection… there are any number of ways to beat marines that aren't as straight forwards and suicidal as "fight them on their own terms".

Kasatka said:

This assumes that the Astartes are willing to walk into YOUR trap! Murder servitors (A form of combat or gun servitor) do not function cohesively without the presence of a tech priest which makes him a primary target. Also, While murder servitors are tougher then your average human or alien trooper I would not necessarily say the same for an Astartes! The same is true of Xenos troops. The closest servitor like construct to a SM is a Necron immortal. I don't think you want a bunch of them running around a ship! Space marine power armor is specifically radiation shielded if you read the fluff. I guess the point is that the Astartes are a breed of being that is genetically designed and lives for the making of war! Their Officers have centuries of experience (Individually)! and are VERY versed in both conventional AND unconventional warfare. Tactically Marines will target areas of the ship not easily covered by the Tenebro maze or cloaking systems. Namely, The Bridge and Enginarium. Cloaking and stealth systems are of limited use in close combat because you still have to bring the Marines down once they are in Close quarters. Don't get me wrong Katsaka; No one is Invincible! SM (Though they are pretty **** close!) included! But it is not as easy as your post made it sound. If a single company of SM is enough to bring a world to it;'s knees then your (relatively) puny ship population is not much by comparison!

I'm actually having a hard time seeing where people are figuring chapter serf crews are going to be some of the best humanity has to offer. These are space marine washouts, usually from feral and deathworlds, primitive cultures etc. In short while they might be good stock as far as fighting men they likely make poor ship operators. I could see maybe a standard of crack crews, but anything beyond that seems unlikely as it's just not their focus. Just my two thrones.

According to wh40k.lexicanum.com/wiki/Chapter_Serfs#.T8EVQ79G4as most chapter serfs are going to be descended from failed recruits. Furthermore, they "are educated and trained to a much higher standard than most other servants of the Imperium, and have access to a better range of equipment." Since, at a minimum, they're supposed to be trained up to at least "Imperial Guard" level I'm assuming that the crew of a Strike Cruiser will be equivalent to an experienced naval vessel's crew.

You can assume that, I believe your wrong, again, good fighters != good voidsmen.

Here's what i have so far. I'll work up the strike cruiser a little later as well as some NPC space marines for it.

Invariably the Gladius is the most numerous escort within a chapter-fleet, being one of the few escorts in relatively uniform usage across the chapters. The vessel, although larger than any genuine gunboats likely to be in use, is relatively small by Space Marine standards, seldom carrying mush more that a single squad of Space Marines who are invariably spread across its decks and control centers supervising the much more numerous chapter serfs upon whom the chapter relies for actual operation of its vessels. As such, the Gladius lacks the genuine punch of other Space Marine vessels, and is of little use in their favored planetary assaults, but instead earns itself an admirable place as the foremost ship of the line when it comes to more mundane border patrols and deep space blockades.

Gladius-class Frigate
Hull Type: Frigate
Dimensions: 1.8km long, 0.7km abeam at fins approx.
Mass: 5.8 megatonns, approx.
Crew: 23,000 crew, approx.
Accel: 5.4 gravities max acceleration
Speed: 10
Maneuverability: +15
Detection: +10
Hull Integrity: 45
Armor: 21
Turret Rating: 2
Space: 55
Weapon Capacity: 2 dorsal
Morale 103
Crew 100
Spacemarines: 6(Ship's secuirty Detail plus command staff)*
2x Best Quality (Increase Range Reduce Crit Threshhold)Mars Pattern Macrocannons All Ships 4 2 1 3 1d10+3 Crit 4 Range 7
Extended Supply Vault
Munitorium
Temple Shrine to the God Emperor (Duh)
Drop Pod Launchers (In case the ship ever has to host space marines on special operations work, likely scout squads)
Armored Bridge
Clan Kin Quarters
Armored Plating
Void Shields
Energistic Conversion Matrix (Trade 3 power for 1 speed)
*Smace Marines!: Space Marines are the most devastating troops the imperium can field and arguably some of the best in the universe. They have the following affects
All damage inflicted as part of hit and run raids is doubled.
When space marines win an opposed command test while on defense they add 1d10 to the amount of crew losses to the attacking side.
All command tests involving space marine forces receive a +20 bonus to the roll.
A ship with an active space marine rating never receives penalties to command tests from loss of regular crew population or morale.
Space Marines can be eliminated through damage to their vessel. For every 6 points of damage to crew population the attack may instead elect to destroy one space marine.
Resilient: Once space marines have begun a hit and run action against a target they are incredibly hard to dislodge and will remain rather then evacuating. Space marine forces onboard will attempt a hit and run opposed command test every turn after successfully arriving on the ship only truely being driven off if they loose the command test by 3 or more degrees of success. These "bonus attempts" are cumulative with additional oncomking forces.

