Pedal Boat Design

good luck!

Thanks BG and good luck with your project.
Beppe

 

Full scale testing of paddlewheels in a static state will provide useful information and enable evaluation of different shape blades. So would be worthwhile in this regard. What you will not see is the amount of lift likely when you are rotating at normal speed.

You can get neat little scooter motors that are quite efficient and have in built gearing. They cost USD35 last time I looked and I think controllers are USD18. I used one to do some testing for electric boat. With an AUD16 multimeter, used as an ammeter, you can get useful result. This shows the test set up:
http://www.boatdesign.net/forums/attachment.php?attachmentid=17436&d=1196419301

I think testing in a static state at near to full scale would be better for the paddlewheel. Testing for the foil allows you to see loads and fiddle with mechanisms. Any data is useful as a good mathematical model should be able to replicate the results.

Rick

 

Yup. http://www.streamdancer.com/kayakfishing.htm

Geno, your test tank project sounds awesome. I hope to be informed once the specs are finalized. (This is fun as hell, being on the sidelines watching you guys bring this old ignored technology into the 21st century.)

Btw, adjusting the paddle wheel depth on the fly may not be as important for you as it was for the old freighter captains. I'm guessing your loads will be pretty consistent, with easy-enough adjustments for an extra bottle of wine in the picnic basket as conditions warrant.

 

Ah, the old screw propulsion trick. Good enough, but someone figured out that an ordinary boat propeller is just a thin slice of that long screw -- without all the extra drag that it has.

This is a lot of fun, though, and you're right, we're recycling old ignored technology. Some of it was really ahead of its time, in that it couldn't be practically accomplished with the tools and materials available back then -- whenever 'then' was. I noticed that the first Voith-Schneider propeller was built eighty years ago (it said that on the website). And, it looks an awful lot like a Schmidt Wave Propeller (even older).

As to BG needing adjustment for his wheel, don't forget that one passenger less will make a very large difference in how his boat will float, so I think it will be important to him. And for an extra pound to implement it -- I say go.

 

Alex
I am really suggesting a version of the Voith-Schneider prop with the rotating axis horizontal and perpendicular to the direction of travel. There would be a supporting wheel or spider on each hull and long foils spanning between the spiders. In operation the foils follow a path very similar to an oscillating foil.

I have since reworked the numbers and it is possible to get efficiencies above 80% depending on how the angle of attack is controlled. Some parts of the rotation are more efficient than others so the foil would be just allowed to coast through these points without trying to generate thrust.

The main feature is that you could mount the top of the wheel very close to the surface because the top part of the wheel does not do much. Also an 8" diameter wheel will do the job if it has three blades so will be shallow draft.

I expect the blade control would not be too difficult to work out.

If you think of a fully feathered blade travelling through the water it will just sit with chord horizontal. Once you start rotating the wheel the feathered blade angle will change to the attached curve. The faster it rotates relative to the boat speed the greater the angle changes. What I found is that it will generate enough thrust turning quite slow if you use three full width blades. So the angle does not change that much.

To get propulsion from it you need to actually prevent the blade from fully feathering so the range of angle is less than the chart. My sums are based on 5 degrees as this gives the highest lift to drag for the foil. There may actually be better values but I just stuck with this simple approach.

I think if you use thick foils, like 20%, it would be possible to just use horizontal blades facing forward and rotate them. This would be a simpler form. The 12% foils I chose would stall out at the high angle of attack but fat foils would keep powering. The horizontal foils would also be poor for getting going but with some torque arm they would be like a variable pitch prop.

Rick W

 

Alex, if you're still having trouble envisioning this, look here (if you haven't already):

http://aa.nps.edu/~jones/research/unsteady/panel_methods/anim3/

click on the picture and it will open a Javascript animation. This is the Schmidt wave propeller. Now, imagine three or so of those foils going around in circles. This is what Rick means.

Now, I can see why the Voith people didn't bother with the stationary foil in that example; it (or maybe five of them!) would have to be on a separate journal,; not worth the bother.

