# Could a submarine 'sail' the ocean currents?

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.

• A quick look seems to suggest that ocean current speeds are less than 1 knot. But also, sails give you push against the drag of the water. If you're in the water, using the water's current, then I can't imagine your shape matters if you just want to ride with the water. Nothing is producing drag; you're just sitting still in some water that, itself, is moving. Jan 31, 2023 at 18:11
• I don't understand the OP as saying the Sub would just float along with the current, but would deploy a water-sail, some sort of device to 'harness the motive power of the current'... Feb 1, 2023 at 17:14
• Sails exist specifically to alter your vessel's movement, because without a sail you'd just be dragged along with whatever the current is. It seems like you're really just asking to not have a sail, i.e. the marine equivalent of a hot air balloon. Feb 2, 2023 at 5:14
• The existing answers have covered ballooning, gliding, tacking with different currents, or with the seabed, or with a cable. I haven't yet seen a discussion of dynamic soaring. It's just as impractical as all the other methods, so it would only be useful for completeness. Feb 2, 2023 at 11:45
• @chasly-supportsMonica air gliders travel long distance by conventional soaring, which I bizarrely had forgotten about, but of course could work in the ocean, it involves exploiting a single flow in the energy-giving direction. I was referring to dynamic soaring, which involves exploiting the difference in two nearby flows, not simulataneously by deploying a keel in one and a sail in the other, but sequentially by alternating between the two flows and changing the kinetic energy of the craft - impractically difficult in viscous water. Conventional soaring would work better, if available. Feb 2, 2023 at 16:44

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.

• I believe this is known as the "parallelogram of force".
– Tom
Feb 1, 2023 at 9:14
• thermoclines are locations where there is a sharp difference in temperature within the body of water, and frequently also have different currents on either side of it. It's possible a submarine could "sail" on the surface of a thermocline but, in doing so would be restricted to a 2 dimensional surface rather than being free to move in 3 dimensions like a conventional submarine Feb 1, 2023 at 9:49
• @Tristan Use the temperature difference for power, and you can zoom alternately up and down away from the thermocline. en.wikipedia.org/wiki/Underwater_glider Feb 1, 2023 at 14:11
• I'm now curious about hot-air balloons. You can somewhat "steer" them in a single medium, right? As in moving up and down to catch currents in the same way a submarine like this would. Is it the actual act of purposefully and physically steering against a second medium that makes it "sailing" vs whatever other phrase ("traveling", "dragging", "coasting", "drifting")? Feb 1, 2023 at 23:36
• @coblr, you can't steer hot air balloons. You can raise and lower them to catch different air streams, but you're completely at the mercy of the wind. Sailing specifically refers to using something to catch the air for propulsion. It implies contact with a surface, but most people don't think it out that far. Feb 1, 2023 at 23:52

### Yes, but not quite in the way you imagined

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!)

• "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)." - but he's asking about a submarine. A submarine can easily balance the torques by having two sails on the opposite sides - moreover, it could use them to turn. by temporarily changing their area. Feb 2, 2023 at 18:50
• ""Sailing vessels obviously have the benefit that they need no external power source, whereas the underwater glider clearly needs some power to inflate or deflate its buoyancy device and change the foil angle. " Well sailing ships DO require external power in the form of sailors to change the area and angle of the sails : ))) Feb 2, 2023 at 18:50
• @Maciej At the risk of going off-topic, that's definitely something you can automate. :) The main point is that the underwater glider has the same strengths as a sailing vessel in not needing to be refuelled, compared to a regular UAV which is limited by its fuel tank. Feb 3, 2023 at 11:01
• @Maciej Re the sails on opposite sides, that can't ever work though, because the sail on the opposite side is still going to be in the current. A boat with two sails above water still needs a keel, for the same reason. Theoretically you might be able to "sail" along the boundary between currents, I guess, but the length of each tack could only be at most the horizontal distance between your two masts - and outside of that you'd be dead in the water. Feb 3, 2023 at 11:17
• @DaFi4 That someone was me, in this answer. :) BTW, I spent most weekends as a teenager racing dinghies, and I used to teach kids how to sail, so I know about how to tack upwind. :) Feb 3, 2023 at 22:03

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:

• Does it need to deploy a separate sail in order for the wind to act on it? or...
• Does it simply move along with the air mass that it is a part of?

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:

• Generate a propulsive force by the difference in pressure from one side of the oar to the other? or...
• Would the boat and oar both remain stationary relative to the water, and continue to drift at the same speed as the river?

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.)

