A vessel (1), for example a long-line fishing vessel, comprises a cavity (5) defined by upper, forward and lateral wall members (6, 6a, 12, 13). The cavity has a first opening (7) towards the water (W) in which the vessel is intended to float and a first gate device (4) operable to selectively adjust the size of said opening. A force-generating plate member (11) may be arranged in a lower portion of the cavity. (Figure 1 to be published with the abstract)
