Cavitation vs Ventilation

When referring to propeller issues and problems with boat performance, many people use the terms ‘cavitation’ and ‘ventilation’ interchangeably, and often incorrectly. As industry-leaders in boat training and boat charter, the team at Ocean Sports are often asked for advice on these issues. Below, we explain the difference between the two terms, how the damage is caused, and how to fix it.

What is Cavitation?

Cavitation is caused by an imperfection of the propeller, which then creates a low-pressure environment on the back side of the blade. This low-pressure environment causes the water surrounding the blades to boil because the low pressure has caused a vacuum. The water vapour bubbles implode against the surface of the propeller, causing visible pitting or ‘cavitation burn’ on the blades. These bubbles also stick to the propeller blades and increase the thickness of the blade itself, resulting in the need for more power to maintain speed due to friction on the blade.

Cavitation of your propeller can cause an uncomfortable ride, creating a vibrating sensation and reducing bite, consequently reducing efficiency.

What is Ventilation?

In contrast, ventilation is defined as when the propeller of your motor sucks in air like a vacuum, causing a drastic reduction in thrust and a revving engine. This fault can be caused by having the outboard motor trimmed too high, or by jumping waves and causing the propeller to reach too close to the water’s surface.

How to Troubleshoot a Propeller Problem

If your boat isn’t performing at its best and you suspect damage to the propeller, it is important to find the cause of the damage before attempting to fix the problem. Cavitation can often be caused by water disturbance around the propeller either because of damage to the surface of the prop or inconsistencies on the underside of the vessel. Often, if a rib is not fully inflated, it causes water disturbance around the propeller, which can lead to cavitation. Secondly, debris around the prop wears away the edges of your prop blades, reducing the diameter and affecting their performance.

To prevent air or exhaust gasses being pulled into the prop blades, many boats have an anti-ventilation plate. This surrounds the trailing edge of the exhaust hub, and creates a high-pressure barrier which prevents ventilation from damaging your engine. If an anti-ventilation plate does not solve your performance issues, it is likely that your prop is suffering from cavitation. If this is the case, we recommend that you visit your local marine repair service, who will suggest a long-term solution.

About Ocean Sports Tuition

Ocean Sports Tuition is a Southampton-based company offering RYA-recognised practical and theory courses and boat charters. We have successfully delivered training on the South Coast and the Solent since 2004, and pride ourselves on providing excellent tuition and service to beginner, intermediate, and advanced sailors alike. To find out more about Ocean Sports and our friendly team, click here.