Prop sizing

[jonesthenuke]

The issue will be matching the prop power curve to the motor. Do you have details of the motor? Is it a straight series wound or does it have any compound winding? My first thought are that a shunt wound motor with electronic speed control would be better as you want high torque at max rated speed not reducing torque with increasing speed, which is what a series wound motor will give. It also prevents the overspeeding to destruction scenario that you describe above which could occur if the driven load is removed.

 

[peterboat]

2 hours ago, jonesthenuke said:

The issue will be matching the prop power curve to the motor. Do you have details of the motor? Is it a straight series wound or does it have any compound winding? My first thought are that a shunt wound motor with electronic speed control would be better as you want high torque at max rated speed not reducing torque with increasing speed, which is what a series wound motor will give. It also prevents the overspeeding to destruction scenario that you describe above which could occur if the driven load is removed.

 

Series wound and it torque figure is flat to the rated 1650 after that back emf takes its toll which is why I am experimenting with props.

I am also doing testing with 60% charged batteries as I need real life figures, series motors were the backbone of forklift trucks, and are used extensively in DIY electric cars running over voltage!  Because I have a direct drive arrangement I am hoping that over revving due to broken belts will never be an issue, so this week will see how the first swop works

[jonesthenuke]

17 minutes ago, peterboat said:

Series wound and it torque figure is flat to the rated 1650 after that back emf takes its toll which is why I am experimenting with props.

I am also doing testing with 60% charged batteries as I need real life figures, series motors were the backbone of forklift trucks, and are used extensively in DIY electric cars running over voltage!  Because I have a direct drive arrangement I am hoping that over revving due to broken belts will never be an issue, so this week will see how the first swop works

The statement that it is "series wound" and has a flat torque characteristic appear inconsistent to me.

 

I would have expected something with very high starting torque, falling rapidly as speed rises, and its not the characteristic I would choose for a propellor driven boat. Do you have some details of the motor that you can share?

 

 

The torque requirements of a car/fork lift are completely different to a propellor driven craft.

Edited March 24, 2019 by jonesthenuke
Typos! and final sentence added plus I cannot type today!

[peterboat]

6 minutes ago, jonesthenuke said:

The statement that it is "series wound" and has a flat torque characteristic appear inconsistent to me.

 

I would have expected something like this, and its not the characteristic I would choose for a propellor driven boat. Do you have some details of the motor that you can share?

https://www.google.com/url?sa=i&rct=j&q=&esrc=s&source=images&cd=&cad=rja&uact=8&ved=2ahUKEwiRiMWv45rhAhVK5eAKHdfBDTMQjRx6BAgBEAU&url=https%3A%2F%2Fwww.globalspec.com%2Freference%2F10789%2F179909%2Fchapter-3-ac-and-dc-motors-dc-motors-dc-motor-types&psig=AOvVaw10Vb_H9gfd-jjIn5XaVYER&ust=1553516831142962

 

Reversing a series motor is easy and I have a curtis controller for speed control which works well, Compound motor controllers dont seem to advertised as much and the sizes available were small.

My motor is 12 inches in diameter and about 16 inches long, big enough for the job I hope, it was also nearly new, had the right RPM range for direct drive, the high starting torque is great as it gives you good braking, also its mid torque where you spend most of the time is good, its a drive motor so is set up for thrust, and the price was right 

[jonesthenuke]

34 minutes ago, peterboat said:

Reversing a series motor is easy and I have a curtis controller for speed control which works well, Compound motor controllers dont seem to advertised as much and the sizes available were small.

My motor is 12 inches in diameter and about 16 inches long, big enough for the job I hope, it was also nearly new, had the right RPM range for direct drive, the high starting torque is great as it gives you good braking, also its mid torque where you spend most of the time is good, its a drive motor so is set up for thrust, and the price was right 

The speed controller is a key part of the system, you will need to match the combined effect of the speed controller to the boat and prop characteristics. PS I am still not convinced this is series wound, most speed controllers work with shunt motors......?

 

[peterboat]

26 minutes ago, jonesthenuke said:

The speed controller is a key part of the system, you will need to match the combined effect of the speed controller to the boat and prop characteristics. PS I am still not convinced this is series wound, most speed controllers work with shunt motors......?

 

You had better talk to Mr Curtis then because it is a series wound motor and this is the controller he sold me

Curtis 60V 72V 1205M-6B403 PMC 400A

 

This is the second boat I have done, the first I used a Agni motor bought from Cedric Lynch, on that boat I used a belt gearbox to up the revs and increase the torque.

This boat works but I need to get more revs from the motor so it produces movement and not heat, on the move the motor keeps a steady RPM and at 3 mph draws 50 amp [3.3kw] I would prefer the motor to be spinning faster so that it is operating cooler [fan cooled with a blower for extended periods of cruising] that is why I am playing with propellers. this week I will know how it works with the smaller prop

Edited March 24, 2019 by peterboat

[jonesthenuke]

Yes that will be a series motor. Not ideal in my view, however, with the speed control set to full do you get full battery volts at the motor terminals? If not you are losing in the controller and will not get rated torque/power from the motor.

 

Taking your point about cooling, I can see your problem when you apply full current at less than rated speed. Generally the motor fan will be designed for adequate cooling at rated full speed. If you run at, say, half speed but at rated full current the heat dissipation will be the full motor rating but the cooling will be very very much lower as fans generally have very non-linear characteristic. For this reason most motors used with speed control have separate blowers which run at full speed regardless of the main motor speed. As you intimate its run at full speed by changing the prop or adding a gearbox, or use the blower. 

