Propellor vs. paddle, was Re: Quieter more efficinet wind mill

It's all part of the vast energy-wasting conspiracy.

It is all about the edges, the dipping in and rising back out.

Dear RichD:

A screw prop (and a turbine, and a fan) directs flow for the entire rotation, with "little" air lost off the tips (depending on loading etc.). A paddle wheel directs flow for only one direction, for about 1/3 the total rotation, has to be removed from the fluid to be driven for the rest of the time, and loses fluid off of *at least* three sides.

The paddle wheel has slow speed, and large area going for it, so is good as a "cave man" effort... however graceful and beautiful they might be.

Additionally, I don't think they carve up wildlife quite as randomly as props do, but I don't know if that is due to the prevalence of props...

David A. Smith

Something about vortices and turbulence?

-- Rich

The directed force trying to lift the boat on paddle entry, and trying to pull it under on paddle exit. The significant differences between paddle speed and water speed at entry and exit.

Because props have vortices and turbulence at work as well.

David A. Smith

I was just having a bit of fun with my answer. Keep in mind that, according to Newton's 3rd, you produce force when you accelerate a mass. The paddle wheel is moving with an essentially constant speed with respect to the water, so it accelerates the water caught between the blades, only as it slaps the surface (with lots of useless splashing). From then on, the blades and water are moving at a nearly constant speed, so no acceleration, no force (except for the outside surfaces dragging against slower moving water.) Then the paddles lift a lot of water and it pours and splashes around, none of which energy absorbing stuff produces force to move the boat.

A well designed prop, on the other hand, knifes into a mass of water without much disturbance (the angle of attack nearly matching the angle produced by forward motion of the boat and rotary motion of the prop blade), then the water slides along a changing attack of angle of attack that sweeps that water backward in a smooth acceleration, throwing it behind the next blade's wash. That smooth acceleration given to packets of water produce a smooth and efficient force.

John Popelish wrote in news: snipped-for-privacy@adelphia.com:

"Produce force"?

I think you better revisit your thinking. In addition to gleaning a better understanding what "acceleration" is you might also take a look at centrifugal pumps and consider all the modes in which a paddle wheel moves water. Consider also the volume of water being moved by a paddle boat.

Inefficiency prevails in all mechanical devices. Paddle wheels are rather primitive but they did represent an acceptable low tech solution in their heyday. For extra points calculate the number of Roman slaves at oars they could have replaced using one paddle boat. For social science class discuss the ramifications to European timber had paddle boats been in use from early Roman times onward. Would shipboard slaves have been replaced one for one by land based slaves in the timber and firewood transport industry or would steam driven paddle boats created a manpower disadvantage if the paddle boat was the only comparatively advanced technology available to early Romans?

Please define acceleration.

Look at variable pitch (aircraft) props and the reason for them. Air and water are both fluids. Consider the efficiency of a properly used oar. Would a paddle holding the paddles (consider tank tracks) in a vertical position been more efficient?

"N:dlzc D:aol T:com \(dlzc\)" wrote in news:CqfGe.178927$Qo.53699@fed1read01:

Men and monkeys have things in common too, some more than others.

It is apparent that you felt qualified to respond although that fact, standing alone, isn't very interesting. You have this characteristic in common with many others, some of whom actually are qualified.

Having something in common is often not worthy of notice, possibly more often than not.

Dear Strange Indeed:

"Equal and opposite reaction"

Have you actually seen how a centrifugal pump works? It has much more in common with a propeller than a paddle wheel. Now the silly little pumps that are used to circulate domestic hot water... those *are* like paddle wheels.

And how much of that volume (per paddle stroke) is actually directed rearwards...

...

I assumed he meant the water adjacent to the paddle had the same relative speed as the paddle. A very large "boundary layer", if you will.

Yes, assuming the mechanism that keeps the vertical alignment, and uniform "just a little more than boat speed when in liquid" didn't eat all your gains.

David A. Smith

Dear Strange Indeed:

Ah, so you say "I was trying to get John's goat, and I don't like interruptions". Got it.

I see that you imagine those qualities in others, that you refuse to see in yourself. How sad.

Yeah.

David A. Smith

(snip)

(snip)

I've never owned a goat.

John Popelish wrote in news: snipped-for-privacy@adelphia.com:

To say nothing of the fact that David completely missed every point.

These issues should have been resolved decades or centuries ago.

The de Lavel 100% impulse turbine from gas turbine engines has an efficiency of 80%.

Not 40% like the popular 2 blade prop windmills. Not 53%.

Just add nose cones and stator blading and change the rotor bucket shape and there shouldn't be any difference between a de Lavel and the best/last wind turbine.

It should be quiet and save birds as well.

Even the Reynolds numbers shouldn't be too different.

This is supposed to be a science and here we are making stabs into the dark.

Bret Cahill

So there are no forces at structural joints, because no accelerations are observed?

There is constantly applied force, overcoming viscosity.

This probably captures the essential inefficiency, but I am looking for something more quantitative.

-- Rich

(snip)

Static structural joint forces cannot cause a ship to move.

But a big fraction of those frictional forces are between thew paddle wheel and the nearby side of the ship. A good rower drops the paddle into the water at a speed that nearly matches that of the water. Then he applies force to the paddle accelerating it, and the water around it with a fairly constant force, then reduces the force, so the paddle again nearly matches the speed of the water near it, before lifting the paddle with minimal splash. The paddles are most efficient if they work a significant distance from the hull, to minimize the frictional losses of having the more rapidly moving water near the hull. Paddle wheels are far less subtle.

I good finite element analysis program would be some help.

John Popelish wrote in news:w7KdnZvDHvPzcW7fRVn- snipped-for-privacy@adelphia.com:

Apologies if I managed to goof up the attributions. _Marine Propulsion: Principles & Evolution_ ,Robert Taggart, Gulf Publishing 1969 has a section comparing the open water efficiency of feathering and straight paddle wheels that might be of assistance. He also has a section on propellor efficiency.

Don't forget gravity is an acceleration.

Regards,

Bill Ward

Yes. Is that solvable analytically? Probably not. Numerically? What would the model look like? I'm far from an expert in fluid mechanics.

So water is lost around the edges. Let's consider further - how about reshaping the paddle? I'm thinking of something concave - that would 'cup' the water. Intuitively sensible, yes/no?

I was thinking about this also...

??

I realize the paddle wheel was replaced by the screw prop for good reason. But if we look at the history of engineering, we see discarded ideas later resuscitated for various reasons, e.g. as new materials became available. I wonder if the paddle wheel might be another...

What is the efficiency of a screw propeller under water?

-- Rich

Thanks. I will try to find this when I get the chance.

-- Rich