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Old 17-09-2010, 14:15   #46
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I think the drag situation is greatly dependent upon the type of propeller you have. A two blade or even a three blade has significant lateral "open space" between each blade. So the "Flat Plate Drag" area is smallest with the two blade when the prop is not spinning. However, with my 4 blade propeller there is virtually no "open space" between the blades so the "flat plate drag" area is near 100% when the prop is not spinning. Allowing the prop to spin introduces the longitudinal open space between blades into the equation and effectively reduces the flat plate drag.
- - But the first and foremost determinant in letting the prop spin or not is the manufacturers stated limitations in the operating manual or service bulletins.
- - The next level determinant is as Ex-Calif said: "The 10,000 miles of freewheeling wear on the system trumps the speed issue anyway."
The wear and tear on your bearings, stuffing box, cutlass bearings, and induced vibrations can add up to a lot more money than buying a proper shaft brake. Not to mention the down time getting repairs for the aforementioned worn out parts.
- - If you are really into maximizing sailing speed without the engine running buy a feathering/folding propeller - I think it would be a lot cheaper in the long run.
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Old 17-09-2010, 14:15   #47
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In "Ted Brewer Explains Sailboat Design" he states that tank testing proved a free wheeling propeller produced more drag than a stationary one. The difference was slight but should not be ignored. Another test, while driving your car, stick a pinwheel out the window and feel the drag increase as it begines to spin. There is physical science behind this but I would be very poor at explaining it and I must get back to work.

Peace,
I have no idea who did that testing or what he was referring to so can't comment on his statement.

There have been three studies SPECIFICALLY looking at sailboat props, one by MIT, one by the University of Strathclyde Ocean Engineering Department and one by Practical Boat Owner/Yachting Monthly where they towed actual boats through the water and measured with a strain gauge, All three studies of SAILBOAT props all agree a locked prop causes more drag.

For those still in doubt feel free to watch an experiment I conducted myself to finally try and put this question to bed.

Prop Drag Video (LINK)


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 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, freewheeling and throttle response.

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, quite discernible and coincide with those of the MIT study, the University of Strathclyde study and some other prop drag tests like the one YM Magazine.

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.

13) If we extrapolate the data and remove the test jig, 12 lbs. of drag, and use the high side numbers of both fixed and locked we have 38 pounds of drag in fixed mode (50 Lbs. - 12 = 38) and 13 pounds of drag (25 Lbs. - 12 = 13) in freewheeling. This is nearly THREE TIMES more drag. Again, we're not talking .001 fractions here we're talking nearly a three times difference in drag when this three blade prop was locked.

I'm sure there will still be doubters... Despite all this I still lock my prop, unless I want to go faster..

P.S. Before you do either consult your gear boxes owners manual..
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Old 17-09-2010, 17:02   #48
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Maine Sail - one cool post. I love it!

Now how do you spin in your prop when the boat goes slow and there is not enough flow to induce the spin? This seems to coincide with the time when the power from the sails is low enough for the boat's speed to be significantly affected ...

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Old 17-09-2010, 17:47   #49
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Bill and Ted's Excellent tank test adventure

I'm pretty sure it was Ted Brewer and Bill Ludders doing the tank tests but I would have to go back and read that section again to be certain. There was nothing to specify date, place or methodology, just the results and his recomendation. After reading about your test results I would be interested to see how B and T went about getting their data.
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Old 17-09-2010, 17:52   #50
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Maine Sail - one cool post. I love it!

Now how do you spin in your prop when the boat goes slow and there is not enough flow to induce the spin? This seems to coincide with the time when the power from the sails is low enough for the boat's speed to be significantly affected ...

barnie
My prop begins to spin at .8 - 1.2 knots. If I am going that slow I usually employ the Japanese genny..
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Old 18-09-2010, 13:02   #51
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Eddie currents

I went back and re read the section on drag under sail in Ted's book, and the explination was that the spinning propeller creates more turbulance aft and makes for a slower boat. So, as Main Sail has proven, the spinning prop in it's self has greatly reduced drag, but as noted by some highly accredited naval architects, the downstram effects can still be in the negative, though I would expect the degree of negativity is entirely dependant on hull type, rudder type and configuration, boat speed and many other factors making each and every boat an individual experiment.
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Old 18-09-2010, 15:54   #52
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G'Day all,

Maine Sail -- a well done home-brew experiment.

But, seems to me that anyone who wants to know about their own boat should simply sail in reasonably stable conditions and repeatedly shift in and out of gear whilst watching the knotmeter. If there is sufficient difference in drag to worry about the speed should reflect it.

Meanwhile, I'm happy to have a folding prop!

Cheers,

Jim and Ann s/v Insatiable II lying Cairns, Qld, Oz
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Old 18-09-2010, 16:31   #53
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At the end of the day

You're right Jim, at the end of the day, that is what is important, or not. It's just that some of us suffer from an incurable sickness called "Gotta know what/why". If the experts make a claim and are correct, I want to know all about it. If they are full of guano, I still want to know where they went wrong and why. Life is a mystery.

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Old 18-09-2010, 18:50   #54
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One point that I do not recall being put forth, is the effects a water milling prop on the water flow over the rudder. A milling prop creates a large disk of turbulence behind it and that would tend to interfere with the rudder control surface. If i shut down the engine on my plane, and put it into a glide so the prop windmills (old style non adjustable pitch), the air flow behind the prop is so disturbed as it passes along the fuselage and over the tail surface, that control becomes very difficult. If I then flatten the glide so that the prop stops or only turns very slowly, control comes back to reasonable levels.

