Fishes and dolphins are powerful propellers

The real reason why marine mammals have horizontal tails or flukes and not vertical ones is their need to bee able to escape from the enemy attack. The need to come to the surface to breath put them in a critical situation. With a vertical tail they are unable to produce the fast escape fishes are well none to use when they are in danger. I have been able to simulate horizontal and vertical “Take of “ in my test benches. The vertical “Take of “ is loosing its efficiency do to the ventilating problem.

 

Thank you very much. Your explanation means the boat and ship designers must use a different strategy for creating travelling wave’s propulsion devices. I see for example the amplitude of fishes and dolphins body oscillations increases from the head to the tail. It is easy to realise in propulsion devices. Thank you again and HAPPY New Year!

 

Whales have forked tails, so do gold fish and many of the small varieties. None of them are speed record breakers. In fact just about every type of the fish on my fish market have forked tail.

 

Although I think there is something to what mrdancer claims, fact is whales have a mammal's spine. This means just like when I go in the water it is easier to do a so-called dolphin kick when keeping the legs together than a side to side body motion.

Perhaps a chiropacter could set this disscussion straight.

 

I think marine mammals (whales, dolphins, manatees, etc.) have horizontal tails because they evolved from land mammals. It is easy for a human to swim the same way too. But it is time to fill a glass. Happy New Year to everybody.

 

Vlad,
maybe you already know this:
Review of Fish Swiming Modes for Aquatic Locomotion
Happy New Year to everybody.

 

I met with something like this before but it is more interesting and detailed. Thanks and Happy New Year to you.

 

Have a look at this "Bionic Dolphin"
http://www.bionicdolphin.com/cms/

 

Thank you. I knew about "Bionic Dolphin" before. "Dolphin" with very powerful engine and propeller

 

It is perfectly valid to argue that cetaceans developed horizontal tails due to evolving from (possibly) terrestrial mammals, as much as they have for vertical movement in the water column. Current thinking in research circles just happens to lean toward the latter, simply because of the vast adaptability of living organisms. IOW, if a mammal can evolve terrestrial limbs into powerful fins, who's to say it cannot develop a vertical tail position if that would better serve its purpose? In fact, flatfish (flounder, soles, etc.) will mutate their bodies throughout growing into adulthood in an even more radical fashion (granted, they are a simpler organism to begin with).

In the end, I think it is safe to say that cetaceans have evolved the mode of locomotion that suits them best, and they were just fortunate to have their vertebrae start off in the correct plane!

As for the forked tail argument, again, the vast empirical evidence suggests that form follows function. There will always be exceptions, particularly in fishes that have evolved their tails for other pertinent duties, such as attracting mates or fanning nests, as well as for locomotion. If we limit the argument to purely locomotion, however, then the forked tail wins the design argument for most efficient propulsion at higher speeds (not necessarily the speed of the fish, but the speed of the water moving past the tail - this is an important difference!) and/or prolonged propulsion.

Happy New Year!

 

is it relevant

to this discussion, that some of the fastest underwater fishes (pelagics like tuna's for example) have "pockets" in their outside torpedo shaped body form that fins fold into to reduce resistance?

I mean - one might assume that extra fins would provide more propulsion hence speed - yet the biology suggests the opposite...putting fins out of the way of drag increases speed ...

Is the propulsion solution we seek (mimicking nature) telling us something about the importance of minimising drag over increasing propulsion?

Think about it!

 

Could you express your opinion about traveling wave propulsion and bring this idea into line with fish and dolphin locomotion?

 

Ambulocetus, the walking whale!
http://dsc.discovery.com/convergence/beasts/photo/photo2_zoom9.html
http://en.wikipedia.org/wiki/Ambulocetus

 

I think the idea is valid. I don't think we currently have the technology to implement it in a satisfactory fashion. Perhaps the final propulsor would be akin to a large long rubber fin that has a natural ability to oscillate in a sine wave, or maybe something of a more mechanical vein, say, a coil spring that has insulated runners on either side that can be manipulated electromagnetically (or another similar fashion) to induce a sine wave motion. I think the former is more applicable on a smaller scale, whereas something of the latter may be more applicable on larger vessels. But I digress...

I would lean toward using a vertical wave (side-to-side) as opposed to the up-and-down wave. My reasoning is a more stable vessel, particularly at speed, where the bow has no extraneous input on its entry into the water. As for side-to-side wave motion, effect on vessel can be minimized with twin adjacent drives, each canceling the other and possibly enhancing overall output (think frog kick). I think this would be much easier to design then an up-and-down system that produces no unwanted effects.

As far as forked vs. straight fins? That would be easily tested in the final working version(s) of such a propulsion system. It is too early in the game to worry about that, but something to consider when tweaking efficiency in a finished product.

In terms of fast-swimming fish, most have smaller, thin fins that serve as efficient locating (steering) mechanisms at high speeds. IOW, small thin fins can help steer the fish efficiently at high speeds, whereas large fat fins would be extreme overkill and unnecessarily increase drag. Imagine running across a lake in a powerboat at, say, 30 knots, then sticking a paddle in the water. It is nearly impossible to do with the paddle broadside, but much easier with the paddle placed on edge. Same concept applies to fins at high speed.

Someone mentioned recessing fins to enhance hydrodynamic efficiency. I don't think that is much of an issue for fast-swimming fish, as their fins are small enough and thin enough to not cause undue drag. The protrusions from the scales serve more to enhance their hydrodynamic slipperiness than anything else (notice that all fast-swimming fish have scales). The scale protrusions prevent snowballing vortices that would increase drag (as I mentioned in another post).

 

Since nobody is addressing your question I'll give it a shot.

Your wave propulsor is bounded on each side by the walls of a catamaran and the wave sheet is moved with a fixed amplitude. I think as a first approximation you might consider the cross sectional area corresponding to the peak-to-peak amplitude of the sheet and the inside beam of the catamaran. You have a certain mass flow through this cross sectional area, and you are accelerating it by the wave sheet. The same volume of water then exits the catamaran at a higher velocity.

The thrust is equal to the change in momentum of that fluid. If you put a screen loaded with sensors across the entrance and exit, you could calculate the thrust just by measuring the velocity and density of the fluid moving through those areas. This is simple momentum theory, and you can find it in any fluid dynamics textbook.

Momentum theory will tell you a lot about the potential efficiency of your propulsion system without having to know anything about the details of what is between the entrance and exit. It could be a wave sheet, or it could be a flapping hydrofoil, or it could be an array of propellers. For example, momentum theory will tell you that the higher the difference in the exit velocity compared to the freestream velocity, the lower the propulsive efficiency will be.

The next thing you might do is to estimate the skin friction of the fluid flowing along the two sides of your wave sheet. Basing it on the average velocity between entrance and exit might be a reasonable start. The power you have to put in will have to cover both the power predicted by momentum theory and the viscous losses due to the skin friction.

The next step would be to compare the power required as calculated above with the power required for some baseline propeller configuration. That will give you an initial idea as to how efficient your concept is compared to a conventional design.

Once you've done this basic engineering, I think you can come back with a basis for a more focused discussion on the concept's merits.