Fishes and dolphins are powerful propellers

I don’t meet with information about working craft but I have made and tested a model myself. This idea is pretty old. Insects, snakes, fishes and others sea creatures use it for millions years but a human being doesn’t yet.

 

Could you make clear your question please? What do you mean saying “two different mediums”?

 

Wow.......this could be a first, I'm impressed.

Did it just flounder about, perhaps just go in circles randomly, or did it work anything close to what you wanted? I don't mean to insult you, just hard to accept that anyone would be so lucky as to get it right the first time. That is with my luck.

I think what Gonzo meant was water and air .

 

Why is it that some people seem to take such joy in belittling other people's ideas when it is clear they have not lifted a finger to do any investigation of the matter that they are so off-handedly belittling? Come on guys, let's keep this on a high tone and encourage creative thinking.

The issue of sine wave propulsion is documented, instrumented and proven in more than one venue.
This is probably the most well known of these efforts done at MIT.
http://web.mit.edu/towtank/www/Tuna/tuna.html

Also Robopike done at MIT:
http://robosapiens.mit.edu/wanda.htm
http://web.mit.edu/towtank/www/Pike/pike.html

Some pictures of the mechanisms are attached. I personally think the MIT approach a bit over-engineered and cumbersome, but they were also trying to directly model the living mechanism. I think there is much to be said about engineering simplified versions of this form of hydrodynamic propulsion. Just as the Wright brothers did not try to exactly emulate a bird, the elements of sine wave propulsion can be utilized without exactly emulating a fish.

 

JonathanCole, I hope you don't think I was "belittling other people's ideas". I stand by my statement that there is no amount of luck, and not with my luck that a first effort would be an instant success. I say this based on having tried things just outside of my ability before and sympathize.

The impressive work of MIT grad students going back to 1996 press releases indicates some progress has been made. It's unclear to me if the first link you gave that the mechanical fish did anything more than get pulled by a cable down a tow tank. I'll have to read more on it to say for sure, but so far the feeling that we are "not there yet" stands in my mind.

I would expect anyone's first effort to require tweaking of some sort. Anywhere from adjusting cables to modifying servo circuits to get it right and not have it flounder about or worse go backwards.

If anyone tells me it's easy and they hit it out of the ball park the first time, then they make the likes of Thomas Edison look like a bumbling clown, and my poor self far worse. It's only through failure that we learn what does not work, and that's almost as important as what eventually does work.

http://robosapiens.mit.edu/wanda.htm

Wish they said more about how well it did work and their trials and tribulations.

 

I think the technology may work with a SWATH type vessel. The propulsion pods woud be completely submerged.

 

Kach22i,

No one made any claims for getting it right the first time, but clearly, the principle is easily demonstrated. It is a well known swimming technique for racers.
http://www.quickgetaway.com/technique/9stages/stage09.htm

And there are numerous interesting studies, including:
http://releeps.tripod.com/cgi-bin/
http://www.ece.eps.hw.ac.uk/Research/oceans/publications/Michael_Sfakiotakis/ISAB_2000_abs.html
[font=arial,sans-serif][size=-1] http://engineering.physics.ubc.ca/~roboray/files/Proposal.doc
http://www.ebel-k.de/Loco1/loco2/loco2.html

[/size][/font]

 

The idea of using a Dolphin type fin for propulsion is certainly not new and indeed it was patented back in the 1980's.
I saw the device being demonstrated on the now defunk British TV show "Tomorrows World", a popular show back in the 1970 and 80s.
It consisted of a swept back horizontal fin fixed at the center of its leading edge to a streamlined vertical pillar via a pivot pin and it was lightly sprung loaded to keep the fin horizontal.
In "fixed" mode a craft that had the pillar and fin fixed to the keel would be driven forwards by the rising and falling motion imparted by waves and ocean swell, even against the tide or prevailing wind!
The inventor demostrated it by fixing his device by fixing it to the bottom of a small sailing yaught, that had had its mast removed, and then sailed it out to sea under passive fin power alone!
There was a second mode though: Driven mode, in which the pillar could be driven up and down by an engine or the sliding pivot point could be driven up and down on the back of a fixed pillar.
On the upstroke the trailing edge of the fin moves downward and on the downstroke the trailing edge of the fin moves upwards, thus imparting a smooth continuous thrust.
The latter design (The fixed pillar with sliding pivot point) is mechanically much simpler to construct.
Compared to a propeller, driven fin propulsion is much more efficient and can each well over 90% efficiency!
The one problem with the design that I can forsee is that it would be difficult to reverse the thrust of the fin mechanism without the fin flapping about uncontrollably.
In other words, to reverse the thrust you could simply rotate the pillar around its axis but this means the sweopt back fin would suddenly become a foward swept fin and against the flow of water caused by the foward motion of the hull the fin would be subject to lots of hydrodynamic stress....So its perhaps best used for unidirectional thrust applications.

