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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I am particularly interested because this is my exact prop. 
