Propeller Theory - upstream

I found this to be interesting and related to the topic.

Thunderhead19
http://boatdesign.net/forums/showthread.php?t=8426

 

Kach, I think that the only reason a water jet is so effective is because it throws out a narrower beam, at faster speeds. Meaning it can be directed much more precisely.

 

On a set flow volume, narrowing the exit increases velocity.

What happens to the pressure?

Can I call this static pressure, or because it's water I just call it pressure?

Does the pressure remain constant?

Does it change with narrowing of the exit?

I know this one, just need a refresher.

 

I would assume that they indeed would call it pressurised, because for a given square inch/cms the pressure is higher. The narrower the nozzle relatively to the intake, the more "pressurised" it would be.

However, the water itself cannot be pressurised, and because of this, the exit speed grows.

I'm saying that the flow itself is pressurised, so to speak, but the molecules themselves aren't.

The through-flow (volume-wise) remains constant, though ..

Hmm, I propably don't make any sense.

 

If the nozzle is half the size, then the water will exit at twice the speed of the speed of the intake.

Pretty simplified, but if everything is perfect, that is how it will do. Hence the "jet"-term. A simple jet is just that. A huge intake, with a much smaller exit hole. Meaning that the volume that is pushed through will have a much higher exit-speed, although the volume going through doesn't change.

 

A "normal" propeller doesn't have that "pressurising", nor does it have a "channel" to work in, making it much less effective than a water jet.

Take a small scoop and glide it through the water. Quite a lot of the water can escape sideways, but if you put it in a channel so it cannot go sideways it can push much more water in front of it (or in the case of a propeller: behind it).

 

1) Air and water compressors may look and function similarly, but the physics are all different because of compressibility effects. A hovercraft lift fan design will not work as well with water due to mass to energy/viscosity effects (which is how centrifugal "pumps" work anyway). Water losses through a plenum are high because you pay an energy loss with every turn that not perpendicular to the axis of thrust and radialy balanced.

2) Read what DanishBagger said carefully, there are some little nuggets of insight there, especially concerning Bernoulli's law. When using energy methods, it is important where you draw the energy boundries.

3) Finally, look up White Gill thrusters. I think that this is where you are heading.

 

WOW!

I did not know these existed in such a configuration.

Link:
http://www.teesgillthrusters.com/

I have something similar in mind for a single bow thruster on the front of my hybrid hovercraft. Except it is upside from the water version of course. On the model it is represented with a 35mm film container fliped upside down and cut in half with a slot cut into it.


http://www.elliott-turbo.com/new/products_gillthrusters.html

 

heh, and that without knowing Bernoulli

Seriously, I'm glad because you mentioned the name of that thing, plus number 1 and 3. I can get some more knowledge from there.Evidently - and my struggle to "explain" this prove it - my knowledge is "thinned out over a huge area".

Anyways, since I consider myself an idjit (i.e. everyone is stupid, much to learn, and you can always learn), I just wanted to say thanks for providing some "real" input here.

Thanks again,

Andre