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I’m designing a world where fully enclosed vehicles that can handle complete submersion into water are commonplace. While amphibious vehicles exist, a different class of vehicles has been introduced: drivable across the bottom of waterways and natural rivers.

Just like with amphibious vehicles there are limits to how rough the waterways can be, but what I’m interested in is how the vehicle’s wheels/tracks/whatever would be designed to allow decent locomotion across the ground of the waterways, while also providing a good locomotion when on land.

Things that the vehicle should be able to perform:

  • it needs to be fully submerged and deep enough not to bother regular boats floating on top. No floating on the surface!*
  • It needs to be able to drive extended distances up or downstream, as well as just crossing the river.
  • It needs to be able to drive on roads and cross-country
  • It needs to be able to enter and exit water at places where a regular amphibious vehicle would be able to enter and exit.
  • It needs to be able to handle most surfaces of rivers, like mud and rocky ground.
  • The aim is around 305km/h speeds when submerged. 12050km/h on roads.
  • Having different modes of locomotion in water and land is allowed as long as its not mutually exclusive.

The question: what type of locomotion would fit the best to achieve these goals?

Note: I am not asking for what you might think is more efficient. That is why I already recognize the option for regular amphibious transport modes. Tracks for vehicles is also an incredibly inefficient mode of locomotion compared to wheels but its designed for specific situations. That is what I am asking here: a mode of transport for this specific situation.

You can pick the type of vehicle yourself (personal car, small truck, big truck, full blown tank), but if you really feel the need to be stuck on one specific design:

  • 15 tons in weight
  • 7m in length
  • 3m in width
  • 2m in height (removed to focus on the locomotion rather than questions on the vehicle. Pick a density and dimensions for the vehicle that suits the locomotion you propose.)

*for those that spot the option, having it float at a specific depth and use jets to move as a mini sub is possible, but would still require you to figure out how fast the vehicle would sink to that depth, if it sinks too slow you reach the other side before you reach the desired depth. Also it still needs to drive the parts that aren’t deep enough when it enters the water.

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  • $\begingroup$ Comments have been moved to chat; please do not continue the discussion here. Before posting a comment below this one, please review the purposes of comments. Comments that do not request clarification or suggest improvements usually belong as an answer, on Worldbuilding Meta, or in Worldbuilding Chat. Comments continuing discussion may be removed. $\endgroup$
    – L.Dutch
    May 11, 2023 at 2:05

7 Answers 7

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Well, the vehicle dimensions you mention gives an internal volume of about 30-40 m3. Which means it will always float.

The only rl vehicles for which driving on the bottom of a river are heavy tanks in the 50-60 ton class. For example, the Swedish Stridsvagn 103 tank had a screen that could be lifted on top of the tank to increase its ability to float and cross rivers without sinking. Other tanks have a pipe that extends 1 or 2 meter above the tank to let the engine breath and cross rivers. None of this is a fast or elegant solution. (Google for some videos and see)

Most other vehicles don't have the density to sink. So first, for this concept to work, the vehicle needs to have a reason to be heavy as a rock.

Then there are two mutually exclusive options.

  1. Sink and drive on the bottom.
  2. Float mid water like a submarine.

If the vehicle is designed to sink it needs to be a lot heavier than water, to gain enough traction in the bottom. At least 1.5x I would say, but maybe 2x. This would make it impossible to float.

Tracks are the best propulsion in mud and are therefore preferred in (Northern) European warfare. They do less well in dry sand, so countries like South Africa tend to use heavy wheeled vehicles for the role of main tank. (The Rooikat)

However water currents will interact badly with the mud around a vehicle driving on the bottom. It will dig away the mud around it and make the vehicle sink in the mud. The action of the tracks will only accelerate this process. I don't know of any project that has tried to investigate this, but looking at excavators that get themselves stuck in water suggest that this is a major problem.

Submarine style will therefore probably be the most realistic option. At least in muddy rivers where the mud can be several meters deep.

