kjell said:
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.
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.
