---
title: 'The ''Impossible Torpedo'' Was Real'
source: 'https://youtube.com/watch?v=qvtZIdSI1Yk'
video_id: 'qvtZIdSI1Yk'
date: 2026-07-26
duration_sec: 992
---

# The 'Impossible Torpedo' Was Real

> Source: [The 'Impossible Torpedo' Was Real](https://youtube.com/watch?v=qvtZIdSI1Yk)

## Summary

This video explores the Brennan torpedo, a 19th-century weapon that appears to defy physics by using cables pulled backwards to propel itself forward. The presenter builds a working model and explains the mechanical advantage that makes this paradox possible.

### Key Points

- **The Brennan Torpedo Paradox** [00:12] — The Brennan torpedo has no internal power source; it is propelled by pulling a cable backwards from the torpedo, which somehow moves it forwards.
- **Proof of Concept Model** [01:37] — A working model demonstrates that pulling the cable backwards causes the propeller to spin, moving the torpedo forward, though inefficiently.
- **Sponsor Segment: Odoo** [04:37] — Odoo is a suite of business applications including invoicing, project management, and inventory. The first app is free forever.
- **Comparison to Veritasium's Wind-Powered Vehicle** [04:52] — The Brennan torpedo is analogous to a downwind vehicle that can go faster than the wind, as both seem to violate commonsense physics.
- **Intuition on Friction and Inertia** [05:24] — If friction and inertia are ignored, once up to speed, very little force is needed to maintain motion, similar to guiding a rope.
- **Propeller Analysis** [06:35] — The propeller pushes water backwards, creating thrust. The cable pulls against this resistance, transferring energy to the water.
- **Lever and Pulley Analogy** [07:44] — Using a lever and pulley system, the presenter shows that a small force on the cable can produce a larger force on the torpedo via mechanical advantage.
- **Mechanical Advantage Explained** [09:51] — By moving the pivot point of the lever, the force distribution changes, resulting in a net forward force on the torpedo.
- **Demonstration of Mechanical Advantage** [11:55] — A real lever setup shows that pulling the cable with a small force moves the torpedo forward, but requires pulling a long distance of cable.
- **Fluid Dynamics and Propeller Design** [12:43] — The propeller blades are almost as wide as the torpedo, and the cable wraps around a drum. The exact mechanism involves complex fluid dynamics.
- **Treadmill Analogy** [13:25] — The Brennan torpedo in water is like a wind-powered vehicle on a treadmill: the relative motion of the medium (water/air) is key.
- **Idea of a Cable-Driven Vehicle** [14:19] — Imagining a platform on a cable in the desert illustrates how pulling the cable can propel the platform forward, similar to the torpedo.
- **Steering Mechanism** [15:27] — The real Brennan torpedo used two cables; varying their speeds allowed steering. It was never used in combat and superseded by guided torpedoes.

### Conclusion

The Brennan torpedo is a real example of counterintuitive physics, where mechanical advantage allows a small pulling force to propel a heavy object forward, despite seeming impossible.

