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Comment by todd8 | Hacker News Reader

As I recall from my class work in a hydrospace vehicles course I took decades ago, submarines have the interesting property of being more efficient that surface vehicle because they leave no wake. A wake is a set of propagating waves that require energy to generate and this, naturally, is felt as drag by conventional ships.

The question of course arises, why aren’t submarines used for shipping? We were taught that the problem was buoyancy control. The weight of a ship carrying freight can vary greatly on a round trip.

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I wonder if there is some scaling factor here. That is in certain range submarines are more efficient. But then if you get big enough normal cargo ships do better. I would think that surface area plays big part, with big ships only certain area is under water, for submarines it's the whole thing. And for cargo ships it start to be big.

> submarines have the interesting property of being more efficient that surface vehicle because they leave no wake. A wake is a set of propagating waves that require energy to generate and this, naturally, is felt as drag by conventional ships.

This sounds off. Conventional ships experience drag when they try to go forward and there's a bunch of water in the way. When they shove that water out of the way, it has to go somewhere else, and meanwhile some other water has to flow into the space where the ship used to be. That motion of the water generates the wake.

But all of that applies just as well to submarines moving below the surface of the sea, or cars driving around within the atmosphere. If a car passes by you, you'll feel wind blowing after it. That is the car's wake. And when a submarine tries to move underwater, it runs into exactly the same problem as the surface ship -- there's a bunch of water in the way. How can it possibly move without generating a wake?

It sounds off, but is true, but at least partially because the hull of modern submarines is optimized for submerged use.

That’s why the bow of modern submarines looks more like the nosecone of a rocket than like the bow of a surface vessel.

See https://navalpost.com/submarine-cavitation-drag-underwater-s...)

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.

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.

I can imagine loading and unloading a sub full of containers to be more difficult than an open-top cargo ship

That's surprising to me... is it because handling the variance in buoyancy would require massive ballast tanks? I was thinking the primary blocker would be the sheer complexity and cost of the technology required to go under the water rather than on it for a long period of time.

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