Electric Boat

[Aiden1015]

In my engineering class we're building electic boats to drag race. We can only use a alkiline 9v battery and a small motor. what can we do to maximise on the power for it. i was thinking use an inverter and send the 9v through a transformer and then a rectifier to convert back to dc to transmit to the motor. is there a better way? schematics would help aswell

 

[Boncuk]

Any kind of engergy conversion involves losses.

In your particular case these losses will be so huge that you might as well forget about driving an electric motor.

You can't transform DC. Prior transforming it has to be converted to AC, then transformed and rectified again for the motor.

As far as I know drag races are carried out on a limited track distance, so the best acellerating vehicle will win.

Since you can't change the engergy source you should work at an outstanding propeller design which will give you maximum thrust.

Boncuk

 

[bobledoux]

Your boat needs high current over a short period of time. Look at a capacitor for short term peak power. It may take several seconds to charge, but it can discharge very quickly.

Consider a low voltage motor, perhaps a motor rated for 1.5 - 3 volts. The ability to give up heat is the general limiting factor for motor ratings. With your extremely short duty cycle you can push the motor almost to the point where the current melts the windings.

I'd consider a prop with enough pitch that the motor rpm is loaded down. DC motors typically provide full torque at zero rpm. A slower turning prop may be more efficient than a high speed prop.

Power to weight ratio is critical to good performance. So keeping the boat light will improve acceleration rate.

 

[bfree]

u dont need to boost ur DC voltage u need more current u can use DC to DC converter such as Buck converter to boost the current and reduce the voltage
u can see the details of calculation here Buck converter - Wikipedia, the free encyclopedia
once u have more current u can use a motor with may be 2 or 3 volts and high current to win the race

 

[Willbe]

Your boat needs high current over a short period of time. Look at a capacitor for short term peak power. It may take several seconds to charge, but it can discharge very quickly.

Consider a low voltage motor, perhaps a motor rated for 1.5 - 3 volts. The ability to give up heat is the general limiting factor for motor ratings. With your extremely short duty cycle you can push the motor almost to the point where the current melts the windings.

I'd consider a prop with enough pitch that the motor rpm is loaded down. DC motors typically provide full torque at zero rpm. A slower turning prop may be more efficient than a high speed prop.

Power to weight ratio is critical to good performance. So keeping the boat light will improve acceleration rate.

Look into the (stored energy)/(weight) ratio for a flywheel vs. a capacitor. Pick the winner.
Power to weight ratio for a boat may be swamped (pardon the pun) by drag in the water, making the boat weight relatively less important.
Water drag goes up with the cube of the speed.

And you can abuse the daylights out of the motor. I ran a 1.5v toothbrush motor on 25v for about 30 seconds. You could hear this little muvva' screaming, all over the lab. I had to hold it by the wires. This was before I knew about I^2 T ratings.

I guess I should have worn a face mask. . .? Nah!!!

 

[stevez]

Aiden - if you've been told that the battery is something very specific - at least in terms of size/chemistry then you've established the total power available - if you assume that you draw power from the battery at a rate that allows maximum power to be suck out. The discharge rate will determine how much you can get out of it.

If the track or course has been established then you have a sense of how long the time period might be for power delivery - if you model possible acceleration rates.

What you then need to do is apply this power over shorter or longer periods of time, to move your boat. The ideal situation would be to draw directly from the battery at an ideal rate by matching motor/prop or pump or fan - so that you avoid converters that add to the weight.

Sounds like a fun problem - makes me wish I was a student at times.

 

[SMUGangsta]

do you need any kind of speed control? if so go for PWM for maximum power at lower speeds, if no then its going to be down to engineering rather than electronics, and there are quite a few good ideas above.

 

[Leftyretro]

Your boat needs high current over a short period of time. Look at a capacitor for short term peak power. It may take several seconds to charge, but it can discharge very quickly.

Consider a low voltage motor, perhaps a motor rated for 1.5 - 3 volts. The ability to give up heat is the general limiting factor for motor ratings. With your extremely short duty cycle you can push the motor almost to the point where the current melts the windings.

I'd consider a prop with enough pitch that the motor rpm is loaded down. DC motors typically provide full torque at zero rpm. A slower turning prop may be more efficient than a high speed prop.

Power to weight ratio is critical to good performance. So keeping the boat light will improve acceleration rate.

Considering that the power source is fixed by rule to be a 9v battery I do think that bobledoux's idea has the most merit I've read so far. A couple of super caps, rated say at 1f at 5volts, in series would in effect lower the impedance of the power source for the motor. You would have to experiment with the variables involved, weight or boat, length of course, amount of capacitance, prop pitch, motor voltage and resistance ratings, etc. The power switch would have to have three positions, off, capacitor charge and motor on. I can't think of a more efficient way to go as any additional circuitry would just add weight and introduce losses.

