An interesting study of alternative prop designs.

[Nos4r2]

An interesting study of alternative prop design, particularly of toroidal props:-

https://www.youtube.com/watch?v=KeTh-TIhL9M

[willk]

Interesting. Looks like we won't be burning our props just yet?

A lot of the early steam ships ran a two blader - but much slower obviously. Clearly they were onto something, efficiency wise.

[User name]

Love that channel. He makes some crazy nerdy stuff and makes it sound simple.

[Matt]

Very predictable that on that test rig, the 2 blader would win, as long as it had enough blade area to carry the load without cavitating.

The most efficient would in theory be a one blader. But that presents some challenges.
In the high performance (surfacing) regime, 2 blade props are hard to use - since the changeover between one blade being submerged vs the other creates all sorts of nasty loads and harmonics. Hence 3 blade props.

Blade area, tip speed and tip vortices are the key things. You want enough blade area so it just doesn't cavitate, but no more since that increases drag. And you want enough tip speed that you've not got loads of slip (which is also inefficient), but no more than necessary since that also causes drag and cavitation. And as few blades as possible since you lose efficiency on the tips - just like aeroplane wings.

It's a very non linear thing to model since the same prop & engine on 2 different boats can perform widly differently. That said, for typical use cases, there's a fairly small list of props that will be pretty good.