Spinning Props

[Ex-Calif]

Originally Posted by Marksman

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.

To expand the statement - Main Sail's test clearly indicates that the spinning propeller has less drag than when fixed, for that hull, that prop and that test rig configuration.

Assuming the strain gauge is calibrated, validated and repeatable and I have no doubt of that, the results for different props, hull shapes and conditions will be different.

The effects of scaling have not been addressed. Not to pick an old wound but those who have followed the Beau Boat development could surmise that the results of scale models not repeating on a full size boat indicate variables not considered.

The error in science is not to be incorrect. The error is to assume correctness and ignore significant variables and valid theories that don't fit the original hypothesis.

[David M]

Originally Posted by Liam Wald

For those who believe that there is LESS drag when the prop is spinning, please consider and explain why the following is true,

1) Helicopters that have lost all power will fall at 1100 feet per minute maximum when the prop is turning. This is called a forced rotation. You land really hard but will most likely live. You might break a skid but the helicopter will fly again.

2) Helicopters that have lost all power will drop like a rock when the prop is stopped. You land REALLY REALLY hard and will most likely die in a mangled firey crash. The helicopter will never fly again.

I was thinking the same as you and I think you nailed it. If you have ever stalled any sort of blade you know that it spins up when the laminar flow is suddenly lost. The faster speed means it is taking less power to turn at that speed. Propellers that are not stalled are taking more power because more of the mechanical energy is being converted into propulsion energy. This equation works backwards as well. We see this with steam turbines.

A blade that is stalled is doing a less efficient job of converting energy than is a blade that is not stalled.

Stall your boat propellers blade by locking it and I think it will provide less drag than a blade that is not stalled efficiently converting the flow of water into mechanical energy which is then converted to heat energy by the friction of the boats drive train.

Also, I don't think you ever want to let a prop freewheel because of the unnecessary wear and tear on the drive train.

[Marksman]

"To expand the statement - Main Sail's test clearly indicates that the spinning propeller has less drag than when fixed, for that hull, that prop and that test rig configuration."

Yes, thank you, well put.

"The effects of scaling have not been addressed. Not to pick an old wound but those who have followed the Beau Boat development could surmise that the results of scale models not repeating on a full size boat indicate variables not considered."

Indeed. When tank testing scale models sandpaper has been used to better simulate the laminar flow and boundry layor turbulance characteristics of the full size hull.

[Maine Sail]

Originally Posted by Ex-Calif

The error in science is not to be incorrect. The error is to assume correctness and ignore significant variables and valid theories that don't fit the original hypothesis.

The hypothesis I was working with at the time was because of a challenge by a friend who is an engineer and who worked in the aerospace industry, he is a good friend but was also a STRONG disbeliever.

He was adamant, like many, that ANY prop spinning in the water created more drag than when locked. That was my hypothesis to beat. ANY PROP SPINNING IN THE WATER CREATES MORE DRAG THAN WHEN LOCKED. It went the other way by a long shot and was not even close.

I AM however a believer that at some pitch their could be a cross over or at least no discernible difference as I have noted less variability in drag numbers the lower the pitch goes but still more drag when locked. Most sailboats however have rather high pitches and mostly seem to be some where between 8 & 12+ pitch because many are limited on diamter. I have tested an 8 and 12p three blade and a 12p two blade and all still create more drag locked. I'd be very curious to test a lower pitch, perhaps say a 4 pitch. If anyone cares to send me one, in 1" shaft diameter, I would be glad to throw it on the jig.

Over many months I sent him the MIT study, the Strathclyde paper and then did my own design and test. He sent back lots of equations and theories regarding wall vortexes of the tank etc. etc. as to why MIT said what they said, same for Strathclyde. I then built a rather crude jig:






and sent him photos. He surprisingly agreed that the jig was a fair test and the methodology was satisfactory to prove HIS theory. When I sent him the video and he even accused me of faking it. I then had him come with me in the dinghy and he saw it with his own eyes. He was left speechless and it was pure entertainment. After months of theories and mathematical equations back and forth via email he was left with jaw on the floor. We even tested a two blader that day and had the same result less drag spinning.

All I know is from what I have physically tested myself. My testing surely makes me question the hard and fast "rule" that a locked prop sails faster. My boats with three blade props had never accelerated when locked and almost always accelerated when unlocked. All those years I thought I was just nuts, may still be..

Still not sold on the whirly gigs, eddy's and vortexes on my rudder overcoming a 26 pound drag deficit but keep writing and I'll keep reading.. Perhaps I will have to bust out my digital strain gauge and have my buddy tow my boat through the water. I think I actually have enough cable to reach the helm from the load cell but I still need the data capturing software to really measure it well.. If anyone has a good scale/load cell software program they want to send me please let me know via PM. A digital read out for this type of experiment is only good when exported to a Excel but I don't have an analog load cell only digital..

[Marksman]

I would tend to believe on more modern hull forms the drag created by prop induced turbulance is minimal and restriced mostly to the rudder. However, my mid 60's design hull has a 4 ft. overhang on the stern which becomes wetted surface at higher speeds as it begins to squat and/or heel. This is a prime candidate for parsitic drag from vortaces reacting on many square feet of hull.. It doesn't take much disruption of the laminar flow to reduce efficiency. This is probably apples and oranges, but Nigel Calder of Sail magazine did a extensive study this summer on props while developing an electric drive system and found that just a few barnacles would boost fuel consumption almost 50% at any speed. This is mentioned in the Sept. issue if anyone cares to check it out and it was covered in more detail in an earlier issue.

