Cavitation

Several of the articles in this edition of ABB Review relating to ABB Dynafin™ mention cavitation and stress its importance in marine design. But what exactly is cavitation  and why can it be detrimental to marine systems?

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_{Anthony Byatt External contributor}

In the realm of fluid dynamics, cavitation is a phenomenon characterized by the formation and implosion of vapor bubbles in a liquid when the local pressure falls below the vapor pressure. This process occurs when a liquid, such as water, experiences a rapid change in pressure, typically in regions of high-velocity or low-pressure gradients. The formation and collapse of these bubbles create intense shock waves that can have significant detrimental effects on ships, particularly on their propellers.

In marine propellers, the bubbles are formed by the creation of low-pressure regions at the back of the blades as they move through the water at high velocity.

Breaking and shaking

Most marine propellers are actually designed to allow cavitation, as it is then possible to reach higher efficiency. Cavitation must, in such cases, appear as a stable sheet, rolling smoothly over the tip of the blade. When cavitation is unstable, erosion can occur: When vapor bubbles collapse near the surface of the propeller blades, they release energy in the form of small, high-velocity jets. These jets can impinge on the material surface with a significant force, causing pitting and erosion. Over time, these effects can degrade the integrity of the propeller blades, leading to reduced performance and increased maintenance costs for vessel operators.

Furthermore, cavitation-induced vibrations can also degrade the structural integrity of the propeller and the rest of the propulsion system. The pressure pulses created by the collapsing vapor bubbles propagate through the surrounding fluid, inducing vibrations in the propeller blades and other components of the propulsion system, leading to fatigue and potential failure over time. In addition to the pressure pulses, the noise generated by cavitation can also be a concern in terms of its impact on marine life in sensitive areas and on the comfort and safety of the vessel’s crew and passengers.

Cavitation mitigation

Engineers employ various materials and design techniques to mitigate the adverse effects of cavitation on propellers. Propeller designs may feature modified blade profiles or cavitation-resistant materials to reduce the likelihood of cavitation and minimize its effects when it does occur. The computational fluid dynamics (CFD) simulations described elsewhere in this editi on of ABB Review are a good example of how engineers analyze and optimize propeller designs to minimize cavitation-induced performance losses.

Cavitation is a complex fluid dynamic effect with significant implications for ship propulsion systems. While cavitation can enhance propulsive efficiency under certain conditions, its effects are mostly adverse, in the form of erosion, vibrations and noise, leading to performance and integrity degradation over time. The ABB Dynafin team has invested much time and effort in understanding fully the mechanisms of cavitation and its effects. The knowledge gained informed the advanced design techniques and technologies they used to ensure the optimal performance and longevity of the ABB Dynafin propulsor. 

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