Fixed vs. Free Wheeling Prop - Data !

[Maine Sail]

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 still don't know the answer but I will, at least in relation to fixed three blade props, soon.

I've spent a few late nights in the barn, over the winter, listening to good tunes and plugging away on this design. I think this test jig will tell us what we need to know. It will 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 a sailboats fixed prop.

I will measure it both locked and freewheeling and I will 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 sail boat so from that perspective all should be good.

The drag measurements will be captured with a 50 Lb. digital fishing scale 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 should not affect any readings what so ever by changing the angle of the test jig in the water..

The props I will be using are standard fixed sailboat props. One is made by Michigan Wheel. I call it the "Dumbo ears prop" and the other will be my new Campbell Sailor prop. Unlike Dumbo ears the Campbell Sailor has blades that are shaped like an airplanes wings (creates lift) so this will be a good test of the "helicopter theory"..

I don't actually know when I'll get to this but hopefully it will be in the next week or two when I have time. It took me a good three months just to get to this point..;)


This is an age old argument, with a relatively easy test, surprisingly no one has done it yet, 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:


The Campbell Sailor Prop:



Edited 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 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.

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. 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.

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 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 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 Video Below:
http://www.youtube.com/watch?v=jI-UG9RSlJo

[Loren]

I'm impressed with your setup. I thought of doing something along those lines but it was as far as it got. Thanks for doing the work - I'm looking forward to your results. I'd make a prediction but will just wait for your testing.

Fair winds,
Loren
SV Whippoorwill
CD 27

[Joe Myerson]

MainSail:

If you apply scientific study to this problem, John Vigor will have to come up with something else to generate long wintertime threads--like whether or not to carry firearms aboard.



--Joe

[Dick Villamil]

Does your study imply that a 2-bladed prop will perform similarly? Did you test the effects of a free wheeling vs fixed prop in the turbulent water aft of the keel? And lastly, will this thread take until winter to solve?

[Maine Sail]

Does your study imply that a 2-bladed prop will perform similarly?

After doing this I would make NO claims on a two blade without first testing it or any other prop. This test ONLY relates to the fixed three blade Michigan "MP" prop.

Did you test the effects of a free wheeling vs fixed prop in the turbulent water aft of the keel?

No and that is not the answer I was looking for. I wanted to answer the age old question of does a fixed three blade prop cause more drag fixed or locked when traveling through the water.
Hopefully Carl Arlberg or Dieter Empacher, or who ever else, depending on your boat, designed it to have minimal turbulence as turbulence in and of itself can reduce hull speed....

And lastly, will this thread take until winter to solve?

Hopefully not but you never know. If it was a 10% difference I could see room for argument but with this prop is not even close!!

[Mathias]

Ok, this just confirms what kind of effect winter can have on sailors. It is sad, but it produces impressive results.

Maybe rocket scientists should be taken out of their window-less, fluorescent-lit labs and forced onto a sunny island in the Caribbean for five months and they would figure out how we can solve the space-time continuum and fly to distant galaxies.

Bravo on such a set-up and such thoughtfulness. My comment is this: Is it possible that the difference is even greater than what you present?

With the prop fixed, the meter seemed to be reading at the top end of its scale and sometimes even at its maximum. Scales aren't always accurate at their extremes.

You should consider writing up your results for one of the magazines. But you probably have already submitted.

Nice job.
-Mathias

[Neil Gordon]

Does your study imply that a 2-bladed prop will perform similarly? Did you test the effects of a free wheeling vs fixed prop in the turbulent water aft of the keel? And lastly, will this thread take until winter to solve?

Don't forget that sailing boats don't track exactly in the direction they are pointed. Accordingly, water flow isn't the same down the port side as on the starboard side. Also, the prop, whether free wheeling or locked, is somewhat offset from the direction of travel. (All of this changing based on point of sail, wind speed, angle of heel and other factors, by the way.)

