Fishes and dolphins are powerful propellers

Let us have a look at how propellers work. A propeller can be said to ‘push’ itself through the water. To understand this concept, let us consider a propeller, with one of its blades projecting out of the page (see Figure 1 below), and rotating (from bottom to top). So the propeller is moving from right to left. As this single blade rotates it forces (pushes) water down and back. At the same time, (because every force has a reaction) water will move in behind the blade to fill the space (low pressure) left by the upward moving blade. This results in a pressure differential between the two sides of the blade. This is also just how an aircraft wing works. The same action occurs on each propeller blade as the propeller shaft rotates. The pressure differential causes water to be drawn into the propeller from the front (due to the low pressure, behind) and accelerated out the aft (due to the higher pressure, ahead). As the propeller turns, water accelerates through and around the propeller creating the stream of higher-velocity water behind the propeller.

Figure 2 shows that the flapping wings work similarly. When propeller moves, the suction and pressure faces do not revise one's stand. The distinction between flapping wings and turning propeller consists in change of the suction and pressure faces position of the wing. During a down stroke the pressure face of the wing is below but during the up stroke it is on top of the wing. A fish swims by moving its head from side to side, which sends waves down its body. These waves increase in intensity with the help of body muscles and finally reach the tail, which then swings from side to side and propels the fish through the water. A dolphin moves its head up and down and sends waves down its body in a vertical plane (see Figure 3).

Swimming fishes and dolphins change the suction and pressure faces position on its body and tail all the time. The Forces and Reactions Diagram Figure 3 shows that every part of fish/dolphin body in every moment of the time takes part in pushing them through the water. That is why their stroke is so powerful.

A dolphin is a fast marine mammal, which is capable of great feats of speed and agility. The dolphin can travel at high speeds in many body positions such as on it side or upside-down for a long time without a break. It can start and stop instantly. It can make sharp turns easily and can jump to great heights from shallow water. The average speed of a dolphin is about 35 km/hour with a top speed of about 50 km/hour. Swimming dolphins do not injure anyone and do not make noise. Why not to create propulsion means using the dolphin’s locomotion? You can see on the picture there is nothing mysterious in dolphin swimming and I think a human will create many kinds of vessels using dolphin ‘know how’ in the nearest future.

The general view of the Catamaran is an example of a rational disposal of the propulsion device on this kind of boats. A flexible band M under the bottom of the Catamaran simulates dolphin locomotion. The mechanical movement of the flexible band moves water located in a volume between wave crests/trough in the opposite direction of the Catamaran movement.

 

You are correct that this "sine wave motion" used by marine creatures is the most powerful propulsion method. The trick is how to make a durable mechanical version for boats.

In my Manta submersible, mentioned in a different thread, a cast silicone wing with fiber reinforcement is activated by flexible carbon fibre rods, somewhat like a fishing rod. The rods are sequentially flexed to make a controllable sine wave motion for propulsion.

There are other practical concepts. A series of fins similar to divers swim fins could be individually flexed with a simple lever in an up and down motion. If this were accomplished with an electrical actuator such as a solenoid, different frequencies could be used and the whole thing could be computer controlled. There is a flapping hydrofoil wing mentioned in a thread in the boat propulsion forum that I wrote. This bicycle-like craft propels a person through the water at 16 knots using only human power.

 

There has been considerable work on this form of propulsion. Sir James Lighthill published several papers on the subject about 30 years ago. For more recent work Google around for "Robo Tuna" and "Robo Pike". I would like to see more work on a Robo-Seahorse which can hover as well as move in any direction.

One of the great difficulties in mimicking animal propulsion is that animals have compliant surfaces and excellent feedback systems which allow them to change shape "on-demand".

Have fun!
Leo.

 

See MIT's "penguin propulsion" projects, and, as always, "Hydrodynamics in Ship Design"

The common thread on most of this high efficiency propulsors is large loaded area.

 

This “sine wave motion” is not only the most powerful but the most efficient too. I think it is not so difficult to make a durable mechanical version for boats. One of my mechanical versions you can see here. It is a kinematics scheme of the pedal powered propulsion device.

 

Dolphins and fish are efficient with that kind of propulsion because they are completely submerged. They wouldn't be able to do it in the surface.

 

Looks like a promising technology for human powered water crafts.
The Chinese Yuloh (tail sculling oar) offers another example for how efficient sine wave motion can be.
I wonder whether in this context there's any advantage to such a tail "propeller" moving in a left-right motion "fish style" vs one moving up and down "dolphin style".

The human part of such propulsion systems contributes some feedback, which indeed helps a lot, as Leo mentioned.

Yoav

 

Yes, good point.

There has been some recent research involving paddle wheels with whale-like flukes, as well as flapping flukes at the stern. I'm not sure what regime the flapping fluke undergoes, but no doubt they used a sinusoidal regime for mathematical convenience. I'm also pretty sure the paper was in either Schiffstechnik or the Journal of Ship Research. If I find references I will post them here.

 

Somebody mentioned they are only efficient because the whole animal is submerged.. I disagree.

If you wanted such a drive system on a consumer craft, I would say a Catamaran with two vertically mounted "fins" operating in unison. Both flexing in opposite directions. This would illiminate the side-to-side motion of the whole vessel. I can also see there being potential squeezing the water out the back between the two fins.

Think about how your food pipe works when you ingest food. It might be possible to simulate this motion to create a water jet with no side to side motion, that would also illiminate the need to decide between horizontal or vertical.

Not working on physics here, just my hunches.

 

Am I wrong in assuming that all of the sine-wave designs are theory only with no working craft or models to back them up?

 

Surface vessels have added resistance and speed restrictions because of wave-forming. They are travelling through two different mediums.

 

My pictures show a direct analog for all dolphin body motion. Of course it works under the surface of the water. In order not to complicate the description, the dolphin’s head, body with flippers and tail flukes are represented on the Forces and Reactions diagram by one curved line. This line passes between the upper and lower jaws, through the center of the neck bones, backbone and tailstock. This line is a cross-section of a working model that is a wide band M made of a flexible material.

 

I am looking forward to see these references.

 

It is a good idea to create a water jet using means like a food pipe. You can try.