A device for guiding a boat onto a trailer, the trailer having at least two longitudinal side rails, the device including a mounting assembly for attachment to a side rail to provide for transverse adjustment relative to the side rail's longitudinal axis and a bumper assembly attached to the mounting assembly and having at least one bumper mechanism that moves between an engaged position in which the device guides the boat into an aligned position on the trailer and a disengaged position in which the bumper mechanism is moved away from the boat to avoid contact with the boat.