"Peter" wrote
Cavitation is caused by the prop spinning too fast for the forward
motion of the vessel.


Gilligan wrote:
Cavitation is caused by water not being able to move fast enough to fill low
pressure spaces created around such things as a propeller.

Which is caused by... ?
You're both right. Peter is talking about the cause on a
macro scale.

... A spinning disc
has no forward speed regardless of RPM yet there is no cavitation. A
propeller is a rotating wing and the low pressure side causes the water to
"boil", hencce the creation of cavitation.

But what makes the water boil? Increased temps due to
friction? Tiny devil-spirits waving their pitchforks?


.... Higher RPM gives higher
tangentail blade speeds and greater pressure differential between the faces
of the prop.


Somewhat right, tip speed above a certain linear velocity
will cause cavitation no matter the dP, and if the dP gets
high enough the tip speed matters less.

You could also mention the dread voodoo curse "viscous
shear" which means that somebody is trying to force those
lazy water molecules to move faster than they are willing.

Fluid flow does a lot of things counter-intuitive to people
familiar with normal physics. One of my favorite engineering
profs used to say that fluid molecules are like basketballs
smeared with syrup, which helps intuit the behavior.

DSK