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  #11  
Old 03-10-2012
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Re: Propeller drag test under sail.

A fixed/locked 3blade prop has more drag than if left to rotate - see

http://strathprints.strath.ac.uk/567...ints005670.pdf
... on page 24. "The experimental results confirm that a locked propeller produces greater drag than does a freewheeling screw (up to 100% ... in a strong tidal flow."

Further information/results on fixed or rotating prop drag - Test Results - Flexofold folding Propellers - about halfway ...


For info, Volvo Penta state to allow the prop to rotate with MS2 gearbox.
The Volvo 2000 series with MS2 gearbox Owners Manual states -
"When under sail, the control lever should be in the neutral position if the propeller is a fixed propeller.
If the propeller is a folding propeller, the control lever should be in the reverse position. Start the engine and run it for five minutes every ten hours when on long-distance cruises."

The reason is that if left in forward, it can wear the clutch cones, and if left in reverse, it can jam the cones.
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  #12  
Old 03-10-2012
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Re: Propeller drag test under sail.

Quote:
Originally Posted by Maine Sail View Post
That is my test so if you guys have any questions I can answer them....
I'd say the question has been answered definitively. Now you have to do a similar comparison test for folders, feathering, fixed 2 blade, fixed 3 blade, fixed 4 blade. Also, locking a 2 blade vertically behind deadwood.

That would finally put to rest all the various opinions about what's best or lowest drag.

Then you can move on to anchors.
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Old 03-10-2012
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Re: Propeller drag test under sail.

Thanks MS.
This does make sense:
"1- It is not as hidden as you thought

2- Is taking the path of least resistance"


Quote:
Originally Posted by dabnis View Post
Assuming you have fairly constant wind, course and so on, GPS speed seems pretty accurate?

Paul T
True that ^^ but I'm sailing in a river that has about a 2 knot current in either direction. Still, I can use the GPS to see if there is a slight variation if a) hidden or b) spinning free.
We have to be moving pretty fast (for our boat) to get our prop spinning so I'd bet that in lighter winds it would still be to our advantage to hide the fixed 2 blade prop.
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Old 03-11-2012
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Re: Propeller drag test under sail.

Gee MS, you could have spend thousands more $ to get the same result. You must not work for the Goverment .

Great test and rig and confrimes the arguement I had last summer.

The bottom line is that if the boat is going thru the water and there's enough energy to spin the prop thats how much energy you loose stopping it from spinning

John
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Old 03-12-2012
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Re: Propeller drag test under sail.

I have never done a test in the water like this. I just have practical experience with airplanes. When we talk fixed pitch prop (non feathering variety) a spinning prop has more drag through the air than a not spinning prop. Example; a single engine airplane. At cruise. Engine failure. Reduce speed to best glide then keep that speed in decent. The engine (recip type) will keep windmilling. As apposed to; cruise, engine failure. Raise nose to slow aircraft. Prop will stop. Lower nose to attain best glide speed. Prop remains stopped (Harder to get a prop/engine spinning then to keep it spinning) glide angle is better with non spinning prop. Both instances at same indicated airspeed. Weird I know. The only variable is the airmass movement which will affect track and speed over the ground. However the IVSI (Instantaneous Vertical Speed Indicator) will show a higher rate of decent with the spinning prop, again with indicated airspeed the same. It is significant. Again I am talking fixed pitch. NOT feathering.
I don't know why air reacts different than water since both are fluids so to speak.
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Last edited by jerryrlitton; 03-12-2012 at 11:06 AM.
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Old 03-12-2012
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Re: Propeller drag test under sail.

on an airplane the free wheeling prop is not fully free wheeling it is attached to an engine that is at idle or still spinning aganst the engine compression. there is no transmission to put in neutral. also a lot higher differencial in speeds between the still fluid and the moving craft. the drag increase with speed is not linear
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Re: Propeller drag test under sail.

Quote:
Originally Posted by jerryrlitton View Post
I have never done a test in the water like this. I just have practical experience with airplanes. When we talk fixed pitch prop (non feathering variety) a spinning prop has more drag through the air than a not spinning prop. Example; a single engine airplane. At cruise. Engine failure. Reduce speed to best glide then keep that speed in decent. The engine (recip type) will keep windmilling. As apposed to; cruise, engine failure. Raise nose to slow aircraft. Prop will stop. Lower nose to attain best glide speed. Prop remains stopped (Harder to get a prop/engine spinning then to keep it spinning) glide angle is better with non spinning prop. Both instances at same indicated airspeed. Weird I know. The only variable is the airmass movement which will affect track and speed over the ground. However the IVSI (Instantaneous Vertical Speed Indicator) will show a higher rate of decent with the spinning prop, again with indicated airspeed the same. It is significant. Again I am talking fixed pitch. NOT feathering.
I don't know why air reacts different than water since both are fluids so to speak.
Jerry, Big differance between air and water, one is a compressable gas, the other is a incompressible fluid.

John
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Old 03-13-2012
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Re: Propeller drag test under sail.

Quote:
Originally Posted by jerryrlitton View Post
I have never done a test in the water like this. I just have practical experience with airplanes.
Which has nothing to do with propellers in the water dragged under a submerged hull.

It's antidotes like this that confuse new sailors and perpetuate the old wives tales.

It doesn't matter if we reference the Sailboat propeller drag article in The Journal of Ocean Engineering - Scotland ... Or The MIT paper.... Or the Practical Sailor propeller test, and finally this test we are speaking about as all of these tests came to the same conclusion.

There is no data published anywhere that disputes any of these tests. It didn't matter if these tests were in tanks or in salt water or fresh water the results are unerringly the same. As a matter of fact, Michigan Wheel Propellers now references the Main Sail test as it's definitive reference source. I know, it's my website.

It's time to stop the wives tales and accept the data. Spinning props under sail have less drag than fixed Propellers.

The only mitigating factor is whether or not you are allowed to free wheel your propeller under sail or whether the engine manual specifies the prop must be locked in reverse. Some diesels use the engine sump for lubricating the transmission and these can not be left to free wheel when the engine is not running. All Yanmars are fine with it.
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Last edited by TropicCat; 03-13-2012 at 05:05 PM.
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Old 03-13-2012
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Re: Propeller drag test under sail.

Quote:
Originally Posted by Minnewaska View Post
MS, you are a marine mechanical stud. I'm surprised the lady posters here haven't suggested you pose for a calendar..
Who says he hasn't.. Hanging up above my workbench
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  #20  
Old 03-13-2012
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Re: Propeller drag test under sail.

Good experiment that one.
I have always locked the gearbox when sailing. It is less noisy that way and it won't make that rumbling noise when it spins.
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