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Guest625101138

07-20-2008, 11:16 PM

Curtis
I started this thread for such questions:
http://boatdesign.net/forums/showthread.php?t=23345
Irrespective you have started thread here.

There is some debate over the efficiency of flapping devices. I did a lot of experimentation with them as I felt nature had something to offer. I got close to the theoretical maximum possible efficiency under certain conditions. Kept trying until an observer pointed out that nature has not developed a shaft capable of continuous operation so cannot produce a propeller. I guess if flappers were really that good you would see modern aircraft using flapping wings instead of jets.

The Hobie mirage is a very clever system:
http://www.hobiecat.com.au/support/kayaks/miragedrive.html
The best thing about them is their marketing hype. In reality they give efficiency around 30% with low power and up to 50% at high power. With a heavy boat they would be even worse.

Flapping systems that are surface exposed are less efficient because the foil constantly ventilate. Here is the most developed version I know:
http://www.tailboats.com/propulsion_technology.html

Here is my oscillating foil boat:
http://boatdesign.net/forums/attachment.php?attachmentid=22566&d=1213860730
It is very efficient to 8kph before the boats starts bouncing badly. This speed is almost ideal for operation at 60rpm. It is a nice system for a boat with a large waterplane and reasonably heavy. I can give you design parameters for this.

Here are more flappers:
http://boatdesign.net/forums/showthread.php?t=21025&highlight=oscillating

THe most flexible and efficient set up is a prop. I can help you design a system if you want to try this.

So something to think about.

Rick W.

clmanges

07-20-2008, 11:40 PM

Rick,
Thanks for the input so far. I'll have to get back with the long list of limitations and other particulars; don't have time for it all now.

I've been looking at this topic for quite a while; thought I'd seen it all until I found this forum, but there isn't much here that's new to me, either. Still, I'll state my case presently. I've got to get ready to bring my Mom home from the nursing home tomorrow, so it may be a couple days.

peace,
Curtis

clmanges

07-22-2008, 12:00 AM

Okay, well, this is going to take a while, so please bear with me.

First, a little personal history is needed to explain some of the requirements that follow.

Several years back, my hip became arthritic. When the hip went south, it took my lower back with it. I now walk with a cane.

Bottom line: I don't bend in the middle very well. That is one of the reasons I'm in my present situation, and why I'm here seeking feedback.

Previously, I had an Ocean Kayak Scrambler that I was perfectly happy with. After the hip went bad, I could not safely get in or out of it, had no place to stash my cane securely, and couldn't even sit comfortably in it. Also, I found that I could no longer even get the boat onto the top of my car unassisted (I now have a pickup truck).

I sold the 'yak, but was determined to get on the water somehow. I needed a boat that was small and light enough to wrangle into my truck, and stable enough for me to get in and out of safely. Short, wide, light, flat bottom. A dinghy. I bought the smallest, lightest, (and cheapest) one I could find, this:

http://plasticboats.com/

It is 7.5 ft long and about 4 ft wide overall, weighs 50 lbs. I weigh about 160.

Oh, btw, before I got the Bathtub, er, Backwater, I designed a boat for my own requirements, and built a successful quarter-scale model of it, but figured out that I could not build the full-size version by myself, and shelved that notion (before I spent $$$ on material, thankfully).

Anyway, so far, so good. I'm back on the water. I can drag the boat to the truck and hoist one end onto the tailgate and shove it up on. It needed a modification to improve tracking, so I built a kick-up stabilizer, like a rudder, except that it doesn't turn. As to performance, well, you can guess from the pictures (more to follow). It will never be fast, but I don't care; I'm just out to enjoy being on the water.

Rowing is okay with me; I've always liked getting places under my own power, but I don't like going backwards. My neck is stiff from an old whiplash injury, so it's difficult and painful for me to look over my shoulder. A bicycle mirror helps some, clipped onto my glasses. I've looked at every forward-facing rowing device there is; they're mostly outrageously expensive, and I'm not sure how well they'd work with the boat that I have.

Since my hip doesn't work, anything with pedals is out of the question. I used to love bicycling, and had to give that up, so it's down to arm power.

To summarize, I have a boat that works for me; it accommodates my limitations. I'd just like to work it a little differently.

Now, then . . . (which is it -- now, or then?) . . . how'd I wind up here?

