It's because nobody ever uses the prow. It's just "fire everything as soon as we're in range and continue to fire everything until the reactor blows and kills everything." Yes, I've read a few books where the prow is used, but it's never as awesome as you'd want it to be. More of a "hey, there's a broken ship in our way. Might as well just hit it instead of wasting a shot from my GIANT FUCKING LASER"
from my experience, if you were getting in range to ram, you were also getting in range for me to plaster you with bombardment cannon fire and un-avoidable torpedoes long before you would make contact.
Exactly why chaos never misses it. They don't need it to break apart crippled ships since they're just going to scavenge it anyway and breaking it more is counter productive.
Chaos ships mostly date from before the heresy in design... it's actually explained quite well in the battlefield gothic books as to why they don't having them... at the time the imperial army (the combined imperial guard/navy/space marine entity) had relied heavily on lance batteries and torpedo strikes to attack and small marine detachments to board ships via boarding torpedo and assault craft. These ships are designed more similar to marine battle barges then the later ships. The addition of prows was a regression and tactical decision which allowed massive armored plating to be put to forward to deflect incoming fire as weapon batteries came more into use. The "ram" isn't so much a mere weapon as armor which gives imperial ships the best forward armor in the game (6+ to damage a ship). It's pretty interesting all told. I can post more if people are interested when I get home.
I rammed all the time, every one laughs at my Gothic cruisers. Right up until they go full speed ahead and hit you in the face for 8 4+ damage rolls from 49cm away. After doing that, if my gothic survives, it can still shoot its 4 lances per broadside and the torps up front. One hell of a fire ship.
Really the key difference between the two is still their originating doctrine. The modern Imperial Navy has established a torpedo line doctrine and built their ships to emphasize maximum protection on the way in to broadsiding whilst the older doctrine, involving vessels still in use by traitor forces, focused on bombarding the enemy from a mixture of range and close in fire, like a pack of wolves. All these points are moot though, as Orkzez iz best. What's that you say, a line of cruisers? Meet Inigo Ramship, you killed his buddy, prepare to die.
Damn right its on a collision course. There is only two modes for a proppa' Ork, rammin' and smashin' and ya smash before ya ram!
Can't wait to show the traitors the fury of this fully armed and operation battl- I mean, WAAAAAAAAAAAAGH!!!!!!!! Gameplay looks pretty good and detailed too, so here's hoping it stays as tasty as it looks when it gets released.
Again the Tau get ignored..... oh well.... orkz iz bests.... WHAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAGH!