Interesting Technical Question

[Gludy]

All that matters is the end result of that fuel burn.
In the case given so far which was measured fuel consumption - the two engines running were 5% more efficient than using just one engine to achieve the same 7 knot speed.... so the measured results go against the point you are making.

Comparing running two engines at say 7 knots against one to produce the same 7 knot speed the two engine solution was more efficient. However had the prop been locked the single engine solution may have been more efficient....in fact ,I guess it would have been because there is a fair amount of inefficency in running both engines ... its balancing that against the extra drag of the free prop - its seems that this extra drag is so large as to be more significant than the cost of running both engines instead of one.

 

[Nauti Fox]

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So application of the simple rules of physics together with observation demonstrates the truth of what I am stating. It is also a known fact to those skilled ship designers within the industry.

[/ QUOTE ]

Gludy, I still wait to be convinced, as I said facts are needed so to say what you do still does'nt provide any facts.
To my way of thinking (and thats all it is) dragging static blades through the water would leave much larger eddies behind it than a blade that is allowed to rotate and therefore cause more drag.

 

[wakeup]

But the prop once spinning has momentum and is allowing water to pass it more easilly no???

I don't know if this applies to a prop in fluids but on an aeroplane (light air craft single engine) when the engin stalls in the air the prop keeps spinning becasue it has momentum and doesn't create as much drag and hence upset the aerodynmaics of the aircraft. In the case where the prop jams on a light aircraft it creates more drag and makes the aircraft more difficult to control.

I don't know if there are direct parallels between aerodynamics and fluidics but I would guess that a free spinning prop that once has gained momentum would create less drag even if it was also driving a gear box. This is not a statement of right and wrong just my observations from the light aircraft world.

 

[tcm]

um, well subsonic air and water fluid dynamics are not dissimilar. I agree that stopping the prop wd upset not only the pilot (erk!) but the dynamics of the plane. However, as far as i could see, the subject of aerodynmics in pilot training manuals is flimsy in the extreme.

Think about it - when stopped, the prop pushes the bit of air in front of it forwards, accelerating it to the same speed of the plane, or at least some of it.

But if the prop spins, it has to get hold of a new bit of air all the time, and try to accelerate that stationary air. Olympic freestyle swimmers can be seen to use this effect - instead of simply moving straight down through the stroke, they sway the arm from side to side to effectively grab more of the stationary water.

Anyway, i'm a bit bored with this now. It feels a discussion about going to a posh restaurant and finding the very cheapest meal possible. I reckon the way to save fuel on a big boat is not to go anywhere, ever.

 

[Gludy]

"But the prop once spinning has momentum and is allowing water to pass it more easilly no??? "

No .. this is the fallacy, the momentum is only coming from the power supplied by the other engine - a spinning prop produces more drag in exactly the same way as a spinning prop produces more thrust than a static one.

"I don't know if this applies to a prop in fluids but on an aeroplane (light air craft single engine) when the engine stalls in the air the prop keeps spinning because it has momentum"
I dispute that, i think it offers drag and spins because of the momentum of the aircraft. if you feathered it, then it would offer less drag.

"and doesn't create as much drag and hence upset the aerodynmaics of the aircraft. In the case where the prop jams on a light aircraft it creates more drag and makes the aircraft more difficult to control. "

I think you will find that a jammed prop makes the aircraft more difficult to control because you cannot keep it rotating by passing air through it swopping height for prop spin.


" I would guess that a free spinning prop that once has gained momentum would create less drag even if it was also driving a gear box"

Props have little momentum - if you are travelling in a boat and could stop the drive to the props dead freeing the props - they would only do a few turns before losing all nergy and stopping - the momentum in a prop is very little indeed. try stopping an outboard quickly going into neutral - the prop stops almost instantly. The very momentum you talk about gaining is a momentum that would have to come from the power of the other engine anyway - its not a positive force but a negative one acting against the direction of the boat.


What you are explaining so well are the common fallacies surrounding this subject - for that world to work the free spinning prop would have to have created energy against the basic laws of physics - all the energy that makes that prop spin is coming from the other engine which is having to drag that prop through the water as well as drive its own prop. The very spinning of a prop increases the drag in just the same way that an engine driving a prop makes the prop increase thrust.