A lot of things on the internet catch my interest, and one of them was the topic of ornithopters. About half a year ago, I found the forum at ornithopter.org, and looked around quite a bit. What really got me excited was the work being done by Kjell Dahlberg. I saw his videos and models of boats propelled by various flapping foils, and thought that this might be a solution to my quest: a retrofit for my existing boat; something I could make inexpensively at home, and would do what I wanted done.

The configuration I liked best and focused on is that of two vertical foils. This has been demonstrated by Kjell and others, so we know it works, and it's especially amenable to a simple application of arm leverage. Also, with a proper drivetrain design, it can serve as forward propulsion, steering, and brake. Elegant. (See drawings.)

I know that the drawback of this is inefficiency, but that doesn't bother me. The shape of my rowboat is already inefficient, and I'm never in a hurry to get anyplace with it.

More to come . . .

clmanges

07-22-2008, 12:02 AM

I didn't stop with pitching foils; I continued to look for other possibilities that might be better. One link sent me to the work of Kevin Jones:

http://aa.nps.edu/~jones/

which sent me in at least two new directions. One of those was the "Schmidt wave propellor," and the other was -- eventually -- to the Trampofoil (now "AquaSkipper").

And this is where I am now.

First, look at:

http://aa.nps.edu/~jones/publications/aiaa97-0826/

and open the PDF at the bottom. Figure 3 shows the Schmidt wave propellor, but notice what it says in the paragraph just above: "Obvious mechanical difficulties arise from pure plunging motions . . ."

"Obvious mechanical difficulties" -- ? Now, I don't pretend to be able to follow the math on these pages, and some of the physics is a little above my head (by only a few fathoms or so), but I don't see any mechanical difficulty at all in using my arms and a simple lever to create a nearly pure linear oscillation. And I get free extra thrust from a second, stationary foil? I've been moving my arms already anyway! Where do I sign up??!

Now, back to the Trampofoil for a moment. Check this out:

http://www.instructables.com/id/SP3EGW1F2297BYM/

They show you how to make one of your own, and they seem to have stolen the measurements straight off the high-priced production model. The main wing is 87.25" long (span) x 5.5" wide (chord), which gives an aspect ratio of 15.86, and the make-it-yourself folks seem to think that just about any profile will perform. Jones' studies seem mostly to deal with NACA 0012 profiles.

The AquaSkipper page has cheerfully given up the secret to the device's operation, namely, a very slight pitch change coupled to the plunging motion -- the very same combination that keeps popping up in Jones' papers.

I haven't developed that idea very much yet, but it isn't complicated at all. The drawback with it is that it needs a separate rudder.

So . . . here's where you folks come into the picture.

If I go with the first idea, I need help figuring out the size and shape of the fins. I suspect that I could cut the back ends off of a pair of ordinary swim-fins and attach them and they might just work, but I'm not sure. They may need a peduncle, and if so, how long? Also, how deep should they be below the surface?

As to the second idea, it hasn't aged much in my head yet, so I don't have a lot of specific questions so far. One thing I noticed from Jones' work is that the pitch changes are quite small, about plus-or-minus 5%. I thought that a bit more (user variable as needed) might be good for getting it moving from a standing start, but it seems to me that very little is needed once you are up to speed. This might be thought of as a kind of trim control, and may be adaptable for reverse thrust (not that my boat really needs much help slowing down).

Anyway, that's about it for now. Let the fun begin!

Cheers,
Curtis

Guest625101138

07-22-2008, 01:14 AM

Curtis
Here is a starting idea. It consists of a flapping foil that works under the boat and is activated by a hand crank. The whole thing could be dropped into place using wing nuts to lock it down. The blade would sweep through about 500mm.

Making the crank would be the most difficult part. I have not shown this but it would require a shaft running across the boat with hand grips on crank arms off the shaft. THis would be decidedly more efficient and controlled than rowing.

There is some detail not shown but it is not too hard to make. The wheels shown at the side would not need to be wheels either; just arms but the wheel shows the path. There are ways to engineer this to go reverse but it will lose a bit of efficiency. You would need a rudder of some sort to steer. This would usually be foot controlled.

You could go to a half size foil on each side and decouple the blades but the action would not be as smooth. There are many variations on this.

You should not rule out the possibility of a prop either.

There are also forward facing rowers.

