Calculating the Pitch on a Propeller (Screw)

http://continuouswave.com/whaler/reference/propellerPowerCurve.html

http://continuouswave.com/ubb/Forum4/HTML/005903.html

A displacement type hull trawler has a 7-9 knot optimum speed....so if you know the rpm you have to work with, which should be a sweet spot for vibration minimization, an engine will run smoothly at certain rpm, usually more than one place....so if you have the sweet spot rpm then you can figure out the diameter and pitch required to propel the boat at that speed....

"... displacement type hull trawler has a 7-9 knot optimum speed..."

'Optimum" for what goals? While trawlling or cruising?

Seems like there should also be some LWL caveat in there somewhere as well. For a displacement hull to have a fuel efficiency focussed optimal speed of 7-9 knots, it would need to be around, at least, 50 feet or longer.

Ya ~50' ...The OP I believe has a ~50' trawler....but the joke's on us it's a different application....In any case he can figure out the proper screws for the boat if the need should arise....haha

From the drawing, it looks like 25 degrees...or am I missing something?

No, in a boat propeller blade pitch is measured in the number of inches of forward propulsion through the water for one complete revolution of the prop. Restated, a propeller with a 12" pitch will propel the vessel 12" per rev.

For more read here: http://www.boatus.com/boattech/articles/propellers.asp

That is on the assumption of 0% slip. I think slip is high at low rpm, and low at "middle" rpm, but high once again at highest rpm...is that correct?

This implies the engine has reserve power to arbitrarily increase rpm.

There would be a point on a completely over-powered propeller screw where the speed results in cavitating the screw, a drastic loss of propulsion versus applied power.

That is correct - the theoretical travel is the pitch. There are losses (slip) to consider.

If I read the OP correctly, this is for a fixed machine agitating some fluid (like a mixer?), actual use here is not really a boat screw.

well then, he could encounter the high slip limits at both the high rpm and the low rpm.

There have been complete seminars put forth on mixing optimization. There are instances in homogenizing a blend where some of the ingredients will not be in solution, but merely suspended, where cavitation is a desired result, in order to produce the finest gradations in colloidal droplets suspended in the bulk solvent.

We aren't mixing different materials in a lab. he is propelling a boat through saltwater.

KISS.

A'hem! All he said was "machine", I know he is into boats as well as you do, and I also know he is a formulator.

KISS.

Sorry, you may be right. I can't see the dimensions.

Based on what I see in the drawing by clicking on it, Rixter's value of 158 mm radius is about right. This is pretty large for a mixing tank impeller, unless we are talking truckload quantities in a batch.

Tie goes to the runner - Lyn is probably correct in saying this is a boat anchor propeller.

Agreed, seems sort of large for a mix blade.

Might simply be a new screw for the boss's boat... uh, propeller... I mean.

And, if it were a mixing blade, it would be an impeller, because it is stationary and moves the fluid.

A propeller, propels something through the fluid. Like a boat.

OK, Lyn, now you are just taking victory laps. LOL

From the OP

"I contacted a company in the US to request the casting of a propeller for a machine."

The drawing title block indicates it is from Subiton Laboratories.

This machine remains undefined. I wildly (perhaps incorrectly) guessed a material mixer of some sort.

OK, I will assume no slip and assume that the propeller has a "wash out" so that it becomes flatter at larger diameter so that the "pitch" is equal for different radii and that the 25 degrees angle applies to the outer diameter of radius 157 (mm?).

Circumference C at 157 radius = 157*2*pi. P is pitch or forward motion per one revolution (no slip).

tan(25) = P/C or P = C*tan(25)

That would make P = 460 in the same units the radius of the propeller.

That drawing doesn't show much of "twist" to account for a smaller angle at larger diameters....just musing it doesn't look like a typical prop.

Agreed.

The drawing seems to indicate some negative twist. Definitely going to be a mixer, as there will be a lot of mixing going on, even if it was intended primarily as a prop.

http://n-lemma.com/indoorrc/propcalc.htm

I worked with a fellow that did prop repair as a side business,... and the Diameter and pitch should be stamped on the barrel of the prop or on the end of the barrel.

And at times, where this number was covered over from previous repairs, he had a set of pitch blocks that he'd used to determine the pitch of the prop.

I find it odd that its not on the drawing,... was the drawing reverse engineered?

These were used ones on ebay and of course this is for props being repaired.

On your drawing,... when you find out, you should have the diameter and pitch on your drawing for it to be cast in.

It does appear that there is a standard,... this may not help much,... but its more then what you have now.

A prop whose pitch equals its diameter is said to be "square"; more than that is a pretty steep pitch.

If a prop is angled 25º at the circumference, it will be "oversquare" by a factor of ~1.46. [Pi x tan 25º] See Rixter's note on washout.

Rather than custom-making this, you may be able to find it at a boat prop shop.

Prop efficiency may be around 70%; i.e., it will actually move through water ~70% of its calculated pitch; more if enclosed in a shroud or nozzle.

This is how to measure the pitch of an airscrew.

same calculation.

the question asked pertained to the drawing that was posted as to the pitch of the blade, not what pitch has the highest efficiency in a fluid with X viscosity. Heck, we don't even know what the fluid is let alone the cavitation rheology.

Fluid, not air or gas propellers.

The objective is to calculate not just measure at pitch.

There is a little bit of difference. The degree of cavitation comes to mind.

If the use is for a mixer machine, the medium/density will dictate the blade profile, cross section and relevance of pitch.

A fine edge may not be too good an idea for cement slurry. Curve or cup in the blade may be of little help.

If the blade drives (thrust/lift) a boat/body, then one should consult the library for the following book.

If you do not have this $20 book from which to work, I would say that anything else is a bit of a larger guess.

It is easy to read, understand and applicable to all sorts of fluid (watery type) applications.

My book is dog eared.

Following SolaEagle´s advice, we used the method proposed on a website.

Hope it is accurate!

Doesn't look right...

The travel, or movement is not being grabbed from the same spot, what's with the top view... it's a bit redundant.

I sent this drawing to the guy who does the casting yesterday. We may get some feedback later today...

Is this for a dadgone mixer or a dadgone boat?

I´ll reply in german: sis is for se chemikal reaktor

Then it is referred to as an impeller, not a propeller.

The US manufacturer of the impeller called it propeller... but I do not care on how this screw is called

As long as it suits your needs and you are paying for it, you can call it whatever you like.

It will be a screw to you, once manufactured, yes?

This type of impeller is called a propeller....haha

http://hydra-tech.com/9-different-impeller-types/

This proves that the impeller propeller guy was right!

Dan er ist ein Gross kemical reactoren. (loose translation: He is a large reactor, then.)

An impeller propeller that size is meant for large batch operations, I suspect.

Yes, it is a quite large chemical reactor (1000 liters = appts 250 Gal) driven by a 20 HP electric motor we use for the manufacturing of acrylics.

BTW: a while ago I got the quotation for casting it in 316L SS: 1,100 U$S a piece...

What is the reason for the lack of twist in the blades? As designed, the propeller/impeller will be trying to push fluid around times faster with the outer ~20% of the blade than the inner ~20% of the blade. That seems like setting up for needless expemditure if the additional mixinng/heating/wear are not rnecessary.

Hi, this propeller impeller is used in a 1,000 liter chemical reactor. There is a 20 HP electric motor that turns a 60 mm thick shaft at variable speeds (depending on the phases of the process) @ max 1,200 RPM. The shaft has TWO propellers attached to it: one at the tip, and a secondary one at mid-height. The propeller impeller we are talking about is the latter. The entire vessel and the mixing system in it have been in operation for about ten years now, and were designed and manufactured by a well known company in that field.

At last! The actual application is revealed!...

Is the original company still in business, but unable/unwilling to give you the desired information?...

Do they sell comparable replacement and/or refurbished parts?...

Are you trying to fabricate your own replacement in-house?...

or, could you make-do with a second-hand one?...

Do the more recently manufactured machines use similar, or improved, propeller/impeller designs?...

If your machine broke, what company would you call to fix your machine?...

