I designed the jig bearings to have a similar resistance to the prop shaft on my own sail boat so from that perspective all is quite comparable in terms of freewheeling.
The propeller drag measurements were captured with a 50 Lb. analog scale (I had to ditch the digital scale shown in the photos as analog showed better on video) and GPS was used to measure SOG so as to more accurately compare between the same propeller 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 propeller I used is a standard three blade fixed sailboat prop. It is made by Michigan Wheel. So this post focuses on the Michigan Wheel three blade prop which is perhaps the most common fixed prop used on sailboats in the US.
This is an age old argument, which can be resolved with a relatively easy test, yet surprisingly no one has done it, not even Practical Sailor.
Michigan Wheel DataThe results of the Michigan Wheel MP propeller were...well surprising to say the least. 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, particularly my CS-36, 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 propeller attached, created about 12 lbs. of drag in this configuration at WOT (wide open throttle) on my 4 hp Johnson outboard.
3) Because the test jig is exactly the same in both fixed and freewheeling tests and the ONLY difference between the fixed propeller and the freewheeling propeller test was a 2.5 inch roofing nail, I can definitively state that the only differences in the propeller drag tests comes from the propeller 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 the propeller being locked or 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, fixed/freewheel.
6) The difference between the fixed and freewheeling tests was LARGE, so a pound or two here or there means very, very little. The test jig measured the average drag at WOT (wide open throttle) in freewheeling mode, including the strut, at 20-25 pounds. The average drag in fixed (locked) 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.
For those worried about whirly gigs and vortexes and .0001 differences I then turned the test jig around, with the propeller facing forward, and ahead of the struts "interference wake", and reran the test. 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 I designed 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 three blade prop are quite clear, and quite discernible, and coincide with those of the MIT study, the University of Strathclyde study and other prop drag tests like the one in a the UK's Yachting Monthly magazine.
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 only to itself in both fixed and locked mode.
11) Before anyone gets all fired up because they 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 in place. There is NO possible way that 2.5" nail caused a nearly 300% 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 and the 4 knot range is less than hull speed for most sailors. In light winds, and under hull speed, with a fixed three blade Michigan Wheel, you will see less drag when freewheeling!