The boomer USS Georgia (SSBN 729) in drydock, with her propeller shrouded from sight. But what's that peeking out from under the cover at around 9:00? Courtesy of the U.S. Navy.

The fast attack submarine USS Jacksonville (SSN 699), in drydock in Norfolk, Virginia in October 1998, showing her propeller removed for servicing and/or replacement. Photo by the author.

The propellers from USS Nautilus (SSN 571), the world's first nuclear submarine, are on display outdoors at the Submarine Force Museum in Groton, Connecticut. Much like the prop of a surface ship, it lacks the silencing technology of later screws. Courtesy of the Submarine Force Museum.

Spinning propeller showing cavitation bubbles. Courtesy of Sonalysts, Inc.

It's amazing what sorts of things you can find in storage at museums. One of USS Seawolf's (SSN 575) propellers, out at the Naval Undersea Museum at Keyport, Washington, is covered with a protective white coating and edge guards. Photo by the author.

The author's sketch-proposing to cut down a fast attack's propeller in order to make it fit into the exhibit-met with less than wild enthusiasm. Sketch by the author.

This sketch of a 637-class fast attack submarine was traced from an enlarged photograph. Courtesy of J. Fuchs.

The elevation sketch was cleaned up by an illustrator, but at that point we didn't know much about the blade shape, so that aspect of the original sketch was eliminated. Courtesy of T.G. Ormsby.

Available on the web, this drawing of the stern of a 637-class fast attack is from Chapter 561 (Submarine Steering and Diving Systems) of the January 1992 edition of the Naval Ships' Technical Manual. Courtesy of the U.S. Navy.

Working model of a 637-class fast attack screw, now on exhibit at the Smithsonian's National Museum of American History. Photo by the author; object courtesy of the Hydromechanics Directorate, David Taylor Model Basin, Naval Surface Warfare Center, Carderock, Maryland.

The stern of a Los Angeles-class (SSN 688) submarine at Holy Loch, Scotland, showing a mid-1970s propeller with edge guards; it probably was being installed when this photo was taken. Most of this class remains in service. Courtesy of Norman Friedman.

John Pike, "Run Silent, Run Deep," (Washington, DC: Federation of American Scientists Website (December 1998)) www.fas.org/man/dod-101/sys/ship/deep.htm.

Naval Ships' Technical Manual, Chapter 561 - Submarine Steering and Diving Systems - 114 pages. See p. 1-5 (16), Fig. 561-1-1. Typical Stern Diving Control System for SSN 637 and SSBN 640 Class. Downloadable pdf document @ www.fas.org/man/dod-101/sys/ship/nstm/561v0r2.pdf

Marek Behr, "Models of Spinning and Translating Geometries," www.ruf.rice.edu/~behr/spinning.html

Norman Friedman, United States Submarines Since 1945 (Annapolis, Naval Institute Press, 1994), esp. p. 163.

Glen S. Fukushima, "Japan's quiet power," Asahi Evening News, 4 March 1997, www.interesting-people.org/archive/3611.html

E.V. Miasnikov, "What is known about the character of noise created by submarines? " The Future of Russia's Strategic Nuclear Forces/Discussions and Arguments (Center For Arms Control, Energy And Environmental Studies, Moscow Institute of Physics and Technology, 1995, revised 1998), Appendix 1. www.fas.org/spp/eprint/snf03221.htm

Neal Stevens, "AKULA! The Soviet Shark," SUBSIM Review (November 1999), 6pp. www.deepdomain.olm.net/ssr/akula2.html

Roderick Seeman, "Toshiba Case-COCOM - Foreign Exchange & Foreign Trade Control Revision," The Japan Lawletter (April 1987). www.japanlaw.com/lawletter/april87/fdf.htm

Chris Warren, biography (click on images) web.mit.edu/mhl/people/warren.html

Mary Zoccola, "Signatures Accomplishments Highlighted," Wavelengths (December 1998). www.dt.navy.mil/pao/excerpts%20pages/1998/signadec.html http://www.warship.simplenet.com/images/Flagship/FM%20350-6.jpg