[Steve Laume]

Incredible job as always. You were scarring me on the video though. I couldn't see where we were going as we were starring at the instruments while the docks were getting pretty close. Then there was all those mooring sticks I imagined we might hit, tear the entire rig off the inflatable and sink in the cold Maine waters. All this in the name of science.

This does prove to me beyond a doubt that a rotating prop causes less drag. I am not sure how much less or how important it is. I think your idea of a more powerful boat to check the drag at higher speed is good. I would suspect the gap in performance would widen as speed increased. The scale appeared to be pegged so a bigger one and maybe even stronger lines might be needed if experiments are to continue.

What I don't know is how much 25 or 50 pounds of drag equates to in boat speed. Maybe a comparative amount of sail area in average winds. The big transducer "fairing" block on my boat always bothers me as did the uneven bottom paint before I striped it all off. Is a big 3 bladed prop in fixed mode canceling out my staysail in 10 knots of wind?

I tend to let my prop have it's own way. The question is weather this decrease in drag is going to cost me a transmission overhaul at some premature point.

Is a free wheeling prop more likely to catch a pot wrap?

Every answer tends to create more questions, Steve.

[darmoose]

Mainsail my friend, you are to be commended for your openmindness, your devotion to seeking the truth, and your enginuity.

While this fact of life has been proved over and over ( I myself did similar tests with my own boat which has a new engine, propeller and shaft, and most importantly very little built-in friction in the drivetrain. I took the boat to a given speed under power, then killed the engine and allowed the propeller to freewheel and then later did the same with the propeller locked, measuring the distance and the time it took the boat to come to a stop. After numerous runs, it never failed that a freewheeling propeller would allow the boat to coast farther and longer than a fixed propeller.), alas, no amount of proof is enough to convert the naysayers,aka, proplockers from their dogma.

Seems, in Boston, they find that a bit of sideways motion will reverse your conclusions in favor of locking one's propeller, imagine that!

As I have said from the beginning, several years ago (seems in another life...) No experienced sailor unlocks his propeller and expects his boat to slow down while under sail.

It will be interesting to see if any of "them" will step up and acknowledge your efforts, and the obvious fact.

Thank you for your effort, good job.

Darrell

[Maine Sail]

Not just my findings.

The MIT study done by Todd Taylor & Beth Lurie is also in line with my findings.

The University of Strathclyde Ocean Engineering Department Study finds similarly:
http://strathprints.strath.ac.uk/5670/1 ... 005670.pdf

And in the May 2009 issues of Yachting Monthly they conducted yet another in depth test where an actual boat was dragged through the water with a strain gauge monitoring loads.

http://www.yachtingmonthly.com/ym/any_q ... anyqs.html

Here were their findings:
http://forums.sbo.sailboatowners.com/at ... 1240173326

I still suspect there will be doubters who apparently know more than the collective wisdom of MIT, The University of Strathclyde Ocean Engineering Department and the investigators at Yachting Monthly who put this test to "real world" conditions by actually dragging a real sailboat....


I don't want to burst your bubble BUT...

But wait until you see the Campbell Sailor video.. I won't have it done for a little while but it's a shocker compared to the Michigan Wheel video. It has very different result that do lend some credibility to the helicopter/lift theory... maybe..

The Campbell Sailor was so close in both locked and freewheeling I need to watch the videos again to see if I can even tell a difference.

My hypothesis thus far is that traditional props, like those made my Michigan Wheel, are not shaped like a foils/wings, helicopter blades or airplane prop but instead more like a screw. This is not to say they can't create lift but not as much as a foil shaped prop like the Campbell Sailor can. The Campbell Sailor is one of, if not the only, sailboat prop shaped like a wing thus it creates more lift.

The bottom line is this, in relation to fixed three blade props. If you have a standard prop like the three blade Michigan Wheel prop freewheeling has less drag, every study done has concluded this, but if you have a foil shaped prop like the Campbell Sailor freewheeling it gains little to no benefit by freewheeling. So far I think my experiment is the only one to actually measure this.

