Propeller designs have been closely guarded secrets since the days of the Cold War. It is still common for them to be draped with tarps or removed and covered when a submarine is out of the water. The propeller design is an integral part of a submarine's ability to remain undetected during operations, ensuring that it can patrol the seas in stealth without giving its position away to surface ships.

The find has triggered a debate over whether online mapping services offered by the likes of Google and Microsoft should be allowed to snap and publish images of sensitive US military installations. Reporting the discovery, the Navy Times newspaper quoted military analyst Nathan Hughes as saying that exposing the propeller was a major blunder that had compromised "sensitive naval technology". The paper quotes a Pentagon public affairs officer as saying that the Defence Department does not have a policy - or the legal authority - to demand the removal or blurring of commerical aerial or satellite photography.

The discovery was made by Dan Twohig, a deck officer on a ferry service in Washington State. He made the discovery in early July when he was looking at real estate near Seattle using Virtual Earth, a mapping service similar to Google Maps and Earth. Twohig lives in North Bend in Washington State. Situated about 50km east of Seattle, it was the setting for David Lynch's landmark TV series Twin Peaks in the early 1990s. Twohig was looking for a place closer to his work.

He subsequently posted the find on his blog, MonsterMaritime, and the story found its way into mainstream media late last month. "You can also use the zoom in and out keys and move around the Bangor Sub Base taking a close up look at the bunkers and magazines where they keep the nuclear weapons," he wrote in his blog. "You would think the US government would keep better tabs on this stuff." Twohig's discovery was made around about the same time that Hans Kristensen, a nuclear weapons analyst for the Federation of American Scientists, spotted an aerial image of China's new Jin-class nuclear-powered submarine on Google Earth.

The Chinese sub, which is capable of firing intercontinental ballistic missiles that can reach the US mainland, was snapped at the Xiaopingdao Submarine Base south of the city of Dalian - a facility named in honour of the late paramount Chinese leader Deng Xiaoping who died 10 years ago. An article written by Paul Forsythe Johnston, Curator of Maritime History at the National Museum of American History, and posted on the museum's website, explains the significance of submarine propeller design and the "tip vortex flowfields" the propeller creates.

"Once [the propellers] reach a certain speed, the blades begin to create a partial vacuum, which results in air bubbles," he writes. "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."