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  #11  
Old 11-11-2011
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Quote:
Originally Posted by nickmerc View Post
Maine,

I have experienced the opposite in fixed wing, single engine aircraft. I assumed it would be the same for a prop in water and am interested in why it isn't.
This comparison is also considerably different. The prop on a single engine plane is not freewheeling when the engine shuts off. Rather, it is spinning the engine which has pumping and friction losses. So anytime it is spinning (with engine off), it is causing drag.

And interestingly enough, just as with a sail, there is an optimum speed of the prop with respect to the wind where the losses are higher than at zero speed.

The equivalent comparison to the single engine plane is to allow the prop in the water to work against a generator. There will be an optimum rotational speed (for every boat speed) where it will produce the maximum amount of electrical power. This speed will be the speed of greatest drag because we are producing maximum power.

That speed cannot be the theoretical speed of the prop (maximum speed) in the water defined by the pitch as that speed would result in no slip, therefore no torque.

It cannot either be zero speed, because that speed produces no power. So there will be some speed between zero and maximum where the prop with generator can produce maximum power; hence maximum drag. And that happens because we apply a load to the prop and not allow it to freewheel.
Bryce

Last edited by BryceGTX; 11-11-2011 at 08:21 PM.
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  #12  
Old 11-15-2011
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I would still like to read the reports. A prop installed on a prop shaft that is supported by a cutlass bearing (or two) and connected to a transmission in neutral will still have friction to over come. It will require torque to turn the prop so it will not be freewheeling.
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Old 11-17-2011
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So that clears that up......NOT!! lol
Think I'll just leave it in neutral
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  #14  
Old 11-17-2011
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Quote:
Originally Posted by BryceGTX View Post
Excellent utube test "Maine Sail".

An additional test without a prop would show the drag of the fixturing. This fixture drag subtracted from each test would give a more dramatic difference between fixed and rotating.

Here's some more detail from a post back in 2009:


Question: Does a traditional sailboat fixed style free spinning prop, of typical pitch for reduction gears, create more or less drag than one that is locked when moving through the water?

Over the winter there were a few discussion that lead to no hard conclusions on whether a fixed prop or a locked prop causes more drag when dragged through the water. There have been two studies that have both concluded that a freewheeling prop causes less drag but these studies were done in test tanks and some sailors argued that vortexes created within the tanks throw off the results.

I don't like not knowing.:doh:

I've spent a few late nights in the barn, over the winter, listening to good tunes and plugging away on this design. This jig will tell us what we need to know. It was affixed off the side of my dinghy and dragged through the water ahead of the motor, to avoid vortexes & whirligigs and what ever else, and at a depth similar to that of my sailboats fixed prop.

I measured it both locked and freewheeling and I also measure the drag of the apparatus alone to subtract it from the actual drag of the props minus the test apparatus. I designed the bearings to have a similar resistance to the prop shaft on my own sail boat so from that perspective all quite comparable in terms of freewheeling.

The drag measurements were captured with a 50 Lb. analog scale (ditched the digital as analog showed better on video) and GPS SOG so as to more accurately compare between the same prop in both fixed and freewheeling modes. The range of motion on the scale (movement of the hook) from 0-50 lbs. is about 1/8" so this did not affect any readings what so ever by changing the angle of the test jig in the water..

The prop I used was standard three blade fixed sailboat prop. It is made by Michigan Wheel. I call it the "Dumbo ears prop".

This is an age old argument, with a relatively easy test, yet surprisingly no one has done it, not even Practical Sailor..


The Test jig:



The Shaft Mechanism (the nail is the shaft lock):



The Drag Measurement Assembly:



The Hinge Mechanism:



The Digital 50 Lb. Scale:




Michigan Wheel Data Updated 4/18/09:

The results of the Michigan Wheel MP prop have been completed. I want to reiterate some points below so there is less confusion.

1) This test was only to determine if a standard Michigan Wheel three blade fixed prop causes more or less drag when towed through the ocean at a similar depth to that of a sailboat and with a comparable shaft resistance to a sailboat (namely mine). It is not to give accurate numbers or data on how much drag the specific prop creates.

2) Drag is relative to the the drag jig I used. The drag jig alone, with no prop, created about 12 lbs. of drag in this configuration at WOT.

3) Because the jig is the 100% the same in both fixed and freewheeling and the ONLY difference between fixed and freewheeling was a 2.5 inch roofing nail the only differences in drag come from the prop not being able to spin and spinning.

4) The motor was always run up to wide open throttle to totally minimize any variability between locked and freewheeling.

5) The pin point accuracy of the scale means little because it is only a control. The same scale was used for both fixed and freewheeling and it was only compared to itself in an A/B situation.

6) The difference between fixed and freewheeling was LARGE so a pound or two here or there means very, very little. Average drag at WOT in freewheeling mode was about 20-25 pounds including the test jigs strut. Subtract the test jig strut drag of 12 pounds and you have a free spinning drag of roughly 8-13 pounds of prop drag.

Average drag in fixed mode including the strut was about 45-50 pounds. Subtract the test jig strut drag of 12 pounds and you have a locked drag of roughly 33-38 pounds. As you can see .001 differences in accuracy do not matter when trying to answer this question. The locked prop resulted in an additional 25 pound difference.

With strut drag left in:
Free Spinning = 20-25 Pounds Drag
Locked = 45-50 Pounds Drag

On the low end of both drag range numbers, 20 pounds free and 45 pounds locked, that is a 125% increase in DRAG by locking the prop!

On the high end of the drag range numbers 25 pounds to 50 pounds that is a 100% increase in drag cause by locking the prop!

Strut drag removed:

Free Spinning = 8-13 Pounds Drag
Locked = 33-38 Pounds Drag


On the low end of both drag range numbers, 8 pounds free and 33 pounds locked, that is a 312.5% increase in DRAG by locking the prop!

