Abstract: A crane barge mounting a boom crane aft of its longitudinal center of flotation is provided with a plurality of box-type outrigger barges coupled at its sides to limit list and trim of the crane barge and so protect the crane boom from collapse. The outrigger mounting structure for each outrigger barge is comprised of a pair of removable spaced-apart parallel beams extending horizontally from moment-resisting connections at beam seats on the edge of the crane barge deck to structural hinge or pivot mount connections over the transverse center of flotation of the outrigger barge, spacing the outrigger and crane barges by such distance as to avoid contact of their sides on listing of the crane barge, so that the deck of the outrigger barge may provide a level working surface. Each outrigger barge is so ballasted that the mounting bores for the beams on the beam seat and on the pivot mount are at the same level, when the crane is unloaded, providing easy insertion and removal of connector pins.