A vessel barrier system to prevent vessel movement through a passageway includes a flexible vessel barrier element coupled to one or more energy dissipating units. The vessel barrier element may be a single, typically large diameter element, for example a large diameter, high strength synthetic rope, or alternatively may be a barrier net suspended on a number of floats. The vessel barrier element is coupled by a cable to the energy dissipating unit(s), which may be large weights suspended from a frame. When a vessel contacts and moves the vessel barrier element, placing it in tension, the weights are moved from a first, lower position, to a second, elevated position. The kinetic energy of the vessel is transferred to an increase in the potential energy of the weights, and the vessel is brought to a stop. The system is particularly suitable for Large Displacement Vessels or LDVs.