Propeller Ventilation vs Cavitation

These two propeller issues are highly misunderstood, and often mistakenly used interchangeably. But they are dramatically different. Ventilation is fairly common in small and medium boats, is easy to detect, and does not cause physical harm to your propeller. Cavitation is much less common on recreational boats, is much less apparent when it's happening,  and unlike ventilation, will cause physical damage to your propeller.

Ventilation is by far a more common occurrence on small to mid-size boats. The most obvious sign of ventilation is the sudden and random spiking of motor revs and lack of thrust. The propeller has “grabbed air” and has lost contact with the water and is spinning freely. Sometimes the motor revs wildly, or maybe just modestly, depending on the circumstances. It happens most often because the propeller “grabs air” from being too close to the surface of the water, or because something about the hull or  lower unit creates air bubbles that get into the path of the prop. The air bubbles surround and envelope the propeller, resulting in loss of contact with the water and creating ”water slippage”, resulting in the free revving of the outboard motor.

The sharp turns during watersports often trigger propeller ventilation.

I’ve often heard newer boaters wonder if the over-revving is the result of a propeller hub that has failed and is slipping. In our experience, propeller hubs rarely ever fail. Ventilation is far more common than a bad hub. But to an inexperienced boater who does not understand propeller dynamics, its hard not to assume that they are dealing with a failed propeller hub when the motors starts revving and spinning freely.

Some typical causes of ventilation:

• Sharp turns

• Outboard motor is trimmed too high

• Propeller mismatched for motor and hull

• Outboard motor is mounted too high (not so typical)

• Poorly positioned trim tabs or transducers (not so typical)

• Hull or lower unit imperfections (rare)

Many boats will experience some brief ventilation on hard turns which usually clears once the boat comes out of the turn. It’s usually manageable, and with a short enough duration that boaters will look past it. Often, it can be cured by dropping the trim on the motor, but sometimes not. A brief occurrence of ventilation while in a hard turn might not bother most boaters. But if water skiing or other similar activities are involved, it could certainly get in the way of a great experience on the water. If lowering the trim on your outboard doesn’t cure the problem, then it might be time to test other propellers.  Going to a propeller with slightly more pitch (and less diameter to compensate for the increase in pitch) might give a better result. Another option would be to try a 4-blade propeller. We had a certain tri-toon that gave us fits with ventilation. We knew the motor height was correct, and we ruled out other factors and causes. We fixed the issue by going to a 4-blade propeller that had a touch more pitch than the original 3-blade propeller but also had 3/4in less diameter than the original.

A few years back, we had another boat that gave us persistent ventilation problems. This one was actually a pontoon boat (not a tri-toon) that had a “wave shield” in front of the outboard motor. The issues were caused by the floating seagrasses that are typical in the warm months in southwest Florida. The seagrasses would get caught on the anti-cavitation plate of the lower unit, and create turbulence in the water, resulting in air bubbles into the propeller. How about that for irony? There were two things we could do to prevent the seagrass from accumulating on the lower unit:

• Run a heavy aft ballast (heavy stern) by having more passengers to the rear, making the motor sit lower in the water. (Yes the motor was already trimmed all the way down).

• Setting the throttle to run more than 3500 rpms would also prevent the seagrass from being able to catch onto the lower unit, probably from the resulting water turbulence from the higher speed.

  
  

This ventilation from the seagrass on the pontoon was a major factor toward our decision to running tri-toons only. The center pontoon in front of the motor prevents any seagrasses from being able to collect on the lower unit.

Cavitation presents a very distinct issue compared to ventilation, and presents a pretty deep engineering theories. While ventilation involves air entering from the water surface, turbulence, or exhaust gases, cavitation occurs due to the formation of vapor bubbles caused by low pressure on the back side of the propeller. The reduced pressure leads to localized boiling, producing vapor bubbles that subsequently collapse and result in pitting on the surface of the propeller blade.

Mild propeller cavitation

Severe propeller cavitation

It's much more difficult to detect cavitation as compared to ventilation. You don’t get the obvious fluctuations in motor revs, but rather the symptom will be a subtle vibration along with a slight loss of thrust. If you believe you are feeling a vibration that might be associated with cavitation, immediately reduce throttle. If the vibration disappears, and the boat seems to perform more to expectations, you may likely have a cavitation issue with the propeller. The only way to confirm a cavitation problem is by inspecting the propeller. Any pitting or erosion or a marring of the blade surface confirms cavitation.

Causes of cavitation can be pretty complex, and there are in fact 8 defined types of cavitation that describe the behaviors of the vapor bubbles, and how they affect the propeller. Discussing these different types of cavitation is beyond the scope of this article, but the point is that cavitation is a broad and complex engineering issue.

• Tip vortex cavitation

• Sheet cavitation

• Cloud cavitation

• Bubble cavitation

• Root cavitation

• Face cavitation

• Boss vortex cavitation

Cavitation on a brand new boat would be troubling, and might point toward an engineering or design issue  with the hull. But the more likely scenario, it would be caused by an aggressive choice in propeller, with too much pitch, or too much diameter, or both. If the cavitation is affecting a propeller that you have been running for some time, the problem could be a damaged or dinged propeller.

Cavitation is a complex and well-studied science, because of the way it can adversely affect large ships and submarines. Cavitation creates noise, and that flies in the face of the need to run efficiently and smoothly. And for military vessels, the mission is to run silently. There is a huge engineering and military community researching fluid mechanics and naval hydrodynamics just to eliminate cavitation and the noise generated by ships and submarines. But for us regular boaters, we just experiment with different propellers, testing different propeller diameters, blade shapes, blade sizes and blade pitch. The right propeller will make the difference.

 WHERE TO GO FROM HERE:

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