Sine wave propulsion

Oriental Scull was studied by 4 Japanese in a paper "Hydrodynamic Analysis of the Sweeping of a RO-an Oriental Scull" published in the Journal of Ship Research, Vol.33, No.1, pp.47-62, 1989. The authors are Azuma, Furuta, Iuchi and Watanabe.
They even measured a real boat's Ro. Its max efficiency is 0.43. The thrust is not continuous. During the reverse action of a blade (a short time period), the thrust is even negative because the blade's angle of attack is negative.

Beside, a Voith-SchneiderPropeller (VSP, vertical propeller) operating at high speed also make its blade traveling on a sine wave trajectory with varying attack angle. So, in a way of viewing, VSP is like a set of sculls operating together to provide continuous thrusst plus good maneuvering capability.

 

[QUOTE Why I say I'd stick with a propeller? Because propeller turns 360 degrees and is in uninterrupted movement, constantly providing the power. Kjell's experiments CLEARLY show that every flap has two zero moment positions where there's no work done. In a system of two flaps, there are four moments like that. Waste of time and energy because to move a flap into the opposite direction you must push it from zero. That requires a lot of energy. Not to mention braking energy loss from reaching the top of sinusoidal curve..[/QUOTE]

The limit of propeller efficiency is when the cavitations start.

The tail propulsion don’t have problem with cavitations. In the moment cavitations start on the suction side of the tail, do to the oscillating, this side is converted to pressure side and the cavitations stops. This gives the tail the opportunity to work with more efficiency than a propeller

 

I agree with Kjell that cavitation should not trouble flaps, particularly not in a jet-type configuration. However I was refering to the power loss resulting from the movement, not from cavitation.

As I said, see quote, flaps have two dead/zero points where they come to a total halt and continue moving in opposite direction. Flaps draw water behind. To sustain the force of the water that keeps pushing them forward when they reach such a zero point and fight their way through that mass in opposite direction requires a lot of power. Twice as much. And then they face the same problem in the second zero point on the other side.

That is what I was talking about. And loss of power is the true measure of efficiency. Propeller is a lot more efficient because it does not have zero points. It only starts losing efficiency when cavitation sets in and separates it from the water. And we know that that happens at high speeds only. So for all practical purposes, on speeds below cavitation speeds, propeller works at its' optimum (almost, but that is negligible in comparison with zero points of the flaps).

 

Just to add to the above: because of that loss of energy in zero points, fish bend their bodies and tails to minimise the impact of water trailing behind their tails. That is fine for fish, but applying the same solution to flap propeller would result in a very high level of movement in a vessel. Movements result in sea/air/car sickness as is, and being in a boat that bends left and right fast and with lots of force...I am not stopping you, but you might not have many passengers on your ferry.

 

Propeller propulsion efficiency has reached its limit and the efficiency of tail propulsion is still to discover: I am agreeing that an outside flap propeller is not the solution. That is the reason why I have started to make tests with what I call Tail-Jet Propulsion. It is some thing new and surprising.
http://www.dahlberg-sa.com/kd/WJet.htm

 

Any movement type creates an equal and opposite reaction. Even a propellor has rotary forces with the opposite reaction. In a sine wave propellor, any reactive motion can be cancelled by means of two devices operating in opposite phase, just as two props in opposite rotation would effectively cancel their induced reactive motions. This general principal is best demonstrated in a helicopter with counter rotating blades which allows for the elimination of a tail prop.

 

chenjh, sharp observation. Do you have the document handy to share with us (a link to a page, or a copy of it somewhere? JC will enjoy me asking for a link...)

Kjell, I do agree with you. In fact that is the whole point that some are missing. Reading my earlier posts, I see how I may have been seen as aggressive against it, but I did try to explain that i was not against flaps per se. I tried to point out difficulties earlier serious researchers identified, and point out these to those who enthusiastically jumped on a bandwagon without giving it some analytical thought beforehand. Voight propeller is in some respects better than standard propeller but is not replacing it anyway. Will be for a reason.

And it does work along the flap-propulsion idea lines as chenjh rightly points out.

