Freewheeling propellers: the chopper theory

[Parfait's Provider]

But, he is wrong.

A screw is exactly what a prop is. It might be shaped a bit differently than a wood screw to take maximum advantage of the medium, but it essentially trying to work it way through the water by turning into it. If you look at the prop from a fixed point on the seafloor, the prop is definitely acting just like a screw does when driven into wood. The difference between the media results in slip in the case of the prop, but that is about the only real difference.

And yes, the screw has "lift." If it didn't have lift as a result of its pitch, it wouldn't do anything but stand on the surface of the wood when rotated. You might have noticed that a wood screw sometimes needs a little help getting started. Screws pull themselves into the wood by rotating unless you use a large hammer. Props pull themselves into the air better than a simple screw because they are designed to work more efficiently in fluids and gases, but the principle is the same.

[Dalton]

When I bought my boat the builders told me to put it in reverse when sailing, The transmission manual says to also. So I do. I must say the transmission is a lot quieter when it's not free wheeling. And I do not notice any boat speed change in or out of gear when sailing. So I go with the MFR's suggestions. If I get concerned enough about this I'll buy a feathering prop.

time to pull his thread

[tartansailor]

FORCES ON A PROPELLER


Drag and Lift
When a fluid impinges on a propeller the undisturbed fluid pressure p and the velocity V change. Writing Bernoulli’s equation for two points on the surface of the propeller, the point S (leading edge) being the most forward point and point A being any other point, we have, for horizontal flow,
p + pV²/2 =ps + pV²s/2 =pa +pA + pV ²A/2
At points S, V+s = 0, so that ps = p + pV²/2. This is called the stagnation point, and ps the stagnation pressure. Since point A is any other point, the result of the fluid impingement is to create a pressure pa = p + p(V² - V²a)/2 acting normal to every point on the surface of the propeller. In addition, a frictional force Ff = taooAs tangential to the surface area As opposes the motion. The sum of these forces gives the resultant force R is resolved into the drag component Fd parallel to the flow and lift component Fl perpendicular to the fluid motion Depending upon the pitch of the propeller a wake may be formed which sheds eddies with a frequency of f. The angle alpha is called the angle of attack. From dimensional analysis:

F(Cp,R,M,S) =0
The formation of a wake depends upon the Reynolds number, or S =f(R). This reduces the functional relation to f(Cp, R,M) =0. Since the drag and lift forces may be considered independently,
Fd = Cd pV²(A)/2
Where Cp = f(R,M) A = Characteristic area
F-l = ClpV²(A)/2
Where Cl = fR,M)
It is evident from looking at a propeller that Cp and Cl are also functions of the angle of attack. Since the drag force arises from two sources, the pressure or shape drag Fp and the skin friction drag Ff due to wall shear stress tao0 the drag coefficient is made up of two parts:

Fd = Fp + Ff = CdpAV²/2 = CppAV²/2 +CfpAsV²/2
Or CD = Cp + CfAs/A
Where Cp is the coefficient of pressure. Cf the skin friction coefficient, and As the characteristic area for shear.
Skin Friction Drag As water approaches a smooth propeller with uniform velocity profile of V, the velocity at the plate surface is zero and increases to V at some distance y from the surface. The region in which the velocity varies from zero to V is called the boundary layer. For some distance along the propeller the flow within the boundary layer is LAMINAR with viscous forces predominating, but in the transition zone ie. LOCKED PROPELLER as the inertia forces become larger a turbulent layer begins to form and increases as the laminar layer decreases.
Boundary-layer thickness and skin friction DRAG for incompressible flow over a smooth propeller may be calculated from the following equations, where Rx = pVX/u

Laminar

Sigma/X = 5.2Rx-1/2 less than Rx which is less than 5X105
Cf =1.328 Rx-1/2 is less than Rx which is less than 5X105

Turbulent

Sigma/X = 0.377Rx-1/5 5X104 is less than 106
Sigma/X = 0.220Rx-1/6 106 is less than Rx which is less than 5X108
Cf = 0.0735Rx-1/5 2x105 is less than Rx which is less than 107
Cf = 0.455(log10Rx)-2.58 107 is less than Rx which is less than 108


Give it up Darmouse

[Al Levesque]

n/m

[Dick Barthel]

Give it up Darmouse

I can hardly wait for the moose's reply!