 

[Gludy]

tcm - I know you fully appreciate the facts here - you know that my statements are true so why not stick your neck out a bit?

 

[DERF]

Gludy,

Your references and comparrisions to helicopter blades are nonsense.

A helicopter is a ROTARY WING and NOT a propellor. It is governed by areodynamics and not hydrodynamics.

You are trying to compare apples and pears.

 

[tcm]

tcm sticks his neck out

Rightyho! But I need help in how to copy and paste so i can go through every post line by line dealing with every tiny point, and also I am not sure if i have the stamina to stay with a single subject until everyone gives up the argument or dies of boredom

 

[Gludy]

I disagree.
"It is governed by areodynamics and not hydrodynamics. "
Both are very similiar - its the viscosity that is different. the physics are the same.

I would point out that what I am stating about the drag of a rotating prop is fact - I will be posting some maths etc later.

It is not me that came up with the facts about the drag of a roating prop, I amde the same mistake as others until I had it pointed out ant thought about it.

In effect you are making the free prop move a lot of water because of the very nature of its design - it is made to do a lot of work, work that is taking up power form the one engine. This work in moving all that water is far more than the simple drag of the prop when fixed and dragged through the water. When you really think about this issue you will see that it is common sense.

I am not nuts I have an B.Sc and and M.Sc. .... I am not inventing this stuff and as I have said my intial reaction was wrong because I too fell into the trap of thinking a free prop offered less drag. I did however quickly realise that a free prop dragged through the water to produce a certain speed of rotation must be doing the same work as is needed to drive a prop to that speed and that this work is a measure of the drag of the prop - whereas a fixed prop simply offers its frictional drag and is not being made to work as a prop.. hence the fixed prop offers far less resistance than a free prop because it is not acting as a prop.

 

[Gludy]

Re: tcm sticks his neck out

I can sell you a certain drink that will make you feel 20 years younger and keep you going. Also helps you to join in and persevere through these Gludy threads. I actually think you enjoy watching me alone in the ring ... are you a born again Roman?

 

[Gludy]

"It is governed by areodynamics and not hydrodynamics. "

I think I owe you a better anser on this point.

I agree that the rotor is based on the lift of a wing whereas a prop is based on a screw pricniple of pushing water - so in that sense they are different - however this does not inavalidate my example.

 

[tcm]

Re: tcm sticks his neck out

I think you're doing fine as you are! I hope me agreeing hasn't put you off your stride too much?

 

[Gludy]

Re: tcm sticks his neck out

Well yes it has really.... I can imagine you sitting as a sort of emperor watching the lonely gladiator battling alone - probably eating some juicy grapes as the blood flows freely over the arena ......

The alternative is to get in the ring and join in the cause but the entertainment value is just too much, so you sit there eating the grapes even though you are on my side in this, you simply cannot resist watching and waiting for the lions to arrive ….. ;-)

 

[andyball]

Sycamore seeds - nature to the rescue ?

Are praps a good example - they certainly appear to fall more slowly than a similar sized/weight leafy thing that doesn't rotate. So the spinning sycamore thing creates drag, else how could it fall so slowly?

So does a spinning prop being dragged through the water.

 

[tcm]

Re: Sycamore seeds - nature to the rescue ?

Maybe ! I was going to say that the spinning simply allows them to maintain the flat-down orientation, hence the drag - but then ordinary leaves floating fairly flat build up speed much more. So yep, the sycamore seed is constantly coming up against staionary air - it's spinning does not allow it to acceelerate as much if any air up to its own speed as would be the case with a leaf that fell straight down.

 

[Gludy]

Re: Sycamore seeds - nature to the rescue ?

Yes... you are so right!

You are asking a thing called a prop to do work be dragging it through the water forcing it to move large quantities of water - the only place the energy can come from is the one working engine and so the rotating prop offers a large amount of drag on the only working energy source.

The sycamore seed spins to increase its drag, take more time to get to earth and so hopefully be blown further way from the tree because of this increase in opportunity time.

