Prop sizing

[peterboat]

So you engineers out there consider this, having fitted my tacho to the electric motor its spinning at 933 rpm going forward, and 980rpm in reverse, this is with the boat moored, so I would expect moving the prop speed to increase slightly. Its direct drive and it really needs to spin at 1650 rpm otherwise it creates to much heat. The propeller is 19 x 10 and on the old diesel it would have rotated at 1500 rpm, so I am going to repitch the prop and am thinking 19 x 6.5 if it can be done, this is removing about one third of its pitch so I am hoping for a third gain in motor rpm.

I do not want to go down the gear box route as they rob power, any thoughts people?

 

PS The motor is only 6 KW but torque is huge of these DC series electric motors, so traditional prop sizing charts dont work

 

[Boater Sam]

If you want a third, how about sawing one blade off?

[Alan de Enfield]

5 minutes ago, Boater Sam said:

If you want a third, how about sawing one blade off?

25% surely ?

 

[peterboat]

I did ask for engineers to reply.................................?

[Boater Sam]

3 minutes ago, Alan de Enfield said:

25% surely ?

 

That's a fan not a prop.   Could Crowthers give you some guidance? 6kW  is a bit more than 8HP.

[dmr]

34 minutes ago, peterboat said:

So you engineers out there consider this, having fitted my tacho to the electric motor its spinning at 933 rpm going forward, and 980rpm in reverse, this is with the boat moored, so I would expect moving the prop speed to increase slightly. Its direct drive and it really needs to spin at 1650 rpm otherwise it creates to much heat. The propeller is 19 x 10 and on the old diesel it would have rotated at 1500 rpm, so I am going to repitch the prop and am thinking 19 x 6.5 if it can be done, this is removing about one third of its pitch so I am hoping for a third gain in motor rpm.

I do not want to go down the gear box route as they rob power, any thoughts people?

 

PS The motor is only 6 KW but torque is huge of these DC series electric motors, so traditional prop sizing charts dont work

 

 

You can't really have low power and huge torque, at least not on a like for like basis, power is simply torque times speed, no exceptions, not even for vintage diesels. Your electric motor will likely produce full torque at zero speed, which a diesel can't, but at maximum speed the torque is just power/speed.

 

I'm still struggling to fully understand props but on a moving boat I would have thought the prop would be working better so would the speed not actually fall a little as it will transmit more torque at the same speed????  I suspect the relationship between pitch and speed is not linear so don't think removing a third pitch will map directly to a third increase in speed.  Can you get a second hand prop, or even borrow one, to see how it goes, or bribe a prop company to work it out for you??

[Alan de Enfield]

5 minutes ago, dmr said:

I suspect the relationship between pitch and speed is not linear so don't think removing a third pitch will map directly to a third increase in speed. 

I suppose, on that basis, remove the blades completely and you'd get infinite speed ?

[peterboat]

Electric motors are given a size by how much power it consumes spinning a fan a a certain speed mine is 6 KW at 1650 rpm but torque is developed at very low rpm which is what makes it ideal for boats. However if the motor doesnt spin at the right speed it makes heat instead of propulsion which I dont want so that is why I am going to repitch the prop to take load away from it

3 minutes ago, Alan de Enfield said:

I suppose, on that basis, remove the blades completely and you'd get infinite speed ?

It would explode Alan!!! I knew I shouldnt have asked the question?

[Alan de Enfield]

6 minutes ago, peterboat said:

Electric motors are given a size by how much power it consumes spinning a fan a a certain speed mine is 6 KW at 1650 rpm but torque is developed at very low rpm which is what makes it ideal for boats. However if the motor doesnt spin at the right speed it makes heat instead of propulsion which I dont want so that is why I am going to repitch the prop to take load away from it

It would explode Alan!!! I knew I shouldnt have asked the question?

On a more serious note, I would suggest that if you are planning DIY 'cutting down', do it in small incremental steps (1/2" ??) and re-try at each step, better than taking off 3.5" and finding you have gone too far.

