Cavitation and Ventilation are not interchangeable terms; they refer to two distinct problems encountered during impeller operation.

To understand cavitation, you must first understand the relationship between pressure and the boiling point of water. At sea level, water will boil at 212 deg F. As pressure increases, such as within an engines’ closed cooling system, the boiling point of water increases – it will boil at some temperature higher than 212 F. The opposite is true. As pressure decreases, water will boil at temperatures lower than 212 F. If pressure drops low enough, water will boil at typical ambient temperatures of 50-60 F.

We have said that, during normal impeller operation, low pressure exists on the blade back. Normally, the pressure does not drop low enough for boiling to occur. However, poor blade design, or blade damage can cause an unusual pressure drop on a small area of the blade. Boiling can occur in this small area. As the water boils, air bubbles form. As the boiling water passes to a higher pressure area of the blade, the boiling stops and the bubbles collapse. The collapsing bubbles release enough energy to erode the surface of the blade. You will often see this on severely damaged impellers.

This entire process of pressure drop, boiling, and bubble collapse is called "cavitation". The damage caused by the collapsing bubbles is called a "cavitation burn". It is important to remember that cavitation is caused by a decrease in pressure, not an increase in temperature.

Ventilation is not as complex a process as cavitation. Ventilation refers to air entering the blade area, either from above the surface of the water or from a leak in the pump tunnel via the driveline seal (or through-hull seal), a cracked or poorly sealed intake shoe, or simply a bad seal at the front of the pump. through-hub exhaust system. As the blades meet the air, the impeller momentarily over-revs, losing most of its thrust. An added complication is that the impeller over-revs, pressure on the blade back decreases and massive cavitation can occur.

Here is a nicely written article by Scott Fleming that supports the topic, as well as giving you a real-world example on a Sea-Doo Jet boat…

A propeller on airplane, boat, or submarine is a kind of fan. It works in the open air or open water. An impeller is more like an auger or screw. In order for an impeller to work efficiently, it must have a precise-fitting hole to move material through. On the boat, as the impeller turns inside its hole (the wear ring), it spins water through. After passing through the pump, there's a Venturi (the nozzle) that narrows the outlet (increasing the pressure & exit speed of the water) before it's released through the steering nozzle.

If the wear ring is worn or grooved, water pressure in the nozzle will overcome the pump. Pressure will travel back through the pump resulting in a feeling of a slipping clutch. When this happens, the impeller is turning faster than it's able to push water through the nozzle.

Cavitation is what happens when bubbles are formed (water boils) in a low-pressure environment. A bent or dinged-up impeller (or propeller) spinning at a high rate of speed produces bubbles that can erode metal and aerate the water. As the impeller spins in its own bubbles, it will also create the sensation of a slipping clutch.

Ventilation is what happens when outside air is pulled into the pump from a leaky seal, traveling over aerated water, or exposing the water intake to the air by jumping or a hard turn. Again, it produces the same feeling of a slipping clutch.

The jet pump must be sealed to the hull to prevent ventilation. Forward of the impeller, there's negative pressure as the pump pulls water inward. When the boat is in motion, the pump is out of the water, exposing it to the air and allowing it to ventilate if not properly sealed. In 1995, silicone was used to seal the pump. In 1996, SeaDoo switched to a neoprene seal. Sometime around 2000 they switched to a reusable gasket.

Another common source of ventilation is the carbon ring assembly which seals the driveshaft penetration. (Attached is a diagram of a 1990's driveshaft.) It keeps water from entering the bilge and it prevents air from getting sucked out into the pump. When installed, the boot is compressed, pushing the carbon ring firmly against the support ring, or top hat. The carbon ring does not touch the drive shaft--only the top hat. The ring and top hat are cooled and lubricated by the water under the boat.

Now picture you're at a stand-still with the engine idling. You've got minimal water passing through the pump, then you suddenly gun the engine. You're creating an immediate and significant negative pressure in the water intake. That negative pressure (vacuum) pulls on the carbon seal. If the boot is weak and can't withstand the suction, air will flood through, into your pump, and ventilate it with bubbles. (the "slipping clutch" feeling) Once you get some speed and water is being forced into the intake from your forward motion, the suction on the carbon ring assembly is greatly reduced. The boot stops ventilating, the pump stops slipping, and you're off and running.

A simple fix I've found to fix a weak accordion boot is to put a zip tie around one of the valleys. This forces the boot to extend, increasing the pressure of the ring against the top hat. If your boot is old and weak, you will notice an immediate difference by doing this.