Home » Is A Sailboat A Self-Propelled Vehicle? What About A Cable Car? Or An Elevator?

Is A Sailboat A Self-Propelled Vehicle? What About A Cable Car? Or An Elevator?

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Every now and then, we here at the Autopian take a moment to consider the Big Questions: Who are we? Why are we here? Why do we love all these miserable cars so much? Why does it still burn when I pee? This happened today, as David and I had a conversation this morning that veered into the essence of vehicles. I started thinking about vehicles like cable cars and elevators and their status as self-propelled vehicles, a category that all automobiles fit into. And then I started thinking about sailboats. And hot-air balloons. And then I confused myself. So, help.

Okay, let’s back up: a self-propelled vehicle is pretty much any sort of conveyance that, you know, can make itself move. This includes everything from wind-up cars to Japanese Shinkansen trains to rockets to a hovercraft to your friend’s dad’s old Fiat 128 to that moped you always wanted as a kid and never got. Now, let’s think a little more about self-propelled vehicles. Let’s consider old cable cars.

The famous San Francisco cable cars worked like this: in between their tracks was a channel with a moving braided cable. Here’s how the Cable Car Museum describes the system:

The cable car is pulled on rails by latching onto a moving cable inside a channel beneath the street. The cable is guided by an intricate system of pulleys and sheaves (large pulleys). At the powerhouse, huge winding wheels driven by 510 horsepower electric motors pull cable loops at a constant speed of 9.5 miles per hour.

Through a slot in the street the car grabs the cable with a big vice-like lever mechanism called a grip. To start the car, the gripman pulls back on the lever which closes the grip around the cable. To stop the car, the gripman releases the grip and applies the brakes.

Were you to look at these famous cable cars in operation, they sure would look like self-propelled vehicles, as this 1984 video shows, as it explains it all to you, too. They’ve been around since 1873! Look:

Did you just lose about 16 minutes on that like I did? If so, sorry. But, you get my point: these behave like self-propelled vehicles, though the motive force, a number of 510 horsepower electric motors turning huge winding wheels that actually pull those cables, is not aboard the cable cars themselves.  The cable car itself has a clamp to “harness” the energy of the moving cable. So, does that mean they’re not self-propelled vehicles? Or is the clamp system enough of a mechanism to consider it to be simply using the energy from the motion of the cable for propulsion?

Something like an elevator, which I do think is a vehicle, much like a short-distance vertical railroad, works in a similar manner. Now, stay with me here: If we don’t think that a cable car qualifies as a self-propelled vehicle, what about electric buses and trolleys that have motors on-board but get their electricity from overhead wires? You know, trolleybuses like these:

Really, these aren’t any different than other electric-powered vehicles like subways or trolleys with an electrified rail or a slot car track, even. Like the cable car, these machines get their power from conveyers of that power on a network. In the case of trolleybuses, that power is in the form of electricity, through wires. In the case of cable cars, the power is the motion of the cable. Is it that different? Both need outside energy sources to work; one gets that energy and then converts it to rotational, and then forward motion. The other gets the energy in the form of motion from the get-go.

Now, if we keep thinking along these lines, we get to sailboats. Are sailboats self-propelled in the same way that a cable car or subway or trolleybus is? In all cases, we have a vehicle that gets its energy from some outside network: the trolleybus from the overhead wires, connected to the power grid and then fed to the on-board electric motors to convert to motion; the cable car from the moving cable under the road, the clamp system transferring the motion from the cable to the whole car; and in the case of a sailboat, the power comes from the wind, air moving in the atmosphere of the Earth, which is harnessed by the boat’s sails and the motion of the wind is transferred into motion of the whole boat.


A sail is a mechanism for capturing energy and turning it into motion, like an electric motor can turn electromagnetic energy into physical motion, just in a very different way. The motive network of a sailboat is the whole atmosphere wherever the boat may be, and as such is quite vast and expansive.

So, back to my original question: do any of these count as self-propelled? Is a requirement for being self-propelled that the energy source must be able to be stored aboard the vehicle? The fuel in your car’s tank is really part of an energy network as well, one that started with dinosaurs keeling over, and then lots and lots of time and heat and pressure and microorganisms and probably a bit of magic, then continues with oil companies pumping and refining and trucking and storage and so many steps before it gets to a pump to squirt into the tank of your car.

Is that any different than grabbing a moving cable or capturing wind? Or is it essentially the same idea, just instead of drinking continuously from the spring of energy to move, you’re taking mouthfuls away, moving around, and coming back for more? A battery EV, too, is the same: just like a wire-powered trolleybus, but taking some amount of electricity on-board, replenishing as needed.