I would imagine, that a sailboat will not be that much different, even though the medium it operates in and the pitch of the prop is considerably different. Rudder response may not be as effective on a boat with a milling prop, but the effects would be much less apparent that with an airplane.

Furthermore the disturbed water behind the milling prop is acting directly on the hull--it is actually the disturbed water in the wake of a sailboat that is causing drag. Marine architects go to a lot of trouble to make a hull efficient, depending on what its operating environment is going to be. For the most part, though, you can look at the hull as something of a double foil: water is pushed aside by the bow and hull at a controlled rate until the beam is reached, then accelerates towards the stern as the hull tapers back down again. If the designer has done his work well (with a displacement hull), the displaced water will come back into the wake far enough aft to minimize the drag effect. A milling prop, on the other hand is causing the accelerated flow of displaced water to be interrupted and flowing back into the wake much closer to a more forward point of the hull, thus increasing drag. This would be somewhat analogous to cutting off a portion of the hull so that it was completely flat instead of nicely tapered.

A non-milling prop will also interfere with water flow, but only at two, three, or four points, depending on the number of blades, instead of a milling prop's 360 points. It is thus creating drag also, but not nearly so much.

Now then, I am going to stick my neck way out, or climb out on that very thin limb. Main Sail's very fascinating experiment showed, I believe, not hull drag, but the energy required to actually turn a prop. If a non turning prop (x) was creating 50-lbs of force on the scale, while a milling prop (y) was only showing 30-lbs, then the difference of x - y = 20. That 20 is the extra amount of energy absorbed by the non milling prop compared to one allowed to mill as the boat moves. Yes, that extra 20lbs of force can be extrapolated into thrust or horsepower that is being taken away from your boat, but the effects are fairly small astern of the prop. The milling prop is causing much more turbulence astern to the point that the increased hull drag is not only canceling out any advantage accrued in letting the prop mill, but putting one farther into the red, thus requiring even more sail or wind to overcome it.

The only proper way to test the theory, is to actually use one's own boat and see if one can discern a difference over the course of a long run, and only if one has a proper displacement boat. A non-displacement boat, perhaps a cat for instance, would probably show entirely different drag effects in water and might be more efficient with the prop milling.
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Old 18-09-2010, 18:52   #55
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It's possible to let the prop spin and collect back that energy I think... might make what ever small wear there is worth it.


prop shaft driven generator
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Old 19-09-2010, 12:18   #56
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Aha!

Thank you Astrid, I knew someone else could explain it better than I.
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Old 19-09-2010, 12:36   #57
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Thank you Astrid, I knew someone else could explain it better than I.

Astrid,

I have experimented with this for a long time and have yet to see a spinning prop slow down my boat or other boats I have sailed on. This has included full keelers from a Hinckley to a Pearson 30.

This all started during a PHRF race when the captain locked the prop to go faster and we promptly slowed down. I asked him to unlock and we sped up. He was a disbeliever so he again locked it and we slowed down. We did it about fifteen times on the way back to the yacht club and each time when it was allowed to spin we picked up speed. Yes I race competitively and most racing boats have feathering or folding props but I also do a lot of PHRF stuff on boats people also cruise on that have fixed props.

There will always be disbelievers who's theories and leanings lie in the aeronautical world, this is not new. Many aerodynamicists also argued that DDWFTTW (dead down wind faster than the wind) was impossible, they now have eaten a lot of crow and are still arguing even after an official record has been ratified.

BTW when dragging that jig though the water there was significantly more jiggle, wiggle and fighting from the strut than when the prop was allowed to take the path of least resistance and spin. This to me would seem to be causing more disturbances in the water than allowing it to take the path it wants to.

All five of my most recent boats have sped up when allowed to freewheel and all of them had fixed three blade props. We cruise our boat and have to deal with rivers, tidal currents and lobster pots that can destroy a mechanical prop in a mere second of inattentiveness. One was full keel, three were fin with spade rudder and one was a fin with partial skeg rudder. My experiences match those of the MIT study, the University of Strathclyde, Yachting Monthly and my own physical experiments.

Even Dave Gerr author of the Propeller Handbook and Director of the Westlawn Institue of Marine Technolgy says the following:

“The simple answer is that a propeller creates less drag when free to rotate.”
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Old 19-09-2010, 14:23   #58
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In "Ted Brewer Explains Sailboat Design" he states that tank testing proved a free wheeling propeller produced more drag than a stationary one. ..... Peace,
Just goes to show you that even an expert can be dead wrong.

Look at our member, Main Sail's results. the conclusion's are inescapable and puts all the hokey science where it belongs.

His test is here: Watch the video
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Old 19-09-2010, 17:40   #59
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Hokey Science

"puts all the hokey science where it belongs."


You mean the same "hokey science" that designed the air foils that propell your boat?

Science belongs in the hands of those who understand it. And when it is found to be wrong, it gets corrected. Main Sail's test clearly indicates that the spinning propeller has less drag than when fixed, and no one here is disputing that. But that's only half the equation relating to total parasitic drag, the other half contains many variables and is not as easy to nail down.
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Old 19-09-2010, 18:55   #60
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More Hokey Science

A controlled experiment showing propeller drag/pitch relationship. A steep pitch propeller has less drag when milling, a shallow pitch propeller has more drag when milling.

Stationary and Windmilling Propeller Drag
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