Pedal Boat Design

Curtis--

Trying to kill a few birds with the same stone. If I build just any hull for the model to test the wheels on and they work, then I have to build another model with the better hull design?

Yea, the reason for the deep draft was to avoid the drag penalty for the down struts on either end of each wheel. Apparently that penalty is a crafty devil though...in reading through Greg's blog on his V12 one fairly streamlined rudder cost him a few kph of speed. It was about .25" or so thick and faired reasonably well. My thinking was that if little thing cost him so much speed, imagine what four down struts an inch thick 12" or so long sticking about 10-12" into the water would cost...it seemed logical to make them come directly from the boats side.

I seem to have a hard time with the fluid nature of the rules for boats. Long and thin is supposed to be faster...yet a guy in a kayak can go faster then Greg in his long thin boat. If that is not because of the hull but more the propulsion being paddles...yet paddle wheels are a lot less efficient then a prop...it seems like the kayak breaks every rule. It inst longer and thinner...it basically uses a 2 paddle wheel...

Rick--

Sorry, was only kidding around because I felt good about finally figuring out how to get SW to make variable radius's on the hull--was a bugger to get so when I finally figured it out I was a tad giddy lol. Your right of course I should take the time to figure out FREEship, sorry again.

 

My thinking is that you're building the model to test the wheels, not the hull -- and building it at one-third scale, so this hull can be thought of as a throw-away (once the wheel works). No harm in refining the hull shape at this point, I guess, but if the wheel needs a radical redesign, you might have to start from scratch with a new hull anyway.

I guess generalities can only be taken so far.

By the way, I'm beginning to suspect that there may be a problem with the wheel, but I don't know yet. It's a long explanation, so I'll save it until/unless it's needed. I could be all wrong, and I don't understand the hydrodynamics well enough to be sure, so I'll wait and see how this works out.

 

BG
Some clarification required:
1. You are factoring Greg's rudder loss about 10-fold actual. It was costing him 0.1 to 0.2kph. Having small fins below your hull at the side of the wheels would not cost very much. Irrespective, the slender hull is quite a lot faster than the catamaran. I cannot recall the monohull but it was heavier displacement. Overall I think the concept would be better to just have the fins and keep the hull wider but the penalty is not much.

2. Not sure what kayak you are talking about being faster than Greg's CP2. The previous 24 hour record was set by a surf ski and was 242km. The best a kayak has done (required 2 boats because the first one was so light it broke up) was 237km.

3. I have pushed my old black boat over 18kph and I can produce around 600W. A sprint cyclist could get up over 20kph. If you put a top time trials cyclist on CP2 I calculate he could do better than a rowing scull and these are faster than kayaks. Fast kayaks are just wide enough to fit a narrow bum into. They are like a glove and take some effort to keep upright. The skill involved is impressive.

Point is that there is an optimum minimum drag shape that tends to be long and slender for the speeds of interest. Your 6" wide hull is on the narrow side of optimum for 4m/s.

CP2 is optimised for 12kph and Greg decided to hold 11kph as he knew he could break the record if he pushed along at this rate with the occasional reduced effort while eating, drinking or answering dumb questions from someone in Australia.

The only kayak faster than CP2 is the flyak and it uses a foil and there is no way the effort required to keep it flying could be sustained for more than sprint type distances. Maybe an hour with a really athletic paddler.

You may have some misconceptions that are influencing the design.

As far as quick and nasty goes. The proposed hull, however it eventuates, would be suitable for paddle wheels as well. You will have to decide if you both want to face ahead or sit face-to-face. The latter makes drive system easier.

Rick

 

Rick--

POINT TWO KPH? Well that is a relief! (not the part about my shoddy reading skills lol). I have been over here dreading the down struts like the plague. No idea what the difference is between a surf ski and a kayak is. I was referring to the video Greg posted showing his flat out 1,000 m time against the distance record holder's time. Greg seemed to think they were both outputting about the same power. The other guy sure LOOKED like he was working harder though.

I got your file open in both FS and SW...is the end with the sort of blisters the front? Any idea how you would go about actually building that shape? The foam cutter can't really cut it the way it is. I could carve it by hand and get reasonable close I suppose.

Curtis-- Thats the way to instill confidence lol. Parachute should b fine...though I was wondering about one issue...nah, go ahead and jump and if its a problem I can tell you why after lol.

 

Actually there are several patents that overlap this tech. There are also so many published discussions that getting a patent squeezed in between the existing stuff would be a loosing and expensive fight.

Those Geodesic designs are quite lovely indeed. The snowshoe arrow (or a boat like it) might make an interesting PPB. I actually think Ricks spring steel prop more fits the spirit of those designs though, or something like it. Need to keep it from a hull strike of course.

 

BG
Greg's boat is really not set up for sprinting. The shaft needs to be a bit thicker. He could not get 5/16" spring steel so had to settle for 1/4". It is also geared for higher revving at lower power to conserve leg muscles. Higher aerobic cost but less force on the muscles. Still the same amount of work.

