State of the Art of Jellyfish Design

226 days until the 10th International Submarine Race.

When I started doing research on how fish swim for Sand Shark and Faux Fish there were many articles, papers and thesis written on the subject. In addition, many models had been built to try and develop the fish swimming motion, some very simple and some very complex. Faux Fish falls somewhere in between as far as complexity goes but it was by far the largest of man-carrying fish submarines.

For Bogus Batoid I found only a fraction of the papers that there had been for fish swimming and a number of models that had been built to try and duplicate the swimming motions of the rays. What I found interesting though was that flapping wing motion has been the subject of much research and model building in the ornithopter field, flapping wing aircraft. Rays swim using one or both of the following motions, undulating and oscillating. Oscillating is flapping of their fins while undulating is generating a traveling wave down the length of their fins.  I built Bogus Batoid with an oscillating motion because it was not only simpler and less costly, but the mechanism for a man-powered drive for undulating motion of the fins looked to be very inefficient mechanically and expensive to build.

Both drives on these subs have a long way to go as far as optimizing the motion they produce. Both subs had about one hour of testing before departing for their respective races. I guess the fact that they worked as well as they did must say something. Maybe when I retire I will have time to work on them although the upcoming motorizing of them may show some major performance improvements.

What proves to be most challenging in building these swimming subs is trying to mimic some complex motions while being limited to mechanical drives powered by a single human operator. I just need to get some computer controlled linear induction motors and I bet I could get both to swim circles around the real fish.

On to Mock Medusa. I haven’t found as much research on jellyfish swimming as there is for rays but some of the papers are excellent. It’s really too bad that so many papers and articles are available only by subscription to certain journals or for purchase. The costs for these really add up fast. As far as working models go I have only found a few and they are very limited as to their capabilities. The one that shows up most on searches is the model built by Festo but it looks like the propulsion is created by flapping fins at the end of tentacles rather than by the motion of the jellyfish bell. With Mock Medusa I am going to try and get a more accurate representation of a the swimming motion by having a bell that opens and closes for propulsion. As I mentioned in a previous post, steering is still under review. It would be easiest to just put a big rudder on the sub but that would be just too simple. Let’s see what we end up with in June.

I still don’t have sketches to show yet though I have much of the design worked out. The big question right now is how much force it is going to take to open the bell. I have to make a segment of the bell and test it with different materials. The material has to be slightly stretched on the bell frame in the closed position in order to not have loose flaps of material as on an umbrella. When the bell opens all of the material needs to be stretched to the open position. I am currently evaluating casting the bell out of silicone or using wetsuit neoprene. Like Bogus Batoid, since I don’t know how much force is needed, I will need to build the drive mechanism with the capability of having a large drive ratio between the pedals and the operating mechanism. It’s funny because in a real jellyfish the muscles are used only to close the bell to provide propulsion while elastic forces in the body are used to reopen the bell, just the opposite of what I will be doing. I will also have to make provision to control the closing of the bell since there will be much tension in the skin. I don’t want the elastic forces in the bell overdriving the operators pedals. If necessary, I will use an overrunning clutch to prevent this. I’m not too concerned about the closing speed of the bell in the water, it’s trying to do testing on land that may be an issue.

I’m still hoping to get a 1/4 scale working model in the water before the lakes freeze up. I’m not sure whether I will use an electric motor or air cylinders to operate it. I have to see what I can scrounge up.

Next post should have the construction schedule and concept sketches.

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