If the vehicle has small wings or horizontal fins it can control its depth very quickly. Water is very heavy compared to air and therefore when you make some speed it is very easy to control movement horizontally and vertically with the use of fins. Look at tuna, dolphins and penguins for inspiration. They move to fast for buoyant forces to play an important role.

This would require a powerful water jet like propulsion. The most difficult part is the shallow water near the coast. The wheels or tracks don't have enough down force to get a grip on the soil. And the water jet intake might be to close to the surface and starts sucking in air. Not insurmountable, but it requires some good engineering.

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    $\begingroup$ not just sinking even if you sink if your not dense enough the force of flowing water can just drag it downstream. $\endgroup$
    – John
    May 9, 2023 at 22:52
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    $\begingroup$ Minor nitpick to the first paragraph: the supplied dimensions don't necessarily imply a completely enclosed volume - some or much of it might be open for water ingress, thus reducing the buoyancy. $\endgroup$
    – zovits
    May 10, 2023 at 8:28
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  • The aim is around 30km/h speeds when submerged. 120km/h on roads.
  • 15 tons in weight
  • 7m in length
  • 3m in width
  • 2m in height

I'm not sure if it is concidence or not, but the dimensions and weight your suggested are remarkably close to the real world OT-64 SKOT, a (temporarily) submersible cold-war era APC design.

enter image description hereenter image description here

(image credits A. Łuszczewski, J. Sobieszczuk, via wikimedia)

The notable exception being that the SKOT-64 had a max speed of 94 km/h and 9 km/h in water using a pair of propellers driven by the main engine and a couple of rudders for steering. Perhaps modern powerplants could fix the speeds... they're not hugely far away from your goals. And incidentally, "in water" here means "like a boat", not "like a submarine". It meets almost all other requirements of your vehicle, with the notable exception of

it needs to be fully submerged and deep enough not to bother regular boats floating on top. No floating on the surface!

That's a slightly non-trivial challenge, because of the volume of your vehicle relative to its weight: if it were an ellipsoid with those diameters it would be displacing a good 21 tonnes of water. If it wasn't an ellipsoid you might need to start worrying about drag, because travelling at 30 km/h underwater is going to be a challenge for a boxy shaped thing like a van.

The SKOT-64 could submerge briefly, if you had a sporty enough entry into the water, but it'll pop back up onto the surface quickly enough. Floating on top was considered OK though, because being a submarine adds a load of extra engineering challenges with fairly minimal reward... the surface of a body of water tends to be relatively flat and easy to traverse (exceptions for fast flowing rivers, of course) whereas the bottom could be muddy, rocky, lumpy, whatever. Its hard to see obstacles down there until you hit them, too. Churning up the bottom isn't going to do the local ecosystem many favors either. Getting stuck down there present some significant safety hazards that floating on the top does not, and so on.

what I’m interested in is how the vehicle’s wheels/tracks/whatever would be designed to allow decent locomotion across the ground of the waterways, while also providing a good locomotion when on land.

Assuming you somehow did still need to drive along the bottom, with it being slower and messier and more dangerous, then wheels and tracks will work for you. Lake and sea beds can be deep, soft silt and mud so you'd want broad tracks or chunky tyres (or rolligons, I guess) in order to avoid getting bogged down, but those things will definitely hurt your above-water performance.

If you took the (fractionally more sensible) submarine option you can avoid the terrain issues and retain surface-optimised tracks or wheels, but you need to make density adjustments somehow in order for your vehicle to be negatively bouyant. Doing this with ballast weights will ruin your speed and fuel economy, and flooding the passenger spaces will spoilt the upholstery and necessitate things like passenger drysuits and air supplies which are a different kind of compromise. Your top speed will be limited by the drag of any protruding bits like the wheels... having them retractable is going to massively increase engineering complexity (remember, the Lotus Esprit in The Spy Who Loved Me cheated!) and reduce the strength of the mounting system which will reduce all-terrain driving performance.

enter image description here

(Image credit wikipedia. Don't think too hard about where the hydroplanes came from)

In short, everything is a compromise. Your wheeled submarine is a very big compromise, and as such should probably only be used for specific purposes. General above-surface operations are best left to regular vehicles, which will be cheaper, safer, faster, and probably a lot more reliable.


