Cavitation and the quiet prop's.

[thudpucker]

There was a real good TV program on Sub's this evening.
I was particularly interested when they got to the new "Pennsylvania" class sub.
The newest and quietest Killer on the seas, they said.
The Props were the noisest things on the old boomers.
So they put on a good graphic showing the Cavitation behind the latter leading edge of the Props.
Cavitation caused the water to boil, which formed bubbles. When the bubbles broke, they transmitted sound waves which travel a long ways in the water.

Faster props make more bubbles. So they made more blades, or a different shape, and bigger blades, and ran those bigger blades slower.
Put a stop to that noise

It got me to thinking that maybe the theory of Bigger props turning slower might help with the Horsepower and fuel milage of smaller craft as well.
Are there any qualified speakers on this subject amongs us?

[gonzo]

As you pointed out, the development is about noise reduction. The Navy is not that interested in fuel economy. Also, a fully submerged craft has a different operating mode than a surface craft.

[Submarine Tom]

Yes, slower rotating, larger props are more efficient but fitting a large prop to a boat can be impractical depending on many variables.

-Tom

[Adler]

Every time when started the design of a Submarine propeller there were various limitations.
One of them is the self-echo of the propeller to the Hull.
This factor defines an upper limit to the propeller's diameter.
Have in your mind that the idea of sound-passive sensors on a Submarine is based to measure the partial derivatives of the reflected sound-waves.
The military corporate planing has the asymptotic limit to operate the passive sensors at full speed - if it is possible (Main purpose is searching - recognition - targeting - hunting).

Practically if the design Power to move the Submarine to battle is defined too high, then the permissible pressure to the blades surface obliges the Propeller designer to increase the number of blades instead to keep few blades and to increase their surface, where the cavitation is assured higher.

[daiquiri]

In addition to the above, a boat operating close to a water surface has other constraints to the prop diameter. One, for example, is the necessity to avoid the aeration of the blades. Another that comes to mind is that increasing prop's diameter can put the blade tips too close to the hull, creating another source of cavitation: vortex cavitation between blade tips and the hull.
See this visual explanation:

Then you have a problem of required gear ratios with a consequent gearbox weight etc...

[thudpucker]

Daiquiri, your drawing is very near the TV presentation.
Right behind the blade, at the rearmost part of the blade is that low pressure area where the Water boils and the Bubbles occur (on the TV) program.

Your drawing shows other Bubbles and Adler mentioned the proximity sound waves coming off the Tips to the hull and outward.
It would seem that a Torpedo shaped extension with the large prop would get to the silence better. The Rudder steering coluld be on the sides, ahead of the Prop extension.

Too bad the Sub couldnt control the direction and Frequency of those Reflected sound waves, and make them collect way off in the distance.
Suddenly the Acoustic Torpedoes would be off on a wild Goose chase

I'll bet thousands of hours were put into this idea be several governments.

[daiquiri]

Originally Posted by thudpucker

Too bad the Sub couldnt control the direction and Frequency of those Reflected sound waves, and make them collect way off in the distance.
Suddenly the Acoustic Torpedoes would be off on a wild Goose chase

You know what? That's one hell of an idea... something like a fresnel-lens shaped hull near the prop perhaps (or the same thing applied to a duct around them)?

Keep drinking those blood thinners. Maybe I'll start drinking them too, if they have this effect...

[Adler]

See the attached....

[daiquiri]

Interesting concept, gave it a quick glance. It's a bit late here, so I don't have enough concentration left for reading it carefully. Will do tomorrow. Just one question: the report was written in 1991 - did the project make any advances or see the daylight in the past 20 yrs, or has it been shelved for some reason?
Cheers

[Adler]

It is not shelved.

The Anti-Submarine Frigates like KNOX CLass used the easy solution application with one Propulsion Consumer (Propeller) to avoid mixed echo. That for small ships as frigates are.

Air Craft Carriers is a way different.
They must have less dead reckoning regarding to let noise-clear their substance perimeter to be able act the Anti-Submarine hunters (Helicopters - Frigates - Submarines)

[rasorinc]

A few years back when I visited my son and family here in Tennessee he worked for a high tec copany at the Oak Ridge National Labratories where the fuel for our atomic bombs is made and the fuel for the Satern Rocket was produced. He was tearing down buildings that were radiated. He had top secret clearence and got me a pass so I could go in and see the huge place. Took me to a building that housed a large machine with about 10 arms each with several joints all hydralic of course. I asked what it did? He told me it picks up a 3' by 8' by 30" block of material that weighs a couple of tons and rotates it among these cutting arms and turns out our nuk submarine propellers in one continuous motion. I picked up some fillings on the floor but could not identify any thing. He said the machine is not top secret, but the computer program that runs the machine is four stories underground and under heavy security. I'm sure it got exterior coatings applied to it. Just some info for all of you.

[daiquiri]

Originally Posted by rasorinc

Just some info for all of you.

Too bad they didn't give you a top-secret clearance for the underground stuff too. That would be a really great reading here at BD.net.

Just kidding, razor. A fact that submarine hull and prop surfaces are coated for reduced sonar visibility is not a secret for sure.

By the way - do you happen to have someone from your family at Area 51 facility too? I would have so many questions to ask...

[rasorinc]

I know nothing about area 51, but I have been in the pilots seat of an SR-71 out of Beale Air Force Base some 50 miles NE of Sacramento, California nestled up against the foothills of the Sierra Nevada Mountains below Lake Tahoe.

[thudpucker]

I had a "Top Secret-Need to know" and it got me into a couple of Guard Houses on NUKE bases.

[keysdisease]

Originally Posted by daiquiri

A fact that submarine hull and prop surfaces are coated for reduced sonar visibility is not a secret for sure.

There are "Navy" tiles available in the civilian market. I'm sure quite different than what's on fleet subs, but similar in concept. They are typically used on prop tunnels, over propellers in conventional prop / strut drives, "wave slap" area's like chines forward, stringers in engine rooms, metal decks above engine rooms, etc.

Graphite inpregnated to lubricate the internal structure of the tile that gets excited by vibration and changes vibration energy into heat via friction.

Steve