This is of particular concern for us cruising catamaran owners as we have two propellers in the water and double the drag. We all know that underwater drag under sail is drag we can do without. So, which school of thought is correct? Hint ...you don't have Helicopter rotors on a boat.
First, a note on Marine Diesel Engine transmissions
Unless your transmission specifically says to shift into reverse while sailing, all sailboat owners should be free wheeling their propellers while sailing.
Now .... we know where the transmission should be set when sailing ... but ... RC of Sailnet.com , better known as "Maine Sail" decided to find out if locking the transmission hurt his sailing performance by increasing underwater drag, or if freewheeling the propeller reduced drag and was actually faster. This test has put an end to the bickering and has done what I feel is the definitive work on this subject. He has graciously allowed me to reproduce his work on this page.
This is the first comparison I've read between free wheeling propellers and fixed propellers that has an accompanying video verifying each phase of the testing procedure, as well as photos illustrating the jig constructed for the test. Also, it should be mentioned that this test was done in a harbor, not a test tank and the results are both conclusive and inescapable. Scroll down the page a bit to view the video of the actual propeller test.
Now, on to his test...
Maine Sail's Propeller Drag Test (with his permission!!)
I don't like not knowing.
I spent a few late nights in the barn, over the winter, listening to good tunes and plugging away on a way to test the theories and came up with a test rig. This jig told me what I needed to know. It was designed to be 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 own sailboats fixed prop.
Propeller Drag test
Video of the actual test
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.
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!
Total Jig Drag = measured 12 pounds with no propeller mounted. Jig drag measurement was taken at WOT. (wide open throttle)
Locked Prop Drag = 45-50 pounds: When we subtract the measured jig drag, 50 -12 = 38 pounds measured of actual propeller drag when locking the propeller in reverse under sail.
Freewheeling Prop Drag = 20-25 pounds: 25 - 12 = 13 pounds of actual propeller drag when freewheeling the propeller under sail.
The locked propeller drag is 2.92 times more drag or a 292% increase in drag over a freewheeling propeller when you remove the test jig from the equation!!!!
I'll repeat that I've never seen as much misinformation on a sailing subject as we get on propeller drag under sail.
Since it was mentioned, I'm posting a link to both the original MIT paper on the same subject and The Sailboat propeller drag paper written by P.M. MacKenzie and M.A. Forrester below this paragraph. They are with the Department of Mechanical Engineering, University of Strathclyde, Glasgow , Scotland. The MIT report is an in depth study of 10 Propellers from many manufacturers and both fixed 2 and 3 blade as well as Maxi props.
Actually, both papers are interesting reading and you would have thought these papers would have ended the controversy. In fact every study which was ever done on this subject has come to the same conclusion, a locked propeller creates more drag when under sail, yet that hasn't even slowed down all the misinformation being dished out in sailing forums by 'arm chair' sailors.
I'm hoping that our very own "Maine Sail's" work can do what MIT and the University of Strathclyde could not. Educate sailors and dispel old wives tails and actually change the thinking in the sailing community to understanding that Free Wheeling a propeller instead of locking props under sail is better whenever it's appropriate.
This webpage has been mentioned as the best propeller drag comparison test on the internet and the only place all the propeller drag tests can be found in one place. I will humbly point out that apparently Michigan Wheel Marine (propeller manufacturer) feels this way as they have cited this webpage as the definitive resource on the subject (link to their page).
A huge "Thank You" to Maine Sail for the time and trouble he devoted to answering this question once and for all and for allowing his test to be published on catamaransite.com.