I have made a swing arm drive that is a much friendlier pedal motion than cycling. Attached video is a demonstration of the swing arm pedal system by my late colleague Warren Loomis. Warren also pioneered the forward face rowing system:
http://forwardface.com/

Hope these are a few ideas to get you going.

Rick W

Guest625101138

07-22-2008, 06:44 AM

Here is the twin foil configuration. This set up has steering and reverse because the foils are fixed to the vertical strut. This means their angle of attack is controlled. They work like a fixed pitch prop which is not unusual. Initial load is high but once moving they move freely. If you stop cranking they try to drive the foil to a full up or full down position.

There is still some engineering involved to get it to work nicely and it is not as nice as a wide torsion controlled foil but it will definitely work well. You need to consider the forces involved and balancing might be needed depending on the weight of the foil. A bit of ply with some shaping works fine for the foil. As before the side wheels are only to show the crank path. It could just be a simple crank arm. The angular location of the crank handles is important in relation to the forces. You want maximum leverage when the the foils are mid stroke.

If you stroke the foils out of sequence they will tend to rock the boat. This might create more back movement then you would like.

It would be good to have access to someone with shop ability to make this. It would be quite nice if engineered properly. They may be able to find a market for it.

I can do all the math for the system but it is easy to do it by trial and error. I would start with foils as drawn 2ft by 4" moving plus/minus 250mm.

Submerged foils will work mush better than foils that break the surface because the do not ventilate so you get both sides of the foil working.

There is infinite variation with this type of oscillating foil arrangement. I am not sure if I have contrived the simplest solution for you.

Rick W.

clmanges

07-22-2008, 11:17 AM

Rick,
Thanks for the input. More stuff for my brain to chew on (mm-m, chewy!).

If I go with a foil, it will be full-width, one piece, for simplicity and lightness. I hadn't even thought very much about using two, and it sounds even less appealing after you said If you stroke the foils out of sequence they will tend to rock the boat. Don't want that!

See the attached, it's the first notion I came up with, and isn't fully developed yet, but you get the idea. I'd want it to project off the back of the boat, so it could be swung up for beaching and for loading it into the truck.

I haven't done as well as I should in stating all my needs yet. The places I usually use this boat are quite shallow in spots, and sometimes have big weeds in the water, so whatever I settle on should have as shallow a draft as practicable. Ideally, the thing would kick up by itself upon hitting an obstruction, but that's pretty unlikley; a manual kick-up lever would be okay, but I might just be able to design it with a quick release pin or some such, so that the whole assembly can be swung up as needed.

Which brings up the question, why so much excursion for the foil? I wouldn't think it would need much more than about six inches (15 cm). I base this on an estimate I made after watching this video:

http://www.instructables.com/id/How-to-Fly-a-Human-Powered-Hydrofoil---the-Aqua/

The foot platform is about two feet above the foil, and you can see that the depth varies by maybe a fourth of that. Also, in the papers by Jones, cited earlier, the linear excursions of most of their test models seemed quite small. I may have misjudged that, of course, and their power-extracting device did seem to have a deep stroke.

It would be good to have access to someone with shop ability to make this.That would be me. I have a workshop, with a drill press and some other tools. The necessary skills and equipment shouldn't be much more than what it took for me to make the stabilizer.

That's about all I can think of right now. Hope the weather isn't being too harsh for you down there.
peace,
Curtis

clmanges

07-22-2008, 01:27 PM

Rick,
You know, something just occurred to me about the foil drive -- if I have it off the back, as shown, then it's gonna pitch the whole boat, huh? Some of that action-reaction stuff . . . could be a pretty annoying effect on a boat this size and weight.

I guess that would explain why yours is shown on the midline . . .

Gotta chew this idea a bit more before I swallow it.

Guest625101138

07-22-2008, 05:38 PM

Curtis
Yes the pitching is why you want it under the CoG or close to it. That said the boat has quite buoyant ends so it may not be that bad.

The foil on the aqua skipper is a monster. It is about 8ft wide and 6" long. The drag on the whole thing is also very low and rate of bouncing is quite high. All this works toward having a small sweep range.

The twin foil system I drew could be set up with the cranks 180 degrees out of phase on the foils so your arms work out of phase while the foils are in phase. This should reduce the risk of rocking. These could be made to just drop over the gunwale of the boat and clamp down so not much different to oars. You may be able to get away with about 300mm sweep. They would certainly be worthwhile testing out. You can work in shallow water by only doing a partial rotation. The speed of stroke needs to be faster in the direction you want to travel - reverse or forward.

