Human Powered Sinusoidal Propulsion

[lasasj]

Currently designing a four person catamaran for a local human powered raft race (5 miles). The raft will be powered by a vertical flexible sheet in the center that moves sinusoidally (much like a snake). I've been trying to find something similar to this to get a sense of how efficient it will be but have had no luck. Has anyone seen such a raft?


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[Guest625101138]

It will not be very efficient. It will take a lot of development even to get it to work.

If you have ever seen goldifsh with the large floppy tails you have some idea of performance potential compared with more efficient tails as seen on tuna or dolphins.

You can get quite efficient operation from a high aspect oscillating blade that operates in a similar motion to a dolphin. Theoretic efficiencies around 90% are reported but practical best is around 80% and this takes some design to achieve.

Oscillating foils can be made to operate quite simply and could be more efficient than rowing if set up for a particular speed. Attached is for a single person craft that operated efficiently at 8kph. You would need to double the size to get good results on 4 person craft.

Rick W

[lasasj]

Thanks for your insight Rick,

Do you think it would be better to use one foil or multiple foils with four riders? I could link them together and have them oscillate out of phase (or in phase). I am trying to come up with an innovative, efficient design that can beat a team of rowers that win every year.

An alternate design we came up with (attached) will probably face the same problems you stated before. The idea was to use four bikes to move five cylinders linked to a compressed flexible sheet (PVC or something). This particular design seems very heavy and would operate at a high frequency, so I am hesitant to pursue with building a prototype.

From the research I have done, it seems almost impossible to beat the rowers. They use a long homemade catamaran with long oars. Guidelines state the raft must be at least 4'x8' with at least 3 passengers. If I can't come up with a design that will win, there are awards for most artistic, titantic...etc. I have another idea that involves building a slow but cool looking amphibious army tank raft with a PVC water gun and foam track, but being an engineer, I would like to win the race.

Your insight and advice is greatly appreciated. I hope to hear from you again.

[Guest625101138]

If you want to beat rowers the best option will be a propeller. I did a 4-person pedal powered boat design that could be simply built from aluminium sheet but would not be real cheap. Making the drive frames and buying right angle drives would set you back a bit. You need something like USD2000 to give it a good tilt and you need basic shop facilities.

This would outperform a 4 person rowing scull. Four adult males could expect to hold about 16kph on a long course or 19kph over a kilometer or so if of average fitness level. Four Olympic class cyclists would easily outperform a rowing 8.

I have attached an image of the proposed hull and rough sketch of a cranking station. The four stations would be connected to a single side-mounted shaft and propeller. This is similar to the set up on most of my pedal powered boats. Zoom the image of the yellow boat to see the gearbox and 8mm curved prop shaft.

The best foil system is something like the flappers used on the Hobie mirage. Ideally you would only use one set with the four riders ganged. Each of the two flexible foil would be around 800mm long. The flapping foils overcome the nasty pitching or rolling forces you get from an aggressively stroked oscillating foil on a light craft. The Hobie foils get best efficiency around 50%. You might do better if they were carefully designed but they are complex to analyse. By contrast the prop design I did for the 4-cranks boat would get 89% efficiency. Good rowers and paddlers can achieve about 70% efficiency.

You might be able to get four Hobie flappers and place in a long hull. Their efficiency might be better with an easily driven hull. The efficiency calculations I have done are with the 14ft Adventure hull. I think the flappers are about USD800 a set - clearly 4 will be expensive unless you get sponsorship of course.

Rick W

[sigurd]

Mark Drela in the foiler design thread mentioned he had passed a rowing 8 in the decavitator, a 1 person foiler with air screw.

I have more faith in the flapping foils than the snake motion for such a craft. Maybe it is possible to use the propulsive foils as lifting foils, like the trampofoil? How fast are these rafts going, and how big are they (I assume 4' by much longer than 8').

Oh by the way. does it have to be a mono?

[sigurd]

Here, this should get you started. The 4' minimum is no hindrance for this craft, as the wavemaking drag is very low. Hope you beat the scullers.

[harlemriverman]

don't know how efficient a sinusoid performs but you might consider reducing the number of passengers if you're competing against kayaks.