I think in his original video he described how a tacking boat can go faster than one going straight down wind. As I understand it, that was the inspiration for the propeller. Here is a good site explaining it:

https://newt.phys.unsw.edu.au/~jw/sailing.html

> How can boats sail faster than the wind? Lots of boats can – especially the eighteen footer skiffs on Sydney Harbour. Ask a sailor how, and he'll say "These boats are so fast that they make their own wind", which is actually true. Ask a physicist, and she'll say that it's just a question of vectors and relative velocities.

"The physics of a push-me pull-you boat"

http://202.38.64.11/~cxyu/AJP_pushmepullyouboat.pdf