Gotta say I like the inertia in these two videos: https://gitea.wildfiregames.com/0ad/0ad/pulls/8957#issuecomment-140218  Especially the ships slowly drifting before stopping looks so much more natural than the abrupt stop now.

So for the explanation again, we could now plug any force that could contribute to unit motion and we just sum up the vectors. So it's almost free there, when calculating unit motion. Maybe excepted we still have to check for collisions. The rest of what makes the unit motion looks realistic is inertia, which is also just a ""force"" vector that carry unit velocity over the game turns. Calculating the forces themselves is generally making some dot products and sometimes maybe some trigonometry (kept as minimum for angle of impact etc) so also possible to be kept very cheap. For the current demo of the boat we use 1 trig function which seems reasonable.

Is the computation heavy ?

OP! Thanks!!

I think @Lopess wants to do that, and I am committed to helping him make the Pull Request for it.