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Old 07-05-2018, 10:59   #16
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Stuffing box... Sailing.. prop left to spin

The free spinning prop confusion comes from aircraft I believe.
For them it’s true a windmilling prop is a LOT more drag than if it’s stopped.
So why not a boats prop?
My theory, and it’s only theory is that most boats props surface area covers often more than the entire circumference of the circle, where an airplanes prop look like two or three or more sticks, and covers only a tiny portion.
Both props, but much, much different actually.


Anyone know how to attach a short video clip with an IPad on the app?
https://vimeo.com/268448424
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Old 07-05-2018, 11:24   #17
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Re: Stuffing box... Sailing.. prop left to spin

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Originally Posted by a64pilot View Post
The free spinning prop confusion comes from aircraft I believe.
For them it’s true a windmilling prop is a LOT more drag than if it’s stopped.
So why not a boats prop?
My theory, and it’s only theory is that most boats props surface area covers often more than the entire circumference of the circle, where an airplanes prop look like two or three or more sticks, and covers only a tiny portion.
Both props, but much, much different actually.


Anyone know how to attach a short video clip with an IPad on the app?
I think airplanes feather their props to stop them.

The idea that a free spinning prop creates more drag is a misconception. People think that because the propeller spins, it creates rotational energy from drag. This is correct. But actually a fixed propeller does the same, only that the rotational energy is transferred to the water instead. It creates a vortice behind the propeller. More mass is moved in the latter case, so - more drag.
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Old 07-05-2018, 11:28   #18
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Re: Stuffing box... Sailing.. prop left to spin

I posted the above video after you posted, I edited my post.

Turbines, and multi engine piston engine aircraft can feather, but just a plain ole simple airplane with a fixed prop like the Cessna 140 can stop the prop by turning the ignition off and slowing way down, engine compression will keep it stopped unless you speed up enough so that it will windmill.
You can see on my old 140, that took an airspeed of about 120 mph, best glide speed is about 73 mph.
The difference in glide ratio is awfully large, there is no question.
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Old 07-05-2018, 11:34   #19
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Re: Stuffing box... Sailing.. prop left to spin

You’ll have wear on the packing when ever the shaft is turning. If it is well cooled and lubricated with water there will be less wear.

Why don’t you sail awhile with the prop freewheeling and touch the packing gland. It it’s hotter than it gets while motoring, you have increased wear and need to figure out something else. Maybe a folding prop?

A64 is correct, a stopped airplane prop produces much less drag than a freewheeling prop. I’ve done the tests.

The exact opposite is true of boat props, as Maine Sail’s tests prove.

I think A64 is correct as to the reason. It’s because of the area of the prop blades.
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Old 07-05-2018, 12:00   #20
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Re: Stuffing box... Sailing.. prop left to spin

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I think airplanes feather their props to stop them

The idea that a free spinning prop creates more drag is a misconception. People think that because the propeller spins, it creates rotational energy from drag. This is correct. But actually a fixed propeller does the same, only that the rotational energy is transferred to the water instead. It creates a vortice behind the propeller. More mass is moved in the latter case, so - more drag.
Not all airplanes can feather their props and they also benefit from stopping the prop when gliding.

Many years ago I did tests with an ultralight aircraft equipped with a fixed pitch prop. The engine was connected to the prop with a centrifugal clutch. When the engine was off, the prop was free to spin. I fitted a brake on the clutch so I could stop the prop from spinning. I also had a very sensitive variometer (vertical speed) and air speed gauge.

I would fly to altitude, shutoff the engine and note the air speed and sink rate.
Then I’d stop the prop, the air speed immediately increased significantly. When I slowed the air speed to match that of the spinning prop, the sink rate would significantly decrease. As I recall it made a 200 to 300 feet per minute difference.

At the time, someone smarter than me, calculated that the drag of the spinning prop was close to the drag of a plywood disc of the same diameter.
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Old 07-05-2018, 12:19   #21
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Re: Stuffing box... Sailing.. prop left to spin

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As it's been said many times, a free spinning solid prop creates parasite drag equal to the surface area of a solid disk the diameter of the prop. A prop on a locked shaft creates parasite drag equal only to the combined surface area of the blades themselves. A two-blade prop, not much drag if the shaft is locked. A four or five blade prop with a locked shaft, a lot more drag but still not as much as if it were left free spinning. A lot of people don't believe that but it's true. Check with an engineer. A full feathering prop, doesn't matter because it shouldn't spin much, if at all, when feathered. If you transmission manufacture states not to lock the shaft by leaving it in gear when sailing, it's best to install a separate shaft lock and leave the transmission in neutral. Using your shaft to run an electric generator produces tons of drag. The faster you sail, the more electricity it produces same the greater the drag. Where do you think the electrical energy comes from? Same with a wind driven generator on a boat, sail or power, produces tons of drag too. When sailing it's best to lock the prop on a wind generator too. Unless you really need the electricity, run your wind generator when anchored. There is no free lunch in the world of physics.
I wanted to believe this was true but because I actually am an engineer I was skeptical. The energy that creates the rotation of the shaft is taken from the energy acting on the prop blades and thus reducing the drag energy. Thanks to Sailmonkey for summarizing the test results that prove my engineer instincts were correct.
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Old 07-05-2018, 12:33   #22
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Re: Stuffing box... Sailing.. prop left to spin

my issue was with the stuffing box rather than whether to put it in gear or not...personally i am for letting it spin...

