Pedal Boat Design

BG
Attached is what the link gives on my Mac. I cannot see the track.

There are factors in the prop design that may not be optimum to match the boat. The angles of the blades that I provided are quite aggressive. You might find they are hard to get going or not quite matching the boat drag. The blades might require steeper angles on the up and down stroke and they may require some offset to generate net lift. This means the path of the cams could extend further forward - more offset to the centre of the prop wheel.

It would pay to check what angle can be achieved with the existing envelope.

Once I can see the drawing I will give you a different blade angle to check the geometry for. Different cam plates can be used to change the effective pitch.

Rick

 

No idea about the edrawing viewer for the Mac OS, sorry. The file is an .EASM file which is an edrawing assembly for seeing it as a solid object in 3D space.

I can provide 2D CAD of each plate but that gets pretty confusing.

Yes, I mentioned that just swapping the Cam plate (and possible the control arm plate) will let us try other AoA's.

Did you try to open the file on your Windows laptop?

 

Man, I'm missing this. Downloaded the SW viewer that (I think) was supposed to work with Mac, and it didn't.

 

What we have here gentlemen, is a failure to communicate.

I checked in the file save dialog options and there is no setting for OSX based readers. The files are supposed to be viewable by either platform.

It's an occupational requirement that I have nothing to do with Mac's. Recently while on vacation in San Francisco I refused to even walk into the Mac store to ask for directions despite heckling from the wife.

If you can't get it to work Curtis I can just post a bunch of images, though you won't be able to manipulate it in 3D space it is better then nothing. I did see several posts on using the Mac to view edrawings when I googled it however. You might try that.

 

BG
I did it on the laptop.

It looks neat.

I am not going to get too concerned about scaling. I have already said it should run a little faster than full scale. The drive belt ratios will need to be checked. The top pulley may be a bit on the small size as there is not a lot of clearance. Just means you will need to gear up to the top shaft from the the pedals. Need to check gearing and belt loads are all OK for the full scale.

I like the cam and tracks. It has room for slightly more offset cam if required.

I would like to see the blades with maybe a 0.5 degree upward offset to ensure positive lift. I am struggling to get my model to work with an offset but I will have another go when I am not busy.

Rick

 

Finally, I got the right viewer, and it works. Looks cool, but I don't see the blades -- are they supposed to be there?

Hmm, guess not; they aren't on the parts list.

 

BG
I believe the blade upward offset can be achieved by making the centre of the cam track a little lower than the centre of the wheel.

Right now my model is showing significant net down thrust. I have not worked this out. When I rotate in the opposite direction I get upthrust. So be prepared to reverse the direction of rotation.

Until I work out whether I have a modelling error or it is correct be prepared to make changes to the centre of the cam track or reverse direction of rotation.

Rick

 

I could not really make the blades permanently attached to the control arms because then I could not pass them through the wheel rim. This is a pretty high load area, especially on the scale model, so I will have a set screw on top near the LE of the blades. Slide them onto the square end and secure with a roll pin.

I didn't add the blades to keep the part from getting overly complicated to view...not a big deal to imagine them there lol. There would be one of those assemblies on either end of the out rigger strut arm that has a common shaft that drives the blades from each end. That means the belt would almost have to be toothed, or a chain because regular belts would slip out of alignment too easily.

Are you able to show, hide, and make parts transparent Curtis? I know how you like the little details so being able to do this will really let you dig in--not that it is overly complex, but I think it's still an elegant approach.

 

BG
An animation of one side with the blades on would be nice - if you have a spare minute or two of course.

I can almost see this working. I have to figure out why I am getting the net vertical component!

The lever actuation on the support shaft makes for a neat set up.

I will rework the whole thing based on you chosen foil and a bit more sophistication on the angle of attack. With the skinny hull compared with the catamaran then AoA has dropped down quite a lot.

Rick

 

Yes. I don't have access to the tape measure and I can't drag-and-drop parts or get an exploded view, but it's okay. Tool menu is limited to "Reorder Views." I'd love to get an exploded view, but I think that's just because of old habit. Hiding parts and making others transparent is fine in conjunction with the rotate function.

