Thrust block

The thrust box was built of a box-like cast iron housing with a radial bearing at each end and a number of collars formed on the shaft between them.

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.

Improved understanding of the theory of lubrication films (initially by Reynolds) allowed the development of much more efficient bearing surfaces.

Each lubricant "wedge" can only be of a limited length (in the direction of travel, i.e. circumferential) so multiple pads are needed rather than a single ring.

The need for an efficient thrust block became even more important with the advent of steam turbines, with their higher propeller speeds.