On the high end of the drag range numbers 13 pounds to 38 pounds that is a 192% increase in drag cause by locking the prop!




As I said before we're not talking about .001 differences.

When I spun the strut around, with the prop facing forward, and ahead of the struts interference wake, I was surprised that i could not detect a discernible difference in load despite having to move the line a little higher on the strut. If there was a difference it was clearly less than one or two pounds and not noticeable in the scale of things.

7) Freewheeling is little bit of a misnomer. The shaft was not actually allowed to freewheel with minimal to no friction. The friction bearings were tightened and adjusted to closely mimic the friction of my own sailboats shaft. This test was primarily for me and my own curiosity and then secondarily for the sailing community. This is why the depth of the prop in the water matches my CS-36T and the shaft friction was set to begin spinning at about .8 - 1.2 knots which is what it does on my own boat.

8) The results are quite discernible and coincide with those of the MIT study, the University of Strathclyde study. Dave Gerr and some other prop drag tests like the one in a UK magazine just this month.

9) This experiment is about the prop used, a Michigan Wheel three blade "MP" prop. I make NO claims or suggestions about any other fixed type props including a two blade version of the Michigan Wheel MP. If someone wants to send me a two blade MP in a 1" shaft size I will be glad to test it too..

10) As far as I know this the ONLY video proof that clearly shows a fixed vs. freewheeling three blade sailboat prop being load tested and compared to itself in both fixed and locked mode.


11) Before you get all fired up because you are a believer that fixed three blade props cause less drag, not more, PLEASE remember that the ONLY difference between the fixed and freewheeling modes was a 2.5" nail passing through both the jig and the 1" shaft to lock it. There is NO possible way that 2.5" nail caused a nearly 100% increase in drag or 25 additional pounds of resistance.

12) I need a bigger motor! I was only able to attain a max speed of about 4.2 knots with the jig and prop in the water freewheeling and less in locked mode. I'd like to hit 6.5-7. Most sailors though are concerned about prop drag at less than hull speed. In light winds, and under hull speed, with a fixed three blade Michigan Wheel like this one, you will see less drag when freewheeling.

Edit: I used a 10HP motor and hit 7 Knots but the scale then needed to be bigger as in locked mode it spun past where it should. Free spinning was still less drag by a wide margin. I have not been able to track down a 100 pound scale. If anyone knows of a reasonably priced one please let me know.

13) I also load tested the jig alone, without a prop, at WOT and it had about 12+/- pounds of drag so you would need to subtract approx 12 pounds from the 25 or 50 pound numbers to get the actual prop drag in this test. I think it is safer to say something like "more than double" it is actually closer to 2.9 times more observed drag. I leave the .001's up to MIT but they are really not needed here.

If I subtract the jig drag from the numbers this is what we get.

Locked = Drag 50 Lbs. - Jig Drag 12Lbs. = 38 Lbs. of drag
Free Spin = Drag 25 Lbs. - Jig Drag 12Lbs. = 13 Lbs. of drag

That is a BIG difference!!!

I actually double checked the MIT study again they show the Michigan 3 blade at about 13 pounds of drag at 3.5 knots which is close to the max speed I could get out of it when locked. If you subtract the 12 pounds of test jig drag from the 25 pounds measured on the scale you are within a pound or two of the MIT study. Pretty close and not bad for a back yard hack who did not go to MIT...

Interestingly enough Yachting Monthly just did a similar project and their data also agrees with the University of Strathclyde and MIT as well as my findings:


Prop Drag Video (LINK)




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  #15  
Old 11-18-2011
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Thanks for that, very interesting.
What I was actually bothered about though was if leaving it in neutral would cause any damage and nothing to do with speed/drag.
Remember we are taking about a Hurth/ZF box here with no hydraulic pumps etc. As I said in my original post the maual gives no guide whatsoever yet a ZF dealer told me it MUST be put in reverse or else damage to the clutch plates will result. Who to believe ??
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Old 11-18-2011
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How could leaving it in gear in either forward or reverse harm anything? Nothing is turning!

With a helicopter the blades are pulling the helicopter up not down. If you turned the blades upside down and left one free to spin then compared it to one with upside down blades that are locked spinning it would be a better test.
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  #17  
Old 11-18-2011
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Thanks for that, very interesting.
What I was actually bothered about though was if leaving it in neutral would cause any damage and nothing to do with speed/drag.
Remember we are taking about a Hurth/ZF box here with no hydraulic pumps etc. As I said in my original post the maual gives no guide whatsoever yet a ZF dealer told me it MUST be put in reverse or else damage to the clutch plates will result. Who to believe ??
Please read post #6. I answered your question and provided proof and detail direct from the manufacturer..
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  #18  
Old 11-18-2011
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Originally Posted by steel View Post
How could leaving it in gear in either forward or reverse harm anything? Nothing is turning!
.

Pretty simple...On the Hurth/ZF boxes the clutch plates slip and wear on each other when left in the same direction of travel or forward gear when sailing forward.. Doing this sheds tiny metal particulate into the fluid and it then wears the thrust bearing out until the gear box starts slipping.. Do not leave any Hurth/ZF box in the same direction of travel eg: forward when sailing...

This pic is the result of a Hurth box being sailed while in forward gear.. Locking in reverse or letting a Hurth ZF freewheel are both perfectly acceptable just not forward.

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  #19  
Old 11-20-2011
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BINGO !
Yes that sounds spot on. I know from bitter experience it's always the thrust washers that go long before the clutch plates and that explanation explains why.
I did actually read your post before but didn't notice the part about not leaving it in forward.

So nuetral it is - less drag - no wear.
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