• The same train of thought would also apply to airborne gliders, though, which do a pretty good job of getting around without simply going the same direction as the air mass all the time. Whenever there are complicated vector fields of motion in the medium (including vertical motion), those can be harnessed without just drifting in them, and additionally buoyancy can be leveraged as in @Graham's answer. Feb 1, 2023 at 17:43
• @KenWilliams, what you say is true, but independent of ocean currents or atmospheric wind so I'm not sure of the relevance, or what your actual point is. Feb 1, 2023 at 18:54
• My point is that objects in a fluid medium are not just limited to remaining stationary relative to the medium, as you suggest with your added "spoiler alert". By using control surfaces (not propulsion) and taking advantage of differential currents they can achieve dramatic effects, like gliders that stay aloft for several hours. Feb 1, 2023 at 19:57
• @KenWilliams, sure, if you had a much faster current within the length of your oar you could certainly tap into its energy. Conversely, it you convert the potential energy of altitude into kinetic energy using gravity and have control surfaces like a glider you can do all sorts of wonderful things! I think that's a bit beyond what is being asked though. See my edits and let me know if you have any additional feedback. Thanks. Feb 1, 2023 at 20:14
• @KenWilliams, gliders use two forces just like a sailing ship does. In the case of the glider, the two are air resistance and gravity.
– Mark
Feb 2, 2023 at 0:40

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.

• 1st paragraph: I think that's called drifting with the current, not sailing. Feb 1, 2023 at 1:54
• OP asked"Could a submarine 'sail' the ocean currents?". You responded "Subs and ships do this already without the need for sails". Emphasis on "this". Feb 1, 2023 at 2:03
• @DKNguyen You're being a bit literal and narrow minded. "...without the need for sails. ... if the sub/ship has no method of propulsion ... then the currents take them where the currents will based on the buoyancy of the ship." But, whatever. Down vote if you don't like the answer.
– JBH
Feb 1, 2023 at 2:05
• @DKNguyen More accurately, nobody who understands ocean currents would misunderstand my answer - so you're really beating a dead horse. Like I said, whatever. Down vote if you don't like the answer.
– JBH
Feb 1, 2023 at 2:10
• Answers only understood by those who already know the answer are pointless. You're basically saying your inaccuracies don't matter because people who know enough will know to ignore them. Feb 1, 2023 at 2:11

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!

1. Anyone who has gone swimming in a lake in early summer, can recall, that the surface is nice and warm, but a few meters down, the water rapidly gets icy cold.
2. A popular photo of an ocean with a clear boundary between fresh water and salt water.
3. Current - underwater current is similar to a river in water - it moves in a different direction and/or speed, compared to the rest of the water.

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.

• this answer is great, and certainly the most complete here so far. I would add that whilst a submarine could sail along a thermocline, doing so would restrict it to remain very close to that two-dimensional surface, making its movement much more restricted than a conventional submarine and so probably of little practical use. You'd also need the distinct thermal layers to be moving at significantly different velocities in order to get much power. This likely limits you to a few specific locations e.g. near where a glacier meets the sea Feb 1, 2023 at 9:52
• Nice answer! Sailing depends on a difference in the speed vector of sail and surrounding medium. Only way to keep that difference from vanishing is to stay somewhere between two media, or to have more than one "sail", and move them with respect to each other, e.g. a propellor.
– Karl
Feb 1, 2023 at 17:43
• @MichaelHall I think the additional component is that the two differently-temperatured bodies of water may also be moving at different speeds, and as such could be used as separate mediums to “sail” along. Ex: if you had a current, you wouldn’t plop yourself entirely inside it, but sail along the boundary of it and the slower moving water. Or at least, that was my takeaway from the answer Feb 1, 2023 at 23:41
• @MichaelHall updated to be more explicit. Feb 2, 2023 at 6:05
• Also, physics works in mysterious ways (not really). A difference in properties of the same material, generally induces flow (current), because a saltier or colder water is heavier/less bouyant than fresh/warm water. Feb 2, 2023 at 6:50

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)

• Neat, but doesn't really apply to an ocean-going vessel, unless you can make that tether a temporary thing? Maybe some sort of harpoon mounted grappling hook gun that hooks into rocks or whatnot on the sea floor, and then you release the tether to move onto the next one? Maybe something that remotely releases at the hook end so you can reuse your ropes at least, though you're still littering the sea floor with a trail of used hooks. I imagine it'd be kind of like Spider-Man web-slinging around a city, only slower and underwater... Feb 3, 2023 at 19:16
• @DarrelHoffman Well, I didn't mean to say it was practical. :D
– Karl
Feb 7, 2023 at 20:23

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.

• Upvote for being the first person to pluralise 'medium' correctly. A third possibility would be a system of cables that generate power from currents and the submarine hooks onto them for a tug. Feb 1, 2023 at 14:24
• The answer is incorrect - You dont need two mediums, you only need to balance torques. HOW you do that is another matter. Sailing ships use the counter-pressure of water, but submarine could simply have two sails on the opposite sides - something a sailing ship cannot do since there isnt much wind in the water. Feb 2, 2023 at 18:50
• @wokopa - And then Maciej shows up and drops "mediums"... Feb 2, 2023 at 21:44

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.

## Yes, but only with a really big submarine

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.