Well, I feel like the ship's too... messy. There's all kinds of things haning off it, when in the sketch, it's all much more... well, intigrated, and flat. Those turrest are much to large. Keep something in mind.. this ship is 1.5 kilometers long. For Use english sytem people, that's about a mile long! Thos turrets don't need to be big... As they are now, I estimate they're almost a football field, or larger... Which isn't right. Try to keep the dimensions from the sketch.
also, I'd like to see an untextures model, before I see one with a texture. It's hard for me to tell the geometry of the model witha texture on it....
Just some thoughts.
--Chris