As an aside, positive bouyancy submarines do exist... there's at least one commercial company, trying to sell them (commercial vendor link). They use the forward motion of the vessel and winglike hydroplanes to produce downforce in the water to overcome bouyancy and will float to the surface following a loss of power or movement. They're an inappropriate shape for a surface vehicle though, and travelling at speed below the surface in confined water like a river or a lake seems like a recipe for a fatal crash in pretty short order.

enter image description here

(DeepFlight Super Falcon, image credit Steve Jurvetson

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    $\begingroup$ For the submarine option, why not have bladder-type ballast tanks, and only operate as a submarine when there is no cargo or passengers to get in the way of the expanding tanks. $\endgroup$
    – Theodore
    May 12, 2023 at 15:03
  • $\begingroup$ @Theodore if there's a big cargo area you could seal it off and flood that. Probably simpler, though it might make any additional passengers a bit nervous in a "you have failed me for the last time mr. bond" sort of way when operating normally. $\endgroup$ May 12, 2023 at 16:03
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It would be a submarine with wheels because of this:

The aim is around 30km/h speeds when submerged. 120km/h on roads.

At your desired speeds, water resistance would still be about 10 times greater than air resistance and as a result of this, the vehicle would need to be optimised for operation underwater, with driving on roads being an afterthought in comparison.

...and as a result, it will drive poorly. The land-driving gear would need to be small and/or retractable to preserve as much of the submarine's hydrodynamic shape as possible, which would put it under great strain once it has to bear the vehicle's weight, especially when also travelling on uneven terrain. Power transmission would be problematic for the same reasons, which could make it difficult for the vehicle to climb hills, including getting out of the water. It would likely break often, which may add an interesting twist to your stories.

But in the water, it could be as capable as the existing submarines.

If you want to make this vehicle more realistic, the easiest way would be to make it slower underwater. Then you can imagine something along the lines of an amphibious APC, beefed up a bit to achieve the 120 km/h on land but plodding submerged at a more leisurely 5 or 10 km/h.

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    $\begingroup$ 30km/h submerged is fast. And without careful hydrodynamics and significant depth, your 'submarine' is going to end up with a wake that will end up disturbing the surface significantly. $\endgroup$ May 10, 2023 at 10:29
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One contraption that comes to mind when reading this is screw-propelled vehicles such as this one from the soviet era.

enter image description here (source)

Those screws basically are large tanks that can be used as floats when empty, but in OP's scenario be filled up with water to become ballast when operating underwater, and meet buoyancy criteria required. (perform like a boat, be neutrally buoyant like a submarine, or sink so that it operates on the seabed.)

Such a vehicle would of course require watertight and pressure resistant cabin. The screws when filled up with water resist water pressure, and still produce thrust to reach high speed under water.

Note that the original soviet vehicle roughly performs equally well on roads, water, mud, forest, ice, snow, sand, quicksand,... (typical asphalt roads being maybe the worst surface to operate, because abrasion.)

Also note that the vehicle in the picture would only perform loopings underwater, some design modifications have to be made so that thrust vector of each screw is coplanar to center of mass of the whole thing.

MPG would be awful in any case, and power requirements tremendous, but still this provides a solution that requires only one kind of engine, and one kind of "wheel" or "track" or "power to motion converting device", that also happens to manage buoyancy of the whole system.

Knowing this vehicle can navigate like a submarine, in practice there would be no reason to travel at the bottom of a lake or river, since it would be far slower. Instead it can use this ability to anchor itself for precise positioning or mining or whatever operation at the seabed.

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It likely cannot be done within your exact parameters.

First, it’s probably too light for your expected usable carrying capacity.

