# Calculating Pitch, DAR etc when diameter is known

Discussion in 'Props' started by Mat-C, Oct 3, 2012.

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### Mat-CSenior Member

Ok - understand all that... thankyou.....

So.. as I don't have the option of a reduction gearbox and based on what johneck said:
...I am actually going to be running most efficiently (with a 3-blade prop) with diameter of about 10", and slightly better off again with a 2-blade of somewhat bigger diameter. Ok... I can figure that.... thanks

Now... to complicate matters somewhat, the manufacturer recommends a cruise of around 700 rpm, with the max of 1400. That is significantly different to an IC engine where the the cruise is often more like 80% of the max rpm. Should this be taken into account when selecting the prop?

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### BertKuSenior Member

Mat, in my view, the propeller has to be slightly more rigid build with electro motors than with IC engines. The reason being, that because full torque is continuous hammering the prop, at whatever speed, fatigue may be a slightly higher and earlier. But again, only a few people can answers such question because very few people have the practical experience with PWM mode power train. Go for your logic and gut feeling and use the formula's as a guidance, until somebody puts his head on the block and tell us that they are the same.
Bert

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### BertKuSenior Member

Matt, the answer lies in the next thread. We have to use a CPP prop. That solves all problems.
Bert

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### BertKuSenior Member

Bad news, there are no CPP for small powertrains of 2 - 8 HP. Back to a normal prop.
Bert

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### johneckSenior Member

There are two separate problems here; one being what the engine can do, the other what the propeller can do. It does not matter how much power the engine can deliver if the propeller cannot absorb it. A fixed pitch propeller can only absorb a certain amount of power at a given RPM and vessel speed. Generally, FPP's are designed to meet the most extreme condition they will have to encounter, which is typically full power, full RPM, or perhaps for tugs a bollard condition. Then you get what you get everywhere else with the FPP.

I was away for a few days and have not read everything here, but if you were to design for a mean condition, or for a cruise condition, the propeller might not be able to absorb all the power available when you might want to use it. It is an interesting situation to have complete freedom in what the power curve vs RPM is for the motor, so perhaps there is a way to design the propeller to operate more efficiently over a wider range than would be possible for a diesel engine, but I think that you still need to be sure that you can absorb all the available power at a reasonable RPM, without cavitaiton issues. Then see what you can get at other operating points.

For a given relationship between RPM and power at a given speed there will always be an optimal diameter that yields the best efficiency (most thrust). Once again generally, a larger diameter and lower RPM will result in a higher efficiency.

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### BertKuSenior Member

Mat, John has very nicely explained it for you (and me). My thanks to John.
Bert

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### BertKuSenior Member

Mat, something to be aware off. Let say your brushless motor controller can handle 100 Volt. If a motor manufacturer is able to state the maximum internal temperature and let say it is 130 degree Celsius. We electromotor fundies can do much more than with an IC engine.

Example:The motor manufacturer states 48 Volt 4,7 KW and 1400 rev.
You have a prop overdesigned and is much larger. Thus you need more torque (power) to turn it. I could thus theoretical put 72 Volt on the motor, but turn the PWM down to 2/3 of maxiumum value. I will now still have an average power of 4,7 KW, but the torque is MUCH HIGHER AND ALSO THE REVS IS NOW MUCH LOWER , no longer 1400 revs, but approx 933. However nearly double the torque power. Due to the fact that you will have 72 Volt x 1/3 more maximum current drawn. Just for a shorter period of time. However, your heat will get higher and therefore you need to make arrangements for better cooling.

Moral of the story,

a) check your motor and controller specifications for maximum values.
b) make sure the battery will be able to give you higher currents.
c) the battery amperehour, as rated and given by the manufacturer is now lower. i.e. a 100 Amphour battery will probably be only 60 - 80 Amphour (A battery manufacturer gives the AH ratings at approx 5% i.e. 20 hour rate. (But you draw higher currents, the battery is proportionally much quicker discharged) no longer the 20 hour rule.
d) your cabling size has to be fatter and be able to handle not the average current, but the maxium drawn pulsed current.
e) your overload sensing circuit must be able to handle higher currents
f) You are in a much more flexable position than an IC engine user.

