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A sail generates propulsive force by the difference in pressure on the concave and convex sides. This principle of fluid dynamics should be true of seawater as it is of wind. And the seas do have currents, including predictable permanent currents.
This got me thinking, could a submarine deploy an underwater sail that would billow in the current and create a force that pulls it forwards?
Thanks.
Sailing actually requires two separate mediums for it to work. Sailing ships tack based on the wind pushing the sail, and that presses the keel of the ship against the water. The direction of the ship is based on the sum of those two forces. Solar sailing involves the sum of the solar wind against the solar sail and the force of gravity.
With a submarine, you only have one medium. You could raise and lower the submarine to catch different currents, but I don't believe it's practical for a submarine to, for instance, scrape its keel against the ocean floor to generate deflection.
Oceanic currents are generally fairly slow, so aren't a great source of power. And as stated in other answers, surface sailing requires the ability to push sideways on the surface to counteract lateral forces, whether that's with a keel (on a boat) or wheels (on a land yacht). I don't think that's possible in this context.
There is something which you may be interested in though, which is the underwater glider. Currents may be slow, but the actions of floating up and sinking down are much faster. Instead of using a vertical aerofoil to produce force from a horizontally-moving current, the glider uses a horizontal aerofoil to produce forwards force from the vertical action of floating and sinking. It angles the aerofoil so that sinking produces forwards motion; then changes the aerofoil angle and floats up, producing more forward motion; and repeats the process. This cycle runs continuously, so an underwater glider can travel in any direction and is not limited to travelling with the current, or (with suitable positioning equipment) can be left to stay on station at a fixed location regardless of currents and winds.
Sailing vessels obviously have the benefit that they need no external power source, whereas the underwater glider needs some power to inflate or deflate its buoyancy device and change the foil angle. However the temperature difference between deeper and shallower water can be exploited as an energy source in its own right, and this keeps the glider powered. Underwater gliders therefore need no external power source, only an onboard battery, and can operate indefinitely in the same way as a sailing vessel. (Barring antifouling, repairs and so on, of course!)
There are a couple good answers already, but in case it helps...
You would have to extract energy from something, so an underwater sail would only work if it "billows" into a current that is traveling faster than the one the submarine is in to "pull it forward". Otherwise the sail and submarine would just continue move along at the same speed.
Picture a hot air balloon drifting with the wind:
Alternately, consider a rowboat drifting along in the middle of a river. If you dipped one or both oars in the water and held them stationary, (not imparting any force or motion to them) would you:
Whichever answers you pick, the same principles apply to a submarine in an ocean current.
(Spoiler alert, the second bullet in each example is what would happen, but this presumes a relatively uniform and homogenous mass of air or water, and is premised on NOT using gravity to create kinetic energy.)
Subs and ships do this already without the need for sails. Sails are for wind currents. If the sub/ship has no method of propulsion (e.g., sails), then the currents take them where the currents will based on the buoyancy of the ship. So, the answer is "sure!"
But why would they deploy sails? I doubt a ship can tack underwater. Tacking into the wind works because you have the ocean's surface to push against (like an ice skater using the edge of their skate to change direction on the ice). I can't imagine the ocean being both wind and surface to push against. Thus, the sails are mostly irrelevant.
The answers provided are almost entirely right.
Sailing requires two mediums. Absolutely correct, however, it is extremely important to define "medium". More specifically, if we take two control volumes, the medium inside - does it have the same density, velocity, composition, energy etc.?
Conservation laws state, in laymans terms: You can't get something from nothing.
Sailing works, because sailboats actually have at least two sails: one in the air, one in the water. A sailboat can tack faster than the wind, because it works on the principle of transfering energy between the two. In the process, part of the energy is siphoned and converted into boat speed.
Adding the two concepts together, in general terms - in order to "sail", we need two sails in different control volumes, which have different properties. Next we initiate energy transfer between the two sails and siphon part of the energy into our vehicle.
Returning to the OP: Can a sub sail underwater?
Lets check. Can we find two different "medium" underwater? Yes!
The point is - water can have different properties with very pronounced boundaries. Sailing requires a difference in velocities, but if you can find that difference in velocities underwater, you can put two sails on a sub an sail these boundaries. The exact speed we can achieve requires calculation.
