||Year of Hell
|Class Name :
Temporal incursion platform |
|Unit Run :
Only a single vessel is known to have been created 
||Length : 818.54 m 
Beam : 197.9 m 
Height : 206.89 m 
Temporal incursion weapon 
x Conventional weapons; details unknown 
|Defence Systems :
||Vessel is isolated from space-time and invulnerable to conventional weapons  hull.
(TNG scale) :
|Normal Cruise : 6 
The Krenim are a Delta Quadrant species, and one of the few species in the galaxy besides the Federation which use temporal technology. The Krenim Empire is located many tens of thousands of light years away from the Federation, and as such the encounter was limited to the interactions centred around the USS Voyager.
Voyager encountered several alternate timelines as a result of the Krenim. In the Zahl-dominant timeline the Krenim Empire had been overrun and destroyed by a species called the the Zahl, and the few remaining Krenim ships were small patrol craft with limited weaponry. These remnants took to accosting travellers through Zahl space, claiming the area as their own and threatening destruction to the trespassers. Such a vessel accosted the USS Voyager, although it lacked the weaponry to make good on its promises.
The catalyst for the creation of the other timelines was the Krenim Timeship. The Krenim are one of the very few species which employ Temporal weaponry on a regular basis. In the Zahl-dominated timeline many Krenim ships were equipped with Chronaton torpedoes which, being in a state of temporal flux, were able to penetrate conventional shield technology. While these weapons did not prevent the Zahl victory over the Krenim, a more ambitious system did offer some hope. The Krenim Timeship was a large craft which was able to perform temporal incursions, removing specific objects from space-time in both past, present and future. This act would generate a temporal shock wave which would rapidly generate an altered timeline. The Timeship operated defensive systems which allowed it to operate outside of normal space-time, rendering it invulnerable to all conventional weaponry.
The Krenim ship was commanded by a scientist named Annorax, a man who was obsessed with restoring his home and wife to life again by preventing the Zahl from destroying the Empire. He spent over two subjective centuries on this mission, conducting incursion after incursion without success. Even when the removal of the Zahl home world was successful in restoring 98% of the Krenim Empire, Annorax was determined to continue because his own colony remained unrestored.
With the restoration of a Krenim-dominated timeline, the Krenim Empire spread across 5,000 parsecs and 849 inhabited worlds. In this timeline Voyager was under near constant attack by the Krenim, whose temporal torpedoes were easily able to penetrate her shields. The Federation ship took heavy damage but after over two months of almost continuous attack, Voyager managed to improvise a form of temporal shielding which isolated the ship from the changes the Krenim Timeship was making to the timeline.
The next Krenim incursion - which destroyed the Garenor home world - left Voyager unchanged, but the new unexpected factor of the ships temporal shielding badly altered the effect of the incursion on the rest of the timeline. The entire Krenim Empire reverted to a pre-warp state, leaving Voyager still badly damaged but without most of her enemies. The Timeships temporal scans revealed the source of the problem, and Annorax intercepted Voyager and kidnapped two members of her crew - Lieutenant Paris and Commander Chakotay - before attempting to destroy the ship with his main temporal weapon. Voyager was able to escape by outrunning the Timeship, whose large mass limited it to Warp 6 at most, but suffered near-catastrophic damage in the process. The ship spent some time in this Krenim-prewarp timeline before another incursion destroyed the Ram Izad system and restored 52% of the Krenim Empire.
Captain Janeway ordered the bulk of her crew to abandon ship  and hid Voyager inside a nebula. Despite further equipment difficulties, Janeway was eventually able to forge an alliance with the Mawasi and the Nihydron, two local species who were eager to help Voyager hunt down the Timeship once they realized the damage it was doing. Voyager led a fleet of three Mawasi ships and two Nihydron cruisers into battle against the Timeship. On board the Krenim vessel, Paris and Chakotay were able to disable the temporal core to allow the fleet to attack with conventional weapons, but they were unable to defeat Annorax and he disabled all of the ships. Captain Janeway ordered the fleet to take their temporal shielding off-line and rammed the Krenim ship broadside, rupturing the temporal core and causing one final incursion.