[matty40s]

v

Edited March 13, 2019 by matty40s

[Tony Brooks]

1 hour ago, peterboat said:

So you engineers out there consider this, having fitted my tacho to the electric motor its spinning at 933 rpm going forward, and 980rpm in reverse, this is with the boat moored, so I would expect moving the prop speed to increase slightly. Its direct drive and it really needs to spin at 1650 rpm otherwise it creates to much heat. The propeller is 19 x 10 and on the old diesel it would have rotated at 1500 rpm, so I am going to repitch the prop and am thinking 19 x 6.5 if it can be done, this is removing about one third of its pitch so I am hoping for a third gain in motor rpm.

I do not want to go down the gear box route as they rob power, any thoughts people?

 

PS The motor is only 6 KW but torque is huge of these DC series electric motors, so traditional prop sizing charts dont work

 

No, the torque at zero revs is huge and getting lower as speed increases. That is because the current decreases with speed and its is also why at lower than design speed the motor runs hot. I would suggest that you use the torque at the moror's rated continuous speed in a conventional prop charge.

[peterboat]

9 minutes ago, Alan de Enfield said:

On a more serious note, I would suggest that if you are planning DIY 'cutting down', do it in small incremental steps (1/2" ??) and re-try at each step, better than taking off 3.5" and finding you have gone too far.

I am taking it to woodwards to have it repitched rather than cutting down, unless it cant be then its out with the angle grinder

[Tony Brooks]

18 minutes ago, peterboat said:

Electric motors are given a size by how much power it consumes spinning a fan a a certain speed mine is 6 KW at 1650 rpm but torque is developed at very low rpm which is what makes it ideal for boats. However if the motor doesnt spin at the right speed it makes heat instead of propulsion which I dont want so that is why I am going to repitch the prop to take load away from it

It would explode Alan!!! I knew I shouldnt have asked the question?

Not really fully up to speed on series motors but I rather doubt it would explode unless it w s a poor design because as the speed increases the torque and current drop so at some point the frictional torque and motor torque would be equal so the speed would stop rising.

[peterboat]

1 minute ago, Tony Brooks said:

Not really fully up to speed on series motors but I rather doubt it would explode unless it w s a poor design because as the speed increases the torque and current drop so at some point the frictional torque and motor torque would be equal so the speed would stop rising.

They explode DIY electric vehicles is full of pictures of unloaded motors that have let go!!

[Mike Adams]

Could I suggest that the motor is overloaded and that is why it is creating too much heat. I am not sure what motor/driver you have but a 19 x10 prop will require a lot more than 6KW to turn it at 1500 rpm. The prop loading goes up as a cube power of the speed that is why it is so difficult to get exactly the correct prop to get full power at the rated speed. I terms of prop size it is much too large and you can't alter the aspect ratio so much without losing efficiency. Look at the prop size for a 8HP stuart turner for example. You can use the free prop calculators on line to find a optimum prop. I would guess about 14 inches max diameter. Otherwise you need a reduction gearbox.

[peterboat]

7 minutes ago, Tony Brooks said:

No, the torque at zero revs is huge and getting lower as speed increases. That is because the current decreases with speed and its is also why at lower than design speed the motor runs hot. I would suggest that you use the torque at the moror's rated continuous speed in a conventional prop charge.

Tony the current to the field windings is reduced which enables it to have a flat torque line here is a link

Speed Torque Characteristics of Series Motor - EEEGUIDE

 

www.eeeguide.com › ELECTRIC DRIVES

 

[ditchcrawler]

I would talk to the likes of Crowthers, you are not the first to have electric propulsion. Think of all the electric bow thrusters for a start. also pod propelled boats. The information is probably  already there without you experimenting.

[peterboat]

8 minutes ago, ditchcrawler said:

I would talk to the likes of Crowthers, you are not the first to have electric propulsion. Think of all the electric bow thrusters for a start. also pod propelled boats. The information is probably  already there without you experimenting.

