Speed and Patrol Screws

[redboat219]

Can somebody tell me more about the diference between the so-called "speed" screw and "patrol" screws used on subs.

[aeroengineer1]

As I understand it one

As I understand it one is designed for speed, being the typical shape that you might see on any ocean liner with little skew. A patrol screw is highly skewed and is not meant to go necessarily fast, but is optimized for quiet operations.

Adam

[tom dougherty]

So-called "speed screws" were installed

So-called "speed screws" were installed at the building yards in the late 1950's early 1960's. They were standard five "fan" bladed designs, and with these, the Skipjack class went very fast (speeds well in excess of 24 kts!) They were highly efficient with large blade surface areas.
Here's a speed screw photo on SSBN 611 during construction:

Speed screw



It was discovered (by the then nascent SOSUS network) that submarines equipped with these screws could easily be tracked for long distances. In fact, it has been said that the George Washington (SSBN-598) was tracked all the way across the Atlantic on her first deployment. Besides machinery noise (which was solved by "rafting" the engineering decks and other measures), the standard "speed" propellers also had a problem caused by something termed "blade rate" . The propeller was turning in the wake of the cruciform rudder and stern planes (the design from the Skipjack class forward). As the fan-shaped blades entered and exited the wake, they vibrated slightly, leading to a low frequency sound. This Hertz range (as opposed to higher frequency kilohertz or megahertz) sound propagated extremely well for long distances through the water. A new propeller design, the damped J-series, with 7 skewed blades and slower revolution rates was adopted. Due to the skew, the slower turning blades entered and exited the disturbed wake zones more gradually, and had other features to reduce vibration. The J-series propeller was placed on all of the Skipjacks, Permits, Sturgeons & first series of SSBNs. The Soviet Union did not catch onto this development for some time, making it easier for both US submarines and SOSUS to track their earlier generations of nuclear submarines.

The J-prop was declassified and featured in the Smithsonian "Fast Attacks & Boomers" exhibit. The LAs & Ohios have advanced blade designs, which are still under cover, as are pump jets.
Here are some J-series props

J-series-Smithsonian exhibit

J-prop- USS Barb

USS Casimir Pulaski J-prop- from Don Murphy

[chuck chesney]

Tom,
Thank you for the

Tom,
Thank you for the quick seminar on the screw blade design evolution. I've always wondered about the reasons for the move to the "J" type screws. I had read that they were quieter, but had no further information on it, and didn't take the time or effort to find out why.
You, sir, are a fountain of information. Thanks again.

[mylo]

The level of knowledge here

The level of knowledge here at the Subcommittee continues to impress. I enjoy the reads.

Mylo

[redboat219]

did the blade chord have

did the blade chord have any influence on the screw's sound emmission?