Propeller Torque

Actually, your question needs more information before anyone can answer it well. Torque is a function of speed and power, and speed and power are functions of a lot of other things. Wikipedia has a pretty thorough explanation of the theoretical torque required for propellers.

Torque is a function of speed and power, ....

Guru Bill,

I thought TORQUE is simply a turning "moment" (not force) and is a function of "Load" times "distance" of application of force. In the case of a propeller, the resistance offered by the medium (density) determines the "Load" on the propeller and "distance" is given by the distance of the centroid of the propeller blade from the center of the propeller hub .

I can stand corrected if you can please explain.

Thanks and regards

D.Ramakrishna Naidu

Torque = power divided by angular velocity. (Of course, as always, it is up to the individual making calculations to keep the units of power, speed and torque compatible.)

Since there's such thing as slip and drag, not to mention anything about propeller design or hull design, the torque required for propellers is not a simple thing to determine.

As I mentioned earlier, Wikipedia has a fairly good treatise on the subject.

http://www.google.com/search?sourceid=gmail&q=formula%20for%20calculating%20propeller%20torque

Source Wikipedia: TORQUE,

Q=(Rho)Va²D³ X f_q(ND/V_a)

Va=rate of advance fq= Froude number D=Propeller diameter N=Number of revolutions, Rho=Density of medium.

Hope this helps you,

D.Ramakrishna Naidu

I had spent some time looking over the equation for propeller torque listed on wikipedia and I have a couple of questions. Maybe someone can help.

Is Va (rate of advance) simply the propeller pitch times the rate of rotation?

Also, I have no idea why N (number of revs) is needed to caluclate torque?

Finally, the units do not seem to be coming out right. I assume the final answer should be units of force times distance.

Dear Goolies,

your observations are correct.

Torque should simply be = Mass {(Rho/volume)} moved per revolution X Mean Dia.

This will give the "Turning moment" required for rotation.Using outer dia of propeller will give a higher figure which is useful for determining the maximum torque.

Hope this helps.

Regards

DRN

Thanks to all for your replies to this point. I wanted to try and explain my application. I build wooden crankbaits for fishing as a hobby. Developing new crankbaits requires testing to see the action of the bait in water at varying flow velocities. I am currently researching ideas I have for building a bench top bait tester. One of my ideas is to build an oval test channel with 6" pvc pipe and fittings and circulate the water through the pipe with a propeller and small AC motor. I know the volumetric flow rate I need through the 6" pipe and I have used equations and charts supplied by manufacturers of industrial mixers to estimate propeller size and motor size for my application. I thought these equations may work since my application requires high flow but very little pressure. When looking at specs for AC motors I noticed that they produce very little torque without the use of a gear head. This made me question if the torque required to turn the propeller (primarily during system start-up before the water starts to circulate) would overload the motor. I have an engineering background but any engineering courses were taken many years ago. Thanks again for your help.

Rather than an AC motor to turn the prop find the smallest DC electric trolling motor you can and use the power foot from it. With the controls you can vary the water speed easily and best of all, it won't be plugged into the wall!

Travis

Hello goolies, I won't say anything about your name..................................'cept to say you do have only one...........ummm, 'L' that is.................

Were you referring to a small propeller to circulate water through the test bed you want to build? It might be worth thinking of an electric water heater pump? They have constant flow, and you can add a potentiometer.

This site will show the full prop' curve with all engines and at all speeds, including the various engine sizes from 1.9 - 223 hp. The site has all kinds of calculations including a 'Speed Prediction Formula'. There is charts with explanations between each.

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

1 KW = 1.34 HP. This is a handy site to check convertions quickly:

http://www.convert-me.com/en/convert/power

Hope this helps. Please keep in touch about your project OK?

Take care and happy holiday.........................