At school, we are taught that water boils at 100°C (212°F). While we tend to take this for granted, “boiling at 100°C” is actually a special case. Water does not boil at a fixed temperature on its own; it boils when its vapor pressure equals the surrounding atmospheric pressure. At sea level, where pressure is approximately 1 bar, this occurs at 100°C.
In fact, the pressure wasn’t chosen to match the temperature. Rather, the Celsius scale itself was defined using water: 0°C was set at the freezing point and 100°C at the boiling point under standard atmospheric conditions (1 atm).
As we climb higher into the mountains, the air becomes thinner and the pressure drops. Consequently, water boils at a much lower temperature – on the summit of Everest, it’s roughly 70°C. The water is boiling, but it isn’t especially hot.
A Cup of Tea at 70°C
If you try to brew tea on Everest, the result is noticeably disappointing: the extraction is weak, the flavor is flat and the process simply doesn’t work as it does at sea level.
“Boiling” does not necessarily mean “hot enough.” It only means that, under the current pressure, the water has begun to turn into vapor. Water can transform into vapor in two ways: by increasing temperature, by reducing pressure, or both.
From Mountains to Marine Propellers
This same principle applies to your boat’s propeller. As a propeller rotates, it creates low pressure on the forward surface and high pressure on the rear surface. This pressure differential is exactly what pushes the boat forward. Because liquids are incompressible (they cannot be “squeezed”), the high-pressure side simply pushes the water back and we see it flying from under the stern.
However, on the low-pressure side of the propeller, the pressure can drop so significantly that the water begins to “boil” and turn into vapor without any added heat.
This phenomenon is called cavitation
A boat propeller is designed to work with liquid water – relying on its density, mass and viscosity to transmit force. When cavitation occurs, part of that water becomes vapor. Vapor behaves very differently: it has far less mass and viscosity, meaning it cannot transmit a load effectively.
A propeller designed to grip solid water begins spinning in vapor; it loses its “bite,” and efficiency plummets.
The Sound of Implosion
Cavitation also causes significant noise. Imagine a pot of water on a stove: just before it reaches a full boil, it makes a distinct hissing sound.
This happens because vapor bubbles forming at the hot bottom rise into cooler layers of water and collapse. That collapse creates sound. Something similar occurs on a propeller.
Vapor bubbles born on the low-pressure side, move toward the high-pressure side and collapse violently. The scale is different and the intensity is far greater, but the underlying physics is identical.
Why It Matters
Cavitation is more than a minor imperfection. It marks the point where the propeller is no longer working entirely in liquid. This leads to loss of thrust, increased noise, vibration and consequently to physical damage of your propeller – over time, the violent collapse of bubbles can pit the metal, leaving the blades looking like a golf ball or Moon.
Efficient hydrodynamics involves a complex balance of many factors: boat speed, RPM, engine power, propeller diameter, pitch, weight and even the salinity of the water you sail through. Achieving the perfect “state-of-the-art” balance is key to performance.
Keep the balance and fair winds!
Share Box
Explore a wide selection of marine propellers at West Marine
Browse spring offers on marine electronics from Gael Force Marine
SamBoat – check rental boat availability in Florida now
Helly Hansen: explore professional-grade sailing gear for your next adventure
Yacht Delivery & VAT Reset
BOAT MASTER
Reach boat owners, skippers and marine professionals worldwide