With 6.34mm (1/4") shaft I can get my boat to about 14kph compared with 18kph with a 9mm shaft. The shaft is a spring and winds up on the power part of the crank and unwinds as you go through the deadspot. It delays the carry through the deadspot. The springier it is the slower the passage through the deadspot and the harder to apply power.

I suggested to him it was not a good idea to do sprints with the 1/4" shaft unsupported as the buckling load is nasty. I expect that contributed to the reason it failed during training although the design is marginal for infinite fatigue life anyhow.

The other thing is Greg held a camera all the way through that sprint and was not dead beat at the end. He gives 10 years to Carter in age. Both Carter and Greg are a long way behind Olympic rowers and kayakers when it comes to sprints. They train as endurance athletes not for power. Long distance events like this are hard on the body and keeping the food intake is a key part. You get to the point where it is hard to look at food let alone actually eat anything.

The PPB will inevitably have a weight disadvantage but the hull is better than a kayak and even the surf skis that are longer with less beam than a kayak. The means of converting body power to hull power is more efficient for the PPB. The numbers I have show a slight advantage to PPB overall. It is just that it not not a well developed concept. Gives me something to pursue.

Rick

 

Here we go again looks like.

http://www.weather.com/maps/news/atlstorm9/projectedpath_large.html

I am down in the tip of Texas. We already had one direct hit this year...

 

BG,

All right, when you put it that way, here it is. I might have injected this earlier in the thread, but I only figured it out in the last few days myself -- and as you'll see, I'm still learning.

:!: WARNING: LONG POST! :!:

First, quick references, so you don't have to spend time looking them up:

http://www.voithturbo.de/545950.htm

http://www.boatdesign.net/forums/showpost.php?p=225377&postcount=374

http://aa.nps.edu/~jones/research/unsteady/panel_methods/anim3/
(click on the picture for the animation)

I've spent lots of time lately with the Voith simulator, trying to figure it out. I haven't yet got all of the kinematics, but I found out something enlightening about the hydrodynamics of it. To get to where I'm at, open the link, then open the animation. Do this now, and you can follow along with me.

In the animation controls, select "Hydrodynamic Forces" in the "View" menu at lower left. Keep in mind that, for our purposes, the view we want is rotated 90 degrees from that shown; the bottom of our rotor is on the right in the sim.

Leave the "Rudder Pitch" control alone. If we were to incorporate this on our rotor, it would result in vertical thrust, which we don't want (well, maybe not; it might come in handy as a trim control for one- or two-person conditions).

Leave the "Revolution" control at zero for a moment, and push the "Driving Pitch" up by some amount. Look carefully at the pale blue lines connecting the foils with the center of force (the little red cross-hair). Notice that these lines are exactly perpendicular to the foils, right through their moment centers. I found this delightful; I can put a center of force wherever I want it and just draw these lines to determine the pitch for a foil at any point in the circle of rotation; it's a graphic solution without the need of math.

(Quick aside: I tried drawing one of these with the center of force outside of the center of rotation, just to see what would happen at say, 130% thrust. It was instructive, but not pretty; the foils waved back and forth somewhat chaotically during a portion of their cycle.)

Now move the "Revolution" control up a small amount. I find that if you take it up more than 5%, it gets jittery and hard to look at. Play with the "Driving Pitch" for a while; what I noticed is that the "flip" doesn't become prominent below a relatively high value. Stop the rotor at various points and study the angles, and you'll soon figure out why "100%" is just that -- and the two rightmost (bottom) foils get into 'articulated paddlewheel' mode.

BTW, another thing I noticed about the Voith sim; it's "Rotation" control seems to have no effect at all as long as it's above zero. Turns out that it isn't really supposed to; they're designed to work at a fixed rpm.

The most important thing I get from this is that the foils are always positioned for their maximum lift/thrust in the desired direction. I think this is the Voith's strong suit, because, as I understand it, an airfoil is much better at producing lift than it is at producing forward (chordwise) thrust, and Voith has utilized this.

One thing that becomes apparent with the Voith propeller is that their foils operate through a very wide range of AoA -- up to a little less than 45 degrees. I'm used to seeing 15 degrees as the maximum AoA before stall, so I don't know what this means, though I think I have read that foils do continue to provide lift past the stall angle.

Moving on . . .

Now open the second link I posted above. It's Rick's diagram of the foil positions for the current wheel design.

I tried applying the graphic solution to these foil positions, to find the center of hydrodynamic thrust, as it works in the Voith diagram. Drop a perpendicular from each of the foils, through their moment centers. These lines don't even intersect at a single point, and if it did, it would be several wheel diameters below the wheel's center of rotation.

