The Brennan torpedo, invented in 1877 by Louis Brennan, was one of the first (if not the first) guided torpedoes of a practical design. Amazingly, it had no internal power source but it did have a very clever and counter-intuitive mode of operation: a cable was pulled backward to propel the torpedo forward.
If the idea of sending something forward by pulling a cable backward seems unusual, you’re not alone. How can something go forward faster than it’s being pulled backward? That’s what led [Steve Mould] to examine the whole concept in more detail in a video in a collaboration with [Derek Muller] of Veritasium, who highlights some ways in which the physics can be non-intuitive, just as with a craft that successfully sails downwind faster than the wind.
Pulling the cable out the back of the device turns the propeller thanks to a pulley-type assembly with the prop shaft connected to a drum, as seen in the animation here. The actual Brennan torpedo was somewhat more complex, but the operating principle was the same.The real thing had two cables coming out the back and drove two counter-rotating props. It could be steered by changing the relative speed at which the two cables were pulled, which caused a rudder to turn and allowed the torpedo to be guided. It really was very clever, and the Brennan torpedo was in service for over a decade before being superseded by designs with internal power systems that could be launched by ship.
The basic concept is explored with the help of a working model in the video embedded below, along with identifying what makes the physics tricky to intuit. If you have a few extra minutes to admire the importance of leveraging mechanical advantage, check it out.