For shallow water operation in calm conditions it is hard to beat paddle wheels. Something like 12 blades off a spider. The blades would be say 12" wide by 4" deep on each wheel. Use the same sort of crank drive I have shown for the foils.

Rick W.

clmanges

07-22-2008, 07:19 PM

Rick,
Yes the pitching is why you want it under the CoG or close to it. I'm wondering now, if having two foils in countermotion would work -- one goes up while the other goes down. I don't see why not. Let me know what you think of that. It could be pretty easy to implement, a simple rocker.
For shallow water operation in calm conditions it is hard to beat paddle wheels.I'm surprised to hear that; I'd have thought them pretty ineffective, but maybe not. One more approach to consider; thanks.
Curtis

Guest625101138

07-22-2008, 07:51 PM

The rotating wings that the Hobie Mirage uses is probably the best design to avoid unintended consequences from the forces. These develop maximum thrust as they sweep through the 90 degree point so create opposing lateral forces.

It is not too hard to get paddlewheels up in the 70% efficiency range. This is better than paddles for "untrained" rowers. For the little dinghy there is probably little point in having better than 50% efficiency unless you want to paddle for long periods. I have not done numbers but I suspect 40W would see it starting to make waves. It is an up hill battle from there. Very little gain for extra effort. Attached photos shows the university teams in 2004 waterbikes. You can see by the boat planing that they are doing a fair speed. The two paddlewheelers are up with the rest. I think paddllewheelers have actually won the overall event on occasions.

Rick

clmanges

07-23-2008, 04:02 PM

Let's go back to the single pitching foil case of posts #10 & 11 again. It's the simplest case of this family of drive, and I'm a big fan of simplicity.

Further thought on this leads me to believe that the induced pitching of the boat would only become a problem at some range of critical frequencies, and could be cured by mechanical damping. Probably one of the easiest approaches would be to set up the linkage such that the induced pitch is countered by my own body movements as I shift my weight back and forth when pushing/pulling the actuator. Since one stroke of my arms = one stroke of the foil, that approach works at all frequencies.

Adding a second, stationary foil would also serve to dampen vertical motion of the boat, especially if the stationary foil was wider than the moving foil. I can't be sure, but I think it might also straighten the flow a little downstream. Is that right? If it does, then that might help as well.

The real enemy here is the length and weight of the boat, in spite of it having bouyant ends, as you mention. I'm guessing at a waterline length of about 82", and it only weighs 50 lbs empty, say about 225 or so loaded, so a little induced motion goes a long way. That's the problem with retrofitting anything as opposed to designing it all from scratch.

Let me know what you think. Thanks.
Curtis

Guest625101138

07-23-2008, 05:46 PM

Curtis
The stern mounted foil is probably worth a test just to get a feel of things. It can all be worked out but there is nothing like actually trying something to learn what the issues might be.

I found making foils was easiest using pieces of aluminium. The attached sectional shape is something to aim for. It does not need to be precise.

Lets say the foil is 2ft wide and 6" long. I would make this out of 0.8mm sheet with a 12mm square tube as reinforcing. This needs to be hinged to the vertical stroking strut (needs to be narrow so it does not have a lot of drag). I placed straining wire through the leading edge to act as a torsional spring. It is a bit heavier than a clothes hanger. I can give more detail if you want.

The lever arm would need to be pivoted in a way to give a minimum of 300mm sweep. The longer the lever arm the better because you have less angular change at the foil. You might be able to see in my foil attached that I have an angle compensating linkage in parallelagram configuration for this purpose. This blade was swept through 600mm and was nice at 8kph (5mph) on a long boat.

You cannot reverse with this set up. There is no steering without a rudder,

If you set up with two of these, one on either side you could get steering. The best place to pivot would be up near the bow with the vertical strut just over the stern. Could be awkward to set up. If the pivot is on the stern then the foil will be further back and this will increase pitching moment.

Rick W

clmanges

07-23-2008, 06:17 PM

I can give more detail if you want.Yes, please. I was thinking of using wood, but the idea of rot bothers me.
Thanks,
Curtis

Guest625101138

07-23-2008, 07:02 PM

Curtis
I hope the attached image is self explanatory.