You can design any flipping propeller in principle and get the best efficiency by fiddling the electronics and battery voltage.
Make a propeller for 400 Revs, increase Voltage and turn the PWM down , instead of running at 700 rpm.
But remember, fatique and stresses are now your headache. (except if we are worried for nothing)
Bert

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### sailor0000Junior Member

Electric motor facts

I really don't want to offend anybody, but I believe there's some confusion here.

Here's the facts: YES, the max TORQUE a permanent magnet motor can deliver is (almost) constant from 0 rpm to max rpm. NO, the max POWER a permanent magnet motor can deliver is definitely NOT constant from 0 rpm to max rpm. Power is torque times rpm, so max power is an (almost) linear function of rpm.

In your case: Using the PWM motor controller to get 700 rpm, you'll see 50% of your battery voltage at the motor. This is exactly the same as connecting the motor to a battery with half as many cells. So, while getting full max TORQUE, you'll only get 50% of max POWER. Using a 2:1 reduction, let's say with a toothed belt system, you'll get 1400 motor rpm and 700 shaft rpm. Because a reduction gear reduces rpm and inevitably also INCREASES TORQUE, your prop shaft will see 200% of max motor torque. So you'll get 100% of max motor power to the prop (minus the losses in the reduction setup, of course).

Wooden Boat Magazine has several articles about electric launches, you may want to look at their archives.

Also, watch out: Your motor may well have high-energy neodymium permanent magnets. They'll loose their magnetism when they get too hot. So don't let your motor overheat, this could permamently damage it!

Hope this helps.

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### BertKuSenior Member

It is not easy to offend a good sailor. We always are open for comments. Sorry for the language, indeed power was not the right word.

Thanks sailor, I may have to register as a member and cough up some dollars , before I am able to see this.

Thanks, do you know at what temperature in Celsius degrees, it start loosing its permanent magnetism?

The only problem one has to watch out for is the Amperehour rating of a battery is no longer applicable and much lower, due to the continuous high peak current during such pulsed method. One think, he has a 100 Ah battery, but find it will only give him some 70 - 80 Ah, this is due to the way manufacturers quote the battery rating at 20 hours.

Thanks

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### sailor0000Junior Member

The critical temperature varies with the alloy used for the magnets. For some magnets, I remember it being as low as 70°C or 80°C or so. This caught my attention because I thought that it's REALLY low. I'd suggest talking to the manufacturer about this.

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### BertKuSenior Member

Wooowhh, that is low. I have my microchip programmed / ntc temperature set for 120 degrees Celsius. Talking to the manufacturer want help. My Chinese is lousy. My original idea to cool the motor in transformer oil , may have to be implemented. Thanks Bert

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### sailor0000Junior Member

Well, 120°C may be perfectly ok. It all depends on the type of magnets.

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### BertKuSenior Member

Thanks sailor, but I had to wait nearly 6 months for the motor, I don't want to take a risk. I will try to ensure good cooling. I will take your advise on 70 degrees C. Very difficult to cool a brushless motor and keep it to 70 degrees internally. But I will try. Are you from the French speaking part , Italian or German part?
Thanks
Bert

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### sailor0000Junior Member

German speaking, near Berne.

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### BertKuSenior Member

I will never forget when I passed Bern once. When I drove from Frankfurt to Zurich, getting close to the Swiss boarder and customs, I saw a sign above the highway in French. My French is useless, as I had the first lesson at 11 years old. I read something about 20 Swiss franc (can't remember 10 or 20) and I assumed that it was a Toll road. Thus my wife (She is a South African) put a 20 Swiss franc in my passport and I approached the Swiss boarder. Boy Oh Boy, the officials thought that I wanted to bribe them for 20 Swiss Franc and they took our hired car and our luggage to pieces. Needles to say they found nothing, but I was flabbergasted and they did not wanted to listen to my apology. Yes, I do speak German, but it is getting rusty. Bert
P.S. Although at that time not very pleasant, thereafter we had a good laugh about it.

Last edited: Dec 26, 2012
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