You mentioned a concern with water in the track; why not just have a couple of little holes somewhere to let it leak out (or get squirted out by the moving parts)?

 

Rick--

Where in the rotation are you seeing the down thrust? As I view it, I see the leading edge of the rotation, call it from the dead band at TDC to the dead band at BDC as exchanging up thrust for forward thrust (like the tacking example) and the reverse of the rotation, moving from BDC to TDC converting it's down thrust component into forward thrust. The system should balance out with the net being good because both convert (either their up or down thrust) into forward thrust. Some losses there of course.

I think the issue is the odd transition areas at the 9 and 10 o' clock area. When I set the model to spin at 50 RPM and just watch it there is a definite bump right there that is not on the down stroke. It seems to me that as we are using a symmetrical foil on a circular path with the dead band at TDC and BDC...well, shouldn't the up stroke, or reciprocal from BDC to TDC be the exact but inverse of the down or TDC to BDC stroke?

When I look at 12 positions for the down leg and 12 for the up leg instead of 6 each, there is a significant under loading on the up stroke (which produces down thrust) compared to the down stroke. All this is assuming a CW rotation of course.

For grins try it with matching angles on the up and down strokes and see if the forces don't balance out.

One other theory. Gyroscopic procession.

I don't think it applies to our application the same way it does on an airplane prop, but it might in the way a helicopter blade experiences it. The retreating blade on a helicopter is a limiting factor in forward flight speed because the faster you go the slower is the relative speed (with respect to airflow) of the relative wind over the retreating blade and it ultimately stalls, just like any wing with no wind over it. Regardless of the direction of rotation, CW/CCW, the loss of lift is on either the left or right side of the forward moving helicopter which is applied (due to gyroscopic precession) 90 degrees later in the direction of rotation, which means the aft section of the helicopter loses lift during a stall and the nose pitches up instead of down as in a fixed wing stall.

I know the relative speeds were at, say 100 RPM make it seem like a non issue, but with something like 8 times the density of air...wouldn't that exaggerate the flow enough to get us into the realm of possibility? It sure fits the up lift the models are showing. I get it in EITHER rotation direction also, which lends even more credence to the theory...course I am pretty slow lol.

 

There is drain space between the wheel rim and the cavity it occupies. I have it at .005" which is in the realm of the laser and would allow plenty of beached drainage. The problem I was talking about is waters dislike of being compressed lol. If I drilled say a 1/8" hole then expected 4 cu in of water being propelled to whatever speed 70-100 RPM translates to...well, your looking at a syringe pretty much. Plus it would make me have to pee all the time =)

I am less concerned now anyway because really the three 3 point rollers form three cavities in the track of the same size that all rotate at the same speed with nothing to push against that would cause hydraulic locking. Not sure what it will do from an energy required to move the water point, but we are really talking a very small amount here. Certainly less then your average oil filled crank case...

 

The animation (as well as the exploded view) are in the eDrawing file. I have no idea how to get those to work for you guys.

The lever style on the LE is an old airplane trick to counter balance control surfaces. If you have never seen an aileron oscillate to the point of exploding you can't appreciate well balanced control surfaces. I got to see some pretty cool test to failure runs when I lived in the land of Big B. Like seeing a triple 7 main spar loaded up until it grenades into a few thousand bits. They are something like 400K to make btw.

For the test model I am going super simple. 6' long and just pinched together at the ends, flat on the top, and a very small rake fore and aft on the bottom. 100% hard chine lol. That way I can give you EXACT numbers to feed into your software so you will know what speeds we should see vs what we are actually getting. I will need some help figuring out a way to determine what the actual wheel RPM is though. Knowing how fast the boat covers say 1,00 feet is not very useful if you don't know the true RPM right?

 

Bicycle speedometer? Strobe light?

 

Strobe/timing light was the first thing I thought of too Curtis lol. I can see my fat butt now jogging along next to the thing in a foot of water shooting it with a timing light while counting lol.

I was hoping more along the lines of a hall effect sensor on the motor. If we know armature/shaft RPM we can calculate the gear train all the way out to the wheels. Just need a way to store the information in NVM on the boat.