Archimedes (Helical) Screw Pontoons

"(How does this Ofira paddle work?) "

It has conventional paddlewheel floats mounted basically on the same shaft as the foot pedals. So you have a human powered paddle boat with just two floats.

 

Petros, when you said that the screw has a very low aspect ratio I'm guessing that's still underselling it.

A prop is like a wing, it isn't just a conveyor to mechanically move water between two points but generates lift too because of its shape. This comes from the whole blade, not just the leading edge of it biting into the water. Likewise, the trailing edge has to be well designed to not spoil what has been gained.

An auger, especially one for dry goods (which can be completely flooded and still move stuff uphill unlike one for pumping water) doesn't provide lift, it's a conveyor that mechanically moves a column of whatever from one place to another, as you well know.

Now, an Archimedes screw used as a propeller has a leading edge and trailing edge where hydrodynamic effects may actually be generated, but the bit in the middle is still just a conveyor. So it may be accurate to say that part of it has an aspect ratio but a possibly large chunk of the thing may have no aspect ratio at all.

With an auger or a land bound vehicle this is no big deal, because these are concerned with moving something over a distance equal to its physical length more than creating a moving flow of water longer than itself to move a boat.

In one sense then, I guess I could say that when Ericsson proverbially cut a slice from a screw and got a propeller he had a mechanism that worked on an entirely different principal for not having that big middle bit that was a mere conveyor (with is associated losses).

Does that sound about right to you folks?

 

I would like to make the drive unit only for a small 6' pontoon . How would it be done?

PC

 

Texas rivers in my area have rocks of various sizes as well as thin water especially with the present drought. Some banks are too steep for even a screw drive. Seems like you would need amphibian capabilities for a heavy vehicle. I get by now with portaging a 25#, 6' pontoon using backstraps.

PC

 

Only if the screw is operating in a tube. And if the screw is in a tube then the peak efficiency will be with a zero length tube.

Without a tube the water does not have follow neatly along the screw. My guess is the flow will be something like a helical vortex wrapped around the screw.

 

That is of course true. Even though an auger may be able to move dry goods uphill when flooded with them it will not do so without containment on the bottom and sides at a minimum.

The helical screw still seems for at least some of its length more of a conveyor than propeller, trying to move a stream of water through mechanical might rather than hydrodynamic effects. I think it may be better to compare it to a paddle track or paddlewheel than a modern prop for that reason: only in this case the proverbial paddle is a surface piercing twisted blade that is rotated around a shaft/pontoon.

One thing about the shapes of these things, as they have mostly been for amphibious use, they tend to have pontoons and blades grow finer at the back end and, especially in the case of the blades, this seems like it would make the effect you are talking about even worse as the thing is shedding what little it has to do anything to do work with after the midpoint of the pontoon.

I see no reason why a helical screw optimized for aquatic use wouldn't have blades end standing proud, at or near their full depth. Probably should shaped like a propeller blade's trailing edge too. The pontoon should probably get finer though. Such a critter should be horribly ineffective in reverse, shedding water as you cite, just more so as if it was made for that.

 

I think that DaVinci proposed this method of propulsion in some of his imaginative drawings. That the concept has not been utilized much, if any, is suggestive is it not?

 

Don't let Dan Brown hear of this.

 

IMHO, if this is to be aquatic-only, with no aspirations of amphibiousness, then the screws should be contained in tubes, or pipes, as DCockey alluded to. This will give you much better efficiency and can be much safer (can put screens at ends of tubes).

The water will exit the tubes in much the same manner as a low-speed jet. The question then becomes whether you can affect performance by using a variable-pitch screw within the tube, again similar to a jet.

Moreoverly, you can use much higher speeds with the screw contained in a tube. To carry the concept further, you can eliminate the hub drive and have a hubless screw that is powered by electromagnets along the length of the tube (much like some modern hubless computer cooling fans). Thus, longer tubes would have more power due to possibility of more electromagnets. The electromagnets and lack of a hub would also reduce issues with bearings.

 

If the screws are going to be put into tubes then there are more efficient systems than a screw in a tube such as a ducted propeller.

 

Here's another possibility for thin water/rocks/rapids, etc.


https://www.youtube.com/watch?v=Yn3lOtabSNo


Not completely infallible as shown..

OE