I stopped talking after about three minutes. Dead silence and utter stillness greeted my proposal to a sizeable gathering of retired U.S. Navy admirals, former submarine captains, experts in naval intelligence, nuclear propulsion engineers, and current submariners of various ranks. Eventually, peoples' eyes began to slide around the table, gauging each others' reactions to what I had said. Maybe they thought I was joking or deliberately provoking, but I wasn't.Nobody said a word, so I started in again, detailing my special interest in the subject and describing its lowly 19th century origins. Finally, one of the Pentagon representatives at the other end of the table raised his hand rather tentatively, cleared his throat and voiced the collective expert opinion I had already anticipated, "Excuse me sir, but all that's classified." Others around the table immediately and unanimously confirmed his appraisal of the situation.I suspected I would have only one chance to sell my idea, and the time was now. We were in the early planning phase of a proposed exhibit on nuclear submarines at the Smithsonian Institution's National Museum of American History - a joint venture between ourselves, the Navy Submarine League and the Navy - and at this meeting, we were discussing various possibilities. At that point in time, all ideas were worthy of consideration, however preposterous they might appear on the surface.We had been told that the Navy might be willing to declassify certain aspects of submarine technology and operations just for our exhibition. Details were vague, but the offer was straightforward and sincere. However, they'd had other things in mind when making that offer, and I had unexpectedly taken aim at one of the most secret aspects of modern nuclear submarines. It had never before been seen by the public and was even hidden at restricted-access submarine bases worldwide, and now I was proposing that we share it with the five million visitors that come to our museum each year. Hence the quiet.Hoping the chap at the other end of the table who'd spoken knew the most about my subject, I directed my next question to him. "Is there any way we could look into this a bit further before giving up?" He smiled, gracefully acknowledging he'd been hooked. I got his name, and we went on to other topics. I was very pleased, as I had a fallback plan and an alternative to that, and I hadn't needed to bring up either one.What I'd asked for was a nuclear submarine propeller, better known in the Navy as a screw. It is so secret that when a sub comes in from patrol its propeller is shrouded with a large covering, and if a sub is drydocked for any length of time, it is commonly removed and stored away from the ship for servicing and prying eyes. It is what drives a submarine quickly and quietly through the water, and it is absolutely critical to a submarine's stealth. It is an extremely high-tech piece of equipment, resulting from extensive Navy research and development, and until recently our propeller technology exceeded the Soviet Union's by a considerable margin. As a result of this and other factors, Soviet submarines of the Cold War were noisier, and therefore easier to detect than our own. Within the context of national security and defense, as well as our nuclear deterrence policy, it is easy to see why propeller research and design have been so carefully guarded. The propellers of USS Nautilus, the first nuclear submarine, are an exception. Essentially World War II submarine propellers, they - along with the rest of the sub - are on display at the Submarine Force Museum in Groton, Connecticut, near where she was built in 1955.But something happened to change all that. Back in the mid-1980s, the Japanese company Toshiba sold propeller milling machinery to the Soviets through the Norwegian Kongsberg firm; this and other submarine intelligence furnished by the Walker spy ring resulted in significantly quieter Soviet subs by the later part of the decade. As writer Neal Stevens wrote about the-class Soviet boats, "The combined results generated a steep drop in broadband acoustic noise profiles."What is so special about submarine propellers? It's really pretty simple when you are lucky enough to have the world's foremost experts explain it in simple terms, and you don't need to be familiar with "tip vortex flowfields." As props turn, the angle of the blade pushes through the water, driving the submarine. Once they reach a certain speed, the blades begin to create a partial vacuum, which results in air bubbles. This is a state known as cavitation. Bubbles are noisy, and submarine propellers are designed and shaped to reduce cavitation and exploit other relevant laws of physics as much as possible and still maintain useful speeds. Of course, balancing all of these factors results in a compromise, and our newest generation of submarines, the-class, does not even use propellers. Neither will the future generation, the-class, still on the computer design screen. Other options offer fewer disadvantages.Before I could go much farther, I had to get a good drawing or photograph of a nuclear sub propeller. Once armed with that, I would have something to show around and generate interest. I learned fairly quickly that our best bet would be a propeller from a fast attack sub (hunter-killer, or SSN), as it would be considerably smaller in diameter and lighter than the screw of a boomer (ballistic missile sub, or SSBN). This would help exhibit space and floor weight considerations. I also approached the director of design for our museum, who luckily had a friend in the submarine service and expressed some enthusiasm. By chance, as part of a design project he was seeking large icons for the entrances to our various exhibit halls, and this might be just the thing for the Armed Forces History Hall. However, the size and floor loading (weight) were prohibitive, so I moved on to another option. More on that later.With the help of our Navy liaison, I obtained contact information for the Navy staff that manages the submarine propeller inventory. After identifying myself, I asked if it might be possible to visit the propeller repository, with an eye to snapping a photograph for exhibit research purposes. They are definitely unaccustomed to inquiries from the public, and inadvertently, I set off an alarm with this request. Only later did I discover why. What I thought was a simple request sent a ripple up the chain of command, causing a series of checks and questions. Nevertheless, the results were pretty interesting.It turns out that there are actually three classification levels of nuclear submarine propellers in the spares inventory, depending upon their condition. The highest level is A, or ready for issue. This group consists of screws that are coated with a white, plastic-like protective surface, fitted with edge guards, and