Darrell is Darmoose a Star Wars thing?

John in your wildest imagings did you think you'd be starting "the place to be discussion" for all cyberspace bound sailors for 2007? We're not hearing much from Randy - he's probably sailing.

You guys are the greatest! Happy New Year to all.

Dick

[Sandy D.]

Which creates more drag in,lets say, 60% humidity 75 degrees?

It's new years eve. boys,lets meet up at the club house,I'll buy the first round of Bloddy Marys. John,the next batch is on you!!

Looks to me that it is aggreed that the best thing for our boats is to lock the shaft. If you are concerned about the drag, mark you shaft,and hide your prop behind the keel. If you have three blades,tuff....have another beer!!

Fairwinds,
Sandy

[darmoose]

i dont think you have proved anything by reprinting a bunch of complicated formulas from the internet.

formulas, by the way, that you have in no way explained how they apply to the question at hand.

if you want me to give it up you will need to first convince me that you understand what you have offered up, and apply it to our question.

can you even fill in the numbers in any of those formulas, and come up with a value, much less explain how it would prove your point. for all we know, those formulas could prove the opposite.

i asked you very direct questions in my last post. maybe you are a politician(who dont answer questions they dont want to)

your postulation in your previous post said that at speeds of 5kts or less, there was no significant difference in the drag on our prop. you said that "at some point" this changes.

where are these points? what causes drag, which is either increasing or decreasing, as our prop speed either decreases or increases, to suddenly change and go off in the opposite direction.

i dont think you have a clue!!

so i expect that your next post will fill us with knowledge as you will explain the application of the formulas you have copied, perhaps run us through an example formula, and answer some of the questions i have asked you

or... you maybe you will be silent, who knows.....

TS.. are you a scientist of some sort or did you just stay at a holiday inn last nite.

if you can truly address these formulas and answer our questions, i will be the first to thank you for resolving the question at hand.

while you work on that, i am going sailing.

Happy New Years
darrell

[Lew Gresham]

There are three kinds of men: The ones that learn by reading. The few who learn by observation. The rest of them have to pee on the electric fence and find out for themselves.

[Didereaux]

Folks,
The fundamental facts are these
1. There are airfoils
2. there are screws
3. Boat props are screws not airfoils.

The curvature to the blades are not for airfoil like lift, they are to smooth out turbulance and to direct the passing liquid stream in such a manner as to extract a wee bit more of the energy that the engine has put into the stream.

Someone in a previous post did grasp this by noting that theoretically the boat prop(screw) has, when perfectly matched with its optimum rpm, an almost zero resistance profile. What that means is that the water being PULLED into the prop is leaving the backside at the exact same rate i.e. no turbulance. So ANY speed of rotation NOT at the optimal rotation causes the prop to virtually increase its profile from the waters point of view.

You must realize that a boat prop(water) is designed to act as an ideal screw, producing its work by PULL through the medium. Whereas an airfoil produces work far differently, it actually through its airfoil shape, SUCKS(not scientific but hey) the medium....it produces a strong decrease in pressure on its upper(working edge. The screw does not.

So in conclusion and summary: to compare ANY airfoil to any boat propeller and then to derive a third factotum or draw a conclusion by so doing, is syllogistic folly.

Okay, just mixed up a truly great butter rum mix(this so rarely happens I am tempted to preserve it for posterity....nahhhh heh) So. it's toodle off to the backroom aka the land of pirates with steaming mug in hand.

[darmoose]

the ones who believe everthing they read.

Hey Lew

i appreciate you input, and i apologize for being such a slow learner.

perhaps you can share what you have learned by reading TSs missapplied formulas, and answer any of the question i posed for him.

i would reeaaalllyyy appreciate it.

still waiting
darrell

p.s. once again, didereaux, a very mind clearing explanation. go enjoy your toddy!

[Dick Barthel]

I think it was Leonardo Da Vinci that said when it comes to thinking there are three types of people: Those who see, those who see when shown and those who don't see.

The interesting thing about any good debate is that both sides can be convinced the other side is clearly in the last category. In fact, those exact sentiments have been expressed by various contributors to this thread.