Is this the fundamental differentiator for self-propulsion? On-board energy storage? Or does that actually matter, and it’s just the ability of a vehicle to turn energy into self-motion, via some mechanism that could range in complexity from a cloth sail to the complex electric motor unit in a Tesla.

I asked some smart people I trust if they felt a sailboat was self-propelled. They didn’t think so, but the more I think about it, the less sure I am. I think now that a sailboat is self-propelled, and it does have an on-board mechanism for turning energy into motion, or at least one kind of motion (wind) into another (ship moving).

If you don’t think a sailboat is self-propelled, you at least have to admit that it is remarkably independently mobile for something that isn’t. Sailboats have criss-crossed the globe for centuries, not being pulled by whales or massive clusters of plankton or anything like that. They’re very steerable, in the right hands, and behave effectively like something that is self-propelled. I think it is?

But then I wonder where the line gets drawn; why does a glider not feel like it should count? Or, say, a simple raft, which, really, isn’t any different as it flows down a river than a cable car is as it gets dragged down Mission Street.


Or a hot air balloon, even? Does the burner to make heated air that rises count as a propulsion system? It carries fuel with it for that, but it’s only active on the vertical axis, and relies on outside wind energy for horizontal motion; so does that make a hot air balloon self-propelled on the vertical axis but not on the horizontal axis? Or is it just self-propelled?

I’ve taken a simple idea and confused myself really, really effectively. Help me figure this out!

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93 Responses

  1. I would personally define self-propelled as being capable of movement without the application of any external force–ruling out sail-powered vehicles and cable cars–with a power source that is connected to, and moves with, the vehicle–ruling out wire or third-rail powered vehicles. My criteria is essentially if you stick it in the middle of the Kansas Prairie on a still, cloudy day, can you get it to go?

    I would argue that there is the implication that a vehicle and its propulsive force are mechanical in nature, not living. A horse does not qualify as a vehicle, nor does a bicycle, push-scooter, rollerblades, wheelies, etc, etc. I would also argue that there is an implication that a self-propelled vehicle has some amount of freedom of movement.

  2. Sorry for adding another layer of difficult philosophy:

    Does self-propulsion imply that if MUST have a sentient being that is AWARE of movement?


    Thus, meaning that a horse can be self-propelled. Any human controlled device can be self-propelled, but a rock that simply falls down the hill is not. A Tesla is not sentient (thank God) but the sentient being inside makes it so.

    How bout them apples?

    1. Adding to my thoughts:

      It’s NOT the propulsion mechanism form that matters but rather the embodiment of SELF that is the key determinant.

      Whoa! I need to lie down now.

  3. I think the problem is “self propelled” of at least the “self” part.

    If you separate vehicles into motorized and non-motorized you get a more manageable taxonomy.

    Cable cars are not motorized because the motor is remote. Electric trollies and busses are motorized, sail boats are not unless perhaps a wind turbine powers a motor. Hmm…

    I will leave this one for you


  4. What about a moving sidewalk?
    If you are standing on it are you self propelled? Why do they call it a moving sidewalk when a person standing on it is moving but the sidewalk is not? Is the person powered but not self propelled? Is the sidewalk a vehicle if it doesn’t move? Is the person a vehicle or a passenger?
    How is a person on a moving sidewalk different from a cable car?

  5. When I bicycle down hill (almost daily) the bike is self propelled, because it harvests the energy of the earth’s centre’s gravitational pull and turns it into motion, no doubt about that, according to my logic.
    Actually the same principle as brake regeneration in EVs. So if my bike isn’t, EVs – partly – aren’t either..

    1. Nah. A bike is not because it’s the same principle as dropping a rock. Gravity is turning gravitational potential energy into kinetic energy and the bicycle is performing no energy conversion. An EV is turning chemical potential energy into kinetic energy. It can also turn kinetic energy into chemical potential energy (Regen).

      1. So conversion is the key? Jason’s cable car “converts” the pull of the wire to pull of the cable car by grabbing on to it, That’s not much of a conversion either.

      2. To clarify, I think your analysis is only valid if you are regenning on say the front axle and using that energy as it is generated to power the rear axle, which would be silly and you would be moving less than just letting gravity pull you.

  6. What qualifies as a vehicle?
    Is the coach element of a horse-drawn stage coach self propelled?
    Or is it merely a trailer?
    How is the cable car different from the horse drawn coach?

    For land vehicles, I’d say if it can move uphill via *onboard* power, then is self propelled.

  7. Here’s another hot take: everything is a boat. There’s a lot of bad boats out there, sure, but a cruise ship and Kevin Costner’s trimaran are boats just as much as Kate Winslet’s door is.

  8. Now study “apparent wind” in which, as the speed of a sailboat increases, that forward vector gets added to vector that describes the wind and there’s MORE wind than there was originally. Stay tuned for the 37th America’s Cup, and prep to do some stories on (not-self-propelled) sailboats that fly several feet above the water at 90+ kph.

