# Generator power estimate

1. Jan 8, 2016

### sea kay

I am trying to figure out whether I can use a small Minn Kota trolling motor as an electric generator.
If I add a 10" prop the max energy from the water passing the prop should be about 700W and if the prop is 50% efficient, it should put half of that on the shaft. I expect the prop to turn about 600 to 1200 rpm.
Can I use the standard Minn Kota motor if it runs 1800 rpm unloaded at 12.8V?
I expect it will need to turn more than 2000rpm to generate 12V. Is that correct? In other words: if a motor needs Y volts to achieve X rpm, does that also mean it will need at least X rpm to achieve Y volts?

Last edited: Jan 8, 2016
2. Jan 8, 2016

### Staff: Mentor

Yes you can use the motor as a generator. It sounds like you plan to hold the motor still as water runs past, correct? What is the velocity of the running water? I ask because the RPM values you cite sound very high, especially when the generator is loaded. Perhaps your plan is to drag the electric motor while you cruise at high speed with your gasoline outboard to recharge the battery.

The voltage produced by a generator depends on the load connected. Making 14v when unloaded, does not mean that you can charge a battery with say 20A of current.

3. Jan 8, 2016

### sea kay

Thanks Anorlunda.
Yes, I plan to use the motor dragging behind a (sail) boat, under water. The rpms stated are correct, the prop has a pitch of 4.5inch and speeds are between four and six nautical knots (2-3m/s). However... I think achieving enough power and voltage will be problematic since dragged rpms are 800-1200rpm and if my understanding is correct, The motor will need to turn more than 1800 rpm (=no load rpm) to generate a voltage high enough to charge the battery, even at very low amps.

Last edited: Jan 8, 2016
4. Jan 8, 2016

### Staff: Mentor

The answers you need are in the motor specs: what is its rated rpm, current and wattage?

5. Jan 8, 2016

### CWatters

Don't forget that when used as a motor the speed of the water through the prop is much higher than the boat speed. So when used as a generator at a boat speed of 4-6 knots the prop may not spin very fast = not much voltage. If the motor drives the boat at say 4-6 knots on 12V then don't expect >12V if you drag the prop through the water at 4-6 knots.

6. Jan 8, 2016

### sea kay

Current max is 30amps, at 12V ->360W maximum power consumed by the motor at 1200rpm (rated under load.)
I haven't got a clue about the internal resistance of the motor though, which I assume is necessary to estimate the power generated by the motor/generator.

One other thing I am curious about is whether the windmill Betz limit (rotor efficiency of 40% max ) also applies to a propeller in water. I think not, but would be nice if someone can confirm.

@CWatters: You are correct and I had thought of that, but I could switch props and play with pitch a tiny bit

7. Jan 8, 2016

### CWatters

I think the curvature of the blades will be the wrong way as well.

8. Jan 8, 2016

### Staff: Mentor

Good idea and good luck, but consider these additional points.
• Minn Kota's control electronics, if they have more than an on/off switch, could block the generator mode. Your question should be posted to the Minn Kota support forum. In fact, I'll wager that they have had that question before.
• The RPM of a dragged motor unloaded, will drop drastically as you add electrical load to the generator.
• Many outboards have a charging output rated at least 5A DC . I used to own a Honda 10HP outboard that had that feature. Why not make use of that?
• Even if you put substantial current in for the short time it takes to get back to the dock, and even if you replace the energy used, the battery will not be fully charged. You should study three-stage battery charger technology. Less than a full charge causes sulphation which rapidly degrades a deep discharge battery unless you have a way to desulphate. Therefore, even if you charge on the way back to the dock, you'll still need a trickle charger for many hours more to fully charge your battery and keep it in top condition.

9. Jan 9, 2016

### sea kay

Don't worry, Cwatters/anorlunda, I have plenty of knowledge to solve the practical issues and I could use a 24V Minn Kota as a basis for charging, since it would only need half the rpms to reach adequate Voltage (14V)

As for the rpm drop of a dragged motor, that still concerns me. Is there anyway to calculate at what slip percentage or tip speed to water speed ratio generating propellers are most efficient? All data I found on the web was experimental or windmill related

10. Jan 9, 2016

### Staff: Mentor

Ok, well I haven't looked at everything, but for the main question of the OP, if the water's energy is 700W and you are assuming 50% efficiency (which is reasonable), you should be fine with the size you have. Others have gone into the other practical concerns and I'll leave that to them...

11. Jan 9, 2016

### meBigGuy

The question is that, given 350W input from the shaft, what will the motor output. That requires knowledge of the motor.
What we know about the motor is

1. Consumes 360W from 12V at some unknown load and unknown RPM (no idea of the output torque and RPM).

Seems to me you need to jury rig another motor to drive the unit as a generator and characterize its performance. I guess they use permanent magnets so you don't have to deal with powering the field coils.

Once you know what the generator will do, then you are faced with what to do with its output.
You will need some sort of output power conditioning and storage to get any sort of constant voltage and current capability.