Since the industrialization of maritime transportation, ships have become increasingly larger. However, as vessel size has grown, a variety of technical problems have also emerged. Propulsion system failures such as propeller fracture, shaft system damage, severe vibration, and even main engine crankshaft breakage have caused significant economic losses and, in some cases, casualties.
Extensive research into these issues has helped engineers gain a deeper understanding of marine propulsion systems. One notable historical example occurred in 1894, when the newly built British destroyer HMS Courageous was discovered during its first sea trial to produce only 3,700 horsepower, about 7.5% lower than its rated power. The ship achieved a speed of only 24 knots, far below its designed speed of 27 knots. For a warship, speed is a matter of survival, so such a large discrepancy raised serious concerns.
The British military was extremely dissatisfied with the performance of the vessel. Engineers repeatedly modified the propeller design, even when severe vibration appeared at the stern. However, the issue remained unresolved.
During the sixth modification, engineers increased the propeller blade area by 45%. In the next sea trial, the ship surprisingly achieved the target speed of 27 knots. This unexpected success led to deeper studies of propeller efficiency and hydrodynamic behavior.
During World War II, German submarines sank a large number of British supply ships. Britain had no choice but to turn to the United States for assistance. The American-built Liberty ships could be constructed in an average of only 42 days, which was an impressive achievement in shipbuilding speed.
However, many Liberty ships experienced cracks and fractures at the large tapered ends of their propellers, and in some cases the propellers even fell into the sea. These incidents raised serious concerns about propulsion system design and durability.
Since the invention of the propeller, marine propulsion systems have faced numerous engineering challenges. In solving these issues, engineers gradually realized that ship propulsion involves far more than simply combining a propeller, shaft, and internal combustion engine. Instead, it requires complex engineering calculations and hydrodynamic analysis.
The earliest boats did not have propulsion systems in the modern sense. Historical records show that early Chinese watercraft were created by hollowing out large tree trunks. These simple vessels allowed people to float and travel across water.
The earliest propulsion tools were likely paddles—simple wooden boards used to push against the water along the side of the boat. Later, more advanced rowing tools appeared, including oars and sculls.
The scull was a more efficient device typically installed at the stern of the boat. One end was fixed to the vessel as a pivot point, while the other end moved through the water. The underwater cross-section of the scull resembled the shape of an aircraft wing. By rotating the handle manually, continuous propulsion could be achieved without lifting the blade out of the water, making use of the lever principle for higher efficiency.
During the Tang Dynasty, an inventor named Li Gao attempted to install wheels on both sides of a vessel to improve propulsion efficiency. This innovation led to the development of paddle wheel ships.
Later, during the 18th century, the First Industrial Revolution introduced steam engines to many industries. Engineers began installing steam engines on ships and connecting them to paddle wheels at the stern.
These paddle wheels were partially submerged in water and significantly increased propulsion power. This marked the first time humans were able to break free from reliance on natural forces such as wind or human labor, using mechanical power generated from fuel combustion instead.
In the early 19th century, European engineers invented the marine propeller, one of the most important innovations in shipbuilding history. The propeller dramatically improved propulsion efficiency by converting rotational torque into forward thrust.
This invention fundamentally changed the way mechanical power was transmitted to move ships forward. Interestingly, when the propeller was invented, airplanes had not yet been developed. Later, aircraft propulsion would also rely on propellers that convert rotational motion into linear movement.
The conversion between rotational motion and linear motion became a fundamental principle in many mechanical systems, including electric motors, pumps, and internal combustion engines.
The combination of propellers with internal combustion engines greatly increased the amount of usable power available to humans. Ships were no longer dependent on wind or manual rowing.
This technological advancement enabled the construction of extremely large vessels, such as container ships capable of carrying more than 24,000 containers and oil tankers with capacities exceeding 300,000 tons. These developments dramatically increased global transportation capacity and trade volume, supporting the rise of economic globalization and industrial cooperation.
As marine propulsion technology evolved, engineers observed unexpected phenomena. After a period of operation, certain areas of ship propellers developed numerous holes, and the tips of the blades appeared heavily eroded.
Further research revealed that this damage was caused by cavitation, a phenomenon in which vapor bubbles form and collapse rapidly in low-pressure areas around the propeller blades. Cavitation can significantly damage propeller surfaces and reduce propulsion efficiency.
Some ships experienced cracks in bearings, broken couplings, and twisted or deformed shafts after operating for a period of time. These failures forced vessels to stop operations for repairs, causing significant financial losses and safety concerns.
In some cases, investigators initially suspected improper shaft alignment during ship construction or weak bearing support structures.
For example, the British aircraft carrier HMS Queen Elizabeth experienced multiple shaft coupling failures. Researchers suspected that improper shaft alignment might have contributed to these failures, highlighting the importance of precise engineering even in basic shipbuilding processes.
As large mechanical systems developed, engineers began conducting extensive research into propulsion system failures. These studies contributed to the development of important scientific fields such as hydrodynamics and gyrodynamics.
A deeper understanding of vibration behavior helped engineers explain problems such as propeller shaft breakage and structural fatigue.
Even after solving many early propulsion problems, new issues continued to appear. Some vessels experienced main engine crankshaft fractures after long periods of operation. Others suffered from gearbox damage or cracked elastic couplings.
Further research revealed that many of these failures were related to torsional vibration within the propulsion system.
As a result, modern ship propulsion system design must consider torsional vibration, vortex-induced vibration, and axial vibration from the earliest design stages. Shipbuilding in the modern industrial era requires far more sophisticated engineering than simply connecting a diesel engine, shaft, and propeller.
In recent years, some ships have adopted stainless steel shafts to improve environmental performance. However, inspections in dry docks have revealed corrosion pits appearing on the shaft surfaces.
If these corrosion pits continue to expand, the shaft could eventually fracture, posing a serious safety risk for the vessel.
With advances in technology, ship electrification has become an important trend. Electric propulsion systems are increasingly used in modern vessels, fundamentally changing the way ships are powered.
However, electrical propulsion systems have introduced new challenges. For example, electrical pitting can occur on bearings, affecting their functionality and the stability of the shaft system responsible for transmitting mechanical power.
In severe cases, bearing damage can become serious enough to require replacement of major ship components.
Modern vessels rely heavily on electronic equipment and electrical systems. Occasionally, ships experience unexplained electrical failures.
For example, a vessel may operate normally during docking and departure maneuvers. However, when the ship is moored and the bow thruster is tested, other equipment such as power transformers may suddenly fail.
Similarly, when using a shaft generator, some electronic systems may stop functioning properly. In certain cases, simply shutting down the shaft generator resolves the issue.
The development of ship propulsion systems demonstrates how complex modern engineering has become. From early wooden boats powered by paddles to today’s sophisticated propulsion systems involving diesel engines, propellers, electrical drives, and advanced control systems, marine technology continues to evolve.
Understanding hydrodynamics, vibration behavior, electrical interactions, and mechanical design is essential for ensuring the safety, efficiency, and reliability of modern ships.
As shipbuilding technology advances, engineers must continue researching and solving new challenges to ensure that marine propulsion systems remain efficient, reliable, and safe for global maritime transportation.