The act destroyed the Timeship and restored the preferred timeline completely. In this timeline the Krenim Empire was largely benevolent, and advised Voyager to avoid their space as it was in dispute. Voyager proceeded on its way as normal. 
Seen in the Voyager "Year Of Hell" two parter, the Krenim Timeship is one of the more intriguing - or improbable, depending on how you look at it - ships we've ever seen. The Krenim and the year of hell concept had been established in the episode "Before And After", but only in a fairly sketchy outline which included no mention of the Timeship itself.
As seen in "Year of Hell", the Timeship is a device which is capable of rewriting history. This is nothing new - we've seen episodes which involve some action that changes the future many times in Trek, right back to the TOS episode "The City On The Edge Of Forever". However, the Timeship works in a slightly different way - instead of going back into the past and effecting some change, the Timeship fires a beam of some kind of energy which removes the target completely from the timeline. The result is a "temporal shock wave" which spreads out through space, changing everything it passes over to the way they would have been if the Timeships target had never existed at all. Hence if you fired the Timeships main weapon at, say, Germany, then the world would literally vanish and be replaced by a world in which Germany had never existed.
On the face of it this is a pretty impressive technology, but it has its drawbacks. Namely, the universe is an insanely complex place and many of its various bits interact with one another in ways which are both subtle and gross. Most people have heard of the so-called "butterfly effect", which encapsulates the idea that a very small change such as the flap of a butterfly's wings can have a very large scale and unpredictable effect such as a hurricane half a world away. This is the basis of Annorax's problem - in order to restore his Empire, he must know exactly where to intervene in the timeline. This process can take months of calculations, and still doesn't work most of the time - indeed, in the episode the Timeship never once achieves the desired results with any of its incursions.
The Timeship itself is stated to operate "outside space time", and this apparently confers several advantages. Annorax indicates that he and his people have all eternity to complete their task; the obvious implication is that they are not ageing, but this is improbable to put it mildly. Since Annorax and his crew are able to move around and breathe, time of some sort obviously does exist on the ship. Their bodies continue to operate, so chemical reactions are still working - surely they are indeed ageing. My guess would be that Annorax meant that he could take a long time to complete his task because once accomplished those restored would not have been aware that there had ever been a problem. As an aside, it's possible that the Krenim are normally immortal anyway, so justifying his "all eternity" phrasing.
Another advantage of this "outside time" effect is that the Timeship is invulnerable to conventional weaponry - Annorax specifically says so when Voyagers fleet attacks at the end of the second episode. The episode gets around this by having the ships 'temporal core' sabotaged to make it vulnerable.
When the Timeship first encounters Voyager, we are told that the ship's mass limits it to warp 6 at most - this is, amazingly enough, the first canon statement to specifically establish that the mass of a ship is a factor in how fast it can go at warp speed!
Scaling the Timeship is a difficult problem. What I did was make a video capture of the scene where Voyager rams the Timeship and compare the width of the saucer section with the width of the circular feature which it hits. After various estimates, I get the circular feature to be just about one half the width of Voyagers saucer section, which is 128 metres across. With the circular feature 64 metres wide the whole Timeship comes to 1,240 metres - about the size of a D'Deridex class Warbird.
One final and faintly amusing note. As British readers will know, the name "Annorax" is something of a play on words over here. There is a kind of coat called an "Anorak", and in the UK at least this coat has become heavily identified with what the US would know as a Nerd - to such an extent that the word Anorak is virtually interchangeable with Nerd (I have no idea if other countries have anoraks, or if they have the same reputation). The word is often levelled at Trekkers over here, and I often wonder if the writers of this episode had this in mind when creating the character of Annorax.