Conical Propeller

Raked Propeller - what is it?

Whilst discussing human powered boats I heard mention of what can only be described as a conical propeller.

Usually HPB's use a propeller which looks a lot more like an aircraft prop than a boat prop, so I presume a conical version would look something like this.

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Or perhaps like this which Google turned up

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Does anyone have any ideas if I'm right? I can see the reason for doing this. If you've got a huge great aircraft like propeller it has to sit well below the hull to give clearance. Something like this could sit a lot closer, but would it work?

Or is there something entirerly different I've not thought of?

 

More surface and less diameter = less efficient.

Efficiency is the name of the game in low power applications.

 

Maybe a horizontal version of Leonardo's design in his 'helicopter'....

 

Conical Propellor

The first Atlantic crossing by a propellor driven ship, in 1839, used a screw invented in 1827 by Joseph (Yosip) Ressel, and later refined by John Ericsson. It was based on Archimedes and Leonardo's ideas, and the voyage took 40 days.

I suspect the conical design alexlebrit is looking for is like his illustrations, rather than Ressel's Leonardo-derived version shown here. Jehardiman is right, of course, the props with small diameter and large surface area are not very efficient at low speeds. There's a great story about the discovery of this principle. Supposedly one of the early 19th century screw propellor ships was being tested in a canal. The prop screw was made of wood and was typical of the early designs, like the one shown. The ship touched bottom, breaking off more than half of the screw, which floated to the surface. The test was cancelled and the ship was ordered to return to its moorings at whatever speed it could make. Everyone was amazed as, with only a remnant of the long screw still attached to the shaft, the ship moved off at twice its original speed.

 

Personal thanks Charlie. I love the tailpiece. Now tucked away should the subject be raised at the pub.