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Old 23-08-2016, 08:56   #16
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Re: free prop or locked prop?

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Originally Posted by Dockhead View Post
Actually it has been definitely proven in several tests that a spinning prop produces much less resistance than a locked one.

So I think there is a consensus -- lock it only if your transmission won't allow you to leave it spinning (some hydraulic ones for example). The idea that a locked prop produces less resistance is an urban myth which has been definitely busted.

But ultimately, yes, of course, folding or feathering is the only really decent solution, if you actually sail.
ha. let's have a cyber drag race. when the light turns green we both fire up our supercharged search engines and see who can google the most "TESTS" either for or against spinning props.
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Old 23-08-2016, 08:58   #17
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Free prop or locked prop?

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Originally Posted by Dockhead View Post
Neither -- you should buy a folding or feathering prop. Best investment you can make if you ever sail at all. . . .

Why bother with the expense, just tilt the motor up and the prop is completely out of the water. That's what I do.

Seriously though, freewheel is the lowest drag. There may be warrantee issues.

A house is but a boat so poorly built and so firmly run aground you would never try to refloat it.
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Old 23-08-2016, 09:01   #18
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Re: Free prop or locked prop?

as an aside, any pilot knows that with an engine failure with a fixed pitch prop if he can slow the aircraft to stop the windmilling he will glide much further. compression will hold the prop. with it spinning it has an aerodynamic effect of a solid disk as is the case with a helicopter's rotor plane. does this relate to sailboats??? not a clue but both air and water are fluids and the topic is drag within a fluid.
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Old 23-08-2016, 09:03   #19
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Re: free prop or locked prop?

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Originally Posted by jrbogie View Post
ha. let's have a cyber drag race. when the light turns green we both fire up our supercharged search engines and see who can google the most "TESTS" either for or against spinning props.
Better yet, why don't you fire up your own search engine, and tell us what you conclude yourself, after you've read what you find?

You can start with this one:

Lock your prop, or let it spin?
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Old 23-08-2016, 09:06   #20
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Re: free prop or locked prop?

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Originally Posted by Dockhead View Post
Better yet, why don't you fire up your own search engine, and tell us what you conclude yourself, after you've read what you find?

You can start with this one:

Lock your prop, or let it spin?
uh, in my original post i tried to make it clear that i've concluded that the only consensus is that there is no consensus at all. and of course i wasn't the one who claimed that several tests PROVE one way or another.
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Old 23-08-2016, 09:09   #21
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Re: Free prop or locked prop?

great page dockhead the only part needed with the system is a clutch so the alternator can be engaged and disengaged when charging is not needed.

thank you all
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Old 23-08-2016, 09:17   #22
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Re: free prop or locked prop?

Quote:
Originally Posted by Dockhead View Post
Better yet, why don't you fire up your own search engine, and tell us what you conclude yourself, after you've read what you find?



You can start with this one:



Lock your prop, or let it spin?

I read the article, liked their methodology and most of their conclusions. The only conclusion I disagreed with was that the stopped prop could cost you 3/4kt speed at 6kt.

The extrapolated their results without accounting for scale effects. I believe the loss of speed would be substantially less though still measurable and possibly significant over long passages.


A house is but a boat so poorly built and so firmly run aground you would never try to refloat it.
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Old 23-08-2016, 09:17   #23
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Re: Free prop or locked prop?

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great page dockhead the only part needed with the system is a clutch so the alternator can be engaged and disengaged when charging is not needed.

thank you all
a transmission neutral with prop spinning will not run an alternator. belt driven by the engine you see.
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Old 23-08-2016, 09:28   #24
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Re: Free prop or locked prop?

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Originally Posted by jrbogie View Post
as an aside, any pilot knows that with an engine failure with a fixed pitch prop if he can slow the aircraft to stop the windmilling he will glide much further. compression will hold the prop. with it spinning it has an aerodynamic effect of a solid disk as is the case with a helicopter's rotor plane. does this relate to sailboats??? not a clue but both air and water are fluids and the topic is drag within a fluid.

