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Comment by thaumasiotes | Hacker News Reader

Sorry, it's true that a passing submarine does not disturb the water it passes through?

I'm not challenging the idea that a submarine might require less energy to move one knot than a surface ship does. I'm challenging the idea that a submarine leaves no wake.

In the photo heading the article you link, a wake is visible behind the submarine.

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Wake can mean two things. I think you're confusing disturbance of the water that it passed through with travelling waves that appear as ripples on the water surface in the case of boats. Submarines don't generate the latter type of wake. Of course they do consume some energy. They're not perpetual motion machines. That some energy goes into the former type of wake.

The surface of the water allows a unique mode of energy transfer (surface waves) which doesn't exist without a surface. Underwater, energy can still be transmitted by sound, but that's not the same mechanism as surface waves.

I'm skeptical that picture is a photo since it doesn't have blueing and darkening with distance. Maybe it's been doctored or is a small scale or something unnatural.

> Submarines don't generate the latter type of wake

They do, and it's detectable through the bioluminescence it generates

That's the former type. Maybe my sentence was too messy to clearly distinguish them. Ships generate a V-shaped surface wave and (deep enough) submarines don't.

> Wake can mean two things. I think you're confusing disturbance of the water that it passed through with travelling waves that appear as ripples on the water surface in the case of boats.

In what sense are these different things? How could you have one without the other?

They're distinctly different physical phenomena. Surface waves are like spring-mass systems where gravity acts as the spring (hence they're also called gravity waves https://en.wikipedia.org/wiki/Gravity_wave). They can't exist in a homogenous medium, like underwater approximately is. Since they're travelling waves, they carry energy when they travel away from the boat, so it's a source of energy loss.

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