Freewheeling propellers: the chopper theory

[Neil Gordon]

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

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

[Dick Barthel]

I know this is off-subject, but I am wondering if there is an existing record for maximum number of posts on a subject, or visits to a thread.

As I type this, this thread has had 4037 visits! Are we going where personkind has never gone before?

Maybe Cathy can illuminate us on this.

Anyway, back to props and choppers...

Dean,

I don't know what the all time record is but I know "Fenix in Panama" that was started on May 31, 2006 has 7874 posts and because that voyage is still ongoing it is likely to have many more.

Should we be giving prizes for the originator of the winning threads? If the fixed v freewheeling thread manages to finish in the money, I would suggest a copy of "Twenty Small Sailboats to Take You Anywhere" as a suitable prize.

Dick

[Neil Gordon]

Should we be giving prizes for the originator of the winning threads? If the fixed v freewheeling thread manages to finish in the money, I would suggest a copy of "Twenty Small Sailboats to Take You Anywhere" as a suitable prize.

Do you think we might arrange to have it signed by the author?

[Tod Mills]

of solving the problem mathematically, the following web pages may be of interest:

http://en.wikipedia.org/wiki/Computatio ... d_dynamics

http://en.wikipedia.org/wiki/Navier-Stokes_equations

Actually, I'm not certain that todays programs have progressed to the point where they could come up with an answer with sufficient reliability. This is a VERY complex problem. I don't know any ordinary engineers who would have a clue as to how to solve it, myself included; this is definitely not bachelor's or master's degree stuff.

I do know that towing tanks are still in operation despite advances in the science and software.

As an aside concerning the state of engineering, my brother-in-law's brother is an engineer with NASA and for a while a few years ago, his task was to try and devise a method of predicting whether an airplane propeller (turboprop) would suffer from "high-speed flutter", a condition where the blades of the propeller begin to vibrate and eventually self-destruct. This problem is a mix of dynamic structural analysis and fluid dynamics. At the time he decided to move to another area of work at NASA, they did not have a way to predict whether a prop would self destruct. The only reliable method was to physically test the prop.

[Dick Barthel]

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

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

Neil,

It was to hard to see what he was doing through the binoculars!

Now I've given you two opportunities to reach your personal milestone.

Congratulations.

Dick

[Dick Barthel]

Should we be giving prizes for the originator of the winning threads? If the fixed v freewheeling thread manages to finish in the money, I would suggest a copy of "Twenty Small Sailboats to Take You Anywhere" as a suitable prize.

Do you think we might arrange to have it signed by the author?

I wish I would have thought of that!

[Tod Mills]

Dick and Neil, that is hilarious! (at least, if John becomes the winner!)

[darmoose]

To the board,

seems that now we dont think we have the engineering capabilities to resolve this problem (that may or may not be), i dont know....

why dont we see just how far we can take this by reasoning and logic (nothing to lose)

i would like to try to ask a few questions, and hopefully get concurrence before we move on to the next question.

here is the premis: boat sailing in the forward direction, 3 bladed propellar, 4 or 5 kts of speed, steady conditions.

first question: as we consider whether a spinning propellar or a locked (stationery) propellar creates more drag on our boat, does it matter what is causing the propellar to spin? yes or no is way prefferable.

who wants to play?

darrell

[Neil Gordon]

... does it matter what is causing the propellar to spin?

If we're sailing, what's available to make the prop spin other than the flow of water along either side of the keel?

[darmoose]

we could spin it by hand or we could turn on the engine and put it in gear

darrell

[Al Levesque]

The engineering problem is solvable but unique for a wide number of variables. First of all the solution is discontinuous. When the speed is insufficient to create the torque that overcomes the friction in the cutless and transmission bearings there is no motion. Each boat will have a different point where rotation starts and it may vary in the same boat depending on temperature and other variables. Once rotation starts in the freewheeling application the friction plays a part as well sea and water conditions. When the boat has reached some level of speed the prop may have trouble keeping up and other factors have to be taken into account. In the case of the locked prop the deadwood shape and the prop angle all play a part in the turbulence that occurs ahead of the prop and the drag that results. This turbulence and the related drag would vary according to the boat's speed too.

If we limit the discussion to the interval where a freewheeling prop is turning at its greatest efficiency and the corresponding situation with a locked prop we can use the analogy that the freewheeling prop keeps its bite on the water and is capable of producing more drag than the locked prop. That is where the helicopter autorotation analogy serves us well. It should also be clear that water is a fluid and functions similarly to air. The major difference between our boats and the helicopter is that our blades max out at about 60% efficiency versus the helicopter's 85% because our blades are so short.

