Help me design the "Poorqeedo", another efficient electric boat

Wow, I was thinking ~20kts, not hurricane type winds. You're certainly braver than I am.

 

Once you have a prop design for your design conditions you can check performance by changing the rpm on the design page and then use the analysis pages to see how it performs. You do not redisign to look at conditions away from your design point. This is a bit tedious but it can be done.

I think the problem with the 10X7 prop will be its strength. The 16X16 will probably be OK for 80N but the smaller one will have blade flex or even break. I believe I have already suggested how you can test their load capacity.

If you can weld stainless and use a hand grinder it takes about 4 hours to make a good prop. A little longer for the first one. If you make a folding prop you do not even need a welder.

I suggest you test the small prop for strength and if OK set up the shaft and motor mount to allow room to use a bigger prop but try the smaller one first.

If you want to operate in heavy weather then the bigger prop with gearing is going to be better. A 16" diameter prop sweeps more than twice the area of a 10" prop so it will produce considerably higher thrust for your power input in a heavily loaded state. You can get an idea of the difference by looking at the thrust for power of the different props at low speed using the analysis pages.

 

Hi Mental,
It was not Bravery, as I said, it was only a Bay chop of about 2 1/2' to maybe 3' with waves about 5' apart. The wind was the only real circumstance, and the ride was fun. Steering off the wind offered some interesting challenges as the boat drifted sideways dramatically while trying to hold course. It really was just a fun ride...better than Disney. It just showed what one can do in a lightly powered boat that shouldn't sink.
John

 

Thanks again. When I upped the load to 80N I believe the prop loading went from "low" to "medium."

I'll buy a couple of 10x7 props and test one, as per your instructions.

I still like the idea of shedding some prop diameter and the reduction drive, although it may be to my detriment.

Also, building a prop is probably wise, but I'm still trying to keep construction effort low. Perhaps in the future....

I'll repost your test method so no one else looking at this will have to dig around:

"3. I have not handled the 16 X 16 APC props but I know they are suitable for pedal power, which can easily exceed 207W. The force will be 50N on each prop. That is say 11lbf for the metric impaired. A simple test is to load the hub with a string carrying an 11lb weight with the blade supported flat either side at 75% of total radius. This will be roughly the flexing you will get in operation."

 

There is quite a bit of work in setting up a nice reduction gear. If you have a lathe and a milldrill it is reasonably easy to do. Without these it can be a challenge.

You will do reasonably well with direct drive using a 10X7 on each hull. If you are prepared to make your own props then these will be suitable for long-term use under high load. So you can then look at the shaft tube coming out at the stern of the hull with a short curved shaft to the motor.

Making props is not hard. Here is an example:
http://www.boatdesign.net/forums/props/seagull-prop-cat-racer-29910.html


The torque constants you quoted in post #62 for the bigger motors is out by a factor of 1000. The torque is miniscule - less then 4Nm at rated current. You could use a 1/4" shaft with direct drive.

 

That's a gorgeous prop...

I'll probably take your advice and build a prop, but not until the boat is in the water and usable. I don't think I'd have much trouble making one, It's just that I want to make sure the boat gets in the water.

For now I'll be recalcitrant and operate under the assumption that I will use a stock APC prop. I'll order the 10x7 and test it with various loads, and then see how much force it takes to break it. If it does not pass the test I will then reevaluate my options. Even if the props do not offer long term reliability they only cost US $2.50 and I can carry spares.

I'm optimistic that the 10x7 prop will be strong enough, since the 16x16 seems very robust. Both hubs use the same ~8mm shaft mount.

I just did a deflection test of the 16x16 prop, set up like you suggested. I didn't bother to support the foil, just a single point on the leading edge.

Deflection with 77N of force (~7.8kg) was ~3mm and deflection with 38N of force was ~1.5mm.

I'm not sure what is acceptable for a prop, but it seems ok to me. Deflection would probably be less if the prop was supported for the length of the foil to eliminate twisting.

I just used Jeremy's conversion factor to get from Kv to Kt, probably my mistake. I didn't use that value for estimating anything, wouldn't know what to do with it.

I have access to a good mill and a lathe and I have made reduction drives before, so I know what's involved. When I started thinking about this boat I just assumed I would be making them.

 

The ability to make a stronger 10" prop gives you a way out if you need something stronger. The standard prop will allow some good testing and may do the job. Not heavy or expensive to carry spares either.

In operation I have deflected aluminium blades on 400mm props to about 20mm and they still operate efficiently. Over time they would fatigue but the load is quite even if they are in clean flow so a very long time before they actually fail. A prop is not a high fatigue environment if they are in undisturbed flow.

One of the reasons I developed the folding prop was to make it easy to remove weed. If you are likely to encounter weed then a folding prop will shed it as soon as it coasts.

 

This is really becoming hard to ignore. Better efficiency, better strength, weed shedding, folding on impact (?), seems like a ton of benefits.

I'll probably still use a plastic prop for initial tests, but now I'm thinking a hand made prop is more of a necessity. If I do a shoddy job making the prop and it performs worse than APC's finest, I'll be sure to blame you =)

Rick, off the top of your head, what kind of efficiency do you think I could gain from a handmade folding prop, direct drive? Please don't spend any time on this, since I haven't yet tested my mismatched motors.

 

The various options for a folding prop are given below. These values are based on something very easy to make and strong enough for the loads you will have. They are all 10% thick foils. The chord values are the smallest that will stand up adequately to the loads if made from stainless steel. It is best to work with ready available sections. For example I can get 30mm wide and 3mm thick or 40mm wide 4mm thick. You could use 1" wide and 1/8" thick but I then need to use data for a 12.5% thick section.