i have seen more studies in this favour eg ybw june 11th 2015 as well as the book by Jean Luc Pallas pg 65 Maintenance for Diesel engines... but anyway one can always try both and compare the results for his/her boat...
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Old 07-05-2018, 13:41   #23
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Re: Stuffing box... Sailing.. prop left to spin

Please note that if you have an hydraulic gear like a BW you can put your shift lever in any position you like without the engine turning. The prop will be free spinning regardless when under sail unless you have a shaft brake.
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Old 08-05-2018, 06:39   #24
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Re: Stuffing box... Sailing.. prop left to spin

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You may put it in gear briefly, just to stop the spinning, then put it back into neutral. You should read your engine brochure to see what they recommend. Yanmar has changed its recommendations in recent years, from leaving it in reverse, to leaving it in forward gear when sailing. When I have spoken to Yanmar reps, they say it doesn't make a whole lot of difference.
This is the response I got from Yanmar:
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Old 08-05-2018, 11:42   #25
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Re: Stuffing box... Sailing.. prop left to spin

Your theory is nonsense. Airplanes now have six and seven blade propellers for the same reason many new boats and ships have multi-blade props. That's to more efficiently utilize the power of the engines. However, most sophisticated airplanes are equipped with auto-feathering systems in the event of an engine failure. Why? To reduce drag of course. Air is a compressible fluid, water is an incompressible fluid. Ig anything reducing drag is even more desirable with a boat prop in a high density, incompressible fluid like water. Why do you think most marine engine manufacturers give specific instructions on how to stop a propeller from water-milling? Their advice ranges from leaving the transmission in forward, leaving it in reverse or leaving it in neutral and employ a separate shaft locked. The primary reason is to reduce drag of course.
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Old 08-05-2018, 13:41   #26
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Re: Stuffing box... Sailing.. prop left to spin

Meanwhile i contacted my engine dealer ( a well known brand) they said...Let the thing Spin Man...
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Old 08-05-2018, 13:51   #27
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Stuffing box... Sailing.. prop left to spin

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Originally Posted by jmschmidt View Post
Your theory is nonsense. Airplanes now have six and seven blade propellers for the same reason many new boats and ships have multi-blade props. That's to more efficiently utilize the power of the engines. However, most sophisticated airplanes are equipped with auto-feathering systems in the event of an engine failure. Why? To reduce drag of course. Air is a compressible fluid, water is an incompressible fluid. Ig anything reducing drag is even more desirable with a boat prop in a high density, incompressible fluid like water. Why do you think most marine engine manufacturers give specific instructions on how to stop a propeller from water-milling? Their advice ranges from leaving the transmission in forward, leaving it in reverse or leaving it in neutral and employ a separate shaft locked. The primary reason is to reduce drag of course.


I assume you mean me with the nonsense answer?
Just about everything you said is wrong, aircraft props have more than two blades primarily due to ground clearance, two blades is the most efficient, more blades can be smoother but will develop less thrust at typical prop speeds.
Of course an airplane prop feathers to reduce drag, as does a boat prop too, and it’s not a modern thing, it’s almost 100 yrs old.
However less drag is produced on an airplane fixed pitch prop when it’s stopped, and on a typical fixed pitch boat prop if it’s left to spin. Too much testing has been done to prove that, you can’t deny the testing.
Well, I guess you can, some still believe the Earth is flat
Stopping a prop is desirable among other things to reduce noise and possibly prevent wear, but it is NOT to reduce drag.
What testing can you show to illustrate a stopped Boat prop is less drag? Or do you say of course because it’s what you want to believe?
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Old 08-05-2018, 14:22   #28
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Re: Stuffing box... Sailing.. prop left to spin

[QUOTE=a64pilot; the Cessna 140 can stop the prop by turning the ignition off



You can also blow the mufflers off doing that. You meant the mixture,
and isn't a 1 blade prop with a counter weight is the most efficient?
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Old 08-05-2018, 14:28   #29
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Stuffing box... Sailing.. prop left to spin

You can’t kill the C-85 that is used in a 140 by mixture, you could turn the fuel off though. However I’m not worried about the mufflers, as when the C-140 was made in 1946 , it had straight pipes, no mufflers and mine is original in that dept, except for the metalized wings and the alternator, it’s close to original, nobody put C-150 mufflers and other C-150 parts on as many did.

I think a single blade prop may be the most efficient, and they have been built, I assume you knew that.
I also think that Hughes helicopter may have built a single blade helicopter, I think.
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Old 08-05-2018, 14:32   #30
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Re: Stuffing box... Sailing.. prop left to spin

[QUOTE=jmschmidt;2628701]Your theory is nonsense. Airplanes now have six and seven blade propellers for the same reason many new boats and ships have multi-blade props. That's to more efficiently utilize the power of the engines.”

Actually your statement is nonsense. The fewer blades a prop has, the more efficient it is. The only reason to add blades is that there isn’t enough room to swing a prop of big enough diameter to absorb the available horsepower.

The problem is that as you add blades, the blades are working in air / water already disturbed by the preceding blade.

I once saw an aircraft fly on a single blade prop. Very clever design with a counter weight that caused the pitch to adjust automatically. Don’t think they sold many of them. It was just too strange looking.
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