Interesting Technical Question

Re: Sycamore seeds - nature to the rescue ?

By coincidence the childs windmill was the point I just came up with as you did.

You are totally correct and the reason is that you are forcing the windmill to work hence this requires more energy = more push.

We are talking about simple science here ... thats all.
 
You are 100% correct in all you have stated.
What I was meaning in responding to the point about the airfoil is that the shape of the roto on a chopper is an airfoil shape whereas I do not think a prop is. However, I stand corrected in my response on this which makes my chopper example even more applicable.

"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. "

Your examples completely support all I have been saying - if you could feather the prop on a boat then you would also do it to reduce drag. Leaving the prop on the plane unfeathered would as you say increase drag - just as it does with the unfeathered boats prop.
 
Your examples completely support all I have been saying - if you could feather the prop on a boat then you would also do it to reduce drag. Leaving the prop on the plane unfeathered would as you say increase drag - just as it does with the unfeathered boats prop.

[/ QUOTE ]

Totally different Gludy, I thought the difference was wether it was rotating through drag or locked stationary and with the same blade angle, and dont forget in the case of aircraft the dragging prop will also be trying to turn the engine as well.
So I dont think it supports your case at all.
Not saying I'm right as like yourself I have no FACTS to prove or disprove what I think.
 
whilst I take onboard all you are saying Gludy, as I said at the very beginning - I still think its down to the boat.

I don't know if you read the T&T list (but I think your earlier figures were from there), but there are folks on there that have tried this with various boats and the answers are always inconclusive. For instance, Bob Lowe has posted his results, which contradict your theory. I see no reason to disbelieve this guys results. Thread starts here.

If you search through the T&T archives, you'll find lots about it - all of it contradictory /forums/images/graemlins/wink.gif

Rick
 
Unit18
I am making a simple claim.

I am claiming that this si simple science that cen be measured and proved as well as theoretically derived. I am also claiming that the actual evidence on this thread supports the point - evidence such as that measured with the 42 footer and that pointed out in the braking of the ships shaft not being possible.

The claim is that a rigid prop when left to turn on a twin screw boat without power will generate more drag than when it is prevented from turning.

If this was not the case props would not work. It is actually common sense, although I like others fell for thinking it was the opposite at first glance.

Where do you think the energy is coming from to turm the prop and have it move all that water that it is forced to move? Can you not see that if it could not move it would have less work to do?

I would also point out tha a dragging prop on a boat will also try to turn the hydraulic gearbox although this is not being relied upon for the truth of what I am claiming.

I will be boring you all with the maths etc later.
 
Thanks for the links and the contribution which was contructive.

The purpose of this thread for me was to find out if the extra drag from the rotating prop made it worthwhile to spend the money on a brake to stop it turning. The thread so far has not got past some simple science.

There is no doubt whatsoever that like for like a fixed prop offers less drag than a frewheeling prop - that can be measured and proved and is common sense when really thought about.

Yet the purpose of the thread is more than that - its to find out to if in general there is a benefit to fixing the one prop. Sure there would be less drag but would that lower drag reduce drag such that there was an overall efficiency gain?
That depends on the engines and how they perform throughout their range etc.

I shall read the thread links you posted and get back onto it with any comments.
 
Paul - thanks for the great read.

I have rarely seen such simple issues weaved into a tapestry of confusion with such consumate skill - awe inspiring.

I will pick 2 rather obvious, but extremely simple, observations from the evidence given so far.......
(1) you compare the helicopters 'free fall' with the drag on a free propellor yet they are spinning in opposite directions under the forces driving them! How is this?
(2) you keep mentioning retaining the second engine at tickover to reduce drag but as anyone familar with small OB engined craft knows you take the throttle back to tickover to slow quicker than taking it back to neutral as the slow driven prop has more effective drag than the freewheeling one created by going into neutral. So if a slow driven prop is more drag than a freewheeling one, regardless of the actual drag of a freewheeling one, it is extremely likely not to be a better solution!

Keep it up!
 
A guy at the University of B. Columbia researched this, with regard to using a small prop. dragged to produce electricity.

Sadly the online report is no longer available, but refernces to it suggest that he measured drag when stopped vs freewheeling as a part of the research & found of course ,that the freewheeling one had more., but poss. not enough to make a huge difference.

http://steamradio.com/pipermail/multihulls/2003-February/016544.html
 
Duncan
its amazing how you can so misread the facts ... awe inspiring in fact.

(1) you compare the helicopters 'free fall' with the drag on a free prop yet they are spinning in opposite directions under the forces driving them! How is this?

Both are spinning in their designed directions and functioning as they should.

The helicopter is falling and in so doing swopping it potential energy for the dynamic energy in the rotors so as to slow down its fall.

The prop on the boat is also turning in its designed direction and in so doing is everting a drag on the boat. The force driving the prop is the water flowing past it - it direction is in the opposite direction - exactly the same as the chopper.

Think about what forces are driving what and it will make sense.

"you keep mentioning retaining the second engine at tickover to reduce drag"

Not really, I have mentioned it - in fact you would have to speed the prop up to the same speed that it would be when free wheeling in order to counter the drag - that may be well above tickover.

"anyone familiar with small OB engined craft knows you take the throttle back to tickover to slow quicker than taking it back to neutral as the slow driven prop has more effective drag than the freewheeling one created by going into neutral."

This is an interesting point - comparing a slowly powered prop to a free wheeling neutral one. I frankly do not know the answer - I have a lake full of such craft so I shall test your point and come back to you.