[darmoose]

SUPERFLUOUS POPPYCOCK

why do folks insist on inserting real life infinite variables and maladies into a discusion of a HYPOTHETICAL engineering problem that seeks to determine one thing and one thing only (assuming all other things are equal, which, yes, i know in the real world they are not)

lets discuss prop drag on a perfect boat with no friction in the drive train, no deadwood effect, no whatever that tends to make the discussion impossible. i thought J.V.s analogy and subsequent eureka moment was predicated on an imaginary helicopter and an imaginary boat, not anyones in particular. if we dont do this , those who claim this is unsolvable are absolutely right.

"we can use the analogy that the freewheeling prop keeps its bite on the water and is capable of producing more drag than a locked prop. that is where the helicopter autorotation analogy serves us well"

my dear friends and fellow cders, do we not realize that when freewheeling a sailboat, the propeller"s leading edge is forward and biting into the water so to speak, as al has said above, but the helocopters blade, while falling, has its leading edge pointing up, or in the opposite direction of the helicoptors travel, which is down. the pilot has altered the blade from its normal flight setting in order to catch the wind and make the rotor blades spin without power.

that is why the analogy fails.

forgive me if i sound frustrated
darrell

[Al Levesque]

The leading edges in both cases are on the leading side. Both props are freewheeling for the most part, limited by friction, in both cases. They really are very similar. Hope this helps.

[Dick Barthel]

The leading edges in both cases are on the leading side. Both props are freewheeling for the most part, limited by friction, in both cases. They really are very similar. Hope this helps.

Al,

Your last two posts are a much better statement of what I have been trying to say about seeing the analogy (although admittedly not a perfect analogy by any means).

I think we should all agree to stop trying to prove whether the copter analogy is apt and just stick to trying to show whether a sail boat propeller creates more drag when freewheeling or fixed. I think variables should be permitted to illustrate particular points. Mixing the two issues (copter = sailboat analogy and fixed v free wheeling) makes any solution a near impossibility.

After all John's original point is that free-wheeling props create more drag than fixed props. He only saw the copter analogy as proof. Say he's wrong on the analogy - that doesn't prove or disprove his ultimate premise. If variable ultimately come in to play he might end up being only half right or half wrong.

So let's all agree to just proceed with the real issue:

Issue:

Can a free wheeling propeller cause more drag than a fixed propeller?

Notice, the way I've stated the issue permits variables that could change the outcome.


Food for Thought:

If an engineer were asked to design a propeller that was not going to be used to power the boat but just to create the least possible drag, how might it be different that the typical sailboat propeller which I assume is designed primarily for drive, smoothness, etc? Of course no one would ever do this but I hope it illustrates a point. Powering the boat efficiently and creating the least drag when not in use I'm guessing are competing considerations. For example, isn't the Campbell Sailor propeller supposed to be a unique design which allegedly separates it from say the traditional Michigan Wheel propeller? Why? What design characteristics are different? To what end?

It seems to me that the ultimate authority might be the engineer's that design propellers. I suggest that experienced sailors are not even close to being qualified authorities because they're not using empirical data. The best racers in the world don't have to deal with propellers do they? Naval architects might be closer because they probably do study these things but not with the focus of say an engineer that works for Campbell. Isn't it conceivable and probably likely that companies like Michigan Wheel and Campbell have all the fancy equipment that Tod alluded too?

In fact, when I repowered last winter I had lot's of back and forth with the people from Michigan Wheel. I am going to send them an email with our issue posed for them and see what they have to say.

Jeez, I feel like Dider after this rant!

Dick

Quote:

A foolish consistency is the hobgoblin of little minds, adored by little statesman and philosophers and divines. With consistency a great soul has simply nothing to do. He may as well concern himself with his shadow on the wall. Speak what you think now in hard words and to-morrow speak what to-morrow thinks in hard words again, though it contradict every thing you said to-day. Emerson's essay on Self-Reliance

[Didereaux]

Jeez, I feel like Dider after this rant!

Dick

Nahhhhh!, no way Dick. a mere hint of snit! heheh

Wait until your highly reasoned and reasonable proposal is pooh-poohed by one or to of the granite cranial set....THEN you'll rant! LOL
<big grin>