Ship stats taken from dark reign. I've mostly just eyeballed what i need to not get bogged down on the petty details so not a lot of squeezing for best quality componetns unless they made sense (like weapons). I'll likely add

The much more canon BFG rules for Space Marine vessels (available elsewhere for free), also emphasise the fact that the Astartes are absolute murder in hit and run, and boarding actions. Aside from planetary assaults, close actions against enemy ships tend to be one of their talents (greatly facilitated by their Power Armour).

You talk about Chapter serfs being "washouts" like they're pathetic wrecks. Aside from lack of physical ability there is also genetic compatability that can disqualify a potential recruit. Those kept on as Serfs will largely be in peak physical condition, and with women around they can always make more. Not to mention, Space Marines aren't going to keep people around without heavily indoctrinating them, and generally holding them to high standards (for mortals). This may be through conventional means, of ritual and the general prestige of serving the legendary Space Marines, or through hypno-doctrination. Maybe a bit of both. Access to better equipment than usual wouldn't be out of the question for Space Marines, either.

Alasseo said:

Honestly, this always seemed like a weak argument to me. The marines are meant mainly for planetary assault, true, and the Navy is supposed to be able to outgun them if worst comes to worst, also true. But Macrobatteries are too imprecise to use for orbital bombardment, and torpedoes would cause too much collateral damage. Orbital bombardment--when not done with the aptly named Bombardment Cannon--is almost always done with a 'Lance Strike.' Thus Lances would seem to be the most mission-relevant weapon to outfit their frigates with (since ships that small count mount a bombardment cannon).

As for the Fleet v. Fleet aspect, Lance weaponry is certainly powerful, but I'd argue not as useful as macrobatteries. The Imperium's main foes in space are countless Tyranid swarm ships, an endless Ork tide, and those perfidiously Holo-field protected Eldar pirates. In each case, being able to perform target saturation with massed macrocannon fire would seem to be infinitely more useful than trying to take the hordes on one ship at a time with lances. So the argument doesn't really make sense because most Navy ships aren't equipped with Lances in the first place.

Finally, torpedoes are the weapon designed for ship-to-ship combat. Lances are powerful and accurate, but a single torpedo can cripple a cruiser whereas a single Lance will just bounce off that same Cruiser's shields turn after turn. This is balanced somewhat by the fact that torpedoes can be intercepted, but Space Marine frigates, with their increased speed, maneuverability, and armor are infinitely more likely to get into a good firing position than an equal number of Cobras. Not to mention Thunderhawks being a superior platform for running torpedo cover duties than Furies. You will note that SM Strike Cruisers do not have torpedo launchers. If anything, I'd think the Navy would be most upset that the marines are using a torpedo gunboat.

Besides, Bombardment Cannons are already like better Lances anyways

I haven't gotten around to pitting my PCs against loyalist marines, but if I ever do, they'll encounter Nova/Gladius/Hunter escorts at a 2:1:0 ratio.

Firaxin said:

Actually, i think you'll find that Eldar, be it corsairs, craftworld or dark varieties, are a fairly small portion of the Imperiums star-bound foes… Chaos reavers and regular pirates, renegade naval ships and independent human realms are a large number of the vessels in the stars, then there's also the burgeoning Tau empire and their assorted allies (Kroot, Nicassar, Human Auxilliaries etc).

As for the Tyranids, the Imperium haven't done anything to adapt their naval vessels to fighting them yet - timeline wise they've only been encountered in the last few hundred years and that's about a 5th of the time needed for forgeworlds to turn around and come up with new stuff!

Kasatka said:

Ultramarines have, they have a Battle Barge, "Sedito Oprimmaire" (or something like that) where they stripped out the weapons batteries for massive Lance arrays to deal with Tyranid ships at long range. It's incredibly powerful in the Battle Fleet Gothic game and I'd imagine would be in RT.