Grav plates are wierd. I treat them as magic, and move on because the grav field they create is the illusion of tens of thousands of tons mass, without the inertia. Theoretically, if you roll over so you can look "up" at a planet, you should be drawn in at about, oh, 10,000 m/s2.
And attempting to explain congragrav and grav plates as one phenomena is pretty useless: the grav plates are at least 4-5 orders of magnitude more difficult to do. And trying to explain by talking about gravity waves just reveals you never studied field theory. Deck plates, not jump drives, is the real fantasy element in traveler.
Contra grav is comprehenible. For example, you can "drag the metric" to create a negative gravity field of about -5,000 Kg (without the 5 tons of inertia) that is repelled by the planetary gravity a little bit harder than a 4,000 Kg structure and payload is attracted, so the air-raft is lifted in the air. Of course, as we currently understand it a -1 ton grav pod will require control of a black-hole to build and contain a couple of gigatons of potential energy, but that's just engineering.
Maneuver Drive
Back in the seventies and eighties, I treated this as a
Davis Mechanics device. Davis mechanics are a correction to the 3rd derivative of Newtons laws that allows angular momentum to be converted to linear momentum. These are real, from the 1960s, never proven but never disproven equations. (If you can convert the momentum, change in kinetic energy becomes minor if non-trivial.)
An alternative would be a
Woodward drive based on Machs Principle. NASA has been looking into this, but I think it is going to require a lot more energy to be useful than people expect.
http://inetarena.com/~noetic/pls/woodward.html
And from NASA's dying Breakthrough Propulson Physics office:
http://www.grc.nasa.gov/WWW/bpp/summ.htm#Cramer%20Task