Abstract: A boat trailer centering accessory device for fitment to each rear corner of a boat trailer and which is a pair of boom gate frames which can be swung single-handedly and independently from a forward rest position for road transport alongside a trailer and clear of a boat and then rearwardly out over the water, with the trailer on a launching ramp out of the water, to form a centering causeway for a floating boat supported by the water. Each boom gate is formed as a generally quadrilateral shaped closed frame pivotally attached at one corner to a bracket which is mounted rearwardly on the side member of the trailer and is formed as an elongated U-shaped bracket with two short arms connected by a longer arm, one short bracket arm forms a pivot axle for a sleeve bearing mounted on the corner of the boom gate frame, while the longer arm also serves as a stop for limiting the drop height of the frame when it is swung to the forward transport position from the rearward centering position.