wiggle drive propulsion application

That diagram is correct for an entire airplane.

Lift is also used in the context of an airfoil and the standard definition is given in my previous post.

So I'm not arguing that fin system produces "lift" in the context of a entire airplane. But lift is also used by engineers to describe the forces on airfoils whether they be aircraft wings, propeller blades, turbine blades or the blade(s) of an oscillating fin system.

 

I agree with you, not just on this point, either.

What concerns me in this thread is that some have made statements that are patently incorrect, presumably to try and show that their preferred system is "better" than another.

If we could see some true efficiency data, in terms of mechanical power input for propulsive power output then it would help sift the chaff from the wheat here.

In my case I did some still water, no wind, tow testing on my boat so I know pretty well how much power is needed to run at a modest range of speeds. I could then compare propeller and drive system performance by measuring the input power needed to make the boat go at a particular speed and by dividing this into the know propulsive power needed I could derive a pretty good efficiency figure. Similarly I measured the parasitic losses in the drive system by removing the propeller and measuring the power over a range of rpm. Subtracting the parasitic losses from the overall power enabled an accurate determination of the propeller efficiency (which was closer to 87% than 86%, as it happens).

If one of the proponents of fin systems could do the same then we would be able to get a better feel for the true efficiency of this propulsion method. My gut feeling, supported by some of the research I've read over the last few years on the subject, is that flapping fins can be pretty efficient, probably around 85% or so. What concerns me more is that the drive system required may well have higher parasitic losses both from the need for a fairly high reduction ratio and the inevitable frictional losses in the mechanism to convert rotary to oscillating motion. If those losses can be reduced to an acceptable level then I can't see any reason why a fin system shouldn't be as effective as a propeller.

 

I have been working with propeller propulsion for many years. I know that the propeller is one of the simplest ways of marine propulsion but it has reached its limit. The propeller is made, based on HP, RPM and for one boat speed. If you alter any of these components the propeller size is wrong.

The reson why I am working with tail propulsion is. The Tail propulsion is new and its limite is not known. The natur has demostrated how good and economical propulsion system it is.

 

Exactly the same hydrodynamic limits will apply to oscillating fin type drives though, as they are using the same operating principle as the blades of a propeller, especially the pivoted fin arrangements where the fin tip velocity is greater than the root velocity.

By way of illustrating just how a propeller can be correctly designed to operate efficiently over a fairly wide speed range, I've just completed some work this morning on a propeller that I will be using in a cordless power tool powered boat race this weekend. The boat hull resistance varies from about 20 N at 2 m/S to about 45 N at 3 m/S. The propeller efficiency varies by 0.3% from 84.4% at 2 m/S and 412 rpm to 84.7% at 3 m/S and 617 rpm, so is fairly constant with a 50% increase in boat speed.

I believe there is promise in the type of fin arrangement where the whole fin moves at a near constant velocity across the whole fin effective area, like the Tailfin and similar arrangements that have the fin, or fins, mounted on a lever/linkage that is out of the water and so free from causing additional parasitic drag. This arrangement offers the opportunity to use a constant and well-matched section across the whole fin effective area, rather than have to compromise with thicker sections near the root in order to meet the structural requirements.

 

Sorry to say. You don’t understand how the fin is working. The fin is not part of a propeller. The propeller blade moves at constant angular speed and constant angle of incident. The fin moves with constant angular speed but the angle of incident is changing. In the moment of changing direction, something is happening that never can happening with the propeller. Stall delay and wake capture.

 

Ok, i will try once more to explain the detail of the fin path of one that is manipulated by a "swashplate"
It does not follow the path that you would think, there is no need for any progression of pitch, or as you say, a difference in water velocity over the root to tip of the fin, a straight fin is almost perfect. Its almost the perfect path !
I know this is extremely hard for anyone to interpret as truthful without physically seeing it, but surely if it was the case as you have stated then there would be all sorts of aeration issues, complete lack of thrust and huge power consumption....but thats not the case, its the complete opposite !
Maybe you could take another look http://www.youtube.com/watch?v=DtD09MTUsnE

Heres my example, this exact same hull fitted with the same motor, same battery, same data logger but fitted with a fairly efficient prop, or even better a jet in a planing condition is drawing in excess of no less than 500watts of input electric power. The exact same setup with my oscillating fin devise can be doing the same thing using a little less of input electric power, surely this must provide a small suggestion something special is infact occurring within this devise.
Again, its not the fin that is doing anything special but rather the devise controlling it !
regards
Craig
Swashdrive

 

Sorry, but I do understand well.

A propeller is a moving foil in an incompressible fluid, just as a moving fin is. They are both using exactly the same fundamental method of turning mechanical power into propulsive power, although there are clearly some differences, such as the differing velocity distributions across the working surfaces.

The varying velocity distribution across a propeller blade means that the effective AoA (the vector sum of the rotational velocity at each station on the blade and the inflow velocity) changes across the blade, as does the Re. Both are accounted for by varying pitch, foil section, chord and thickness across the length of the blade to ensure best efficiency.

If a moving fin is designed to operate with the whole fin moving at the same velocity at any instant, then the analysis becomes somewhat simpler, in that there need be no AoA variation across the length of the fin. The downside is that the fin clearly needs to accelerate to speed, decelerate to a stop and then accelerate again in the opposite direction, so ideally would need a variable foil section and chord with stroke position in order to maximise efficiency. Clearly this isn't practical to implement, so I suspect in reality it's only AoA that is changed as the fin velocity changes.

If a moving fin is designed to work in the manner of the Hobie Mirage fins, then the analysis is very like that for a propeller, albeit an oscillating propeller rather than a rotating one.

 

Looking at your video, the tips of the fins seem to be moving over an arc of greater radius than the root of the fins. Is this the case? If it is, then clearly there will be a varying AoA across the length of each fin. If the tips of the fins move through the same distance as the root then what you say holds true. It isn't clear from the video that this is actually the case though, as the tips certainly seem to be transcribing a larger radius arc than the root.

What is the hull propulsive power requirement at the test speed? Have you measured it? It would be nice to see the efficiency in terms of propulsive power divided by that 500 W input power.

500 W sounds like a high power input for the size and speed of that model to me. It's actually about 4 to 5 times more power than my 17ft two seat riverboat takes to cruise at about 4 kts.

 

If you understand how the fin works, than you can explain why the water is accelerated on the lee and not on front side

 

http://en.wikipedia.org/wiki/Lift_(force)

 

What is Lift?
Lift is the most powerful, most secure web framework available today. There are Seven Things that distinguish Lift from other web frameworks.

 

When the supposed experts who've invented these "new" fin propulsion systems cannot even grasp the basic nomenclature used to describe the forces acting on moving foils in a fluid, and they insist on ridiculing those who simply want to point them towards a better understanding as to how what they seek to do can be better understood and modelled, then it's time to duck out and leave the debate.

 

Lift is what Wonderbra does and that's good enough for me.

 

The easiest way to not answer difficult questions is to be upset.

 

Several of us have been trying to politely explain standard terminology. What's the difficult question which isn't being answered?

The picture you attached shows the force on a propeller blade slice resolved into components labled "Thrust" and "Propeller Torque Force". The latter is really the component in opposition to the propeller torque. You will also find pictures with the forces on a propeller blade slice resolved into components for a different set of axis which are "lift" and "drag" of the blade slice. Both diagrams are valid.

http://www.boatdesign.net/forums/attachments/propulsion/71270d1339087589-wiggle-drive-propulsion-application-propforce.jpg