Yes, but if it were to do anything other than to use the sail to provide initial acceleration to overcome its inertia and travel with the current in a straight line it would either need to operate at the boundary between two currents or find some way of generating drag.
For example, if it deployed a sail in front of itself that would try to move with the current, but if it also deployed a steerable drogue to much deeper- i.e. colder and denser- water it might be able to tack.
Yes, that's possible, and it's being done since a long time. The catch is, you need a catch somewhere, i.e. a steel line that connects your boat to a fixed point, or to a rail or wire with a traveller on it.
With the line stretched by the current, you can change the angle between current and hull (=sail), and observe a perpendicular force.
https://en.wikipedia.org/wiki/Reaction_ferry (check the picture of the Rhine ferry at Basel in there)
We need two media to sail, such as sails in the wind and the keel in the see, as people have pointed out.
There is one overlooked possibility, which is to have a submarine with wheels on the ocean floor. This might work like the 'downwind faster than the wind' machines but underwater. I doubt it is useful - the sea floor goes up and down, and the abyssal seas are quiet. The submarine would need a good grip on the sea floor for the process to work. I doubt whether this is a good way for getting from A to B with any speed, but a slow crawler that investigated the ocean depths, could have a tethered float with a turbine that generated electricity to turn the wheels.
As others have pointed out, you will have a very large challenge doing anything but floating along in the direction the water is already going.
But even if you accept that, it's of limited use.
In some rare situations, ocean currents are forced between obstacles. This can concentrate the current and produce higher velocities. The problem with that is, it requires the current to be constricted. This is very unlikely to be helpful to a sub. Imagine you are letting yourself be dragged by water between two sharp obstructions. The chances of a good night's sleep are minimal.
You will see such things in certain bays and inlets due to tides, for example. You might get some help crossing through the tidal zone. Then you are in deep water again, having risked scraping the bottom.
Other than such restricted locations, the fastest ocean currents are only a few miles per hour. The Gulf Stream, possibly one of the fastest in the world, is only about 5.6 mph, slowing to 1 mph in the middle of the ocean. So even accepting that the water determines your route, it takes a very long time to get there. Absent the ability to "tack" somehow, you are pretty much limited to the speed the water is moving. It might well take several months to cross the Atlantic.
If you remember Pixar's Finding Nemo, there was a scene where Crush and fellow sea turtles ride an underwater current. While the current was dramatized for the movie, it still is an example of a current. Real animals use currents such as this. Fish do not have sails, so your sub wouldn't have to, either. However, in specific, predictable currents, sails would help with propulsion. Airliners use high-altitude jet streams to their advantage, for fuel efficiency. If your world has large, predictable subsurface currents, and there is sufficient technology to make the sails open and close underwater, this could work. However, another question is, why? If a civilization understands that a sail would work underwater, they would probably also understand that water is more viscous than air. It is harder to move through water than air, so most people would stick to surface ships with sails. However, if a civilization is located entirely below water (think Atlantis or the Gungan City from The Phantom Menace) something like this would most likely develop in some fashion. Either way, it's your world, so "Why" questions can be answered however you want.
One detail that every answer so far has missed is that oceanic currents move slower the deeper you get. The Gulf Stream for example moves at 6.4 to 9 kph at the surface, but that speed tappers off to nothing at a depth of 800-1200m deep. So, if you have a really big submarine, something a few hundred meters tall, then you'd have enough gradient to experience the 2 vector forces required of sailing. Instead of "sails" what you really want are 2 giant rudders that go over and under your sub to create that interaction between opposing currents.
This said, your best bet is generally going to be to just go with the flow, and not bother sailing much at all. Oceanic currents are way more predictable than the wind, they move in relatively fixed paths and will bring you more or less where you need to go all on its own (as long as you are planning to follow the stream) and if you follow it long enough it will bring you back to where you started making it viable for unpowered trade route propulsion.
Consider this, if you are moving at an average of 154km per day following the Gulf Stream, you could drift the distance between Spain and the Bahamas in about 50 days. That is faster than it took Columbus to cover the distance using traditional sailing before the discovery of the gulf stream. Unfortunately, you will still need to have some other form of propulsion to help you get from the ocean currents to ports and back again, but while in the current you could just use your "sails" to help you keep on course and navigate to find the best parts of the current as you go.