Using less than 1/8" thick will be marginal for stainless.

If you get around to make a folding prop I can help you with the detail.

Dia (mm) 200
Chord (mm) 30
RPM 1089
Torque (Nm) 1.8
Power (W) 211

Dia (mm) 250
Chord (mm) 25
RPM 789
Torque (Nm) 2.4
Power (W) 197


Dia (mm) 250
Chord (mm) 30
RPM 745
Torque (Nm) 2.5
Power (W) 198

Dia (mm) 300
Chord (mm) 30
RPM 544
Torque (Nm) 3.3
Power (W) 188

Dia (mm) 350
Chord (mm) 30
RPM 416
Torque (Nm) 4.3
Power (W) 185

Dia (mm) 400
Chord (mm) 30
RPM 328
Torque (Nm) 5.3
Power (W) 182

To get the best from the motor you would design it to run into its speed limit around the same point where it runs into its current limit.

Have you got the actual part number on the motors? This should give the torque rating as the figures you have given are way out. The sort of torque you are showing would require be from a little V8 car engine.

 

I'll openly admit to being a bit concerned at the amount of work needed to make a simple folding prop, using the knowledge Rick has shared with us. When I finally bit the bullet and decided to make one I was really surprised as to how easy it was. I think that Rick's estimate of time to make one is, if anything, a bit pessimistic. I think you could make a prop in a bit less time easily enough, with nothing more than hand tools and a small angle grinder.

Jeremy

 

(I'm thinking about using different motors yet again, read further down to bold text)

Ok, went back and looked at the numbers and I multiplied instead of dividing. Here are the real motor numbers:

Electrocraft E712 - 34.5rpm/v - Kt = 0.28Nm/A
Electrocraft E713 - 32.8rpm/v - Kt = 0.29Nm/A

The motors are old (80's?) and rare, so the datasheets are not available online. I called them and they said they need to get the info out of a filing cabinet and they'd call me back. I do have the specs for the E712:

Reliance Electric/ElectroCraft
Model E712
DC Brushed Servo Motors
Maximum Peak Torque 1500 Oz-In.
37.51 Oz-In/A
25.0-30.5 V/KRpm
Motor Terminal Resistance .75-1.02 Ohms @ 4A.
Maximum Terminal Voltage 60V
Maximum Continuous Speed 2100 RPM (No Load)
Maximum Peak Current 44.5A
Maximum Continuous Stall Torque At Max Motor Temp 300 Oz- In.
Maximum Continuous Current 8.9A
Size Nema 42
Length approx. 147mm, 5.75"
Diameter approx. 100mm, 4"
Shaft Diameter 0.5".
Approx. 4.2 kg, 9 lbs

Anyway, I opened up the two motors and the stators look very different. The E713 stator is about 50% longer and the windings are about 80% thicker. It's strange that that it is rated at only 1A higher and 240W instead of 560W for the E712. These motors are all OEM so maybe they derate them to the clients requirements.

So I then opened up the first motor I was thinking of using, the E722. It's rated at 100V and 5.6A. I attached a photo below, the E722 is on the left and the E712 is on the right. I have 3 E722's.

To compare:

E712 ________________________ E722

60V _________________________ 100V
9A __________________________ 5.6A
34.5 rpm/V ___________________ 26.3 rpm/v
Kt = 0.28Nm/A ________________ Kt = 0.36 Nm/A
Winding Dia : 0.43mm __________ Winding Dia : 0.50mm

All of these motors are sealed. My current thought is to use 2 E722's, run them at 36V volts and exceed the rated amperage (say 10A instead of 5.6). This would be 64% of the motor's rated power, and similar to the rated amperage of the E712 which has the same stator and smaller windings. For insurance I'll drill some holes in the endcaps and add a fan to keep the temperature down and the efficiency up.

One advantage of these motors is the max RPM is reduced by 20% so I can switch to a larger prop with a 9 inch pitch.

I have three of these motors, so I can test one of them at 10A for a couple of hours and see what happens.

I'm by no means certain this will work and I am also uncertain that this is good for efficiency.

Thanks for the info on the folding prop, I will almost certainly build one once the boat is up and running.

This wouldn't exactly work with the motors above, unless you call the current limit whatever number I choose. =)

 

Good to know. Thanks Jeremy.

 

Typically half of the losses in an electric motor are due to winding resistance at rated load. If you double the current you quadruple the winding losses. Hence running a motor at 10A when it is rated at 5.6A will push losses up about 3 times those at rated condition. This will cook it if it is sustained.

The winding resistance is very high so they are not very efficient on low voltage. The little Turnigy 42SK motor I used on the yellow boat has a winding resistance of 0.032ohm compared with the 1ohm on the E712 you have. The E722 is likely to be higher. Even at 1 ohm the winding loss at 10A will be 100W - a huge waste.

Unless you are getting these motors for nothing or already have them I would be looking at the scooter stuff or even gearing one of the model aircraft outrunners. I have one of the 250W scooter motors with inbuilt gearing and it worked well with a big prop. See attached.

I got over 9kph with about 200W from the two little 12V batteries - the voltage dropped to about 20V at 10A. This boat weighs all up 130kg. I was able to do 7km at 7kph from one charge of those batteries.

Rick W

 

Arrrg, thanks. Clearly I haven't got a clue =)

Yes, the motors were free. Still, it doesn't mean I should use them.

The E713 has the fattest windings (3x as much copper as the E712) and is rated at 10A, 24V so it is probably the most suitable motor for 36V, 10A (right?). There's a slim chance I'll be able to get another E713 for free, but it could take a week to find out.

For now it's back to the drawing board....

 

Also, I may consider using a chinese scooter motor like you mentioned, although I'd like to avoid it for the moment.