What Is Propeller Cavitation?

Propeller Cavitation Basics: How Does Propeller Cavitation Work?

Propeller cavitation is the creation and dissolution of tiny vapour bubbles in propellers, ultimately affecting how the propeller performs. But what exactly causes this phenomenon? How important is it to recognise? And how can you avoid it on your boat?

Propeller Cavitation: The Basics

Cavitation itself 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 that down. The liquid here is water. Water has a specific vapour pressure – the pressure that’s typically exerted by a gas under equilibrium conditions when water is in a liquid or water phase. It’s a fancy way of indicating the tendency for water to evaporate.

When an instance of static pressure reduces, it encourages the water to locally evaporate; small bubbles of gas are produced, not unlike the small bubbles of gas at the bottom of a boiling pot of water. When introduced to a higher-pressure environment, these bubbles (sometimes called “cavities,” hence the term “cavitation”) collapse, resulting in a pressure differential that results in a shock wave.

There are examples of cavitation in many different manmade environments, including not only propellers, but pumps, impellers, and control valves as well. We can also see examples of cavitation used in the animal kingdom; for example, the pistol shrimp’s famous “shock wave” style attack to stun prey utilises cavitation for its force.

Why Is Propeller Cavitation a Problem?

In a propeller, cavitation is undesirable for several reasons. Primarily, boaters are concerned about a loss of efficiency; when cavitation occurs, it creates a force that interferes with the efficiency of the propellers. Your boat will not be able to move as quickly, and it may expend more energy to move the same speed as before.

Cavitation is also noisy. Propeller cavitation is usually a relatively low-energy event, so this noise isn’t ear-piercing – but it’s still annoying, especially if you’re trying to fish quietly.

Even more importantly, cavitation can cause erosion and long-term damage to your propeller system. If you’re thinking that tiny air bubbles couldn’t possibly cause much damage to a heavy-duty boat propeller, you’re partially correct; individual instances of cavitation aren’t going to wreck your boat. The problem is that this damage accumulates over time. Without proper prevention and care, excessive frequency of propeller cavitation can eventually ruin your propellers.

How to Prevent Cavitation

Now for the practical takeaways. What can you do to prevent/avoid propeller cavitation?

Some of the work has already been done for you. With better designed propellers, made from better materials, engineers can limit the potential of propeller cavitation in a given environment. For example, in slower boats (with speeds at or below 35 knots), propellers have “low-loaded profiles” that feature a flatter shape and smaller angles of attack. Engineers also use multi-blade screws with a large diameter to achieve the amount of thrust necessary to compensate. As for faster vessels, cavitation is much harder to prevent through shape design.

Unfortunately, moulding propellers into different shapes can only impact performance efficiency in a modest way. A better solution is to change the materials that propellers are made from. Until recently, most boat manufacturers focused on an alloy made from a combination of nickel, aluminium, and bronze that resists erosion without costing much – but this material isn’t especially good for preventing or dealing with the impact of cavitation.

These days, boat manufacturers are increasingly experimenting with better materials for limiting propeller cavitation. Composite materials, like resin or carbon fibre, are capable of achieving the same performance efficiency as their older material counterparts, but they’re also flexible enough to withstand the low-energy cavitation impact. This is called a “hydroelastic” effect.

If you’re interested in minimising the potential for cavitation, consider replacing your older propellers with these newer, more flexible varieties.

Also, you should understand that the intensity of the cavitation effect tends to increase with speed – so you could always reduce your speed if you want to minimise cavitation.

Less Efficient Solutions for Preventing Cavitation

Other solutions to cavitation have also been explored. For example, Samsung Shipping in South Korea designed and introduced a nozzle system, which uses a small set of nozzles to counteract the effects of cavitation. These nozzles are positioned directly in front of the propeller, mounted on the hull of the ship. They produce a jet of compressed air over the propeller, creating a large bubble designed to completely surround the operating propeller. In some tests, the nozzle system was found to reduce the effects of cavitation by up to 75 percent. However, it comes with a few important drawbacks – including the fact that these nozzles require a significant expenditure of extra energy. Purchasing, installing, and operating the nozzles is often considered too expensive for the benefits they bestow.

Similarly, engineers have designed an air-filled rubber membrane, which uses the same core principle as the nozzle system. This thin membrane creates a pocket of air without requiring any additional power, making it cheaper to operate – but not quite as effective.

Can Cavitation Ever Be a Good Thing?

In extreme cases of cavitation, a submerged object can become completely enclosed in a large bubble – a phenomenon called supercavitation. When supercavitation applies to the nose of an object, it prevents contact between the object and the surrounding water, thus reducing skin friction drag on the object in question.

Supercavitation can be achieved by producing an object with a very specific shape and propelling it at sufficiently high speeds. It’s been researched and utilised by a variety of organisations, including the Russian Navy, German weapons manufacturers, and even the U.S. Navy. While primarily used for torpedoes and similar projectiles, the principle has also been applied to prototype ships as a method of propulsion.

Looking for a New Boat?

Are you in the market for a new boat? Or are you hoping to upgrade an older boat that suffers from frequent cavitation? You’ve come to the right place. Check out our wide selection of new and used boats for sale today!

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