Adeptus Astartes Fleet

Each Space Marine Chapter have it's own fleet, in which they can send their troops & support the Imperium in any campaign or conflict. Because of the role the Space Marines have overall in the Imperial war-machine (To support the Imperial Guard Astra Militarum, by fast deployment of Space Marines, either by drop pods, or by Thunder-Hawks), the ships the Astartes receive are different, not just technically, but also in doctrine. Space Marine vessels have the primary role of transporting Space Marines, with the delivery and suppression designed to facilitate planetary assaults. Only the smallest of vessels is at least in theory permitted to act exclusively as gunships, with the larger battlebarges and strike cruisers remaining predominantly as aids to ground support & planetary invasions, that way ensuring the Space Marines would never present a threat to the Imperial Navy proper, in case of another Horus Heresy.

Astartes ships are very well armored, having tons of armor, to resist any planetary defenses the enemy have, and so be possible to deploy and/or bombard the enemy positions on the ground; But also are fast, so they can move rapidly to where is needed, and striking before the enemy can act, and in void warfare, they could go close to the enemy ships, and then make massive assault boardings of battle-ready Space Marines ready to take the enemy vessel or make it empty of any life. Why do you need weapons, when your own crew of soldiers (specifically 8 feet tall super-humans wearing a walking tank as armor) is enough to kill the enemy? In any case, Astartes ships, besides escorts ships, always have macrobatteries as dorsal weaponry, while depending on the model, spinal weaponry is generally macrocannons or bombardment cannons (a turret-mounted linear accelerator that fires magma bomb warheads of much higher calibre than regular torpedoes at a much higher velocity, so its very useful for planetary assault, but its ship-to-ship capacity is appreciated too); And in the prow, either they have launching-bays, heavy lances to strike the enemy from afar & in front of them, or torpedoes launchers to launch either normal torpedoes or boarding torpedoes. Also Battle Barges are frequently used to enact Exterminatus, so they usually also carry a lethal cargo of world-killing weaponry such as Cyclonic Torpedoes and Virus Bombs.

The weakness of these ships is that compared to normal Imperial ships, they aren't as armed as normal ships, and the weaponry they have, being focused in ground support, means that is less efficient & focused on void warfare. Then again, a Space Marine fleets main function is not that, and still a Space Marine fleet is dangerous for any enemy void fleet who dares to fight them, because of the strong armor, relative good speed, and the compliment of Space Marines inside the ships.

Crew

Unlike the vessels of the Imperial Navy, a Space Marine ship has a relatively small crew. A Space Marine is far too valuable to waste in manning a gun or watching a surveyor screen, and so only the officers aboard a vessel are likely to be Space Marines, as well as the few Techmarines who oversee the engines and perform other mechanical duties. Almost all the ship’s systems are run and monitored by servitors, but there are also a few hundred Chapter Serfs to attend to other duties, such as routine cleaning and maintenance, serving the Space Marines during meal times and other such honoured tasks.

Although human, they still benefit from remarkable training and access to superior weaponry than is usually found on a naval vessel, and are quite often augmented in various ways beyond the human-norm; making them a fearsome prospect in a boarding action too! So its not only Space Marines the enemy must fear during an assault. Just because this guy failed the test to become a Space Marine doesn't make them weak or non-dangerous; To merely survive the deadly tests required to become an Astartes is a feat few people have the constitution or mettle for, so it would be a grave error to take those Serfs lightly. Nevermind the other voidsmen and armsmen no doubt on board.

Ship Classes

Battleships

• Battle Barges: The largest class of warship used by the Adeptus Astartes and most Space Marine Chapters control two or three of these potent vessels. The term itself is more of a ship type than a specific pattern; the Adeptus Mechanicus does produce a special pattern of ship just for Space Marines, but any battleship can be used as a battle barge so long as it has the capacity to launch vast quantities of Thunderhawks and Drop Pods and enough weapons and armor to protect them. A standard battle-barge has the facilities for three entire companies to operate out of, including support staff and logistics, such as vehicle repair. The vessel is extremely heavily armored and well-shielded for breaching planetary defenses while also protecting its cargo. It is also a dangerous enemy, especially with boarding actions but also contains enough firepower to destroy all but the most powerful of warships. Battle Barges are some of the most powerful ships the Imperium has at it's disposal, due to both the power of the ships and their contents.

Cruisers

• Strike Cruisers: The Chapter's Strike Cruisers are more common although still rare compared to Imperial Navy ship classes, and weigh slightly less than the Imperial Navy's Dauntless-class Light Cruisers, but the arrival of a single Strike Cruiser is usually enough to quell a rebellious planet, because no one which is not stupid would want to call the wrath of a Space Marine chapter. Strike Cruisers can carry up to one full company of Space Marines and their equipment, and they are primarily used for rapid deployment and are often the first arrive in orbit of a threatened planet and capable of deploying their cargo within twenty minutes of arrival.
• Vanguard Cruiser: A variant of the standard Adeptus Astartes Strike Cruiser that has been refit for long-range operations without the benefit of Escorts or other naval support. This is a variant that is primarily used by fleet-based Chapters of Space Marines that often need to send detachments of Astartes on long-duration missions far from the main Chapter fleet.
• Punisher Strike Cruiser: Not an Adeptus Astartes ship, but a Strike cruiser variant that is for the Imperium FBI, The Adeptus Arbites. Because sometimes implanting the the law inside a solar system requires 5-6 kilometer long space ships to make sure no one breaks it. The Punisher possesses less armour and heavy weaponry than its Astartes counterpart, but possesses specialized scanning arrays designed to locate pirate bases and heretic strongholds in isolated regions of space. Adeptus Arbites ships are few compared to their Imperial Navy counterparts and thus rarely participate in major engagements or joint fleet actions. Nonetheless these ships boast an array of Torpedoes and Bombardment Cannons in addition to a limited amount of Attack Craft which are used primarily for scouting and point defense.
• Grey Knights Strike Cruiser: Obviously, the most special & secret of Astartes chapters would require their own special snowflake ships too! Possessing impressive firepower, heavy armour, tremendous speed and the most advanced technology and the finest engineering of the Adeptus Mechanicus, the Grey Knights fleet of starships would be the envy of the wider Imperium if their existence were known. Their advanced technology is augmented by powerful enchantments to protect them from the perils of the Warp. Grey Knights ships are also piloted by the most trusted and finest Navigators of the Navis Nobilite, capable of steering the most efficient courses through the unpredictable Warp.

Escorts

• Gladius Rapid Strike Frigate: A modified version of the Sword-class, used by the Space Marines as an escort for their strike cruisers. Has 20% more powerful engines than a Sword, making it more mobile, which suits the Marines strategy perfectly. No prizes for guessing why it's called the Gladius.
• Nova Rapid Strike Frigate: A super-special version of the Firestorm for the Marines. As Lance armed ships are optimised for fighting other ships instead of attacking planets, the Navy considers the Nova to infringe on its turf and gets uppity when a Chapter has too many, thus they are uncommon.
• Hunter Destroyer: A Space Marine knockoff of the Cobra. Mostly used by the Dark Angels as their mistrust of other factions means they can't acquire actual Cobras or ask the Navy for help. +++the Dark Angels simply do not want to bother the Navy with their hunt for traitors with no connection to the First Legion