I'm new here

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

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

 

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.

 

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.

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

 

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.

 

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

 

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.

 

Finally, a problem I can solve (some of) myself!

One question: what is "plate Cd"?

Thanks,
Curtis

 

Paddle wheels. NOW we're talkin'! Check this out: http://www.boatdesign.net/forums/showthread.php?p=181695#post181695

 

Thanks, Tinhorn. Lots of stuff to look at.
Curtis

 

Rick, is there something wrong with the spreadsheet? You have the wheel diameter as 0.9m, and it shows peripheral speed as 2.66m/s.

pi x 0.9 = 2.827

What caused the discrepancy?
Curtis

 

Cd is coefficient of drag.

No. There is allowance for the blade depth.

Rick

 

Curtis, Rick - I've been following this, and it's a great,fascinating thread. I'm really mowed out here at the moment, so have not found time to make contribution, but here's a little and interesting one, albeit slight O.T.

actually, there is one, though it is very tiny, used by e. coli, and other bacteria.

bacterial motor driven screw propellers.

Curtis - good luck and good energy with the project.

 

Hi, Turnip, welcome to the fun. Seems I've actually overestimated the capabilities of science to provide quick answers to everything . . .

. . . now, how did I not notice that before?

Anyway, here's something I found on our microscopic screw-propeller friends (powered by sub-atomic particles, yet!) quite recently, while I was trying to get some better comprehension of Reynolds numbers. Seems they really don't have many options for getting around . . . it's a delightful article.

http://brodylab.eng.uci.edu/~jpbrody/reynolds/lowpurcell.html

Stay dry,
Curtis

 

Someone else described my situation quite well:

"In the beginner's mind there are many possibilities. In the expert's mind there are few." - Shunryu Suzuki