[TheBiscuits]

7 hours ago, peterboat said:

This boat works but I need to get more revs from the motor so it produces movement and not heat

I still think that overdriving the voltage isn't going to help long term.

 

Is it 72V you are feeding to a 48V motor?

[peterboat]

11 hours ago, TheBiscuits said:

I still think that overdriving the voltage isn't going to help long term.

 

Is it 72V you are feeding to a 48V motor?

I run it at 60 volts [5 x 12v batteries in series then paralleled to the same again] which in reality gives me 69 volt at 13.8v per battery. These motors can run at 144 volts, running it slightly over voltage gives me protection against any voltage losses and may give me extra power at full throttle, the rest of the time the controller [a PWM based item] makes sure that the motor is operating correctly

[peterboat]

So today I changed the prop! Has anybody else ever changed a prop through the weed hatch? using a 3 legged puller underwater is good for the soul, but not the knees.

So a 16 x 14 gained me an extra 100 rpm but it will have to wait until tomorrow for the readings as I have had enough messing around for one day

[ditchcrawler]

1 hour ago, peterboat said:

So today I changed the prop! Has anybody else ever changed a prop through the weed hatch? using a 3 legged puller underwater is good for the soul, but not the knees.

So a 16 x 14 gained me an extra 100 rpm but it will have to wait until tomorrow for the readings as I have had enough messing around for one day

Biggles did

[LadyG]

21 hours ago, peterboat said:

So today I changed the prop! Has anybody else ever changed a prop through the weed hatch? using a 3 legged puller underwater is good for the soul, but not the knees.

So a 16 x 14 gained me an extra 100 rpm but it will have to wait until tomorrow for the readings as I have had enough messing around for one day

well done you .................... it's only half a working day ........ no excuses!

[peterboat]

So I butchered adjusted my 19 x 10 prop down to a 16 x 10 fitted it and with nearly flat batteries managed 1200 RPM. I have also discovered that my 60 volt controller is really a 72 volt controller so the next run will be with 6 batteries in series and paralleled with another 6 in series if that makes sense, they will be fully charged so I am expecting great things. The blower is also connected now and the motor is hardly warm after nearly 1 hours running, fingers crossed that tomorrow gets me the results wanted

[yabasayo]

On 07/04/2019 at 16:57, peterboat said:

So I butchered adjusted my 19 x 10 prop down to a 16 x 10 fitted it and with nearly flat batteries managed 1200 RPM. I have also discovered that my 60 volt controller is really a 72 volt controller so the next run will be with 6 batteries in series and paralleled with another 6 in series if that makes sense, they will be fully charged so I am expecting great things. The blower is also connected now and the motor is hardly warm after nearly 1 hours running, fingers crossed that tomorrow gets me the results wanted

Guess what - you have reduced the prop area by about 30% and the rpm has increased by about 30% ie from 900 to 1200 rpm.

I think you said the motor should run at 1650 rpm. On that basis you need to reduce the prop area by about another 25%  or down to about 14 inches diameter to get your desired full load rpm.

As I pointed to in post #41, your prop was overloading your motor by about 100%. Reducing the prop area by a total of 50% or so fixes that.

[peterboat]

11 hours ago, yabasayo said:

Guess what - you have reduced the prop area by about 30% and the rpm has increased by about 30% ie from 900 to 1200 rpm.

I think you said the motor should run at 1650 rpm. On that basis you need to reduce the prop area by about another 25%  or down to about 14 inches diameter to get your desired full load rpm.

As I pointed to in post #41, your prop was overloading your motor by about 100%. Reducing the prop area by a total of 50% or so fixes that.

You didnt read what I put! with nearly flat batteries it increased to 1200rpm.

With fully charged batteries and 6 in series to get the voltage correct for the controller I got 1500 rpm on the next test, I then reduced the bank back down to 60 volts again with fully charged batteries and I was getting 1400 rpm plus.

On a one hour test on the 60 volt bank, at 575 rpm which equals 3mph the motor draws 37 amps 2KW or so, I have 2.48 KW of solar assigned for drive, which I can increase to nearly 3.68KW if needed.

Yesterdays test included full throttle testing drawing between 200 amps and 350 amps, the motor and controller both ran cool, it has a blower through the motor with an outlet pointing at the controller, which is bolted flat with heat sink grease to aid cooling. Slow speed testing at 350 rpm which is between 1 - 2 mph, drew nothing from the batteries at all, the solar was charging the batteries as well as powering the boat, [high cloud reduced incoming solar]

So it would seem that on a sunny day solar will provide the needed KW for 3 MPH cruising.

So as long as I have the wheeled steering sorted I will take the boat to Sheffield next week for its first proper run

 

[yabasayo]

Sounds like you're nearly there. I'm surprised you can get 2.48kw from solar panels though. You must have something like 20m2 of panels.

Presumably you've cut your prop down on a lathe ? If so you must have some pretty horrible blade profiles. Once settled on an optimum size you should get an increase in performance by buying a new prop.

Edited April 12, 2019 by yabasayo

[peterboat]

23 minutes ago, yabasayo said:

Sounds like you're nearly there. I'm surprised you can get 2.48kw from solar panels though. You must have something like 20m2 of panels.

Presumably you've cut your prop down on a lathe ? If so you must have some pretty horrible blade profiles. Once settled on an optimum size you should get an increase in performance by buying a new prop.

I butchered the prop with an slitting blade on an angle grinder, I profiled one blade to a shape I liked and then made a template of it, I copied this onto the other two blades and that was that! I have a spare prop which can be sorted at a later date but at the moment this one is as smooth a silk!! no vibration or cavitation pure luck rather than skill! I am happy with the results and dont think cutting the prop down anymore will achieve much better, but you never know I might try it