If you can get some heavy straining wire about 4mm diameter it will do both the torsional spring and the pivot. There are two nose pieces that are a bit less than half width. This allows the wire to be bent out from either side and clamped to the vertical strut.

You form the noses pieces over something like 1/4" steel rod to get a nice curve.

The tail piece is best bent very hard by clamping between two pieces of wood.

You only need 0.6mm thick aluminium for this. Our local hardware in Oz, Bunnings, stock this in small sheets.

The stiffener is 1/2" (12mm) square tube with 1/16" wall. You can pop rivet it together or screw it with self tappers.

You do not absolutely need the plastic bushes but they do stop the wire flogging about and it is quieter.

Hope it makes sense.

I would make one to test out so you get an idea of the forces and its effectiveness. We will look forward to your posts on building.

Rick W

clmanges

07-23-2008, 08:59 PM

Thanks, Rick. Finding the materials for it is no problem. It will be a little while before I'm quite ready to build this part, but I've printed the drawings and related text. Fabrication seems pretty straightforward.

Curtis

Guest625101138

07-23-2008, 09:26 PM

Getting hold of the spring wire is the hardest part. A coat hanger is just too tin.

I have a spring maker near me and I can get straight length of the best quality spring wire. 4mm wire has a yield around 1600MPa. This is super stuff. Despite the strength it has good ductility and can be bent in tight radius. They cold form it for springs to retain the temper.

Ideally the foil will flap through about 90 degrees when at rest and will be less once moving. It is best if the vertical strut stays vertical but if it is mounted on a pivoting lever arm then this is not possible. Any added angle on the lever arm reduces efficiency.

It is possible to get the vertical strut to stay vertical by setting it up as one leg of a parallelagram. This could be a refinement.

Attached shows something that could be made to clamp on the stern of the boat. Use one on each side. You need to pump in unison to avoid roll. If you went hard there might be a point where pitching is a problem but it will not take much power to get moving. Operating one unit and leaning into a turn should give acceptable steering.

Rick W

clmanges

07-24-2008, 10:43 AM

Looking at this again, I have to ask about the spring. What is the purpose of it? How does it perform work in the system? I know you mentioned torsion, and your sketch is clearly that of a torsion-spring arrangement, but I would have thought that the foil should be rigidly mounted, probably at mid-chord. Is the spring only needed when the foil is supported at one end? it would make more sense to me in that case, to prevent failure at that attachment point.

Guest625101138

07-24-2008, 05:32 PM

In operation the foil will have a low angle of attack. Maybe 2 to 3 degrees. So if you consider the situation when the boat is at rest the foil must range through almost 180 degrees to keep that angle of attack. As you increase speed the velocity vector changes such that the foil might only need to range about 90 degrees to achieve the best angle of attack.

Hence the torsional spring plays a vital role. It is similar to the reason foot flippers are made from rubber. Same reason the Hobie flappers are rubber. You can achieve a similar action with a rubber blade but it is not as efficient as the torsional spring on a rigid, high aspect foil.

The system works very nicely if the vertical strut stays vertical. It is works as a variable pitch prop as the force on the blade is almost constant irrespective of boat speed and irrespective of how fast you oscillate the blade.

Rick W

clmanges

07-24-2008, 09:26 PM

Cool! One more question
You can achieve a similar action with a rubber blade but it is not as efficient as the torsional spring on a rigid, high aspect foil.
Is the torsion spring more efficient than rubber on low-aspect foils?

Guest625101138

07-24-2008, 09:57 PM

High aspect blades are better than low aspect blades for any given surface area.

Defined shaped, achievable with rigid foils, are normally better than a flexible shape achieved with rubber. The exception is where you create a variable camber foil that comprises a flexible membrane with a rigid frame leading edge. I have seen something shaped like a porpoise tail using a curved steel rod frame with a membrane glued around the rod. This can make a very efficient foil as it forms a thin, highly cambered foil in operation. It will work reasonably well at speed without a knuckle pivot; hence the torsional spring can be avoided. Without the pivot it stalls out at start but would be good enough to get you moving. The tilting blade has very high thrust at low speed.

