The "Rainbow" is a fully submerged hydrofoil catamaran and has a motion control system named APF (Auto Pilot on Foils). Although the original APF could eliminate wave-excited motions, there remained a certain level of vibration. To reduce the vibration, some improvements were enforced on the APF for the "Rainbow 2". The effectiveness of the improvements were verified during the sea trials of the "Rainbow 2", and it was clarified that the vibration levels of the "Rainbow 2" were reduced to about 10% of those of the original "Rainbow". Copyright (C) 2001 IFAC.