Freewheeling propellers: the chopper theory

[Stan W.]

...I will keep my propeller locked for reasons that have nothing to do with performance.

Everything else being equal, do what Stan did!!!

FWIW, I'm a locker, but so far I haven't bothered trying to line my two-blader up with the deadwood. Saving that for next year.

[Matt Cawthorne]

Darell,
Any engineering problem that has had the important factors tossed out to simplify it is not an engineering problem. It is a junior high school science problem.
I'll repeat this again as it seems to have missed it mark earlier. Depending on key things like pitch, bearing drag, barnacles, shaft log friction, velocity, blade section number of blades etc, a freewheeling prop prop can be made to produce more drag or less drag than a fixed prop. I think that a previous post mentioned test data of a boat in a current used to prove this point. You can hypothesize for 1000 years and get a wrong answer, but a few tests on your boat will get the correct answer for your boat. Your boat may well have a different result than my boat.
As far as the helicopter analogy, both the autorotating helicopter and the freewheeling prop have pitch angles that point the nose of the blade into the flow relative to the disk plane, and both have pitch angles that create an angle of attack that provide lift in the opposite direction from the direction of motion. Most sailboat propellers have a profile that allows for the development of lift with the flow either direction provided the angle of attack is not too high. Both extract energy from the flow and both have the potential to generate more thrust in the opposite direction from the flow than stalled, non-rotating blades. The analogy holds in these respects. What John failed to recognize in the original post was that he was comparing a powered state on his model to an unpowered state of a freewheeling prop. The powered state on his model is equivalent to sailing with the engine running and the transmission in reverse.

I am tired of this post and will not be reading it any more. If anyone has a specific question, send me an email.

Matt

[darmoose]

to all the contributers to this thread,

before i start i only hope you realize how hard this is to write, the only thing harder has been the past 24 hours trying to decide whether to admit this or not. in the end i must.

i want to apologize for my stubbernous, and for my overbearing foolishness as it relates to my not being able to see Johns analogy.

i dont know what to say, except that i truly didnt recognize it until this past Sunday when i read ALs post,and went back to reread Matts. i was in shock.

i have reread this entire thread, and for what its worth, i think i got off kilter on 12/29/06, and never recovered. i couldnt see that the leading edges of both blades and prop were down or forward.

not only was i wrong, i wasnt always nice (some of that was attempted humor, but not all)

i could go on, but this is painful. i have learned a life lesson. i hope all will accept my apology.

darrell

[John Vigor]

Darrell, there's really no need for an apology. You added a lot of vim and, um, vigor, to this debate. You stirred a lot of people to deeper thought, and you went through a process whereby you eventually overcame your preconceived notions and had an epiphany. That's all good and makes you an admirable character. It takes guts to admit a mistake, or a firmly held misconception, and I admire you for it. You were obviously excited and stimulated by the debate, and that's fine. It makes you very human. I hope we will see more of your contributions for a long time to come.

Cheers,

John V.

[tartansailor]

Darrell


Now there is a man I can respect.



Dick

[Neil Gordon]

... i have reread this entire thread, ...

That's punishment enough!

[Oswego John]

Yo Darmoose,

Good man. Keep up with your usual good posts, brother.

O J

[Dick Barthel]

Darrell,

One thing that I just learned is that Darmoose is loaded with intellectual honesty. We all enjoyed the spirited defense of your position and you were a big part of making this a really fun thread.

Dick

I use not only the brain I have but all I can borrow."
--Woodrow Wilson--

[Greg Ross]

I consulted an important publication and a recent addition to my Library, "The Practical Mariner's Book of Knowledge"
and this didn't seem to be addressed?

[John Vigor]

Greg, see page 125, Propellers, Various Handy Facts.

Cheers,

John V.

[Dick Barthel]

Greg, see page 125, Propellers, Various Handy Facts.

Cheers,

John V.

Greg post and John's response got me to thinking (always dangerous). For the heck of it I checked another JV authored book "Boatowner's Handbook" which I quote from page 48:

"There is disagreement among the experts about whether a prop free to rotate under sail causes less drag that one that is fixed. U.S. naval architect Dave Gerr says flatly: 'The simple answer is that a propeller creates less drag when free to rotate'" Boatowners Handbook, page 48.

However, I came across a curious contradiction from the aforementioned Mr. Gerr: On page 108 of his book "Propeller Handbook" in a section entitled Locked Propeller or Free to Rotate for Minimum Drag. I will quote the entire paragraph so no subsequent disputes can arise over context:

"This brings us to the old argument as to whether a propeller produces the least drag when it is free to rotate or locked. The answer is both, depending on the configuration of the hull, keel and propeller. (Note, though, that some gearboxes are not lubricated unless the engine is running: if so, their bearings will be destroyed if the shaft is allowed to rotate.) If the propeller is neither folding nor feathering, and is exposed to the water flow - as with a propeller on a strut well aft of a fin keel - it will generate the least drag when it is free to rotate. If, on the otherhand, a fixed two-blader can be well hidden behind the keel, it will produce less drag when locked vertically. Fully-feathering propellers should be locked vertically, if possible, while folding propellers need not be locked since they show so little area and have no tendency to rotate when folded." (my underlining for emphasis)

Gerr clearly states that with a fin keel it is better to free wheel. He seems to say that fixing is only better if it is hidden behind the keel. This is a clear contradiction to the quote he gave to John. That old saying comes to mind: "I feel strongly both ways." Or maybe Mr. Gerr lives by Ralph Waldo's words about "foolish consistency" (see my last thread). If that is the case, then his Propeller Handbook thoughts are his best guess because it has a later copyright.

This is like having an expert contradict himself on cross. And that brings me to another idea concerning expert opinion - its not just that you are considered an expert that is important to the court, that is a prerequisite just to speak, but more importantly it is what the expert says. And of course what the expert says is subject to disputation.

So you thought it was save to go back in the water! May the controversy continue to rage!

Dick

[Neil Gordon]

"There is disagreement among the experts ..."

Was he talking about us or was he thinking of some other set of experts?

[s.v. LaVida]

I've only been able to peck away at all the posts, but this is amazing.

On LaVida, we lock the tranny in reverse, as Volvo suggested.

Locking stops the whine of the box when I hit 12kts.!

Rit

[Dick Barthel]

I've only been able to peck away at all the posts, but this is amazing.

On LaVida, we lock the tranny in reverse, as Volvo suggested.

Locking stops the whine of the box when I hit 12kts.!

Rit

Rit,

This is what happens when people have too much time on their hands!

Dick

[Greg Ross]

Greg, see page 125, Propellers, Various Handy Facts.

Cheers,

John V.

I've separately extended an apology to Mr. Vigor for not fully reading the Index in the referenced Work. That said;
My logic tells me, if the pressure of the water applied to the curved blade of a propeller, no matter how many fixed blades is sufficient to overcome inherent resistance of clutch drag, bearing drag, viscosity resistance of lubrication in the gearbox, etc, etc then, allowing that propeller to free wheel has to represent less overall induced drag.
And the business about hiding a two blade behind a keel, at 800 mph maybe! I cannot picture any arrangement short of having the propeller retract into the Keel and considering the above logic, the tubulence/ drag aft of the keel would not be improved or lessened by locking the shaft.
My 2 Cents