More food for thought:

Does anyone have any idea what the approximate range of propeller rpm would be for a "free wheeling" propeller (say a two blade) on a reasonably maintained cape dory (ie. minimum shaft friction) when sailing at or near hull speed? I'm guessing the prop is not turning very fast at all but I don't really know. If it is indeed turning relatively slowly, isn't it logical that the difference we are trying to measure would be extremely small? Which might be why the issue has never been conclusively resolved by the so-called experts (propellor engineers, naval architects, etc.)?

Let's try and keep this thing going until the Spring!

Dick

[M. R. Bober]

but not someting that you want to do your boat. The noise and additional wear on the drive drive do not merit the increase in speed, which the reduction in drag affords.

Mitchell Bober
Sunny Annapolis (where moms often say "don't make me tell you again!" just before they do.), MD

[John Vigor]

As the old year drags to an end, and this overlong thread likely with it, I’d like to thank all of you who took part in the Great Fixed vs. Spinning Prop Discussion.

The result was evident from the very beginning, of course, but this polite and civilized board gave us all the opportunity to air our views; that is, to expose to the public glare our individual prejudices, our deep and thorough lack of grasp of the principles involved, and our ingrained resistance to new ideas that might upset our preconceived notions. And we managed to do all that without swearing. (Except under our breath, because, as you know, there are always those #@^$! idiots out there who know it all and who ... well, never mind.)

It was an act of foolhardiness by some to try to disprove the exhaustive conclusions of such luminaries as the great F. S. Kinney, of Skene’s Elements of Yacht Design; the renowned engineering firm of P. Newall Petticrow; the legendary Eric Hiscock, et al – yet I have to admire Darmoose, Didereaux and their cohorts for being such wonderful losers. Their charm, politeness, and finely tuned talents of persuasion almost won me over to their side. Fortunately my brain did not cease to function completely and I needed only a small portion of it to understand that a fixed propeller creates less drag than one free to spin.

Nevertheless, I thank you all for your interesting views, the net result of which has been to move this important discussion swiftly sideways toward a satisfying inconclusivity.

That being the end of it, I bid you all a wonderful New Year, filled with peace and prosperity. And don’t forget to pray for those who do not yet understand. With your comfort and support they may yet see the light and clamp down on those irritating and inefficient freewheeling propellers.

John V.

[darmoose]

dick b,

this is very interesting, aint it. while i would like to secure some agreement from the "yellow liners"(see tods graph), i agree folks become so imbedded in their position, they go blind at times.

as to your question, i would rather stick to a three bladed propellar so the question of hiding behind the deadwood dont come up, you know.

as to the rpms while sailing, i have a brand spankin new volvo D1-20 in my cd30k. it has new shaft, coupling, tranny, cutlass bearing, and PSS dripless shaft seal. i dont think one could have less friction than i have got.

i haved observed my coupling (it is located closest to the cabin on my boat cause of the V-drive) many times while under sail. when i am goin 4,5,6 kts that coupling is spinning very fast ( I do freewheel you know per the volvo manual) you wouldnt wanna try to grab it for sure, just a guess, but its goin many hundreds of rpms. another thought, when i am freewheeling like that, i dont think the trans is engaged at all, so i dont think any gears are moving, the water pump aint moving nor the alternator belt. so i think the only thing happening is the shaft, coupling, and prop are spinning. and yes, i know, that can be puttin wear on the cutlass bearing.

in my simple mind when i see how fast that freewheeling propellar is spinning, and i know that the only thing making it spin is the rushing water hitting it at 5kt or so, and i realize that it takes considerable energy to make that prop spin that fast, i gotta ask myself "where does all that energy go if i lock that baby up like a brick wall with that water hitting it?" i think it goes into slowin down the boat. now dont that make sense?

but, i will be glad to wait for more educated types to explain those fancy formulas, and answer my simpleton questions so"s i can understand.

hope we dont gotta wait tooo long.

restless in So Florida
darrell

[Neil Gordon]

Does a freewheelin prop gather less barnacles?

Yes, the poor little buggers get dizzy and fall off. If you have any human compassion left in you, you should lock your prop shaft.

No, the awareness of the spinning prop only makes them grab on tighter. If you want to be kind to an otherwise homeless, defensless little creature, let the prop spin (even if it costs in boat speed, which I'm not convinced it does).