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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

by todd8 5y ago

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.

jareklupinski 5y ago
I can imagine loading and unloading a sub full of containers to be more difficult than an open-top cargo ship
HomeDeLaPot 5y ago
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.
thaumasiotes 5y 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. 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?
- Someone 5y ago
  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-speed/ https://navalpost.com/submarine-cavitation-drag-underwater-s...)
  - thaumasiotes 5y ago
    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.
    - exporectomy 5y ago
      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.
      - dingaling 5y ago
        > Submarines don't generate the latter type of wake They do, and it's detectable through the bioluminescence it generates
        5y ago
Ekaros 5y ago
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.

by todd8 5y ago

jareklupinski 5y ago

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

HomeDeLaPot 5y ago

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.

thaumasiotes 5y 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. 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?

Someone 5y ago
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-speed/ https://navalpost.com/submarine-cavitation-drag-underwater-s...)
- thaumasiotes 5y ago
  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.
  - exporectomy 5y ago
    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.
    - dingaling 5y ago
      > Submarines don't generate the latter type of wake They do, and it's detectable through the bioluminescence it generates
      - 5y ago

Ekaros 5y ago

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.