 

I disagree. One can tell pretty well what the propulsive efficiency of a tail-jet is just from the geometry. I suggest you look at simple momentum theory. Here are some good sites:
http://www.grc.nasa.gov/WWW/K-12/airplane/propanl.html
http://www.mh-aerotools.de/airfoils/propuls4.htm

Although the web sites are written from the standpoint of propellers, momentum theory makes no assumptions as to what it is that is accelerating the flow. It could be a prop. It could be a flipper. It doesn't matter.

For the case of your tail jet, it's enough to know you have a tube with water coming in the front and going out the back. The simple fact is that thrust is equal to the change in momentum of the flow, which is the difference between mass times velocity in and mass times velocity out. Since the mass flow in has the be the same as the mass flow out, the thrust is also equal to the mass flow times the change in velocity.

For the same thrust, you can impart a large change in velocity to a small volume of water - the narrow diameter jet. Or you can impart a small change in velocity to a large volume of water, which is what a large span flipper does.

But energy is half the mass flow times velocity squared. The energy you have to expend is the difference between the energy of the flow coming in and the energy of the flow going out. For the case of the narrow diameter jet, the difference in velocity squared is high. For the same change in momentum - thrust - the flow leaves with a higher energy due to its higher speed. That energy has to come from the propulsion system, and so losses are high with a small diameter jet.

The most effiicient propulsive device will be one in which the change in velocity imparted to the fluid is small, so the fluid leaving the device is only going a little faster than it was when it came in. The trouble is, if you go a little faster, then the diameter is oversized and you incur drag. And even with a near-ideal sizing of the propulsor there are other losses. But momentum theory sets fundamental limits on the possible efficiency of any propulsive device even if what's moving the water has zero losses in its own right.

So a jet drive will always, always, always always have poor propulsive efficiency no matter what is inside driving the water. It doesn't matter whether it's a prop, or a flipper, or steam, or an undulating sine wave, or antigravity repulsion. There's nothing new to discover here. No surprises waiting to happen. It's a matter of conservation of mass, conservation of momentum, and conservation of energy.

It's not even true that propellers have reached the limit of their efficiency. Propellers are the size they are because if they are made bigger, the tips move faster and encounter problems like cavitation. But our growing ability to predict these effects means we can create designs that avoid them. That allows one to increase the diameter and improve propulsive efficiency.

The only way flipper propulsion is going to improve on the effiency of a propeller is by moving a larger volume of water at a lower speed. The external flipper can do this if the whole flipper moves, instead of being pivoted at the center. A propeller moves at low velocity at the center and high velocity at the tip. It's the high velocity at the tip that's the limiting factor.

So if the whole flipper translates, it's as though it had the average velocity of the prop, but not the same maximum velocity. So the flipper can be made larger without exceeding the speed limit set by cavitation. And larger is more efficient. Provided that the extra wetted area and weight/complexity of the mechanism doesn't get you first.

 

Tom Speer. Your mesage is (as always) an enjoyment to read. Clear well thought physics.
Both convincingly simple and eyeopening at the same time. Thanks

Anders

 

Hi Tom,
The theory is very good but what happen to the propeller efficiency when it receive turbulent water? Than the theory is gone. Flippers don’t have that problem. The pitch on a propeller is calculated on the incoming water speed and if the propeller is receiving a different water speed the propeller can’t give the output it was calculate for. I have over 15 years of experience with propeller calculation and resolving propulsion problems.

 

To Kjell:
Why isn´t flippers suseptible of turbulence?
How do the fippers generate their driving force?

I´m just an interrested bystander that want to learn more about physics and hydrodynamics

 

yeah...tspeer, tread carefully. You are killing the speculative spirit of the forum. If you upset me mate JC, you'll be sculling south of the border on your own log. Mine is to small.

 

To jam007
I am glad that you ask this question. There are very little information how Flipper or Tails generates their driving force. The Tail is a fluid accelerator and works in a different way than foils. They are not working with the lift/ drag principle. The tail produces a fast pressure drop on its lee side and the surrounding water try to fill this vacuum producing acceleration. The mass of water has free way to escape without any obstruction producing a jet effect leaving vortices behind the tail. The drive force depends on the mass of water and its velocity.

 

Sorry this was the picture of the tail accelerator

 

Thanks. Kjell

Maybe I missunderstands but would not the fin create a high pressure on the opposite side as the low of equal strength?