I am on an electric boat forum as well and they are saying the same as me reduce pitch, a chap from the NL has done several boats and he thinks I am in with a shout at 6.5

13 minutes ago, Mike Adams said:

Could I suggest that the motor is overloaded and that is why it is creating too much heat. I am not sure what motor/driver you have but a 19 x10 prop will require a lot more than 6KW to turn it at 1500 rpm. The prop loading goes up as a cube power of the speed that is why it is so difficult to get exactly the correct prop to get full power at the rated speed. I terms of prop size it is much too large and you can't alter the aspect ratio so much without losing efficiency. Look at the prop size for a 8HP stuart turner for example. You can use the free prop calculators on line to find a optimum prop. I would guess about 14 inches max diameter. Otherwise you need a reduction gearbox.

Cedrick Lynch thinks that I am better losing pitch than size, and he is the electric motor guru! as well as building lots of boats, cars, motor bikes etc google him he is an interesting guy

Edited March 13, 2019 by peterboat

[David Mack]

2 hours ago, peterboat said:

The propeller is 19 x 10 and on the old diesel it would have rotated at 1500 rpm

Really? A diesel engine would typically be fitted with a 2:1 ratio gearbox, which implies an engine speed of 3000 rpm. At that speed the engine would be screaming and putting out far more than your 6kW.

[peterboat]

Just now, David Mack said:

Really? A diesel engine would typically be fitted with a 2:1 ratio gearbox, which implies an engine speed of 3000 rpm. At that speed the engine would be screaming and putting out far more than your 6kW.

But not torque and thats what I need and the DC series motor gives me

[Mike Adams]

I was assuming you had a narrowboat. The main problem with such a large propeller with little pitch will be with weed and plastic. Anything around the prop will load the prop much more than a smaller one because it is pushing the rubbish at a higher speed and have more more effect on the prop performance because of the lower pitch. In theory a larger propeller would be more efficient but would need fairly thin blades - not a good idea on a canalboat. If a large prop with a very low pitch worked well you would see more of them about which is not my experience however good luck with it. The method of producing the power at the shaft is not important nor is torque at low speed since the propeller offers little resistance at very low speed.

[WotEver]

22 minutes ago, peterboat said:

But not torque and thats what I need and the DC series motor gives me

That’s not what this site says:

https://www.lainefamily.com/hp.htm

 

Specifically... 

Because of the math, an engine that is tuned to have peak horsepower below 5252 RPM will ALWAYS have more torque than horsepower, and engines with peak horsepower above 5252 RPM will ALWAYS have more horsepower than torque

[ditchcrawler]

1 hour ago, peterboat said:

I am on an electric boat forum as well and they are saying the same as me reduce pitch, a chap from the NL has done several boats and he thinks I am in with a shout at 6.5

Cedrick Lynch thinks that I am better losing pitch than size, and he is the electric motor guru! as well as building lots of boats, cars, motor bikes etc google him he is an interesting guy

If you have spoken to these experts why ask us, non of us have experience of electric propulsion

[dmr]

How can you say "more horsepower than torque" when they are different units????

 

................Dave

[peterboat]

33 minutes ago, WotEver said:

That’s not what this site says:

https://www.lainefamily.com/hp.htm

 

Specifically... 

 

 

But i need a maximum of 1650 rpm which the motor is tested at with a big fan, so take away big fan and insert propeller and you will see what I mean. turning power is what I want, and need at the maximum speed, Mike is correct in that a smaller blade gives me less chance of rubbish slowing me down, but is incorrect because old boats used to have very big props with slow revving engines, its only lately that smaller props have come in vogue and some would say for all the wrong reasons.

My motor will spin at much higher speeds than 1650 but its wasted speed as back EMF starts to become a problem and it reduces torque and efficiency, so for me I am trying balance the size and pitch to what is the best for my electric motor