What's funny, though, is that this all seemed so familiar to me -- we've been going in a big circle, which (for me, at least) started here:

http://www.boatdesign.net/forums/showthread.php?t=23421

(post #7)

and now we're back!

And, as long as we're here, why not throw in that stationary foil for its little extra oomph?

Curtis

 

Curtis
Not quite back to the same place. The concept previously was a single blade. It needs to be large and will have large low frequency forces.

By placing three smaller blades on a wheel you get much steadier force. With three blades it is six force pulses every rev and the force will never drop right off so it will be a steady thrust and steady cycling load. The boat will not bounce around very much.

So effectively an oscillating foil but a better engineered approach. The main difficulty is setting the angle of attack and I have not worked out a good way to do this yet.

Rick

 

True enough, the conversation back then took that direction. What I was referring to was my original inspiration for the idea. That's what we're back to.

I think it would be great if you can figure out how to do this without the feathering mechanism. It would be a very simple build. Here's a thought in that direction: what if you made the blades to have neutral bouyancy, and just allowed them to freely pivot from some point near the leading edge? Would they align themselves to produce thrust?

Curtis

 

The simplest configuration is to get them just to sit horizontal. One way would be to allow the blade to pivot and use a counterweight to hold it horizontal. Still not easy to engineer though.

Rick

 

Attached diagram may help people appreciate the velocity vectors and the way the blade AoA needs to be controlled.

Without control the blades just feather and align with the flow vector so there is no lift and very small drag.

Rick W.

 

That steam dancer actually looks like a fun little boat tinhorn.

 

Rick...

So in your diagram wheel rotation is clockwise correct?

The three blade profiles shown are not one blade in different portions of its rotation but rather a snapshot of three blades in a wheel?

If you could just draw a chord line showing the angle you want the airfoil at in say 20 - 40 degree increments around the rotation I could model cam and followers for that. Then it would be the same path/rotation angles for one blade or three.

I could do the track in the wheel sides with the cams on the axis/axle of the blades. Maybe get it down to profiled ends on the wheel of about .25" (5-6 mm ish)

I just need to know what angle the blade needs to be at throughout its cycle.

Check your email also Rick.

 

BG
The wheel is rotating clockwise.

It has three blades.

I did not show the velocity vector on the top blade because it could be at the surface and in that position it cannot thrust anyhow.

The blade should be tilted at approximately 5 degrees to the velocity vectors.

The curve I provided is the angle of the vectors. So the blade angles falls inside this envelop by 5 degrees. There will be a deadband when the blade will less angle than this.

It is quite easy to draw the vectors at any point but the curve I provided shows it throughout the full rotation.

I am a little reluctant to supply the attached spreadsheet as I have not checked it but it shows the workings. Understanding the vectors (I know I repeat myself) is what is important. It is all geometry. So please read the spreadsheet with caution.

Rick W.

 

Test Tank Notes

Getting back to the test tank idea, how about making the test tank twice as wide, with the walls of the test chute separating the flow generated by the paddle wheel from the return path of the water? Half of the return flow would go back along one side and half along the other side.

Of course, you could probably get away with just having one return channel equal in width to the test chute. This would make it easier for access to the wheel test fixture.

The speed of the water in the return flow could be measured using a flow gauge or an anemometer. You would now be able to obtain data from the wheel thrusting against a moving water flow, and not just a static flow.

If the incoming return stream of water were passed through a set of tubes or straight baffles in the test chute ahead of the test fixture, then the incoming flow would be a bit cleaner and produce more accurate results.

Lastly, you could also place flow restricters in the return channels if you wanted to simulate the additional drag of a boat. Things like wire mesh, framed screening, etc., would probably work well for this.

 

BG
That last diagram is wrong. The attached is better and I have added more points showing the vectors.

The thing to notice is that the blade is not sweeping large angles.

I am not spending enough time working through this. That is why it is always good to check in real life. It does not need to be exact just something that can be related. A single blade would be OK for testing.

Rick W