It is possible to replace the steel torsional spring with rubber bushings that achieve the same result but they need to be quite low durometer rubber. I have found spring steel works best. If you were operating in salt water then you might need to think about more corrosion resistant materials. I paint any spring steel I use. Straining wire is galvanised so it is OK for a while - still best to paint. (Machinable aluminium rod with a good temper, something like 2011 T8, is a good substitute for spring steel and you could go to thicker rod as it has much lower spring constant. I expect 1/4" rod would be good.

The blade with torsional spring is quite easy to tune. If you find it too stiff then you can replace the wire with something heavier for example. Best to use self tapping screws for making the foil until you are certain the spring is right.

Overall I found the torsional spring the best solution. I made maybe 30 to 40 different variations from scale models up to many variants at pedal boat scale so there is a bit of background. I can do accurate force and power calculations but it is easiest if you just have a go. I used to make foils in an hour or two. Making nice mechanisms takes longer. I even got to using roller bearings for pivots and spherical ball joints for linkages to get rid of any floppiness. You do not need these to test something out though.

Rick W.

clmanges

07-25-2008, 12:26 AM

The reason I asked is because, for my purposes, the long foil approach is looking problematic; the necessary mechanism seems more complicated than I first thought, and the addition of a rudder adds more machinery. I think I'd be better just going with my original first idea. It's less complicated all around, and provided it works as I imagined, will give me better maneuverability, plus the ability to leave it permanently mounted and just swing it out of the way for beaching and transport. Also not as likely to get hung up or damaged if I hit a snag. (I've already driven over a submerged log; no damage, but it was a real surprise.)
I have seen something shaped like a porpoise tail using a curved steel rod frame with a membrane glued around the rod.Kjell Dahlberg made some of these at small model scale; I don't think it would be too hard to do. What's best for the membrane? I'm thinking of non-stretchable nylon cloth, like Cordura, sealed with RTV silicone, sandwiched between two sheets of stainless, probably pop-riveted together, as shown below. I don't know if a peduncle would help or not.
Please explain the "knuckle pivot."
Thanks again,
Curtis

Guest625101138

07-25-2008, 12:46 AM

Curtis
I envisaged two of the flappers each mounted off a small bracket that can be clamped over the transom. You could make them so that when the handle is in the lowest position the blade is higher than the keel. The lever handles would be placed about 2ft apart under the shoulders. You would pump the levers up and down about 8 to 12 inches.

Even the aluminium blade can be made to float by squeezing in some foam when it is assembled. The normal rest position would be with the blades up and the handles down. They will not add to draft for beaching.

With two flappers that are either side of the centreline you will be able to steer by being a bit more aggressive on one handle than the other. Leaning into the turn should also help.

The angle sweeping arm with the membrane is effectively the same thing but it has fixed depth so will not get shallower unless you pull it out. You also have to find a membrane and fixed it into place. I simply find working with aluminium so much easier than other materials. Light, easily worked, known properties and strong.

Rick W

clmanges

07-25-2008, 12:07 PM

The angle sweeping arm with the membrane is effectively the same thing but it has fixed depth so will not get shallower unless you pull it out.
If I understand you correctly, the first sketch below may be the answer to that problem. See also the sketch in post #6, it shows (with a note) that the hinge is on the outer bottom edge of the frame.
I don't know just how effective the fins would be in the intermediate position, but they might do something. I'll still have my oars aboard.
The fins and their drive shafts may need to be long enough to cause them to overlap when in the "up" position. If this becomes the case, I can simply make one of the assemblies to project an inch or so further off the back; the asymmetry would probably not be noticed on this boat.

The second sketch illustrates an entrapment scenario for the horizontal foil system, and is a very real possibility where I boat. Tough weeds are a lesser case of the same. The water is murky, so I can't see much of what's below the surface. The vertical fins would be far less apt to get entangled, and can be raised for clearing.

So, I'm pretty much giving up on the horizontal foil approach at this point. If you could help me design the fins, that'd be great.

I much appreciate your time and effort on this, and I'm still open to further ideas and comments. I'm still considering paddlewheels; there's a lot of appeal there for my uses.
Thanks,
Curtis

Guest625101138

07-25-2008, 06:26 PM

Curtis
I like the double waggle system. You could do some trials with a pair of large flippers attached to a tube. Still a bit of engineering involved but it does not need to be sophisticated for initial testing. This will be considerably less efficient than a rigid foil sweeping a good range but, as I pointed out, this is not such an issue with a hull that will only want to so 4 to 5kph easily.

