prop shrouds

One point I wanted to add: in order to double the speed of a submerged object you have to more or less cube the power.
The powerful Otto fueled engine developed by Sunstrand from WW2 German inspired technology has not come with out a price.
This 'envelope push' did result in an accidental explosion for them.long ago in development phases. .
I would hope that this trend toward using a liquid monopropellant for the military would not continue as the *fuel is horrific stuff to handle. Unfortunately its resemblance to rocket fuel is not by chance. By definition it provides its own oxidizeras did the rocket fuel of old rocket(V2). Handling even the improved Otto fuel is tricky,and hopefully never endangers the submariners who perform their duty. I wish that the electric engine could be utilized instead with advanced batteries like lithium or seawater battery technology.

The military wanted the otto fueled engine. Electric was never fast enough.. The ultimate goal was always an engine that would be prototype for a submarine similar to what the WW2 Germans had planned for otto fuel.

I want to dedicate this 'string' to 'all who brave the peril of the sea.'
particularly the submariners

Stewart Loeblich

 

maybe you ought to read the whole thread

By the way its the Navies that build the ships subs torpedoes.....
they kinda' just let the army do land stuff even took the airplanes and made an Airforce instead of Army Air

stew

Why shucks man I didn't know that where des i send de cash?

Are you sure mine says positive 300 pts Rose colored glasses not so you wishful imps...
I am happily married 20 some odd years first wife and no mortgage and an old crate of boat and five acres and half million gallons of pond water me i'm just plain happy

 

smartypants - he is from croatia. Lost in translation - often gneralized in other languages: army=military

 

my apologies for translation jab Alfonso

reading last part of a thread first is like doing the same in a book everyone does it...

 

Not to be a wiseass or anything...I don't work for 'lil' points.'
I saw this written on a bridge I used to pass underneath to tend lobster traps in the Florida Keys long ago:

'The voice of the majority is no proof of justice.'


Cheers.

 

Every time you piss someone off a dolphin dies. Or was it a kitten?

 

Thank you for bringing up this point, that was very intuitive. We are talking about recreational use here. That encompasses a very large range of applications. From race boats to jon boats, people have used and abused propellers for well over a century. Making a comparison where there is a larger prop scenario is indeed unfair.

~Stewart Loeblich

 

Don't worry. SamSam will take Confederate money.

 

Thanks. I'm well aware of army/navy differences. It was not translation mistake, I was just writing fast and didn't think about it. My mistake. It is true, army/military is generalized in most languages, in Croatian also.

 

No problem at all my friend

Now the real claim of the situation is this and let me make it clear to all.
I think that a ducted arrangement - no matter what you call it- is faster than a prop of the same size. I have been called to provide it and here it is.
The armed forces of the world are not worried about power, they use nuclear power for practically unlimited electrical powered engines in subs and rocket-fueled exotic engines in torpedoes...
I do know that the top speeds of all of the 2nd fastest things under the deep blue sea.: the Soviet and American Attack submarines are very fast because of the unlimited power option.
The use of cowled or pump jet propulsion in newer models will be the trend and what is used even right now will be very secreted far more than what is available presently through a web search.

 

I think that some of you can see the proof apparent The presence of the 2nd counter rotating prop to a pump jet for a torpedo does more than simply prevent rotation it provides thrust also, I presume.
Now, I may not be a rocket scientist but having two props is lot having two women life's faster and more fun.
I may take a lot of power but if thats not a problem deep pockets some of the big dogs have yes they do but not as deep as what military money buys and that seems to be fancy fast jet pump not bare prop IMHO

 

That's not a proof of anything, particularly the claim that "a ducted arrangement - no matter what you call it- is faster than a prop of the same size."

 

Thank God! you say he does take confederate forever stamps right? and no its not a kitty it was a bunny that was held hostage by school bus driver in South Park in order to keep the kiddies quiet

Well I got a guppy right here above my hungry cat's mouth.... wanna see?

 

Some general hypotheses/guesses:

1) There is "crossover" surface which separates the region where an optimized shroud and propeller system will be more efficient from the region where an optimized open propeller will be more efficient. This surface is is a function of diameter, input power and vessel speed. Efficiency is defined as net thrust * vessel speed / input power. Net thrust includes the force acting on the shroud in the direction of vessel motion.

2) For a fixed diameter points on the crossover surface will have increasing power as speed is increased.

3) For a fixed power points on the crossover surface will have decreasing diameter as the speed is increased.

4)For a fixed speed points on the crossover surface have increasing diameter as the power is increased.

IMPORTANT - I'm not saying anything about where the crossover in a given situation will be. And I'm assuming optimized designs. All bets are off when comparing designs which are not close to optimized.

Above are my guesses. Other thoughts????

A recent AIAA paper might have provide some answers or at least a systemetic approach to obtain answers. The first page can be found at http://www.flodesign.org/pdf/Ducted Wind.pdf From the introduction:

"The simple but corrected formulation and results presented herein
1) introduce a unified momentum model, based on first principles, applicable to
both bare and ducted/shrouded wind and water turbines; 2) show
how to properly generalize the Betz limit for bare propellers to the
ducted/shrouded propeller case and thereby extract higher power
levels; 3) identify a single critical shroud aerodynamic parameter that
controls the power-extraction level; 4) uncover the appropriate
nondimensional scaling parameters; and 5) provide a firm basis for
further development of ducted-wind/water-turbine technology.
Additionally, this new formulation provides some interesting new
results, insights, and handy relations applicable to ducted/shrouded
propeller propulsion systems."

 

Re-reading the introduction to the AIAA paper above it occurs to me that the paper may not consider the drag of shroud directly since it is for wind and water "turbines" for which the forces acting on the device are of secondary importance. Note that these types of "turbines" behave similar to propellers in that the amount of flow through the device is not determined a-priori.