## Transcript

- That's how it is.
by the British military to launch a type of torpedo Like when you hear how the Brennan torpedo worked, The Brennan torpedo doesn't have an internal power source.
which is already quite unusual. and via some pulley, the cable makes its way at the back of the torpedo.
you pull the cable backwards out of the torpedo. to make something go forwards by pulling it backwards. like a pair of these things.
Obviously, it's really broken, but it was pulled by cables. There should be a location there
I dunno if you can get over there. I have no reason to believe that it didn't work But at the same time, there doesn't seem to be
So there aren't any videos And I really like to see things working with my own eyes. This is what we came up with.
You see, the cable passes through the axle. and goes around these pulley here, it causes the spool to turn.
and so the propeller turns as well. the reality of it makes it seem even more impossible. but it's causing the propeller to rotate in such a way
So these two forces are in a battle with each other. that like maybe under perfect conditions, no energy loss at all,
maybe these two forces will cancel out exactly, But there's just no way you could get more energy before we try to solve that mystery,
And look, there it goes, like it barely goes forwards, These are all the iterations of the propeller.
but it's a proof of concept. when you pull the cable backwards. I'll leave a link in the description for the files
Though, if I was going to sell the designs, but I'd want a system that took care of all aspects I could use the sponsor of this video Odoo.
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To start using your first app for free, So I've come up with an explanation for the torpedo paradox. And I think the explanation is quite satisfying.
Derek from Veritasium made about sailing down wind that can travel in the direction that the wind is going and it's propelled by the wind, which seems impossible.
once the vehicle reaches the same speed as the wind, And surely the vehicle can't go any faster after that. of the Brennan torpedo past Derek
- One of the things that feels intuitive is that like if you could make the friction of that spool then it feels to me like you almost need no force at all
of my explanation, right. We take that away, and let's imagine there's no resistance at all, anyway,
when you first pull on it, there is gonna need to be a force - Yeah. - But once it's up to speed, - If I hold onto it during that acceleration phase,
and then I let go, I can just be then guiding it out, right? that resists being turned, when you are overcoming the inertia of the propeller,
of the propeller because it's pushing against the water In other words, it's only the presence of the propeller So when we're analyzing the system,
It's the propeller pushing the water backwards that pushes the torpedo forwards. of the propeller, which creates tension in the cable.
but you have to be doing work against something which is the tension in the cable. that the propeller is pushing on water.
ultimately goes into pushing water But that's not gonna cut it as a satisfying explanation. - It feels like the amount that you pull here,
to the amount that this is pushing on the water. So it feels like, in fact, you'll have to tug harder
because you're always gonna have some other friction - It's quite hard to analyze when it's a propeller - I had the same thing when I was trying
I was like, yeah, but there's lift and it's like- We switch to levers, and I want to try and convince you So instead of a propeller pushing a mass of water
to push a mass out the back of the torpedo. and the mass is like the water. it's continuously pushing water,
But you could imagine, you know, like a circle of them, - Yeah, exactly. And then like a conveyor belt. of a departure from the propeller,
but now we can more easily analyze the forces. So what forces the pulley feeling? So the pulley is feeling a force of two newtons.
If I have a one newton weight here, I can balance the pulley by pulling down here with one newton also, And so of course, the force I'm feeling up here
In other words, two newtons. but it's anchored to the torpedo, with a force of two newtons.
should be pulled backwards. The lever is also anchored to the inside of the torpedo. of one newton from the cable.
of the mass is also applying a force of one newton. of two newtons forwards. And because it's anchored to the inside of the torpedo,
the torpedo's feeling a force of two newtons forwards. and the two newtons forwards from the lever So you've got two newtons backwards, two newtons forwards.
Now here's the trick. You move the pivot point. right, in the cable.
as the mass. So if we wanted to continue applying a force of one newton And again, that's something we're familiar with
and masses and things like that. to keep the mass on that side from falling. So the total force on the pivot is now one newton
So the torpedo is being pushed forwards But what about the pulley? What's that doing now? with half a newton, that's the tension in the cable,
on both sides of the pulley. of half a newton plus half a newton. with one and a half newtons,
with just one newton. that's half a newton forwards net, So really, it's a mechanical advantage.
here's the lever setup in real life. This is the lever. which goes via the pulley.
that I could, which happened to be a pestle and mortar. of the torpedo. So let's see what happens when we pull backwards
So there you go. and the torpedo moves forward as a result. when you don't have mechanical advantage.
For complete completeness, let's see what happens when we have mechanical disadvantage. But anyway, the point is it's mechanical advantage.
you can apply a small force at the cable And just like in any situation Like it's easy for me to lift this mass up
but I have to move my end a lot to get the mass you have to pull the cable out like crazy, you'd be pulling out four kilometers of cable
that the Brennan Torpedo moved forwards. where the mechanical advantage is happening And that's because, well, fluid dynamics is complicated,
like the angle of the blades, the size of the blades, But one thing that is quite straightforward to see is given a fixed propeller geometry,
that the cable wraps around. at the Royal Engineers Museum, they've cut a panel away and they're almost as wide as the torpedo itself.
as the force, but more, - But there's a particular setup from the video - I'm on the treadmill. - Treadmill.
to the blackbird on land, except you've shifted But in this reference frame, it's just like the Brennan torpedo in stationary water.
coming out the back of the torpedo. and making them spin, just like the cable And in both cases, that leads to the turning of a propeller.
with the sort of fluid dynamics of turbines So at some point, I came up with this idea of a steel cable,
which is just running down the desert And then I imagined having just this platform with some wheels, and I'm just laying on it on my back,
and I'm just holding onto the cable, flying down the desert. okay, what if I had a little generator that just pulls out a tiny amount of power outta the wheel,
and then you stick a little motor on top of this cable, and you just very slowly climb us forward on the cable. that like, actually, we can go faster
So with air, it's kinda flowing around you, And the similar thing with the lever setup,
Just stick my fist down on it, and it's stuck, right? And it's the same thing. You just grip onto the cable. - Yeah, yeah, yeah. - The real Brennan torpedo
and by some really clever mechanism, it was possible to steer the Brennan torpedo towards its target by changing the relative speed of the two cables
I'd love to get into how all that stuff works, By some measure, the Brennan torpedo It was eventually superseded by a torpedo
so that it could be launched from a ship. The cable wasn't for power. Instead it was for sending steering signals
so that that type of torpedo could be guided as well. for 15 years, and it was never used in anger, why there aren't any working models around,
that I dereked you on this one? - [Steve] No, not this time. Not this time.