Imagine a boat in a water tank - the boat has a single prop and the boat is attached to a gauge that measure how much force the boat is exerting in a forward or backward direction. Imagine passing running water past the boat from front to back so that is spun the prop - the gauge attached to the boat would be measuring a force that is acting with the direction of flow, that is making the boat go backwards - even though the prop is rotating the force being measured is backwards. now increase the speed of the water flow, the prop spins faster and the backwards force increases even though the prop is spinning faster. The drag on that prop must be acting backwards not forwards. Now taking the ship where they could not brake the free prop shaft as it required too much energy - all the energy in that shaft came from the water speed over the free prop - energy from the other engine - the force needed to drive that free prop to the same speed is the amount of drag that the free prop is providing on the boat and its drag - acting backwards against the boat.

I am running out of ways to explain this...

 

[wakeup]

Egggh, spinning props don't have much momentum????

have you experience the gyroscopic force created by a prop in a light aircraft, obvioulsy not!!!

Lets take a real simple example,

A paddle wheeled boat, if one paddle was jammed as opposed to free spinning would it go faster or slower and would it vear more towards the jammed wheel.

or an even more straight forward example would be in a rowing boat stick one oar solid in the water and row with the other oar pulling all force through the one oar then try moving the fixed oar in sympathy with the pulling oar without force and you will travel further with less drag, all be it in circles..

 

[Aardee]

Re: Sycamore seeds - nature to the rescue ?

Now that's a work of genius...even I can understand the logic.

The other thing to try is pulling a childs "beach windmill" through the air. Far easier with the "blades" locked than with the "blades" spinning. Don't know the technical reasons why, and frankly don't care...

 

[Gludy]

Sorry but your example totally miss the point.

First of all if you take power of a prop in water it very quickly stops ... you seem to dispute that!

If you power a plane prop and take power off 0 it very quicly stops .....

The amount of energy contained within a boat prop by virtue of its own mass rotating is very, very small and a prop does stop quickly ... very quickly when power is taken of it.

"have you experience the gyroscopic force created by a prop in a light aircraft, obvioulsy not!!!"

That is not momentum.... you are mixing things up.

Now taking your oar example - you a right a fixed oar will offer more resistance but that is not the point - because it is not a prop designed to do work when pulled through the water.

If you drag an old fashioned speed log behind you in the water - in effect a little prop - you could measure the drag by the tension on the rope. If you dragged it with the prop fixed it would offer less resistance that dragging it with the prop rotating - when the prop rotates you are forcing it to do work as a prop and that takes energy.

If a prop when spun did not do work and create thrust it would not propel your boat through the water - likewise when you spin a prop by passing water over it - a free prop - you create the same thrust for the same revs as if you had powered the prop but this thrust is acting backwards not forwards. its the very nature of the beast as a prop that is creating the extra drag - you are forcing it to rotate and do work - the energy for that work has to be taken from somewhere.

Try a kids thing (I do not know what they are called) a stick with a coloured prop on the end. if you use tape and freeze its prop its easier to move through the air fast than when you let is spin and try to move it through the air.

Can I just say that what I am stating is factual simple science here - its not my opinion. I am just expressing what is know to be true. So those opposing it are on a hiding to nothing

 

[spannerman]

Completely wrong!. a prop doesn't work on the same principle as a screw, it works on exactly the same principle as an airfoil, whether it be a wing or rotor blade, it advances through the water due to a difference in pressure between the front of the blade and the rear, if there was no differential there would be no thrust.Thats why you get prop burn on the front face of a blade caused by bubbles forming when there a lower than ambient pressure, as is the case under hard acceleration, i.e. a ski boat. The only time a prop doesn't produce any thrust is under deceleration. And when the vessel is being towed, or propelled by its other engine, when it then produces drag. As those in the aviation world will confirm thrust is always counteracted by drag! You overcome the drag by application of horse power. And when you have an engine failure in an aircraft , if you have a feathering prop you ALWAYS feather the prop to reduce drag and so stretch your glide ratio thereby buying you time to find somewhere to put down.