Moonpools are used on many types of vessels. Waves and the corresponding motions of the vessel or simply forward speed in calm water can induce oscillations inside the moonpool. A very critical case of oscillation is observed in transit when the moonpool bottoms are left open. This can occur even on a perfect calm sea, as the excitation mechanism arises from the flow velocity in the vicinity of the opening. These oscillations during transit cause a tremendous increase in resistance. Despite the research effort conducted on the subject, most of the solutions to dampen the oscillations in the moonpool are still found by trial and error. MARIN has launched a JIP to obtain more insight in the flow characteristics in and around the moonpool and the mechanism behind the excitation and develop a more adequate design methodology.