Few people has had similar problems moving vehicles. The sucject is some what complicated and too long to explain in the forum. It is better to have some working script and move from there. I have not answered because you need a feature the was not esposed in plug-in 1.2, any atent to do it with 1.2 will result in a mediocre implementation.

This weekend we will release a patch 1.5 with vehicles action (partcial implementation), bouyancy and other especial effects.
There are pleanty of demos (most of then by Ventilators) that show how to do what you need.

Sorry for the delay
Newton