Abstract: A semisubmersible trimaran for use in an afloat sea base is disclosed. The trimaran has an upper deck structure 12 supported on a longitudinal center hull 26 and column-stabilized, longitudinal outrigger pontoons 28. The center hull 26 and pontoon columns 38 and 40 have a reduced waterplane area for minimizing motion during operations. The trimaran has a transit draft 37 for deployment at full waterplane area, and an operating draft 36 with a reduced waterplane area for damping motion. The vessel can be deployed with a transit drive 42 on the hull 26, and dynamically positioned at a destination for operations. Multiple trimarans can be assembled to provide an extended upper surface, including a landing area for fixed-wing aircraft.