SNEAKY EDIT! What was the problem with your lighting then? And as Cosides pointed out, maybe it is a little dark... are you using the light_environment settings suggested
here that correspond with your skybox? You need to whack an env_sun in too, if you havent already.
Not so big on the propellers though, I thought the gas inside the derigible was supposed to lift it, with the propellers giving it forward motion and steering? The somehow look wrong pointing that way.