That is exactly why I KNEW that a spinning prop was more drag, cause it took energy to spin it, and where did this energy come from, the airplane of course.

Well it's true for an airplane, but it is NOT true for a boat. Why?
I believe its because if you look at an airplane prop it looks like two or three sticks, the prop blades are very thin and comprise only a small portion of the disk, well less than 10% measuring by eye, so stopped there is little drag.
Look at a boat prop, just the opposite of an airplane prop and in a three blade, some are more than 100% of the disk, meaning the blades overlap, it's why I believe that dragging a stopped boat prop really is similar to dragging a similar sized bucket.
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Old 23-08-2016, 09:33   #25
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Re: Free prop or locked prop?

what does your transmission manual say????
ye would be dummer than dum and dummer to ignore that.
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Old 23-08-2016, 09:36   #26
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Re: free prop or locked prop?

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uh, in my original post i tried to make it clear that i've concluded that the only consensus is that there is no consensus at all. and of course i wasn't the one who claimed that several tests PROVE one way or another.
That's specifically the point I'm making. If you will do some reading, you will see that there is indeed a consensus.

People were probably misled by the fact mentioned by A64 that airplane props do create more drag when spinning. But boat props (because they operate on a far denser medium) clearly do not. The OP has measured a half knot difference himself.

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Old 23-08-2016, 09:52   #27
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Re: Free prop or locked prop?

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Originally Posted by a64pilot View Post
That is exactly why I KNEW that a spinning prop was more drag, cause it took energy to spin it, and where did this energy come from, the airplane of course.

Well it's true for an airplane, but it is NOT true for a boat. Why?
I believe its because if you look at an airplane prop it looks like two or three sticks, the prop blades are very thin and comprise only a small portion of the disk, well less than 10% measuring by eye, so stopped there is little drag.
Look at a boat prop, just the opposite of an airplane prop and in a three blade, some are more than 100% of the disk, meaning the blades overlap, it's why I believe that dragging a stopped boat prop really is similar to dragging a similar sized bucket.
it's not me that claims that it is not true for a boat. as i've said, there is no consensus either way. i've TESTED both. fixed or spinning, airplane and boat and my TESTS have showed that a spinning airplane prop will lessen the glide ratio, lift/drag, of the aircraft while a spinning or fixed prop on my boat makes no measurable difference in boat speed. my guess is the difference is the fluids and speeds each is operating in. measuring drag effect is much easier in thin air and the much higher speeds of an aircraft. but measuring the effect of a boat going five knots in water is a different animal. anyway, i can explain the aerodynamic effect of a windmilling airplane propeller. no testing required, it's simple aerodynamics. i've yet to see anybody explain how the hydrodynamic effect of a spinning boat propeller is any different.
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Old 23-08-2016, 10:04   #28
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Re: free prop or locked prop?

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Originally Posted by Dockhead View Post
That's specifically the point I'm making. If you will do some reading, you will see that there is indeed a consensus.

People were probably misled by the fact mentioned by A64 that airplane props do create more drag when spinning. But boat props (because they operate on a far denser medium) clearly do not. The OP has measured a half knot difference himself.

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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.
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Old 23-08-2016, 10:13   #29
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Re: Free prop or locked prop?

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i've yet to see anybody explain how the hydrodynamic effect of a spinning boat propeller is any different.

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)

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Old 23-08-2016, 10:33   #30
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Re: Free prop or locked prop?

No desire to prove anything to someone even though I was a college teacher for years.......but a couple of thoughts to add to the mix.

There must be a darn good reason boat props don't look like aircraft props.

Intuitively, it seems any force or energy used to freewheel a prop is energy that can't be used to slow the boat. Thinking about vectors of force from teaching statics.

Think about the angle to water flow of a unpowered (dragging) spinning blade. Its apparent angle would be some degrees off dead ahead. Thus absorbing less energy from the flow as the deflection angle would be more shallow.

A fixed prop would need to deflect the flow at a greater angle and have no opportunity to use some of that energy to spin the prop.

Just my thoughts, and all I would like to say on the subject.
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