Submarines actually move through the water more efficiently than surface ships, according to my Naval Engineering class: they don't lose energy supporting a huge wave field on the surface due to a wake and hence experience less drag. [1]
If they are so efficient, why not use them for transporting freight? Buoyancy limitations make them impractical compared to container or tanker ships.
Can you elaborate on the nature of the buoyancy issue? It seems to me that as a container is considerably less dense than water (container ships being mostly above water), you could just add ballast. Is the problem simply that too much ballast would be required to be practical?
I've sometimes wondered why sea gliders are not used for cargo. Sure they're slow but the power requirements are so minimal they can actually run off thermal gradients in the ocean, which must make the operating costs absurdly low.
I'm not an expert in this field, but I did take a class in Naval Engineering while I was an undergraduate (Math/EE/CS). When the class learned of the efficiency of submarines this very question was posed to the professor and he explained that buoyancy issues were the problem. (He was a full professor of Naval Engineering at MIT so I believed him.)
A few back of the envelope calculations are revealing. A very large container ship can carry an enormous amount of freight, around 200,000 Metric Tons. A modern, nuclear submarine, say of the Ohio class, has about 18,750 Metric Tons of total displacement so it would have to find a way to carry ten times as much weight as the weight of the entire submarine. Then on a return trip, when empty, it would have to have enough ballast equal to this weight that would then need to be discarded where it picked up freight. There probably is a way to do this, but I can easily imagine that the design issues result in a submarine less practical than a container ship.
Interestingly, I recently got to visit the aircraft carrier the USS Nimitz while it was underway out in the Pacific. That was an exciting day; subsequently, I looked up more information about that ship. These nuclear aircraft carriers are believed to be the fastest ships in the US Navy. Submarines (although not actually a part of the carrier group of ships that surrounds, travels with, and protects the aircraft carriers) travel hidden and independently along with the group. They scout ahead and hunt other submarines. To be able to travel with the carriers they must be very fast, faster than the official, unclassified numbers (about 25 knots). One source on the Internet says that they can travel at over 40 knots (74 km/h or 46 mph).
I think sea gliders are fascinating. I am curious about where that technology will go.
The CSS Hunley used this design, to no avail. It's not clear what went wrong but I think the larger point to draw is that such a system requires positive action, with more things to go wrong; it is "fail-deadly". Positive buoyancy and dive planes are "fail-safe" - you stop, you float.
They should make the first banner video they show "steady", using some kind of video editing software. It's not super-shaky, but it made me very very slightly queasy for a second, and I can't be the only one. What if one of the sufferers happens has $1.5mil to spend?!
Reminds me of SeaQuest DSV. The "Gazelle" (aka "Stinger"), a personal high-speed submarine craft.