The snag problem is not that bad with the flapper because it only has one narrow vertical strut that will flex quite easily to skip around obstructions. If you know they are there then you just sweep through a smaller range.

Rick

clmanges

07-25-2008, 09:51 PM

Double waggle, I like that. This thing needed a name.
Can you give me a rough guideline on the size and shape of the flippers? I'm guessing they need to be larger than swim fins, but beyond that, I'm -- well -- guessing. :confused:
Here are a couple ideas. The first is what I'd been thinking of and mentioned earlier, but the second is just a shorter, vertical version of your foil, including the torsion spring mounting. Does it even need the peduncle? Might be the best of both worlds.
The dimensions are just guesses; plug in better ones as you see fit.

Curtis

Guest625101138

07-25-2008, 10:48 PM

THe beauty of the swim fins is that the are available and you can get them quite cheap. They are rubber so will just bounce over things without getting bent.

The waggle system will not be very efficient. You would need a control linkage on the foil to get it efficient and that is more complexity. What you have proposed has a lot of moving parts with quite high forces. For you to get an understanding of this it is better if you build something and try it. Ultimately the forces are limited by what you can apply. If you really lay into you could easily break bits but you should get reasonable result with 5 to 10kgf on the handles which will multiply through the linkages depending on length.

You will find it quite awkward to row with the roll up feature because the handles will want to tilt up and down as you row back and forth. It is these sort of things that you have to contend with.

The parallelagram linkage I proposed gives nice force management. It will feel smooth. The applied force is straight up and down.

I could go on forever about the detail but it is better if you build what you think is best. Identify the issues and then redesign to overcome them. I have already done this and shown you what works best for me. You might find something different works best for you.

You could make a simpler waggle device to trial just by fitting the handle to the vertical shaft and setting it at the back of a boat like a rudder. Just waggle it back and forth. It will give you an idea of forces. If it is a flipper it will be more effective than the solid rudder used on sailing dinghies and they waggle or pump their rudder to propel in low winds. Two such devices on either side of the boat at existing oarlocks would be much simpler to engineer.

Rick W.

clmanges

07-27-2008, 10:13 PM

Rick,
Our dialog at this point is really not getting me where I want to go, so I''d like to take it in a different direction. You said I could go on forever about the detail but it is better if you build what you think is best. Agreed, but rather than hit-or-miss experimentation, I'd like to be able to design something that would work adequately the first time out. That was my whole reason for starting this thread -- to get some help from those here with superior knowledge, and thus bypass the grope-and-hope method.

Let me begin with a limitation: that we examine only one approach at a time. This may become a process of elimination, system by system, and if they all get eliminated, I'll just keep rowing backwards.

During this process, I'd like to examine each system by looking at each of the relevant variables, changing only one at a time. This way, I can learn the importance of each, and how to balance them together.

Just for now, let's look at the "double-waggle" system that I've sketched in posts #6 and #27. Assume for the sake of argument that I can build it so that it won't break. I will assume that there's no fatal flaw in the basic mechanism I've already shown in my sketches, since no one has mentioned any.

That leaves the flippers themselves. I need to know how to make a size, shape, material, etc., that will be reasonably effective. I take it as given that flexible is better than rigid for this approach.

These being vertically oriented, span would be measured vertically; chord horizontally.

For the moment, let's set aside the option of swim fins, and try to design a flipper from scratch. First question: shape. Is triangular (as sketched in post #29) better than rectangular? Explain briefly, please.

Curtis

Guest625101138

07-27-2008, 10:57 PM

Curtis
I conclude that you do not have a good understanding of the hydrodynamics involved otherwise you would not present the question.

You will need to have a good understanding of physics and maths and spend maybe a couple of weeks to learn the hydrodynamics involved to analyse the double waggle. My view is you are better off building and testing than spending all this time learning it. I have analysed the rigid oscillating foil because this is simpler. I do it more accurately than any of the university papers I have seen on it.

If you went to Hobbie and said I want to buy a flapper system, but I would like to check the engineering calculations first, that they COULD give them to you? It is not a matter of wouldn't, they couldn't because they do not have them. The flappers are the result of trial and error. As simple as they look they are incredibly difficult to analyse because you are dealing with a flexible membrane that distorts under load. Very complex to analyse. Your waggle system is more complex.