Then, since Rick's foils don't flip over, one of the three will be going exactly backwards at some part of the cycle. Of all the scholarly papers I've seen on airfoils, I have yet to see one describing what an airfoil does in this condition, so -- I don't know. My closest guess might be "stall," but that's only a guess. On the Voith propeller, they are at least giving some thrust throughout most of their cycle.

Maybe it isn't as bad as I've made it seem so far, though. Now open the third link, and click on the picture to display the animation of the Schmidt wave propeller. Here is a foil doing exactly what it does in the current design -- at zero AoA --and these seem to get pretty good results. I don't know squat about how an airfoil produces thrust by moving perpendicularly to its chord and span, but Rick has powered boats that way, and it works (see also "Aqua-Skipper," formerly "Trampofoil").

I found what appears to be an odd difference between the two systems. The Voith seems to give no weight at all to the direction water is flowing around or past it -- just pitch the foils for best lift, and take whatever comes, where Rick's configuration seems very flow-dependant. Maybe I'm missing something.

So, maybe I'm way off somewhere. What I see is two apparently different ways that foils behave, each producing thrust.

I don't think you should stop what you're doing right yet, though; follow through and see how it works.

 

I think (and I am VERY likely wrong) that the voith animation misleads in one way. It shows the airfoils as always vertical in their Z axis when I think they actually are articulated on a ball joint type arrangement at their roots. The tips describe different arcs in TWO planes depending on this Z angle.

If you look at Ricks diagram of the angles each blade/airfoil is at, the flip your seeing (or not seeing) is actually the dead zones TDC and BDC. I know he shows the thrust vectors in relation to the AOC but those really are not the first physical force or reaction in the chain, high and low pressure are and thrust is basically the byproduct because that energy has to go somewhere.

Think of a snow skier...a poor analogy if you have never skied. Thinking of how a sail boat tacks might be a more analogous example. The term "flapping" will get applied to this rotary foil approach sometimes which I think is also a bit misleading. Actually the wave foil is more accurate because you have the slipping component in there. I would say it was even more accurately the tri foil radial tacking propulsion system...but that would be tacky.

It might help you to take the radial component out Curtis. Take Ricks 12 position radial diagram and make it vertical positions for the foils in the 12 to 6 line. Now picture that as one foil traveling up and down with the foil rotated to the AOC shown. Lemme whip up a sketch and add it to this post, one sec.

 

The circles represent the leading edge of the foils which are the black lines.

Very oversimplified but it is pretty much taking the motion of a porpoise tale and translating that force into a more mechanically friendly radial machine

 

BG,

Definitely not. Here's the homepage again

http://www.voithturbo.de/vt_en_pua_marine_vspropeller.htm

a little left of center there's a cutaway view; click below it where it says "Longitudinal Section." You'll see there, as well as from photos elsewhere on the site, that the blades have only one axis of rotation.

Now, the linkages that control them are a different matter; there are definitely two different kinds of motion going on there at the hub, and I've figured out one of them so far, but not the other. To see what's going on, you have to carefully study both the kinematic view in the sim, plus the cutaway. Some of the details are missing from each of those, so it's really hard to figure out how exactly it works. The cam is that yellow vertical thing, Part #10, and in the sim, you can see it as a very subtle pale blue eccentricity at the center hub. Look carefully while adjusting the controls, you'll see it move.

For our purposes, I don't think it matters. Best I can tell, one of those tricky hub linkages is entirely devoted to the right-left control -- that's the one I can't figure out. I think so, anyway. I've spent hours staring at it.

True for both systems.

Also true for both systems. I just have a problem understanding how a Schmidt-type system produces thrust. Maybe it's all due to inverse Karman streets; for all that, I don't know if an airfoil in that mode produces any lift at all. I know how the airfoils produce lift in the Voith model, so I understand it better.

Hahaha! At least!

 

Okay, but I would think you'd want a more pronounced pitch excursion. I have a little familiarity with flapping foils. I should look at that again.

 

Arrg, cross-post. Okay, I got it.

Now, I definitely think it needs more pitch excursion!

 

I still don't see anything that makes me positive the blades cant alter their Z axis alignment. If you look at the kinematics theres actually a green arrow that looks like it is showing this...

Not really important either way. Thats a very nice bit of engineering. Looks hard to maintain though...lots of moving parts to wear compared to your basic propeller.

 

A little update on the 1/3 rd scale model of the drive.

The image shows what I am working with here lol. It is of the 3 point cam follower in UHMW plastic. For an idea of the size of this part, the orange circle is an Aussie 5c coin and the silver is a US dime.

The kerf shown on each leg is for reference. I may have to add several to each arm or none at all depending on how flexible the part turns out. As shown it is .24 mm or .01"

The shaft hole diameter is 1/16" and the cam follower track is 3/16" wide. It will likely be 1/16" deep, or maybe 1/8" if needed. I am thinking basic old styrene plastic for the cam track. Will have plans for the laser cut files for those parts posted as I get to them.

G

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