schlackoff, don't drink so much before you post. tomorrow you read your post
below and you are going to one embarrassed dude.

you can create a thrust vector by diverting non-unidirectional flow
as long as the the sum of the non-unidirectional flow is not 0, which it
is not for the case we are talking about because if it was, the boat
would not move backwards when the engine is put in reverse.


you tipped a bit more and wrote this:

You're probably thinking of an absolute vacuum. In that case, a "vacuum
in air" is an oxymoron also. But since you mentioned a vacuum in air,
here we must talking about a relative vacuum which is simply an area
where the pressure is lower than another area. That is easily created
in water.


you tipped even more and wrote this embarrassing drivel


Actually, the liquid volume can change when the pressure changes.
However, it's a minute amount only measurable for drastic pressure
changes. But that's outside the scope of this thread, where we can
treat the liquid volume as constant when the pressure changes.

onward you went with this:

That depends on how much you reduce the pressure. Is it your contention
that anytime you reduce the pressure of a liquid by any amount that you
must have cavitation? If so, you are plainly wrong.

you wrote the following which makes no sense at all.


You think wrong ... if we're talking about a relative vacuum and not an
absolute vacuum, which is obvious we are from your previous statement:
"so you can't create a vacuum in water like you do in air." A vacuum in
air is also an oxymoron unless you're *not* talking about an absolute
vacuum.

more confusiong with:

But it does flow from higher pressure areas to lower pressure areas.
The lower pressure areas are the vacuum in this case, just like air.
And it does not have to cavitate in the areas under lower pressure.


huh??

Sure they do. The area in front of the prop blade is at a lower
pressure than the area behind the prop. i.e., one definition of a
vacuum.

You can measure a vacuum in water yourself if you want. Just put a
vacuum gauge behind a water pump and you will measure the vacuum of the
pump sucking water through it. I have several of them on my boat for
measuring the condition of fuel filters.

Steve