Free prop or locked prop?

[Dockhead]

Originally Posted by Lanevdl

Dockhead - Where has this 'Urban Myth' been busted? Spinning blades on a gyrocopter offer a LOT more resistance to the air than fixed blades which would cause it to plummet to the earth. Actually I'm pretty sure helicopters also need the blades to spin to offer enough resistance to the air to get off the ground. My prop people say a fixed prop is definately the way to go.

Where has the urban myth about locked boat props creating less drag been busted? In different empirical tests at the Massachusetts Institute of Technology, the University of Strathclyde, Yachting Monthly, and by our own Maine Sail, with unanimous results. The references are in Post #37 of this thread: http://www.cruisersforum.com/forums/...ml#post2195906


What you say is true of aircraft props, and the urban myth no doubt got started because people assumed that boat props behave the same way.

[a64pilot]

No, helicopters that are in autorotation fall slowly because the blades generate lift, there are three regions of a helicopter disk in auto rotation, driving, driven, and reverse flow.
Let's get off of helicopters though as the aerodynamics are not simple, not by a long shot. To quote a famous Russian designer you can take all the aerodynamic problems of an airplane and put them on the tip of a helicopter blade, and have room left over.

A boat prop exhibits less drag when turning due to the chord width is so great, a typical three blade boat prop will cover more than 100% of the disk area, airplanes way less, less than 10% typically.
The flat plate drag of a boat prop is probably ten times that of an airplane prop. It is this flat plate area that is the reason for the drag.
But it doesn't matter, not at all, a spinning sailboat prop will slow you less than a stopped sailboat prop. That is a pretty well established fact.
So if udon the want to have a folding or feathering prop, let it spin if the Xmsn allows, or lock it if required, simple.


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[HopCar]

Do you guys think it is fair to say that boat props produce thrust by pushing massive amounts of water aft, an action - reaction kind of thing.

An airplane propeller works differently. It is actually a wing that creates a low pressure area ahead of it and the high pressure behind it pushes it forward. Of course there is some action - reaction going on from the air moved aft as well.

[Pete7]

Originally Posted by Lanevdl

Dockhead - Where has this 'Urban Myth' been busted? Spinning blades on a gyrocopter offer a LOT more resistance to the air than fixed blades which would cause it to plummet to the earth. Actually I'm pretty sure helicopters also need the blades to spin to offer enough resistance to the air to get off the ground. My prop people say a fixed prop is definately the way to go.

I would have thought the easiest way to find out is to go sailing in a steady breeze. Stick it in gear and what the speed drop. Come back to us with the differences you measure.

Pete

[four winds]

Originally Posted by HopCar

Do you guys think it is fair to say that boat props produce thrust by pushing massive amounts of water aft, an action - reaction kind of thing.

An airplane propeller works differently. It is actually a wing that creates a low pressure area ahead of it and the high pressure behind it pushes it forward. Of course there is some action - reaction going on from the air moved aft as well.

I certainly agree, and said the same things a few posts back.

[Dockhead]

Originally Posted by Pete7

I would have thought the easiest way to find out is to go sailing in a steady breeze. Stick it in gear and what the speed drop. Come back to us with the differences you measure.

Pete

Ha, ha. I first did that when I was about 14 years old. Which is why I never needed to read the M.I.T. study and never cared much about this question. Twas always obvious to me.

[a64pilot]

Originally Posted by HopCar

Do you guys think it is fair to say that boat props produce thrust by pushing massive amounts of water aft, an action - reaction kind of thing.

An airplane propeller works differently. It is actually a wing that creates a low pressure area ahead of it and the high pressure behind it pushes it forward. Of course there is some action - reaction going on from the air moved aft as well.

No, not really they both pull a low pressure area in front, that is why when you put too much power into a boat prop, it cavitates, cavitation is of course enough vacuum in front of a prop that the water essentially boils.

A well designed boat prop is an airfoil, that is why I don't have a love affair with most feathering props, they are flat plates, no airfoil section and are inefficient, they work, just are less efficient.
But an airplane does not need an airfoil to fly either, paper airplane for example or those balsa wood gliders we used to have as kids, basically flat sections, no airfoil.


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[a64pilot]

Not too long ago many "Experts" stated that a rocket would not work in a vacuum as there would be no air for the rockets exhaust to push against, but I guess we all know that rockets in fact do work pretty well in space?


