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Old 04-19-2009
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Fixed vs. Free Wheeling Prop - Test Data

Hi All,

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. 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.

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 is a standard three blade fixed sailboat prop. It is made by Michigan Wheel. I also plan to test the Campbell Sailor that has blades that are shaped like an airplanes wings (creates lift) so this will be a good test of the "helicopter theory". For now this post focuses on the Michigan Wheel.

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

The results of the Michigan Wheel MP prop have been completed. I want to clarify 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 throttle position 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. Average drag in fixed mode including the strut was about 45-50 pounds. As you can see .001 differences in accuracy do not matter when trying to answer this question as related to this very, very popular sailboat prop.

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 big scheme 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 for the Michigan prop are quite clear and quite discernible and coincide with those of the MIT study, the University of Strathclyde study and some other prop drag tests like the one in a UK magazine just this month.

9) This experiment & video below 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 50% difference in drag or a 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, you will see less drag when freewheeling.

Michigan Wheel "MP" Prop Drag Video (LINK)
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Last edited by Maine Sail; 08-27-2009 at 11:10 PM.
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Old 04-19-2009
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so the bottom line - 25lbs of pull saved by freewheeling?
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Old 04-19-2009
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Originally Posted by brak View Post
so the bottom line - 25lbs of pull saved by freewheeling?
No not necessarily. On this jig it related to 25 pounds and this jig showed roughly a 50% difference. Due to the angle of the rope and some other factors it is tough to calculate an accurate drag number.

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 the 12 pounds from the 25 or 50 pound numbers to even get close. I think it is safer to say something like "nearly double" the drag difference rather than apply a number in pounds. I leave the .001's up to MIT...

Actually now that you brought this up I just double checked the MIT study again they show the Michigan 3 blade at about 13 pounds of drag at 3.5 knots which is the max speed I could get out of it. 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 an not bad for a back yard hack who did not go to MIT...
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Old 04-19-2009
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nice I am particularly interested because this is my exact prop.

Too bad I can't let the prop freewheel - neither the transmission nor the dripless seal will like it very much.
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Originally Posted by brak View Post
nice I am particularly interested because this is my exact prop.

Too bad I can't let the prop freewheel - neither the transmission nor the dripless seal will like it very much.
What is the tranny? Your dripless should be fine provided it's not plumbed..

I lock mine anyway and will continue to do so...
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Old 04-19-2009
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Quote:
Originally Posted by Maine Sail View Post
What is the tranny? Your dripless should be fine provided it's not plumbed..

I lock mine anyway and will continue to do so...
Dripless is the Lasdrop "plastic ring" type, it has a tube supplying water from the engine exhaust, though the manual says that this additional supply is presumably only important for high speed craft (which I sure ain't ).

Transmission is some type of Kanzaki, whatever is fitted on Yanmar 4JH3E. It has its own ATF fluid. Still, I kinda feel safer when nothing is turning on the engine that is shut off.

This does underscore the utility of auto/max/folding/prop though - 50lbs of pull is nothing to sneeze at.
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Quote:
Originally Posted by Maine Sail View Post
What is the tranny? Your dripless should be fine provided it's not plumbed..
Most newer PSS seals are plumbed (like mine). What is the downside here if indeed it is plumbed?

I have a 2 blade fixed with an Kanzaki KM2P, but the test still test a compelling story....I've always locked my prop to prevent freewheeling the tranny, but I may change my ways.
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Main Sail - Very very nice experiment and post and thank you for taking time to do that. I know it is around double the drag and around an extra 20lbs or so extra but what does that mean in real terms - is this something that a sailboat displacing 11,000lbs will take effect from or is it so minute that it basically doesn't matter? I mean it is awesome to know which way is more efficient don't get me wrong but I am just trying to picture how much it would effect some larger boats.
Thanks again
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It all depends on a boat. On a full keel heavy displacement sailboat 40-50lbs of drag really don't seem to make a difference. I tested free-wheeling vs. locked prop and did not find any difference in speed on my boat.
On a racing boat with well set sails there would definitely be a difference, but then again - what racing boat has a 3 blade 16" michigan wheel?
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Very clever test. Well done.

A couple ideas:

To convert your scale reading to the horizontal drag the formula would be:

Force_horizontal = Scale_reading X cos(angle of string to horizontal)

So if your string slopes down 45 degrees and you're reading 50 lbs, the horizontal component of the force is 50 X cos(45) = 35 lbs.

Drag force varies with the square of the speed, so if you go from 4.2 to 7 knots (i.e., to 7/4.2 = 1.67 of test speed), drag force will go up about 1.67 X 1.67 = 2.78 of what you measured here.

Will an extra 20 lbs of drag make a difference on boat speed? I expect the biggest gain will be in light wind conditions. If you're running and there isn't enough wind to overcome the weight of the sheets on the sails, 20 lbs would make a real difference. With more wind, once the boat gets up to hull speed, wave making drag goes up so fast that 20 lbs won't make any difference.

If I can jump out on a limb here and make a prediction, I'll guess that the prop with the skinny blades will either have no difference when locked, or might even be better. It's a lot easier for water to get around the skinny blades than the wide ones on the Michigan Wheel prop.

Cheers,

Tim
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