We’ll be generous and assume the vehicle takes up only about 70% of the volume you describe (for a vehicle that size though, this is actually on the low end). That gives a density of only about 0.46285 g/cm³, less than half that of water. To make it neutrally buoyant (not even heavy enough to sink reliably, just enough that it doesn’t bob to the surface by itself), it would have to use almost 16000 liters of water as ballast, which would translate to more than half the vehicle being ballast tanks. Realistically, you probably actually want about 20000 liters of ballast capacity so you can have negative enough buoyancy to stay on the riverbed and have a bit of extra headroom for tuning exact mass.

That then leaves only about 12 m³ of space for passengers and cargo, which is still pretty good (for reference many sedans have less than 3 m³ of space for passengers and cargo), but probably not as much as you are expecting.

This is in theory a non-issue unless you want to be picky about cargo and passenger space, so we’ll just move on to the next issue.

Second, it is very unlikely that it will be able to drive on the bottom of the river, especially at your stated target speeds.

Water has about 12 times the fluid resistance of air, so to achieve forward motion you need to put in that much more work. In theory, this means that whatever wheels, tracks, screws, or other propulsion mechanism you use to try and push against the bottom to move forwards will interact with the bottom by applying significantly more force than they would on land. If we’re generous and assume the bottom of the river is paved (which would completely destroy any ecosystem in the river, as well as being absurdly expensive to do), then maybe this might not be an issue other than limiting effective acceleration.

But most rivers won’t be like that. The bottom will be mud. Very deep mud, and very wet mud. And the thing about deep wet mud is that it flows, and it flows better the longer you try to push against it (have you ever noticed how yogurt or peanut butter seem to get easier to stir the longer you stir them for? most mud is like that too). Or maybe it will be sand, in which case it will either behave similarly to the mud, or it will behave like quicksand (which is even worse than mud in this case).

But this could of course be solved by making it a submersible instead, so let’s go on to the third issue...

Third, moving that fast underwater will have a significant impact on the surface.

And this is the part that really kills things here.You’re dealing with water that’s maybe a few dozen meters deep (any deeper and you almost certainly won’t be able to drive out of it because the banks will probably be too steep). Moving at 30 km/h underwater at such a depth will still result in a clearly visible wake on the surface, and if the wake is clearly visible, it’s a navigation hazard for small boats and possibly disruptive to ships.

At that point, stealth is out the window as well, which begs the question of why you want submersible amphibious vehicles in the first place...

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For a very long time the standard way to do this kind of thing has been vehicle snorkeling. However tanks are the only vehicles that use them to allow complete submersion of the rest of the vehicle i know of, & only some. But it is done on many models. The leopard 2 is the main western example of this. However soviet tanks are where the capability is most common, with every tank from the T-34, T-55, T-72 to the T-80 & T-90 are capable of it. Almost all western MBTs have the capability to snorkel but they do not allow complete submersion of the vehicle.

While this does disturb traffic on the water's surface somewhat with the snorkel it doesn't do so much. While the depth is limited by the snorkel's height you can just make the snorkel taller generally, because you can make them very tall. You probably wouldn't want to travel along the river like this, & it'd just be better to go alongside it, but if you really want to you can.

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Wheels / tracks / whatever ???

What you need, my friend, is a supercharged mechanical hippo.

Hippos can quite happily do everything you want, just not to those speeds.

So grab yourself some mech parts, fire up the old Handwavium Decay Fusion Quantum Power Cube and Bob's your uncle (who was sadly and ironically killed by a real hippo when you were a child)

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  • $\begingroup$ Absolutely nothing in this answer in any way contradicts the not-flagged-as-science-based question, anonymous downvoters. Live a little! Embrace your inner hippo! $\endgroup$ May 11, 2023 at 12:15
  • $\begingroup$ I don't know much about hippos, but this seems right. But, to build such a vehicle, I wonder: How do hippos do this? Are they buoyant or not? $\endgroup$
    – Theodore
    May 12, 2023 at 15:22
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    $\begingroup$ Hippos are too slow for the OP's specifications. The reason for the downvotes is probably because you didn't give the hippo roller skates so that it could reach 50kph on land. $\endgroup$
    – g s
    May 12, 2023 at 19:05

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