 Although lignum vitae wood was used for the radial stave bearings in the stuffing box, cooled directly by seawater itself, this material wasn't capable of withstanding the force needed for the thrust blocks of any but the earliest screw vessels.Each horseshoe was independently adjustable forwards and back, by either wedged gibs or a screwed adjustment.A particular problem with these thrust boxes was in adjusting them so that the force was shared equally between all the collars.