boxed with their accessories. These are ready to be shipped out and mounted on a submarine immediately. The second level is F, or used propellers from ships that need work of various sorts before being ready for use. The third group is H, consisting of those examples that are beyond economic repair.It is probably not appropriate here to speculate upon exactly how submarine propellers achieve H status. For that group, the classified blades are cut off and their hubs are melted down and recycled. The repository contains a pile of classified blades awaiting meltdown, as soon as a facility is located capable of handling such classified material. To the best of my knowledge at the time of this writing, they are still looking for such an operation. While I could visit the repository and see the current inventory, under no circumstances would I - or anyone else - be permitted to take a photograph of classified materials, and it was this aspect of my original request that had sounded an alarm. However, our contact offered to try to obtain a photo from an active ship in port, or a blade drawing from appropriate sources within the Navy.Time was tightening, so I had to widen the net and play a trump card.My first breakthrough came through a casual conversation, and it was a revelation. I learned that I had been asking the wrong questions all along. Of course we would not be able to display a modern nuclear submarine propeller. But what I discovered - incidentally and almost accidentally - was that propeller data up to the-class of fast attack submarines (SSN 637, mid-1960s) was declassified. There were two reasons that no one had ever told me this before. First, although they had been declassified, they had not yet been "approved for public release." So, much of the Navy outside of the hydrodynamic research community did not know this. Secondly and more importantly, I hadn't asked.This was my trump card, and I played it as soon as it was dealt me. Quite literally, it was the key that unlocked the vault (although that event was still off in the uncertain future). Of course we could have a 637 propeller; when could we pick it up? This in turn led to a whole new set of questions regarding the object's exact size, weight, means of transportation, and potential placement at the National Museum of American History.Earlier, I mentioned that the size and weight were too great for placement in the submarine exhibit itself. Two alternatives logically presented themselves. I could try to get it cut down to a manageable size by removing 3/8 of its overall diameter (and lessening some of its weight) and setting it on the exhibit floor, or we could look into an outdoor setting. A single call removed the first option from my plate: although the exterior surface of the-class propeller was declassified, the interior remained under wraps. We could not cut through or into the blades without risking exposure of the blade innards to curious eyes, and my proposed workarounds to that particular dilemma fell on deaf ears.An engineering colleague suggested setting it outdoors in a pit of sand over a sunken concrete base, somewhere out on the museum's lawn. However, when I ran this idea past our design director, he informed me that this would run into the purview of the Washington Commission of Fine Arts and/or the National Capitol Planning Commission, which review and approve anything outdoors on the Mall. However tempting, there simply wasn't time for this option, as the exhibit was scheduled to open in less than a year. Nevertheless, it was intriguing to imagine the reactions to such a proposal from committees who reviewed learned discourses on all aspects of public monuments and statuary on the Mall, and what they might have said about the artistic merits of the stern of a nuclear submarine! With both those options closed, it didn't look like there was enough time left to get a propeller, so I had to return to my original fallback plan. Right around this time, my research suffered two more setbacks.First, our Navy liaison had to redirect his energies to other aspects of our joint venture. Around the same time, the individual who had volunteered to try get a photograph of a working fast attack propeller reported failure, despite repeated and persistent attempts. Cameras simply are not permitted on submarine bases, and I ran again into problems of time, as well as the issue of declassification vs. public release. However, he had managed to locate an unclassified photograph of the stern of a-class submarine, and he enlarged it enough to produce a sketch of its screw. Then I asked an illustrator to re-draft the sketch, smoothing out the rough edges and refining the outline. I also undertook a search for my subject on the web, with somewhat mixed results. Anyway, now I had a marketing tool but no propeller. I had to resort to my original fallback plan - trying to obtain a propeller model, instead of the real, full-sized item. Now, however, I was armed with new knowledge.Because of that, the rest fell into place very smoothly. I contacted the appropriate office at the Naval Surface Warfare Center (yes, that's correct!), identified myself, asked the right questions and received an appointment almost immediately. Although they too were unaccustomed to questions from the outside such as mine, they would be delighted to help out. In fact, they bent over backwards to provide an actual-class propeller model that had been used in high-speed experiments in the mid-1960s. It was identical to the real thing, except for its size. So now it occupies its own case in the Power and Propulsion section of our new exhibit, "FAST ATTACKS AND BOOMERS: Submarines in the Cold War," where this piece of technology - never before seen in public - can be viewed by anyone who walks by. There is only one restriction: we had to mount it in a very specific way to allow it to be removed quickly, should it need to be returned for use by the Navy.There are two bits of irony connected with this story. During the course of this propeller research, I learned that a book written by submarine expert Norman Friedman and published by the Naval Institute Press, an affiliate of the Navy, had actually published a pretty detailed photograph of a classified-class propeller at Holy Loch, Scotland in 1993! The second occurred during one of our visits to a boomer, when we spotted a large Toshiba television in one of the recreational spaces for the enlisted crew. This was the last thing I would have expected to see on an American submarine, given the company's history. I asked the Executive Officer about it and he only smiled, indicating he knew why I was asking. Maybe Toshiba was the low bidder?As might be expected, there is not much public information available about the design of nuclear submarine propellers.Nevertheless, there are a few bits and pieces, some of which are below: