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Old 23-08-2016, 11:46   #31
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Re: Free prop or locked prop?

To A64s surface area thinking.

Consider this.....

A fixed boat prop "stacks up" more water in front of it than a freewheeling prop. The freewheeling prop allows more flow (or less resistence to flow) between the blades as a result of the apparent angle to water flow.
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Old 23-08-2016, 11:51   #32
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Re: free prop or locked prop?

Quote:
Originally Posted by jrbogie View Post
you're making a point while neglecting mine. i'll grant that you've read much on the topic but even after several back and forth you can't seem to grasp that i have read a bunch too. so since neither of us can establish who's read the most can we agree that we are equally read and informed on the issue. as i mentioned in a previous post, nobody has been able to explain to me why the denser medium that a boat operates in with respect to an airplane has a different effect on drag. care to give it a shot? i'm a wiz when it comes to aerodynamics but never considered hydrodynamics so perhaps you can show me with an expananation in FLUID DYNAMICS, of which both are a subset, how the effect of drag is different depending on the greater viscosity in water vs air.
Well, I'll be glad if you can lead me to information which contradicts what I have.

What I have seen is two points of view on the subject:

1. People who guess and suppose that locked props on boats just must cause less drag because that's the way it works on aircraft, and surely it ought to be the same with boats.

And

2. People who've actually tested it. That includes even the Massachusetts Institute of Technology, as well as Yachting Monthly, and our own Maine Sail, and others.


Group 2 are unanimous. From which I conclude that there is no serious controversy on the matter.

If I'm wrong and I missed some important study, then I'll be glad to be set right.



Oh, I forgot Group 3 - those who've been messing with sailboats their whole lives, and who've actually tried locked vs spinning on different boats and have actually felt how a locked prop slows you down, always and on every kind of boat, and so never really wondered much about this and don't know what all the fuss is about. That's my group.

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Old 23-08-2016, 11:56   #33
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Re: Free prop or locked prop?

Totally dependent on the transmission - example: on a Beta engine you MUST leave it in reverse ... Or potential warranty violation.
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Old 23-08-2016, 12:14   #34
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Re: Free prop or locked prop?

One last post here, I promise. Nobody really cares what I think anyway.

We all know that pitch angle determines the "gearing" on a boat prop. More pitch gives more thrust if sufficient power is available. Less pitch less thrust at the same power.

Though it's a bit difficult to visualize this "backwards", without engine power applied the water is the power source. And a rotating freewheeling prop is presenting less pitch to the flow than a fixed prop. This because of the change in apparent direction of flow when freewheeling compared to fixed. And the result is less force felt on the spinning blade.

A diagram to illustrate comes to mind but that would be venturing into a desire to prove something beyond doubt.

I've said enough, carry on.
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Old 23-08-2016, 12:15   #35
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Re: Free prop or locked prop?

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Originally Posted by a64pilot View Post
I think I did, a boat prop takes up nearly or all of the surface area of the swept area or disk as its normally called in the aircraft world, where an airplane prop only a small portion. Look at this picture, see how little surface are the props are compared to the circle they cover when rotating? Now think boat prop, it will cover the whole circle. It's not air density or speed, it's surface area that is the difference. ( I think)

i hardly need look another picture of airplane props. before you said that the higher density of water makes the difference and now you say it's surface area. in reality all three, fluid density, speed and surface area effect the force that drag has on an object be the object is attached to a boat or an airplane. perhaps it will be easier if we can agree that both aerodynamics and hydrodynamics are subsets of the study of fluid dynamics. if that is so;

as regards fluid density, can we agree that within the higher viscosity of a particular fluid vs a less dense fluid, water vs air in this case, the drag will have a greater effect on an object moving through one vs the other?

as regards speed, an increase in drag results as the square of the speed increase. in other words, double the speed=quadruple the drag.

as regards surface area, other than on model airplanes, i cannot think of an airplane propeller that has a smaller surface area than a typical sailboat prop. neat airplane picture but it would take both of us to carry one of the propellers on the b25. and the area of the "disk" that so effects drag is many times that of a sailboat propeller.

so again, without pictures from goggle, explain to me how and why fluid dynamic principles differ between airplane propellers and sailboat propellers ESPECIALLY how fluid drag is proportionally opposite on a boat vs an airplane as you contend.
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Old 23-08-2016, 12:20   #36
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Re: Free prop or locked prop?

i believe the consensus was do what transmission mfg recommends.
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Old 23-08-2016, 12:45   #37
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Re: free prop or locked prop?