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[Djarraluda]

Originally Posted by a64pilot

No, not really they both pull a low pressure area in front, that is why when you put too much power into a boat prop, it cavitates, cavitation is of course enough vacuum in front of a prop that the water essentially boils.

A well designed boat prop is an airfoil, that is why I don't have a love affair with most feathering props, they are flat plates, no airfoil section and are inefficient, they work, just are less efficient.
But an airplane does not need an airfoil to fly either, paper airplane for example or those balsa wood gliders we used to have as kids, basically flat sections, no airfoil.


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You are quoting flat blades for feathering props, but many are not flat. I believe the auto-pitch types are flat, but I have seen many that look "normal" as well.
Getting into the cavitation bit gets interesting, when folks think they understand that, we add the tip vortex issues into the mix. Also look at the fluid flow issues close to the hull (top of prop) and at the lower edge and most sane people reach for the gin or just fall asleep.
We should not really compare aircraft and boat props as the issues involved can appear minor but are not so simple once we move away from the pure maths. It used to take a PhD but now it just needs a computer and an expensive CFD program.

[hsi88]

you keep repeating yourself with your prowess at googling even though it came back to bite you with your area claim that airplane propellers have less exposed area than do boat props. it's on the internet so it must be true. so allow me to repeat myself as well. with all that i've read and experienced there is no consensus on the issue.

I know lots of people have a distrust of science. But at least in my mind, unanimous studies pointing to the same thing is good enough for me. There are still lots of people out there that believe the world is 6000 years old and we never landed on the moon, etc. etc. What's a little sad is the U.S. is way ahead of the rest of the world in its mistrust of science, yet has no problem reaping a profit from it. Belief is a powerful emotion. It allows one to completely negate facts and evidence...

[malbert73]

There is definitely consensus based on fact that the freewheeling prop generates less drag by a good amount.

There is theoretical conjecture that a spinning prop on a boat acts like a prop on a plane- but that is different as plane propellers are directly coupled (right?) to the engine so don't have a true neutral. Their blades are also so thin and high aspect that it's really not analogous.

If your neutral transmission has enough drag, or if you are running a shaft generator, I am sure a spinning prop would be close in drag to locked.


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[HopCar]

"There is theoretical conjecture that a spinning prop on a boat acts like a prop on a plane- but that is different as plane propellers are directly coupled (right?) to the engine so don't have a true neutral."

Malbert it doesn't matter if an airplane prop is coupled to the engine or not. An airplane propeller that is allowed to free wheel creates tremendous drag. Much more than a fixed airplane prop. See my post number 51.

I agree with you that from all the testing that's been done, it seems obvious that boat propellers are exactly the opposite.

[Uncle Bob]

Originally Posted by Lanevdl

Dockhead - Where has this 'Urban Myth' been busted? Spinning blades on a gyrocopter offer a LOT more resistance to the air than fixed blades which would cause it to plummet to the earth. Actually I'm pretty sure helicopters also need the blades to spin to offer enough resistance to the air to get off the ground. My prop people say a fixed prop is definately the way to go.

Hi, if you go to mainesail's site you will find the video of the test that he conducted, if you still doubt the results why not perform a similar test, video it and post the results here.
As to airplane props, I believe that air is somewhat thinner than water, but I may be wrong, so the analogy is moot.

[malbert73]

Originally Posted by HopCar

"There is theoretical conjecture that a spinning prop on a boat acts like a prop on a plane- but that is different as plane propellers are directly coupled (right?) to the engine so don't have a true neutral."



Malbert it doesn't matter if an airplane prop is coupled to the engine or not. An airplane propeller that is allowed to free wheel creates tremendous drag. Much more than a fixed airplane prop. See my post number 51.



I agree with you that from all the testing that's been done, it seems obvious that boat propellers are exactly the opposite.

Right- the reason the freewheeling prop in a plane may cause so much drag but not in a sailboat may be that it's actually spinning the engine as it freewheels whereas a sailboat prop is spinning only the shaft and coupling in neutral...


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[HopCar]

Malbert you're wrong. I have flown a powered glider that when you shut off the engine, the prop shaft was disconnected from the engine. The prop did not turn the engine. It was free to spin in ball bearings. It produced tremendous drag until I stopped it with a brake.