Cavitation - Technical Terms

[sifocalpoint]

Ill be going over boat hull design with the students at school this week, and wanted to explain cavitation in an easy to understand manner.
On a v-drive or prop driven boat, is the prop creating voids in the water trailing the prop? How does a cavitation plate on a v-drive prevent this in a technical sense.
If any of you guys get a sec, explain what exactly is happening,
The kids love this lesson, they get blocks of foam and carve out and shape the hullls they desire. They then vaccuum form the foam to create a mold that they attempt to fiberglass. Works very well and i have fun with it.
Thanks
Ryan

[xracer]

Hi Ryan
At the leading edge of the propeller back (the forward facing propeller surface) the water pressure is depressed below the boiling point (vapor pressure technically). You may have noticed that at high altitude water boils at a significantly lower temperature than at sea level. This is due to the pressure difference. If the pressure is low enough it boils easily at lake water temperature. A corkscrew shaped cavity formed by each blade and trails the propeller until the pressure is equalized and the bubbles collapse.
The pressure depression caused by the flow on the propeller back is large. Cavitation free propellers have been demonstrated in water tunnel tests only through about 12-Knots, far too slow for interest.
On the very large, high speed sport fishing boats I work on the propellers turn less than 1000 RPM and the propeller cavity is very easy to see looking over the transom.
The term "cavitation plate" is somewhat of a misnomer. If the blade back has a path to the water surface it entrains air, develops a larger cavity and "blows out". A plate which severs this air path can be called a "cavitation plate".
Because the propellers are so far forward on most race boats, the so called "cavitation plate" functions mostly as a "trim tab". A device to control pitch attitude (pitch angle) of the hull.

[Highlimit9000]

Hi Ryan
At the leading edge of the propeller back (the forward facing propeller surface) the water pressure is depressed below the boiling point (vapor pressure technically). You may have noticed that at high altitude water boils at a significantly lower temperature than at sea level. This is due to the pressure difference. If the pressure is low enough it boils easily at lake water temperature. A corkscrew shaped cavity formed by each blade and trails the propeller until the pressure is equalized and the bubbles collapse.
The pressure depression caused by the flow on the propeller back is large. Cavitation free propellers have been demonstrated in water tunnel tests only through about 12-Knots, far too slow for interest.
On the very large, high speed sport fishing boats I work on the propellers turn less than 1000 RPM and the propeller cavity is very easy to see looking over the transom.
The term "cavitation plate" is somewhat of a misnomer. If the blade back has a path to the water surface it entrains air, develops a larger cavity and "blows out". A plate which severs this air path can be called a "cavitation plate".
Because the propellers are so far forward on most race boats, the so called "cavitation plate" functions mostly as a "trim tab". A device to control pitch attitude (pitch angle) of the hull.
Excellent Explanation!!! :boxed:

[sifocalpoint]

Thanks so much, that was a great explanation