The one thing I have found is that the common thinking by some that a locked three blade prop causes less drag is not true in either a screw type prop or a foil shaped prop though with a foil shaped prop there is little to no difference between locked and freewheeling. I am still amazed at the HUGE difference in drag with the screw type prop when locked though.

I repeated these test on two different occasions/days, because I was having trouble believing what I saw, and they still put up the same numbers both times. The results are what they are.

I am kind of bummed that my new Campbell Sailor loaded up roughly 30-35pounds of drag in either fixed or freewheeling when I could have sailed faster with an unlocked Michigan Wheel? Of course I will still lock my prop so I do gain quite a bit little over a locked Michigan Wheel just as the MIT study showed..

[Joe Myerson]

MainSail:

Thanks for posting the links to those studies.

The conclusion of the (unedited) Strathclyde study is pretty strongly worded--saying that the drag of fixed shafts can be as much as 100 percent greater than rotating shafts.

I haven't read the others (the math is beyond me, but I understand words pretty good), but I would assume they reached similar conclusions.

So here's my question: Is it safe to let your prop rotate freely? I believe that in the case of older engines/transmissions, like my clunky but faithful 1GM, it's better to lock the prop and suffer through the drag. That's too bad because my 25D is slow enough anyway.

What do others have to say?

--Joe

[Neil Gordon]

>>What do others have to say?<<

I suggest that in addition to the lab science, which sometimes captures sufficient variables and sometimes doesn't, we measure the length of linear feet of trophies won by lockers vs. spinners.

[Amgine]

Not an engineer or mechanic, but based on a lot of reading...

There are two general methods of lubricating a transmission: a pump powered by the shaft, or a pump powered off the gearing. It is much easier to design the pump off the gearing, so it is more common.

If the pump is powered off the shaft, your transmission is lubricated while freewheeling, but is still getting more wear and tear over time than it would not freewheeling. If the pump is powered off the gearing, your transmission is not being lubricated at all during freewheeling; a clearly not safe circumstance.

You'll need to learn more about your boat's specific transmission to know which general method is used.

[Maine Sail]

Most of the ZF/Hurth gears we see in sailboats, most of the Kanzaki's (Yanmar) and also the JS gears (some Westerbeke) can be left in neutral and are "splash" lubricated. Still best to consult your specific manual..


From Universal:

UNIVERSAL DIESEL WITH HURTH TRANSMISSION

Model HBW-50 (2:1)
Used on Models 12, M2-12, M-18, M3-20, M4-30, M25 and M-25XP

Model HBW-100 (1.8:1)
Used on Models 30, 35, and 40

Model HBW-150 (1.9:1)
Used on Model 50

Model HBW-150 V-Drive (2.13:1)
Used on All of our V-Drive Models

CAUTION
DO NOT LEAVE GEAR IN FORWARD WHEN SAILING. GEAR MUST BE IN NEUTRAL FOR FREE WHEELING OR SHIFTED INTO REVERSE TO LOCK PROPELLER WHILE SAILING.

From the C-34 site:
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[darmoose]

Mainsail,

I watched your video, and noticed it is difficult to get an accurate reading on your scale, as it is bouncing around due to the rough water. And in your earlier reply to my post above you said that when you did the test for the campbell propeller you had difficulty reading any significant difference between the freewheeling and the fixed propeller test again due to the bouncing around effect caused by the water.

Let me suggest that rather than try to read your scale, you simply make note of the maximum speed you can attain per you GPS which is easier to read, doesn't bounce, and is more accurate for what is measures (speed). You probably already have the data captured on you video.

Since you are conducting the tests at full throttle (maximum speed ), I would expect that your maximum speed attained was different for both propellers under the freewheeling and fixed conditions.

Please share that info with us. We can calculate the percentage difference in speed loss and get a good feel for what the locked propeller does to our speed.

Darrell