Regards

Aquatek.

 

The claim that fin propulsion or flapping-wing propulsion is inherently more efficient is largely bogus. A propeller without diameter or rpm constraints can exceed 90% efficiency without too much difficulty. And it's also vastly simpler mechanically.

One can think of tail fin as an oscillating propeller blade. Viewed from the front it sweeps a rectangular area equal to the height of the fin times the width of its stroke. This area is roughly equivalent to the disk area of an equivalent propeller --- in both cases the fluid passing through the rectangle of the fin, or the round disk of the prop, will get imparted with axial momentum, at a rate equal to the thrust. The axial momentum can be considered as the induced velocity of the vortices shed by the fin or propeller blades. For any type of such a mechanism, there is an absolute upper limit to the efficiency (i.e. the Froude efficiency) which the propulsive device can achieve. And this limit depends only on the area, not on what the propulsive device looks like.

In practice, a real device will always have an efficiency less that the Froude efficiency because of additional losses, due to 1) intermittent axial flow, 2) swirl flow, and 3) viscous drag. It's easier to minimize these additional losses with a propeller than with an oscillating fin, which is why a propeller is likely to be more efficient than a fin of similar size.

 

As far as I know the best propeller designs to date have a maximum efficieny of only 70 %....Add a Kort nozzle and you can claw back up to 17 % more effiency but thats still less than 90%, and still less efficient than the fin propulsion device in question.

Regards

Aquatek

 

Human powered aircraft propellers have efficiencies of around 90%. I suspect that submarine propellers, which don't appear to have a significant diameter constraint judging by their low solidity, are probably close to 90% as well. Most surface boat propellers have very strong diameter constraints, which reduces their Froude efficiency and thus limits their max achievable efficiency to well below 90%.

My main point was that a fin cannot be more efficient than a propeller of similar size, and the fin is probably worse if you throw in the other losses and practical implementation considerations.

 

The Compliant Wall

For anyone interested in a related topic you might want to look here:

http://aerodyn.org/Drag/compliant.html
The Compliant Wall
Quote:
The complaint wall is a relatively recent idea to reduce the drag by using flexible coatings on the aero- hydrodynamic surfaces. The idea of flexible skins comes from observations on swimming dolphins, and more generally on animal propulsion.

The compliant wall must interact with the boundary layer and influence its development, laminar or turbulent. If this has to be true, the wavelength of the skin flexibility must be of the same order of magnitude of the boundary layer thickness, while the amplitude must be of the same order as the viscous sublayer.

The idea is that, when the surface is affected by fluid flow around it, it will start an interaction surface-boundary layer such that the boundary layer remains attached for a longer length and the drag of the system will be reduced.

The problem, however, is not so easy. The properties of the compliant surface must be well understood, else the effect is ... the opposite (that is a compliant surface that triggers boundary layer separation).

Some energy considerations are necessary:

*

In the case of a rigid surface, the drag produced serves to dissipate propulsion power into the fluid (by means of of viscosity, radiation, etc.)

*

In the case of a passive compliant surface part of this energy goes into the surface itself and is dissipated through internal damping.

*

With an active compliant surface power would be required to activate the surface-boundary layer interaction (for example, heating of the coating to activate its compliant properties).

Therefore, the fundamental question is whether the net energy balance is positive or negative.

End Quote.

Riblets are cool too, don't think they work in water for some reason.
http://aerodyn.org/Drag/riblets.html

 

Cool.....thanks for the links.

 

I am speechless with surprise. Just thank you for answering for Gonzo.

 

I see you can talk over my investigations without me.