Steven Shelikoff wrote:

That's ok. Fluid on the pressure side of the prop is nowhere near
laminar either and will in fact be totally non-uniform around the
rudder. Yet the rudder still has an effect on the boat's direction.


No, it's not laminar, it is unidirectional along one axis.
Unidirectional flow can be diverted creating a thrust vector, unlike
the non-unidirectional flow on the suction side where the rudder
provides pressure drop instead of redirection/diversion. That's the
difference.


Sure you can create a vacuum in water,

You need to check the definition of vacuum if you believe this.
"Vacuum in water" is an oxymoron.

just like in air. The only
difference is that water doesn't change it's volume (as much, but it
does a small amoutn) when the pressure changes.

The *liquid* volume does not change, that's a basic property of
liquids. Their volume is temperature dependent, not pressure
dependent. If you reduce the pressure, dissolved gases will evolve
(that *is* cavitation) but you now have bubbles suspended in a
liquid, i.e. foam.

There's still a vacuum
though.

Don't think so.

And you can certainly create a vacuum in water without cavitation.
Cavitation only occurs if the pressure of the water drops below it's
vapor pressure.

Yes, and you would create a vacuum without doing this exactly how?
Fluid is not elastic. Move it from one point too quickly (what you'd
*have* to do to create a local low pressure area) and you will
liberate dissolve gas (even gaseous water) due to the low pressure
and/or high temperature created by the shear. Water doesn't stretch.

There's a whole art/science of creating props that work
without cavitation for use with submarines.

Quite so. They do not, however, generate 'pockets of vacuum' in
doing so.

Keith Hughes