Virtual Earth Exposes Nuclear Sub's Secret

NewsCloud alerts us to a story a few months old that has been getting a lot of play recently. A Seattle blogger, Dan Twohig, was browsing in Microsoft's Virtual Earth when he accidentally came across a photo of a nuclear sub in dry-dock. Its propeller is clearly visible — this was a major no-no on the part of someone at the Bangor Sub Base. The designs of such stealth propellers have been secret for decades. Twohig blogged about the find and linked to the Virtual Earth photo on July 2. The debate about security vs. Net-accessible aerial photography has been building ever since. The story was picked up on military.china.com on Aug. 17 — poetic justice for the Chinese sub photo that had embarrassed them a month before. On Aug. 20 the Navy Times published the article that most mainstream media have picked up in their more recent coverage. Twohig's blog is the best source to follow the ongoing debate. No one has asked Microsoft, Google, or anyone else to blur the photo in question. Kind of late now.

Comments

From TFA:

[dominious]

You may have to be in Internet Explorer to see this... Firefox shows just fine! just so to let you people of /. know:)

Re:Probably not significant

[TerranFury]

Why three?

I am not a fluid dynamicist. But: To increase thrust at a certain RPM, it seems that you can either (1) increase the diameter of the propeller, or (2) increase the number of blades. The problem with increasing the diameter is that the velocity at the tips increases, which leads to effects like cavitation (which, besides being very noisy, damages propellers). So what you do is increase the number of blades.

Prop-driven airplanes produced near the end of WWII had many-blade propellers for this reason as well: They wanted a lot of thrust, but, if they made the blades any longer, then the tips would have been supersonic. (I think I got this factoid from the History Channel.)

My guess is that a quiet high-thrust propeller would spin slowly and have many, very wide and heavily-curved blades. Let's see if somebody who knows more agrees.

Re:Probably not significant

[mikael]

The secret bit of these propellors is what materials they are made of, how precisely they curve, and to what tolerance they are engineered. The big problem with propellors is that they tend to suffer from cavitation at high speed, where the sudden change in pressure causes bubbles to form and collapse. Apart from being rather noisy, referred to as "singing" (which is a bad thing for a stealthy submarine trying to make a fast getaway) it also causees damage to the blades (much like desert sand on engine turbine blades).

The purpose of having an odd number of blades is no secret - it is to reduce vibration. As the submarine travels through the water it leaves a wake behind it. Above the submarine there is less water pressure than below - so having two blades above and below at the same time is a bad thing. The more blades, the less vibration, but propellors are more efficient with fewer blades. You will see speedboat propellors with three or four blades, and fishing boats can have propellors with only two blades.

There is also the problem that having different metals in close proximity in a salt water environment, can lead to an electrolysis effect where the metals and water act as a kind of battery. Lots of technical papers on Propellor design

"The most frequent cure for a singing propeller is the popular "anti-singing edge". This is a chamfer applied to the trailing-edge to promote separation of the vortices."