Propeller Torque and Propeller efficiency

I am designing an electric motor for sailboats. I am looking for data about propellers actual torque at different speeds. The common calculations around is based on combustion engines and as electric motors have different torque curves I would like to know what torque that a certain propeller must have to be turned at certain RPM. Propeller turning in water and boat not moving would be the max required torque necessary. Usually props are calculated from assumed motor. I work the other way - what motor (power and torque) do I need to turn the best suited prop?

Another question in question is what differences in propeller efficiency we get with higher RPM and smaller prop versus lower RPM and bigger prop. In my case I have the alternatives 2000 RPM and 9-10 inch propeller or 900-1000 RPM and 13 inch propeller.


/johan

 

Hi, Ian Nicholsons Boat Data Book pages 108 through 129 give a lot of information that you might find of use. Generally speaking the prop diameter is related to the length of boat it is driving and the horsepower applied to it. In most cases a larger propeller driving at a lower speed will be more efficient than a small prop at high speed particularly for displacement speeds you would expect with a sailboat.

 

The calculation for the propeller cannot be done in isolation of the boat if you want the best result. You must determine the hull drag at various speeds to determine the prop.

If the boat is easily driven then the 2000rpm and smaller diameter could be better than spinning a bigger prop slower.

Google Javaprop and start the applet. I can help you work through it if you find it a challenge. There is also a number of threads that have discussion on Javaprop. If you want to build an efficient drive system then Javaprop is the best place to start.

Determining the hull drag will depend on the type of boat. If you post more detail on your objective then I can advise the appropriate method.

Rick W.

 

Hi, my preference for a sail boat or displacement motor boat is to go for the larger prop turning at the slower speed. A sailboat is usually underpowered compared to a motor boat and a small prop just does not hack it. I did a study on a 26 foot semi planing hull I designed and found that going from a 1:1 ratio with a small prop to a 3:1 reduction with a larger prop actually increased the speed potential from the same engine.

 

Biggest advantage for a sailboat is with enough diameter a 2 blade can be used.

Least amount of induced drag , and on a long keel boat the prop can be hidden behind the deadwood for very little drag.

Sailboats sometimes attempt to sail , and all do far better without a speed brake always deployed .

FF

 

Thank you all for tips. What I am looking for is measured values for torque demands to drive boatpropellers at different RPMs.

The Javaprop applet does not give an answer to the question. It assume the torque to be related to the enginepower (I am interested in propeller torque, not engine torque). Javaprop actually give you very wrong numbers... If you give Javaprop values for a 1meter diameter prop at 1100 RPM with a power of 500 watt you get a positive result of efficiency about 50% and a thrust value!! (500w will NOT be able to turn a 1 meter prop at 1100RPM!).

I have ordered Ian Nicholsons Boat data book and I hope it will give me some answers...

If anyone have made their own torque measures of props, or seen any propeller curves around, I would certainly be interested...

/johan

 

Johan
I have attached a screen image of the design page from JavaProp to give you an indication of the results you should be getting.

I have selected the 13" prop at 1000rpm. The boat is assumed to be able to do 6kts (3m/s) with 3kW.

You can see the prop efficiency is 60%. The thrust under these conditions is 607N. You would need to check that the hull will do the nominated speed with this thrust - it is an example only.

To get this result, you need to set the options page to water parameters and select a foil profile. The default settings are flat plate section operating in air so I expect you have been trying to design an air prop not a water prop.

The second image is a screen dump of the blade profile. If my drag calculation for the hull is close then a 2-bladed prop would be 13 X 12 with a 5" chord. A 3-bladed prop under same conditions would be 13 X 12 with 3".

Hope this helps you understand how to use JavaProp. It is very simple to use and gives very good results.

Rick. W.

 

Answer to Rick:

I attach values from same as yours on Javaprop but for the diameter of 1 meter!!! Do you think this is a good result?!? Well it is not adequate and it is not what I was looking for.... You have NOT understood the meaning of my thread...

/johan

 

Johan
It is unlikely that you could fit a 1m diameter prop. Also the blades are simply too narrow to take the thrust force. The result you get is correct for the foil section chosen - efficiency is 64.8%. If you could make such a prop then that is the efficiency it would achieve at 3m/s.

The torque required for the propeller is simply the power divided by the rotational speed in radians per second.

So taking the 3kW produced by the motor, the torque at 1000rpm is:
Torque = 3000/(1000/60 * 2 * PI)
Torque = 28.7Nm

So this is the method used to determine torque.

I have made an electric outboard that would suit a medium sized yacht with the appropriate propeller so I have a good idea of what you are trying to achieve. The linked video shows testing on two small batteries. The motor is capable of producing 4.5kW continuously when connected to 48V:
http://www.boatdesign.net/forums/attachment.php?attachmentid=20726&d=1209288392
I produced the propeller used in this application.

What I do not know is how much engineering you understand. I trust you will excuse my ignorance on this aspect, however I did offer in the original post to answer any questions if you found JavaProp challenging.

Rick W.

 

I should add that you can use the same torque equation for the motor if you have gearing but of course you need to use the motor rpm.

Rick W.

 

The exact information you requested in the original post is available from JavaProp in the Multi-Analysis field per attached.

You do need to apply the torque formula given above to determine motor torque at the various power figures determined by JavaProp.

One of the nice features of JavaProp is that you can change the rpm on the Design page and then go to the Multi- Analysis page, re-analyse to look at what power the particular prop will absorb at the different rpm. See second image with speed at 1500rpm.

Rick W.