All Components of the Marine Propeller Assembly

Rubber Bush Assembly

This component acts as an absorber and eliminator of vibrations, shocks, and forces caused by sea waves impacting the propeller and gearbox shaft. It mitigates these vibrations and shocks. In stormy and wavy sea conditions, heavy waves with significant energy collide with the vessel, exerting substantial forces on the propeller assembly. These waves directly impact the vessel’s propeller, transmitting considerable vibrations, shocks, and forces to it. Due to the direct connection between the propeller assembly and the gearbox, these vibrations and resulting forces are transferred to the connecting shaft (between the gearbox and propeller) and gearbox components, which can lead to damage or failure of the shaft or gearbox parts. In such cases, to prevent more severe damage, less expensive components (such as the propeller) are designed to fail first, protecting more critical and costly components like the connecting shaft and internal gearbox parts. The rubber bush serves as a sacrificial component (similar to a fuse) within the propeller assembly, preventing heavier and more costly damage. When the rubber bush fails, the connection between the propeller and the gearbox output shaft is severed, rendering the propeller inoperable (in some cases, due to the high rotational speed of the connecting shaft and its energy, the propeller may be ejected into the water after the rubber bush fails).

It is worth noting that the yield strength of the rubber bush must be such that it fails before the connecting shaft or gearbox components are damaged. This means the torsional (shear) strength of the rubber bush must be lower than that of the shaft or gearbox components.

Spline (Inside the Rubber Bush)

This is a spline (involute spline) component, typically made of bronze to resist corrosion in the humid marine environment. It is installed inside the rubber bush. The gearbox’s torque and power are transmitted to the propeller through the spline on the gearbox output shaft, which connects to the spline inside the rubber bush. This component will be discussed in more detail in subsequent sections of this report.

Thrust Washer

The gearbox output shaft is long, and if not supported by a retainer before entering the propeller, it may experience bending (commonly referred to as “sagging”), leading to deflection. This issue becomes critical when an aluminum propeller, with its assembly weighing over 4 kilograms, is mounted at the end of the shaft and rotates at speeds exceeding 5,000 RPM. Such conditions generate significant oscillations and vibrations in the shaft, which can ultimately cause the connecting shaft to break, the propeller assembly to be ejected into the water, bearing damage, or even partial or complete gearbox failure. Therefore, it is essential to use a retainer, such as a thrust washer, to protect this critical component from bending.

Another reason for using a thrust washer is to protect the rubber bush from the hot exhaust gases, which are located behind the propeller in the back plate area of the marine propeller. The exhaust outlet, positioned upstream of the propeller (at the back plate), causes hot gases, unburned fuel particles, and tiny oil droplets (resulting from normal or abnormal engine leaks) to collide with the rubber bush. This exposure can degrade and damage the rubber bush, causing it to swell over time, significantly reducing its strength and service life. To mitigate this, a protective plate (thrust washer) is installed to prevent or minimize the impact of these hot and corrosive gases on the rubber bush. The hot exhaust gases are directed into the seawater through a small gap in the propeller body after reaching the propeller.

Thus, the primary purpose of the thrust washer is to prevent shaft bending and avoid severe damage to the propulsion system. Its secondary purpose is to shield the rubber bush from direct exposure to hot and corrosive exhaust gases.

(Note: Gasoline, due to its specific chemical composition, can cause corrosion in the rubber bush even at low temperatures. However, unburned fuel particles and droplets, expelled from the exhaust at high temperatures and speeds due to the rapid combustion process, cause even more severe corrosion. These hot, tiny particles and droplets drastically reduce the rubber bush’s lifespan, leading to rapid degradation. Given the rubber bush’s sacrificial (fuse-like) role, this renders the marine propeller inoperable. By using a thrust washer, in addition to preventing shaft bending, the rubber bush is significantly protected from corrosion, thereby extending the propeller’s service life.)

Spacer

This component, like the spline, facilitates the transfer of torque from the gearbox shaft to the marine propeller. Along with the toothed washer, nut, and split pin, it helps secure the propeller’s connection to the shaft.

Bush

Positioned before the nut, this component, along with the spacer, moves toward the shaft’s starting point as the nut is tightened. Since the spacer is connected to the rubber bush assembly, the entire assembly—including the propeller, rubber bush, spacer, and bush—is firmly secured to the shaft’s starting point, creating a rigid unit.

(In some cases, toothed washers, commonly used in industries to prevent loosening due to vibrations, are used instead of a bush. These components have a self-locking mechanism that engages under vibrations, becoming tighter and more secure.)

Nut

In this system, the nut is responsible for securing the connection between the components, preventing the assembly (the entire propeller unit) from loosening, and ensuring integration. Additionally, this connection facilitates the desired torque transfer between the shaft’s spline and the rubber bush’s spline.

Propeller Dimensions: Length: 163 mm, Body Diameter: 104 mm, Circumferential Diameter: 330 mm.