Newton’s third law of motion states – every action has an equal and opposite action, though not always obvious (in a literal sense), it’s the essence of anything that moves us through the world. Ships are powered by propellers and they too use Newton’s third law, as it is the propeller which pulls or pushes the vessel forward by hurling a mass of water behind it.
The Marine Engine
In a marine engine the function of the thrust block, propeller shaft, and stern tube are closely related, being responsible for the efficient transmission of the engine’s power to the propeller and ensuring the control of torque and propeller shaft alignment from the thrust block to the stern tube.
The thrust block is situated just aft of the main engine and its purpose is to transmit the torque produced by the rotating propeller and shaft down into the ship’s structure. The propeller shaft runs between the thrust block and the stern tube and is supported by a number of shaft bearings fitted along the length of the shaft. The stern tube contains supports and several seals for the propeller shaft, as it passes through the ship’s stern before connecting to the propeller.
The different parts of a propeller move at different speeds, the tips of the blades move faster than the parts close to the hub. To ensure a propeller produces a constant force (thrust) all along its length, the angle of attack needs to be different at different points along the blade and that’s why propeller blades are twisted. Without the twist, the propeller would be making different amounts of thrust at the hub and the edges, which would put it under great stress.
Thrust Block Purpose and Operation
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. 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 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-pressurised) 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.
The propeller shaft is bolted to the main engine flywheel, passing through the thrust block then along the shaft tunnel. Here it is supported by the shaft bearings before passing through the stern tube to drive the ship’s propeller. The shaft coupling flange faces are accurately machined and the bolt holes reamed to accept fitted bolts. They are bolted together using high tension bolting, which is tightened using hydraulic tensioning gear.
The supporting bearings are cast in two halves and are usually white metal lined. These have oil scrolls cut into them to distribute the splash lubrication. Nowadays ball bearing shaft supports are being used, but they have been reported as being quite noisy with a tendency to run hot.
Purpose and Operation of Stern Tube
The stern tube is used to support and seal the propeller shaft as it passes through the aft hull. It consists of a cast iron tube welded into the stern frame. Nowadays, the propeller shaft has a CUNI (copper nickel alloy) liner shrunk onto it. Babbitt metal is applied over this and then machined, providing the bearing surface between the cast iron stern tube and the propeller shaft.
This is lubricated and cooled by lube oil supplied from a gravity tank located under the aft peak. The propeller shaft has mechanical and/or adjustable gland seals fore and aft to prevent the ingress of oil to the sea and the aft bilge well.
Copper alloys are widely used in marine applications because of their resistance to corrosion, good machinability, as well as their thermal and electrical conductivity. Bronze alloys, in particular, are the preferred metal for casting large ship propellers.
Friction in Engines
Friction is a necessary evil and it opposes the relative movement between any two objects which leads to loss of energy and hence wastage. Since there are lots of moving parts in any engine including marine diesel engines, there needs to be a method to take care of the same. Friction cannot be eliminated completely but can be reduced by applying appropriate lubrication techniques.
After all mechanisation and technology works in cohesion, it is the round-the-clock vigilance and tireless efforts of the marine engineers which ensures that the ship engines perform at their optimum capacity.
Sea News Feature, February 18