About a month ago, I tried to figure out how fast my 2007 Toyota Yaris would be able to drive underwater. I came up with 15 MPH (23 km/h, 6.52 m/s), which is respectable for a submerged economy car, but not that impressive for an underwater vehicle. Plus, as commenter Azure James correctly pointed out, propelling a car underwater isn’t as simple as having the horsepower to overcome drag: you have to make sure your drivetrain, from engine to gearbox to wheels, can generate enough torque to overcome the drag force.

As usual, I took this as an opportunity to imagine something ridiculous and draw a stupid picture. I present to you, the Hobo Sullivan Cruiser, the world’s most impractical underwater vehicle!

It would be about the size of a large van or a small delivery truck, and that’s mostly because it has to accommodate a spherical pressure hull. Because if I’m going to make an underwater car, I’m for damn sure going to make it able to drive into the Challenger Deep! A lot of the other stuff is self-explanatory. The outer hull has a half-torpedo shape (Or if you prefer, half-cigar shape. Or, if you’re really picky, half a Sears-Haack body.) That’s to minimize drag. I chose to power it with a fuel cell and electric motors because those allow a greater range of rotation speeds (which, as my commenter correctly pointed out, would limit my speed on Mars, since a normal gearbox can’t get the wheel speeds up high enough to propel an ordinary car at 300+ MPH). Plus, with electric motors, exhaust isn’t as serious a problem.

The power for the electric motors is provided by a methanol-oxygen fuel cell. I chose methanol for one reason: space. Hydrogen, even compressed and cooled to cryogenic temperatures, takes up a lot of room. If you look at the hydrogen-burning stages of the Saturn V, the tanks are mostly hydrogen with a little spheroid at the end for the oxygen. Methanol fuel cells don’t provide as much power, but methanol’s a hell of a lot easier to transport. I’d still have to do something about the exhaust from the fuel cell, but for now, let’s assume I run the fuel cell hot enough that the pressure pushes it out a one-way valve.

You know, the more I look at it, the more “screw wheels” makes it look like I’ve got some weird grudge against wheels. In reality, those funny-looking objects are screw-drive wheels, which are so good at all-terrain they make tanks drive home to their hangars and weep transmission fluid into their berths.

That glorious contraption is the 1929 Fordson snow machine. It’s got huge pontoon-shaped wheels that turn on an axis parallel to the direction of travel, rather than perpendicular, with the force provided by many small segments of the helical screw pushing against the snow as the wheels turn. Because the pontoon wheels have such a large footprint, they don’t sink even in deep snow. And there is, of course, the cool factor, which is almost off the scale here. And now, let me send it careening completely off the scale by showing you the Russian incarnation of the screw-drive all-terrain vehicle:

When they say this is an all-terrain vehicle, they mean all terrain. Including the surface of a fucking lake. It’s hard not to be impressed.

Okay. So screw-wheels are cool. Is that reason enough to complicate the design of my submarine car to throw four of them on there? Well, yes. The seafloor is unpredictable. Without vegetation to anchor it or ordinary algae and fungi to bind it, the seafloor can have a wide variety of textures, and almost all of them would be a nightmare for a wheeled vehicle: silt six feet deep, loose sand, gravel rocks, weird clay, mud, sulfide sludge spewed from seafloor volcanoes… A wheel is very likely to sink into a morass like that. I could make the wheels larger, but a pneumatic wheel will deflate at seafloor pressures. I could make the wheels non-pneumatic, but they’re still big and bulky and taking up a lot of room, and they’re going to increase drag, which I’m already spending all my energy fighting. Tracks might work, but they contain an awful lot of joints and moving parts, and would probably get silted up.

Of course, screws as small as the one in the illustration probably wouldn’t fare too well in soft silt, either, so I’d have to make them larger, but nonetheless, I’d say screw wheels are the optimal solution. And besides that, they’re fucking awesome!

Of course, all this is really moot. There’s a very good physics reason we don’t turn all cars into planes (And plenty of other reasons, if you see how people drive on the ground.): it’s hard to generate lift in air. For a long time, many believed heavier-than-air flying machines were impossible, and they only became really successful a century ago. Air isn’t all that dense. An average 80-kilogram (172-pound) human could comfortably stand in a box 2 meters on an edge. That box would be seriously oversized, but the air in it would still only weigh 10 kilograms.

A box of water 2 meters on an edge, though, would weigh 8,000 kilograms, or about the same amount as an African elephant. Water is dense. Denser fluids resist being pushed around, and therefore more readily generate force. In my earlier article, I figured out that my car would theoretically be able to go 135 miles an hour in air, but only 15 miles an hour in water. But when it comes to flying, that resistive force comes in handy, because you don’t need very big wings to turn your submarine car into a submarine…submarine. Here’s a picture to illustrate how much better wings work in water than they do in air:

(Source.)

That is a boat. Being lifted almost completely out of the water by wings that look to me to be smaller than the wings on a Cessna.

And there’s another thing that makes an underwater vehicle confined to a surface kind of silly: buoyancy. Like I said, air’s not very dense, and for a bag full of gas to lift anything, it has to be less dense than the surrounding medium. There aren’t many gases less dense than air. There are hydrogen and helium, of course, but both are made of such small atoms that they leak out of containers no matter what you do, and hydrogen is flammable. There’s hot air, but that’s a lot denser than hydrogen or helium, and after a certain altitude, it gets hard to heat it up enough to provide useful lift.

Now here are some things that are less dense than water: Myself. Wood. Warmer water. Air. Compressed air. Methanol. Ethanol. Propanol. Butanol. Low-density polyethylene. In other words, around half the matter (by volume) in a submarine is less dense than water. Submarines actually have to take on extra water to sink. I had to stick depleted uranium ballast weights in my ocean car just to make sure it wouldn’t float.

But really, why shouldn’t it float? It’s not like I couldn’t stick landing skids or water-filled pontoons on a regular submarine and then have a submarine that can go to the bottom. And with a little creativity and plenty of power (provided by a wide variety of sources), you can adjust your buoyancy on-the-fly.

So forget the submarine car. I just want a submarine.

Actually, that’s a lie. I want a submarine car with screw-wheels. But I’m going to use it on land. Because, holy shit, look at those motherfuckers in the videos up above!