Thrust Block Purpose and Operation
When I was at sea as a first tripper Junior Engineer, I well remember asking an old Auzzie chief engineer about the purpose of a thrust block. He thought for a few moments, and then answered that if there was no thrust block the propeller and shaft would try to push the engine into the foc’s’le head. That was over forty five years ago, and it was not a bad analogy.
A more technical answer would have been that the purpose of a thrust block on a large marine engine is to transmit the torque produced by the rotating propeller through the housing hold-down bolts into the ship’s structure. (Remember this definition when sitting your marine engineer’s exams.)
The structure under the thrust block is reinforced with steel plates and I-beams above and inside the double bottom tanks or cofferdams.
The thrust block itself consists of a housing which contains a number of wedge-shaped white metaled pads with generous helical oil grooves cut into them. The pads are arranged and fixed around a steel support, holding them against a machined collar on the drive shaft.
The pads are prevented from overheating and premature wear by a fluid film of oil between them and the collar, with the oil supply being hydrodynamic (self-pressurized) due to the rotation of the drive shaft.
There is an oil reservoir in the bottom of the housing which may contain an oil-cooling coil through which seawater is circulated.
I have included a sketch of a Mitchell type thrust block of the type used when I was a boy at sea. However, it will not have changed much except perhaps the pads may now be PTFE lined.