"So if a slow driven prop is more drag than a freewheeling one, regardless of the actual drag of a freewheeling one, it is extremely likely not to be a better solution!"

It may not be a solution - I was stating that the drag would be lowered if you could take the speed of the prop up to the speed that it would freewheel so that would apply the same power as the drag and hence neutralise it.

I am almost beginning to enjoy the way some on this thread simply want to ignore the physics of this situation.

Why is it that a childs windmill takes more energy to swash it through the air when free than when fixed?

Take any small toy prop through a bath in both fixed and free forms and you will feel the difference - its significant.
 
Hmmm the in esacapable fact with a free spinning prop versus a fixed one is that more water will pass over it's surface therefore creating more drag regardless of the aerofoil effects etc etc so I have to concede that Gludy probably is right on this one!
 
Thganks for that report.
To save the trouble of going to the link I post the comments on it here:-
------------------------------------------
""Again referring to the performance curves in Figure 6; we can estimate the
propeller resistance to be 26 pounds. The basic resistance of the blade
(i.e. if it is not turning) is 20 pounds, so the added resistance is 6 pounds, or a 30
percent increase over the stopped propeller. "

This is my exact point and any proper measurement will show that difference. The higher the speed, I think you will find, the higher the difference.

I would like someone to explain why a fixed prop measured drag is 30 less than a free prop????

The comment then goes onto state:-

"However, the total resistance of the boat (without the propeller) at 6 knots is estimated to be 400 pounds, therefore the
total added resistance to the vessel as a whole is less than two percent. For a
coastal cruising boat, this will not pose much of a concern. For long
distance cruisers or sometimes racing boats, the benefits will have to be weighed
against the added drag."

We have already had some figures from ShipsWoofy about his gain in efficiency at low speeds (3.5 knots) from single engine operation as against twin engine operation and at higher speeds when a 42 footer gained efficiency using both engines at 7 knots.

The effect is more signifciant at higher speeds and it was to try and get a feel for this that I started the thread that is currently bogged down whilst some flat earthers deny the science of of what is simple physics. :-)

I would like them to now explain how the fixed prop had 20 lbs thrust and the free prop 26lbs thrust? Over to you ....... :-)
 
[ QUOTE ]
I am trying to find out how a twin shaft propped boat performs running on one engine. If you want to really extend range to a thousand or two miles on a trawler style boat there may be a higher mpg if you run on one engine.

However one engine running without locking the other shaft means that the non-working prop is actually having to be turned and is turning the gearbox etc. hence the dragging prop is a hindrance. There is also a slight rudder offset which increases the drag of the rudder.

I know of one boat that has measured fuel consumption and found that running on one engine used more fuel to achieve the same speed than running on two engines at lower speed - no doubt due to the above to factors. However, I cannot find any source that has actually determined if there is a gain in mpg by running on one engine with the idle shaft locked. In this situation there may be an overall mpg gain- as the only drawback is the rudder offset alone.

Does anyone have any experience or figures on this subject?

[/ QUOTE ]

Can I remind you all that this was originally about fuel consumption, not drag.

I may not be a physics major the two may not neccessarilly be related.
 
Thanks for that but it is not so much me that is right as the physics that I am explaining.

Thats why when you measure the prop drag directly as the University did, you get a higher drag with the free prop than the fixed one. At low speeds this may be small - the order of 30% but with the higher speeds it is even more significant.

It makes a big difference to long distance cruising range.
 
Yep I get it but it ain't as simple as you painted. Drag consists of at least to components, induced drag and parasitic drag and as the speed increses the induced drag decreases and the paracistic drag increases and and it happens at different rates so it could be the case that your fixed prop scenario becomes irrelevant at say a higher speed because the parasitic drag increases dramatically versus the induced drag of the prop and you might find that there will be no gain in efficiancy, as someone else saif this is goign to be dependent upon hull type and speed selected also, it isn't a braod sweeping statement that you seem to be applying
 
The only broad sweeping statement I have applied is that the fixed prop has less drag than the free to rotate prop. That is true ... full stop.

I started the thread trying to find out how significant this was - see my opening post - as regards the point about fixed versus rotating that is just fact - as regards how significant this is for a given boat this depends on a host of factors. I have not claimed anything else.

Quote from my opening post:-

"I cannot find any source that has actually determined if there is a gain in mpg by running on one engine with the idle shaft locked. In this situation there may be an overall mpg gain- as the only drawback is the rudder offset alone."

So it is unfair to imply that I am making sweeping statements etc.

What has happened is that a simple fact I stated has been challanged and so the thread has revolved around basics instead of actually discussing the original question.
 
KevB = thanks for that source.

Here is what that site states:-
"The graph to the left shows that after the engine failure the Power required has increased significantly. Most of this is due to the increased parasite drag from the windmilling propeller.

In the situation to the left the power available has been reduced to half and the power required has been increased significantly. The net result is that the power available is less than the power required and this aircraft will not be able to maintain it's current altitude (remember climb rate depends upon Px. Therefore, this aircraft will descend since Px is negative.)

The propeller produces much more drag as it windmills than it would if it simply stopped turning."


I hope that everyone now starts to accept the fact and gets on with the real subject of the thread.
 
Well I was wrong as well and dont mind admitting it.(You swine KevB) /forums/images/graemlins/smile.gif
The hours I spent bashing round the circuit one one engine as well!
 
[ QUOTE ]

"I cannot find any source that has actually determined if there is a gain in mpg by running on one engine with the idle shaft locked. In this situation there may be an overall mpg gain- as the only drawback is the rudder offset alone."


[/ QUOTE ]

Surely it really depends on the hull form of the boat?
 
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