WHAT IS PROPELLER CAVITATION?

Propeller cavitation is the forming and decomposition of small bubbles within propellers, ultimately impacting how the propeller functions. What is the root cause of this phenomenon? What is the most important thing to recognize it? How can you prevent it while on the water?

PROPELLER CAVITATION: THE BASICS

Cavitation as a whole is a “phenomenon in which the static pressure of a liquid reduces to below the liquid’s vapour pressure, leading to the formation of small vapor-filled cavities in the liquid.”

Let’s break it into pieces. The liquid, in this case, is water. It has a distinct vapor pressure, which is the pressure that is typically imposed by gas under equilibrium conditions when it’s in the liquid phase or in the water phase. It’s a fancy method to indicate the tendency of liquid water to evaporate.

In the event that static pressure decreases in a given time, it triggers the water to slowly evaporate tiny gas bubbles are created, similar to gas bubbles that are small in the bottom of the boiling water in a pot. If they are exposed to a higher pressure environment, the bubbles (sometimes known as “cavities,” hence the phrase “cavitation”) collapse, leading to a pressure difference which results in an earthquake.

There are instances of cavitation in various artificial environments, which include not just propellers but impellers, pumps, and control valves, too. There are also examples of cavitation being used in the animal world; for instance, the famous pistol shrimp’s “shock wave” style attack to frighten prey uses cavitation as its source of force.

WHY IS PROPELLER CAVITATION A PROBLEM?

In propellers, cavitation is a problem for many reasons. Most importantly, boaters are concerned about the possibility of losing effectiveness. When cavitation occurs causes a force that hinders the effectiveness of propellers. The boat won’t be able to travel at the same speed and might require more energy to achieve the same speed.

Cavitation can be loud. Propeller cavitation usually is very low-energy, which means that the noise isn’t as ear-piercing, but it’s unsettling, especially when you’re fishing in a quiet manner.

In addition, Cavitation can cause erosion and permanent damage to your propeller system. If you think that tiny air bubbles can’t cause a lot of damage to your heavy-duty propeller on a boat, then you’re right; small instances of cavitation won’t be enough cause a major repercussion on your vessel. The problem is that the damage can build up over time. If you don’t take care of it properly, frequent propeller cavitation could eventually damage the propellers.

HOW TO PREVENT CAVITATION

Let’s look at the practical implications. Do you know how to stop or avoid the cavitation of propellers?

A portion part of this work has been completed for you. With propellers that are better designed and constructed from superior materials, engineers are able to limit the chance of cavitation occurring within a specific setting. For instance, in smaller vessels (with speeds below 35 knots), propellers are fitted with “low-loaded profiles” that feature an elongated shape and narrower angle of attack. Engineers also utilize multi-blade screws that have a larger diameter to create the thrust required to compensate. For vessels that are faster, it is more difficult to avoid through the design of shape.

Unfortunately, molding propellers to different shapes will only affect the performance of the propeller in a minimal manner. An alternative is to alter the material which propellers are constructed from. Prior to recently, the majority of manufacturers of boats used an alloy that was an amalgamation of aluminum, nickel, and bronze that is resistant to erosion without a lot of expense – but this alloy isn’t particularly effective for dealing with the cavitation-related impact.

Today, boat builders are increasingly exploring more effective materials to limit cavitation in propellers. Composite materials, such as carbon fiber or resin, are able to achieve the same efficiency in performance as their traditional counterparts; however, they’re also flexible enough to resist the low energy cavitation force. This is referred to as a “hydroelastic” effect.

If you’re keen on reducing the possibility of cavitation occurring, think about replacing your old propellers with the latest ones that are more flexible.

Additionally, it is important to understand that the strength of the cavitation effect is likely to increase as you speed up – therefore, you can always lower the speed of your travel if you wish to minimize cavitation.

LESS EFFICIENT SOLUTIONS FOR PREVENTING CAVITATION

Other ways to combat cavitation have also been investigated. For instance, Samsung Shipping in South Korea developed and rolled out the nozzle system that makes use of a tiny set of nozzles in order to reduce the effect of cavitation. The nozzles are placed right on top of the propeller and are mounted to the bottom of the vessel. They create a jet of compressed air that is pushed over the propeller, forming an enormous bubble that is designed to completely surround the propeller that is operating. In some studies, the nozzle was proven to decrease the cavitation effects by as much as 75 percent. However, it has some significant drawbacks, such as the fact that the nozzles need a substantial investment of energy. The cost of purchasing, installing as well as operating the nozzles is often thought to be too costly for the benefits they provide.

 

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