You move flat out/t-boost during shooting phase so 18"/36" extra. See moving phase for vehicles. Fast vehicles flat out 12", whilst fast skimmers flat out 18"; normal skimmers just flat out 6" like other normal vehicles.
The changes and exceptions to the normal 6" flat out of vehicles is stated in the individual unit types.
Conclusion, vehicles can only move up to 12" in moving phase, flat out moves are forever made in the shooting phase in 6th ed.
I'm curious to wonder why you would say a skimmer could move 12" and then 18". Unless I guess the interpretation behind was = 12" Vehicle move(movement phase) and then 12" Vehicle move + Flat out move (shooting phase).