Quote:
Originally Posted by Dockhead View Post
Well, I'll be glad if you can lead me to information which contradicts what I have.
either we can agree on the principles of fluid mechanics or we can't. why you need to be led to those principles i've not a clue as you seem very adept at googling.

Quote:
What I have seen is two points of view on the subject:

1. People who guess and suppose that locked props on boats just must cause less drag because that's the way it works on aircraft, and surely it ought to be the same with boats.

And

2. People who've actually tested it. That includes even the Massachusetts Institute of Technology, as well as Yachting Monthly, and our own Maine Sail, and others.
what you've seen, what i've seen, blah blah blah. let's agree that we've both seen a lot for crying out loud.

Quote:
Group 2 are unanimous. From which I conclude that there is no serious controversy on the matter.
hahahaha. group 2 are unanimous and group one is not??? sounds like the supreme court in a 5/4 split. those in the majority are unanimous in their finding and those in the minority are unanimous in their dissent.

Quote:
If I'm wrong and I missed some important study, then I'll be glad to be set right.
not trying so set anybody right. just voicing my thinking on the subject. i will suggest that if you want to continue the topic you might understand my points a little more buy studying the physics that i've tried to expand on.



Quote:
Oh, I forgot Group 3 - those who've been messing with sailboats their whole lives, and who've actually tried locked vs spinning on different boats and have actually felt how a locked prop slows you down, always and on every kind of boat, and so never really wondered much about this and don't know what all the fuss is about. That's my group.

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ha so before you saw but two groups and now you see three. but there are more groups. there are those sailboat messers withers who've found that spinning props have the opposite effect. and for crying out loud you left out my group. the one whose tried it all over about a half century of sailing and have not come to a conclusion either way. hey, let's both broaden our minds and see if we can't come up with even more groups divided on the subject. i mean hell, my point all along is the consensus is is that there is no consensus. other than, of course, those who've come to a consensus that differs with another consensus that is.
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Old 23-08-2016, 13:18   #38
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Re: free prop or locked prop?

Quote:
Originally Posted by jrbogie View Post
either we can agree on the principles of fluid mechanics or we can't. why you need to be led to those principles i've not a clue as you seem very adept at googling.



what you've seen, what i've seen, blah blah blah. let's agree that we've both seen a lot for crying out loud.



hahahaha. group 2 are unanimous and group one is not??? sounds like the supreme court in a 5/4 split. those in the majority are unanimous in their finding and those in the minority are unanimous in their dissent.



not trying so set anybody right. just voicing my thinking on the subject. i will suggest that if you want to continue the topic you might understand my points a little more buy studying the physics that i've tried to expand on.

oh but there are more groups. there are those sailboat messers withers who've found that spinning props have the opposite effect. and for crying out loud you left out my group. the one whose tried it all over about a half century and have come to a conclusion either way. hey, let's both broaden our minds and see if we can't come up with even more groups divided on the subject.

Look, let's simplify this. The question doesn't need home-made theories of hydrodynamics. It is easy to measure empirically. Here are some recent tests:


Here is our own Maine Sail's test: Fixed vs. Free Wheeling Prop - Test Data - SailNet Community

Here is the University of Strathclyde study: http://www.plaisance-pratique.com/IM..._sdarticle.pdf

Here is the study done at M.I.T.:

http://www.catamaransite.com/files/propeller.pdf

Here is the Yachting Monthly test done just last year:

Lock your prop, or let it spin?

"Our graphs show the results, which dramatically confirm that wherever possible you should let your fixed three-blade prop spin. The drag when it was locked was nearly three times the drag when spinning, and the loss of speed was over half a knot at 4 knots, and extrapolated to be 0.75 knot at 6 knots. That means getting to your destination two hours earlier on a 16-hour passage."


Here is a test of propellers done by YM in 2006, where drag was measured, including fixed props spinning vs locked:

http://www.flexofold.com/upload_dir/...onthly_low.pdf


All of these tests produce the same results -- a fixed prop produces double or even triple the drag of the same prop which is allowed to freewheel.

If you want to get into the hydrodynamic theory, the most academic of these is the University of Strathclyde study, which goes into great detail about the hydrodynamic reasons why a spinning prop produces less drag than a locked one.



Not to put too fine a point on it -- the idea that a locked prop has less drag than a freewheeling one is nothing but an old wive's tale which has been conclusively disproved by empirical testing. If anyone wants to continue believing it, based on home-made hydrodynamic theories which fly in the face of all of the facts, well -- that's their privilege.
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Old 23-08-2016, 14:18   #39
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Re: free prop or locked prop?