Food for thought

[Marksman]

Main Sail,

Glad you posted pics of the setup as it wasn't visable in the video. Very nice job and well built. The only thing that colors your results is the angle of the tension line as it will give higher than actual load values. If you know the exact angle you could calculate for true but it doesn't really matter as the percentage would be the same.

Cheers!

[Maine Sail]

Originally Posted by Marksman

If you know the exact angle you could calculate for true but it doesn't really matter as the percentage would be the same.

Cheers!

Was not looking for true just an A/B comparison and the jig and scale are the control. I am sure I could figure out the true based on angle but no real need as an A/B using the same control jig was good enough to show a three times difference when you took away the drag of the jig alone and just left the prop.

As I mentioned earlier I was not looking to measure to the thousandths or ten thousandths and luckily did not have to. I set the jig to mimic the shaft angle, resistance and depth of the prop on my CS-36. Had to adjust the shaft angle based on the dinghy attitude but once that was set it was pretty darn close. Even moved the line slightly higer and spun the jig to get the prop out of the strut wash but noted no discernible difference as a percent.

[Marksman]

Yeah I figgred, I'm just one of "that crowd" so I feel annoyingly compelled to bring it up.

[Dustymc]

Some scary pilots on here - ya'll must fly spam cans.....

A spinning prop is acting like a prop, which is designed for efficiency in moving through whatever media it was designed for. Some of the energy generated by whatever's moving your vehicle is going to turn the prop, and if there's enough energy available to get it up to speed the only thing stopping the prop from moving, leading-edge first with minimal slippage, through the medium is the friction of whatever system the prop is turning.

In aircraft, the windmilling prop is turning over an engine. In a sailboat, it's just turning a bearing or two. In a fairly low-friction system, like a boat transmission, I'd fully expect a turning prop to be pretty low-drag, and a stopped prop to act like a plow (it's not a flat plate, after all). In a high-friction system, I'd expect the opposite - and can demonstrate by noting best glide angle with a stopped versus spinning prop in my airplane. (It's empirically much more efficient to stop the prop since the only alternative is to turn over an engine.)

So, if you want to compare sailboats and airplanes, put your transmission is gear and sail fast enough to get the prop to spin the engine. Until then, you're comparing apples to outhouses. (It also wouldn't hurt the comparison if you'd get tip speeds up to say Mach .98 or so. Probably gonna need a catamaran for that.)

Maine Sail - it would be really interesting if you could repeat the test with various amounts of resistance that the prop must overcome to begin turning. I think a range of results would be interesting and informative, and demonstrate clearly what I probably failed to get across above (possibly due to the current nature of the blood in my alcohol system). I'd expect very low drag with a freewheeling prop, increasing as "system friction" is introduced, and maxing out once you no longer have the energy to begin rotating the prop and whatever it's attached to.

[Tropic Cat]

Originally Posted by Marksman

"puts all the hokey science where it belongs."


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

Ummm...no. The hokey science that is spouted off in this forum that fixed props are faster than spinning props under sail.

I have...count them...2 props on my boat.... If I lock the transmissions in reverse to lock my props while sailing, I might as well have thrown an anchor out.

Main Sail has spent a lot of time, gone through a lot of trouble and has done a great job in documenting and posting his results. Reading his paper, the reader is struck by his objectivity. His results agree not only with my observations, but with MIT, PBO, and any other reputable sailing mag who has ever tried to document the results of fixed versus spinning props.

Once again his paper and the MIT paper (at the bottom of the page) are HERE:

Propeller Drag Test

Anyone who disagrees, either hasn't tried it, or they own a dock queen, or they demonstrate the difference between a sailor and an armchair expert..

Just my opinion...

[Astrid]

Tropic Cat, I don't think you read the caveat at the end of my post:

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.

[GordMay]

I wonder why 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.

[Tropic Cat]

Originally Posted by GordMay

I wonder why 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.

Why Gordo, completely different physics at work of course... everyone knows you can let the dogs out but you have to lock a cat in...

[Astrid]

Most sailing cats I can think of are very shallow drafted and less wetted surface, so their drag effects will be different from a typical deep draft monohull. Cats seem happy to pretty much ride on the surface employing the lift of their sails as opposed to a deep hulled mono riding on the hull's buoyancy. This would create less friction. In addition, I should imagine that the props of a sailing cat are not in as much of the hull's shadow as would be typical of older mono hulled sailboats and there would be less wetted surface aft for prop milling turbulence to have an effect upon. In fact, I would assume, with the shallow draft, locked props, particularly since you have to deal with two of them, would act more as spoilers or brakes in a cat as opposed to the single prop of a typical monohull.

[cfarrar]

Just a quick point about helicopter auto-rotations. When you perform them with the rotor turning the spinning rotor creates an airfoil, hence lift, assuming the helicopter has forward airspeed. You can't auto-rotate straight "down" (ie, perpendicular to the rotor rotation). So the analogy to the sailboat prop is limited.

Great job with the science experiment, Mainesail!