    1. There is not an ounce more wind. Apparent Wind is just a convenient way to think about the effect of having a resistance to sideward motion when traveling across the direction of the wind.

  9. I wonder if the sailboat problem is twofold.

    If the sailboat and wind share a vector the sailboat’s speed is limited to that of the wind’s. The wind is a pushing force, and the sailboat is a hot air balloon, a raft in a current, a car on a cable. And when that sailboat’s speed matches the speed of the wind (again, shared vectors) the sailboat is you and the wind is the elevator – but the cable has snapped, you and the elevator are in freefall and for a few protracted seconds you marvel at the feeling of weightlessness as you experience no other force save, perhaps, for the expulsion from your rectum.

    But if the sailboat and wind vectors aren’t perfectly aligned the wind is a lifting force, too. A pushing force, and a lifting force, simultaneously. Because there’s one wind pushing and a new wind created my moving through the air. Stick your hand through the door of your elevator in free fall and you’ll feel it. To the elevator, it’s drag. To the sailboat, a new source of energy. Because the sailboat has wings.

    “Wings” in that those sails experience a drag force (slowing things down) and a lifting force (speeding things up), and – properly built and set – the lifting force of those sails will exceed the drag. As boat speed increases so, too, does that of the created (or, apparent) wind, which increases the lifting force on the sails, which increases the boat speed, which increases the speed of the apparent wind, which increases the lifting force on the sails, which increases the boat speed, which increases…

    The forecast calls for 15mph winds. You’ve got a fast boat. The wind is blowing due south, and you’re headed due west. You have a date with an old friend to scavenge the shores of a small island for old gravestones slowly exposed by the eroding waves of a rising sea 45 miles away. You’ll be there in an hour.

    Plenty of time to think about whether your boat is self-propelled.

  10. I think the real question is about entropy. Self propelled is about as likely as a perpetual motion machine. All movement results in the inevitable decay of what we know into a cold dead universe.

  11. So…. none of us are self propelled?
    Animals aren’t self propelled because we require calories and gravity to get around? This is too deep for me man.

  12. Clarifying definitions for my classification. In all cases a person is part of the vehicle system = vehicle.
    Self-propelled vehicle = Vehicle that contains the energy for moving the vehicle even if temporarily. Gas, electricity, steam, wood, etc. are examples. Cars, airplanes, bicycles, etc.
    External energy vehicle = Vehicle where the energy for propulsion is external to the vehicle. Includes vehicle where the energy is physically and permanently attached (elevators), or attached with a moving system (electric trolleys). Not self-propelled.
    Subsets of External energy vehicle:
    Earth propelled vehicle – A vehicle that uses the various “things” inherent to being on earth for mobility. Such as:
    Gravity – Surfers and kayakers (rivers, tidal and wind driven waves).
    Wind – Sailboats, sailplanes, and sail cars use earth’s rotation and solar energy that creates wind.

    Hot air balloons are a hybrid as they use both self-contained energy (propane heat) and earth wind energy.

  13. It all depends on how you look at it. If a vehicle is not physically restrained/constrained by its means of propulsion, then you could say it is effectively self-propelled. A couple of examples: a sailing ship is technically NOT self-propelled, but it is EFFECTIVELY self-propelled because its moveable sails and rudder give it a large degree of ‘independence-of-movement’. An elevator, on the other hand, is not self-propelled because it is physically connected to, AND tightly restrained/constrained by its means of propulsion. A hot air balloon is also not self-propelled because it has very little ‘independence-of-movement’; it basically just goes where the wind blows it. An air-ship IS self-propelled because its vectoring-thrusters give it independence-of-movement.

  14. A self-propelled vehicle is one that generates the kinetic energy to move itself on-board. So overhead-wire- or third-rail-powered vehicles are self-propelled, because while they do receive energy from a non-onboard source, the energy is not kinetic but electric, and the conversion to kinetic energy happens on-board. Cable cars and sailboats, however, are not self-propelled because they harvest kinetic energy from their environment and (at most) re-direct it.

    1. Came here to say this. Basically it comes down to where the propulsion force comes from. If the force comes from outside the vehicle, like it with the cable car or sailboat, then it isn’t self-propelled. If the force is created on-board, no matter where the energy source is, like a car, or powerboat, or electric bus, then it would be self-propelled.

      1. That leads back to the bicycle question. A bike’s motive force is created on-board, but by human muscle power which seems to be the very definition of being *not* self-propelled.

    2. I completely agree with this and it is the best explanation, but technically if you stored energy in a flywheel it would be an exception. Maybe not everything can be described with a black-and-white rule.

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