Ventilation and Cavitation

Posted by John Doe 02/02/2018 0 Comment(s)

Ventilation and Cavitation

 

In this two-part issue of our Blog 3a & 3b, we are going to focus on the terms ventilation and cavitation that are often incorrectly used interchangeably. Although there are certainly similarities, and in some circumstances, they can be related yet technically, different phenomena.

 

So, let’s start with Ventilation:

 

Ventilation takes place when air is introduced to water around, or to the flow of water heading to the propeller. This can be caused by many different sources, such as, vented propeller, over-and-thru hub propellers, tunnel hulls, extreme motor elevation, excessively high trim angles, stepped hulls, or even hull attachments (transducers, pick-ups, and the such). As the propeller meets the aerated water, the water's flow to the propeller's blade working surface does not grab the water as it should due to the air bubbles. As the water aerates, the propeller's loading/grip ability is reduced, and the torque applied to the propeller through the motor can more easily spin the propeller to a much higher rpm even at the same given power input. This, not only, causes the motor to rev or over rev easily, it reduces the thrust provided by the propeller, slowing the boat's speed and/or acceleration, as well as, reducing the propeller's control over the boat (blow out on turning, on hole shot, etc.).

 

Another issue causing ventilation can be weeds, grass, foreign objects around the propeller as it rotates that aerates the water cause a lack of grip.  To remove it, you have to stop the boat, apply reverse and/or inspect the prop to remove items.  Once the foreign items are gone, the propeller is ready to start again.  Hope this has helped and have a grrrrreat time on the water.

 

Cavitation, on the other hand, is a bit more specific and decidedly more difficult to explain. Cavitation could be defined as the phenomenon of the formation, rapid collapse, and subsequent implosion of vapor bubbles of a flowing liquid in a region where the pressure of the liquid falls below its vapor pressure. As these bubbles collapse, energy is released in the form of a shock wave that can cause minute damage to the surface of the material. In this case, the flow of water is caused by the propeller's rotational and forward movement through the water, while the surface in question is the blade surface of the propeller, itself. When these microscopic implosions occur, the surface metal can be eroded, and small pock marks can appear, causing "cavitation burns." The damaged area will appear to have been sandblasted at very close range.

 

The true cause of cavitation can arise from several factors, but we see them occur most often, as a result of leading edge imperfections, such as, nicks, dings, scratches, bends, etc. They can, also, be caused by improper geometry, or upstream imperfections related to the hull, such as, hull design/configuration, hull imperfection, incorrectly installed accessories/appendages, etc. Basically, this is what we like to refer to a “dirty or disturbed water flow” and since there are so many possible causes, diagnosing the problem should really be done on a per case basis.

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