The basics of a foil is that the higher the aspect the more efficient it will be. There are diminishing returns after about 4:1.

The angle of attack for an efficient foil is only a few degrees so the length of stroke, speed of stroking, size of foil and hull drag all come into play in determining the best arrangement. The method I use with the parallelagram, rigid foil and torsional spring allows me to separately control the variables.

Flippers are not the ideal foil but then your waggle system is sub-optimal so why bother with anything more complicated and spend time trying to analyse it. Just have a go.

I built many scale models of flapping systems before I started building pedal powered versions. It was much later that I started to actually analyse what was going on. By then I knew what was important to work out. This is the nature of most engineering with a prototype. In fact many people still design and build boats without a good understanding of how they perform.

The other thing mentioned above is your ability to apply force. So you can go through the linkages and resolve the forces based on what you can actually apply assuming the flappers will oppose that force. What I do not like about the roll up flapper is that the flappers will impose a rolling force on the arms. I feel this will be awkward. Your huge foreams might cope with it better but I cannot evaluate this.

Rick W.

clmanges

07-28-2008, 12:26 AM

I conclude that you do not have a good understanding of the hydrodynamics involved otherwise you would not present the question.That's correct . . . okay, I should have understood the level of difficulty with flexible foils. I've had hints; I just didn't put them together. I'm at the steepest part of the learning curve on this; that's the place where I don't yet know enough to ask the right questions.
BTW, the Hobie flippers don't look a bit simple to me.
What I do not like about the roll up flapper is that the flappers will impose a rolling force on the arms. I feel this will be awkward.
The use of that technique was kind of an afterthought anyway, and I couldn't really see trying it except very rarely. Your pointing out that problem with it was useful to me in another way, however; being aware of that sort of torsional force, I now know that if I do use this system, I'll have to latch the frames down, or they might be trying to jump up every half-stroke. And, no, I'm not built like Popeye, so I imagine that could get pretty tiring.

A final question on swim fins, and then I'll move on. I seem to remember from high-school physics (that was 1968) that the average human body has about ten square feet of surface area. I'm guessing that my boat hull presents about three times that or so. Does that mean I'd need three pair of swim fins to push this boat? (At this point, I'm not sure whether I should expect a clear answer . . .)

This double-waggle thing is turning out to be a lot more complicated than I had thought.

On to the next thing: paddlewheels. What's the smallest diameter you'd recommend?

Thanks for your patience.

Curtis

Guest625101138

07-28-2008, 12:34 AM

........
BTW, the Hobie flippers don't look a bit simple to me.

.......
Curtis

I expect they will after you have engineered your own system. Even paddlewheels.

The boat drag for speed is likely to be substantially less than any ratio of wetted area for a human body. Wave drag is the killer for things that are short.

The peripheral speed of the wheels should be around 1.5 times the speed of the boat through the water. I expect you could work on 4 to 5kph as a target range. You should aim for about 1 rev per second.

Rick W.

Guest625101138

07-28-2008, 02:13 AM

Curtis
Without getting into a lot of detail on design, I would suggest a wheel on either side that is 900mm in diameter with 12 blades per wheel that are 200mm wide by 70mm deep. The blades will be just fully immersed with boat loaded and in normal trim.

I estimate efficiency of these to be similar to paddles but considerably easier to operate. The trick will be to design them light.

If they are operated unlinked then the shaft mounting will take a bit of design.

Rick

Guest625101138

07-28-2008, 04:18 AM

Curtis
The attached gives you something to play with for a paddle wheel. You can set the parameters in the yellow boxes in the "Input" worksheet. The hull is the one I drew up in an earlier post. I did not really check to see how closely it corresponds to your hull. It has a total displacement of 108kg.

You can see the tradeoffs for the paddle wheel. The diameter could be more a function of how high the shaft is placed. If you make the diameter bigger to lift the efficiency then remember the torque will go up because you have no gearing. This makes the required force higher so muscle tiredness is a possibility.

If you have typical level of fitness then 80W is sustainable. And maybe 300W from the arms in a serious effort. If you have a fair level of fitness then 120W might be sustainable for an hour or two. If you are typical then to get 2mph you will need blades wider than 300mm (12") on each wheel.

I have never had the opportunity to calibrate this model accurately but it fits with limited data I have from people playing with paddle wheels.

Rick W.