Quote:
Originally Posted by Dockhead View Post
Look, let's simplify this. The question doesn't need home-made theories of hydrodynamics. It is easy to measure empirically. Here are some recent tests:


Here is our own Maine Sail's test: Fixed vs. Free Wheeling Prop - Test Data - SailNet Community

Here is the University of Strathclyde study: http://www.plaisance-pratique.com/IM..._sdarticle.pdf

Here is the study done at M.I.T.:

http://www.catamaransite.com/files/propeller.pdf

Here is the Yachting Monthly test done just last year:

Lock your prop, or let it spin?

"Our graphs show the results, which dramatically confirm that wherever possible you should let your fixed three-blade prop spin. The drag when it was locked was nearly three times the drag when spinning, and the loss of speed was over half a knot at 4 knots, and extrapolated to be 0.75 knot at 6 knots. That means getting to your destination two hours earlier on a 16-hour passage."


Here is a test of propellers done by YM in 2006, where drag was measured, including fixed props spinning vs locked:

http://www.flexofold.com/upload_dir/...onthly_low.pdf


All of these tests produce the same results -- a fixed prop produces double or even triple the drag of the same prop which is allowed to freewheel.

If you want to get into the hydrodynamic theory, the most academic of these is the University of Strathclyde study, which goes into great detail about the hydrodynamic reasons why a spinning prop produces less drag than a locked one.



Not to put too fine a point on it -- the idea that a locked prop has less drag than a freewheeling one is nothing but an old wive's tale which has been conclusively disproved by empirical testing. If anyone wants to continue believing it, based on home-made hydrodynamic theories which fly in the face of all of the facts, well -- that's their privilege.
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.
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Old 23-08-2016, 14:23   #40
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Re: free prop or locked prop?

Quote:
Originally Posted by jrbogie View Post
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 didn't say a word about area. You're confusing me with someone else.


If there is controversy, then let's see it. I found zero controversy in any of the empirical work on the subject. The tests are unanimous, as far as I can see.
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Old 23-08-2016, 14:59   #41
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Re: Free prop or locked prop?

makers specifications/ also fit a pully to the drive shaft and drive an alternator of the spinning prop while sailing
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Old 23-08-2016, 15:44   #42
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Re: Free prop or locked prop?

Air is compressible, water is not.

A boat prop works by moving water aft.

An aircraft prop moves air, too. But the majority of the power comes from low pressure on the front of the blade and high pressure (compressed air) on the rear, from the foil shape and pitch. The low pressure is lift or in this case forward pull.

A freewheeling aircraft prop has the compressed air on the forward face and low pressure on the rear face. So the lift here is a reward pull.

Since water is not compressible a boat prop and plane prop are similar only in that they both rotate. Maybe that's why aerodynamics and hydrodynamic are seperate fields of study.
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Old 23-08-2016, 15:45   #43
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Re: Free prop or locked prop?

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Intuitively, it seems any force or energy used to freewheel a prop is energy that can't be used to slow the boat.
Intuitively I think it's the opposite! The loss of energy from the propeller (rotating or not) is energy that can't be used to speed up the boat...
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Old 23-08-2016, 16:27   #44
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Re: Free prop or locked prop?

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Intuitively I think it's the opposite! The loss of energy from the propeller (rotating or not) is energy that can't be used to speed up the boat...
Actually I agree with that completely. It's just that less energy is wasted on a freewheeling prop than a fixed prop.

The post you quoted is poorly expressed.
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Old 23-08-2016, 18:59   #45
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Re: Free prop or locked prop?

When all else fails, we return to the ultimate master of engineering. To quote Scotty "You can not change the laws of physics Mr Spock." OK, I will accept that this is a translation of the Scottish but.....
We appear to have a few wizards in fluid mechanics here, so I will not enter a lecture but will identify that turbulence is a critical factor not mentioned so far.
Zeehag nailed it more politely that I would. Quite simply "Consult the B..... manual!".
Gearbox manufacturers have their designs that they look after with their requirements. Change that recommendation and you are mad, expect to pay the maintenance costs associated.
Want to go sailing faster and quieter, get a feathering/folding prop.
Want to generate power from a towed beastie, buy a towable beastie that generates power BUT also expect the sailing performance to suffer, as described by Scotty. Take energy out of the movement and you will affect the movement
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