Seems like Computer Fluid Dynamices (CFD) should be able to model the actual mixing characteristics of the specific fluid in the specific mixer to determine the optimal degree of mixing for a variety of blade characteristics, and thereby tell you what more optimally mixing blades should look like, for your operating rpm...

This could give you an actual (picture) of what your miximg efforts actually look like...

And maybe then, such improved blades could replace the existing blades for operation at the same rpm (or even at a lower power rate) ?...

So, this has been, as per usual a FGWoT. Why have cupping on a prop/impeller in a mixing bowl?

I guess the joke's on me.

I didn't devote the time to reading all of the posts. FGWoT!

Cheers,

Stu.

What is the reason for the lack of twist in the blades?

there can be a number of reason would be based on the rheology properties of the product.,... as an example, some product can be shear sensitive, where you want a less aggressive pitch for just mixing, but not to add shear to the product.

This is usually the case for food stuffs where shear will change the texture of the product. Or product that has non-Newtonian properties.

the design of the impeller itself can be based on empirical data (experience)

I would likely be guessing, but I will guess that one wants a very high rate of mixing when polymerizing acrylics to a target Molecular Mass range.

In these reactions timing is everything, and the injection of terminator happens after a very precisely controlled interval, if I remember this batch process well enough.

Not only that, it has to be the precisely required amount, so that reaction stops on a dime, and mixing time has to be short (for the terminator) compared to batch reaction time, especially if good control of properties is required. Usually it is required.

what ever the recipe calls for...

...it ain't cookie dough, although one could try that, with really bad results using the propeller indicated.

I like cookie dough,... but you'd use a blender for that...

and a bread dough hook?

Your guess is correct!

Thanks!

Propeller pitch is almost universally described in 'inches'. A prop such as yours will be described thus: OD x Pitch x shaft major dia., usually all in the old imperial 'inches'.

To calculate the pitch required is easy. You must assume that the blades are flat planes and are fixed to the hub, or 'boss'. you then need to determine how fast in linear velocity you vessel is theoretically going to be driven with the power you have installed.[ there are other calculations you will need to embrace if you have just started with a hull and don't know the velocity it will theoretically reach.] Anyway, imagine the prop will be screwing it's way through butter [not frozen?????], that is with little or no 'slippage' or yielding of the 'fluid'. The pitch describes the advancing of the screw along it's shaft axis in ONE revolution. Therefore it's a simple matter to calculate the theoretical linear velocity of the prop along it's shaft axis if you apply the revolutions [ /min, or sec.] to the velocity of the hull through the water [in inches/min., or sec.] There are charts that will give you the values of relative 'speeds'. ie. miles per hour= inches per minute etc. You do the math.

Cheers,

Stu.

" the drawing below was made by one of our technitians"

That wouldn't be a Hindi speaking graduate skilled in measuring light intensity, would it (?)

Racist slur.

Not acceptable.

The referenced comment lacks qualities of either racism or a slur.

Hindi is a language, not a race.

Any slur is not direct. It would have to be inferred and if it is, it is not clear.

At worst this is a comment making a comment that might be interpreted as less than positive about an individual identified by the language they speak. It is not a negative generalization depicted as characteristic of a group deemed a race.

Let's lower the 'racism' alarm threshold somewhere below crying wolf, back into the realm of utility.

You say " A propeller for a machine" is this a "machine" or a vessel is it for propulsion or mixing?

Bazzer

Ditto!

Although a couple of degree's are sometimes helpful.

Cheers,

Stu.

The exploration of knowledge is never a waste of time...

That's what happens when you ask for an opinion here... we all have one,... some have 8-9 of them.

Mysogynist slur.

Not acceptable.

I saw I already answered...

I developed a specialized proto-type prop for the Navy,... from a sketch... you need to get the pitch angle from the axis.

Off topic... Friend of mine had a prop shop,... and when customers brings in a prop, from hitting a reef or shoal the prop blades were basically beat up and partially missing... he had a customer where the only thing left was the hub... and he wanted it repaired...

anyways, he used pitch blocks to get the angle...