Lefty

 

[audioguru]

Ever shorted a new 9V alkaline battery? I saw a woman with a spare one in her pocket when she gave a speech with a wireless microphone. She was jumping because the battery shorted in her pocket and was burning her.

Energizer says the 9V alkaline battery has 6 AAAA cells inside. Their AAAA cells have an internal resistance of about 0.2 ohms each when new. The 9V/1.2 ohms= 7.5A into a short or 5A at 3V. A 3V motor will provide plenty of power if it draws 5A.

Use an oversize propellor so that the motor has its max amount of torque. The running time is probably very short durring the race so efficiency doesn't matter.

Streamline the boat for the least amount of drag. It probably won't go fast enogh to plane or fly.

 

[crutschow]

Energizer says the 9V alkaline battery has 6 AAAA cells inside. Their AAAA cells have an internal resistance of about 0.2 ohms each when new. The 9V/1.2 ohms= 7.5A into a short or 5A at 3V. A 3V motor will provide plenty of power if it draws 5A.

From the maximum power transfer theorem you would want the motor to have an equivalent operating impedance at speed under load (not it's dc resistance) equal to the battery internal impedance of 1.2 ohms. So you would want a motor that would draw 3.75A at 4.5V under load for a motor power of 16.88W.

Of course the battery impedance goes up as the battery is discharged so, depending upon the running time of the race, the optimum matching impedance may be somewhat higher than 1.2 ohms.

 

[mneary]

What do you have control over? Is the battery provided to you, or do you choose the supplier? Are you permitted to replace it with a new one before each drag race?

Does your team design the hull and/or propeller? Do you design or purchase the motor, or must you choose from available options?

 

[Boncuk]

One word concerning the hull design, especially the finish:

Don't make an even glossy finish. It increases aquadynamic resistance (drag) considerably. Use a relatively rough surface to gain a maximum of dynamic disturbance and minimum drag.

We have made tests in our sailing club with the same class boats (S-class). The boats with a hull finish of rough (tiny pits in the paint) surface made 2kts more under the same wind conditions than the ones with a glossy finish.

The reason is simple: Taking care of many small portions of disturbance the water won't "stick" to the hull (adhesive force) and float smoothly along the shape.

For comparison you might study the properties of shark's skin.

Boncuk

 

[SMUGangsta]

One word concerning the hull design, especially the finish:

Don't make an even glossy finish. It increases aquadynamic resistance (drag) considerably. Use a relatively rough surface to gain a maximum of dynamic disturbance and minimum drag.

We have made tests in our sailing club with the same class boats (S-class). The boats with a hull finish of rough (tiny pits in the paint) surface made 2kts more under the same wind conditions than the ones with a glossy finish.

The reason is simple: Taking care of many small portions of disturbance the water won't "stick" to the hull (adhesive force) and float smoothly along the shape.

For comparison you might study the properties of shark's skin.

Boncuk

I seen something about this on the Discovery Channel, they were researching Water Lilies to find out why they dont get wet when you submerge them, the technology that has came from this is 'Self Cleaning Windows', and 'Honey Spoons' which the honey cant stick too, but the same principle applies to engine tuning, too smooth a surface on the intake runners and the air 'sticks' to the sides.

 

[Willbe]

Don't make an even glossy finish. It increases aquadynamic resistance (drag) considerably. Use a relatively rough surface to gain a maximum of dynamic disturbance and minimum drag.

We have made tests in our sailing club with the same class boats (S-class). The boats with a hull finish of rough (tiny pits in the paint) surface made 2kts more under the same wind conditions than the ones with a glossy finish.

I think that is why golfball surfaces have depressions in them. So turbulence up, laminar flow down, drag down.

 

[Willbe]

The problem with batteries is that the faster you try to drain them, the less total energy stored in them you will be able to extract.
If you can get a math model for this effect for your particular battery technology you will be ahead of the game.

I would spend as many hours as you can before the race transferring the energy from the battery into a storage device.

If the rated motor voltage is Vm, when the flag goes down the storage device should apply 10Vm or 20Vm to the motor.

 

[audioguru]

An electric motor is stalled when it is turned on. Then it is just its internal resistance and uses a very high current. That is why I suggested a motor with a DC resistance the same as the battery's internal resistance and a big propeller so that the motor runs at full torque.

With a big propeller then the boat will accellerate extremely quickly while other boats will have their tiny prop "slip".

 

[Boncuk]

With a big propeller then the boat will accellerate extremely quickly while other boats will have their tiny prop "slip".

100% power adaptation between source and load - should be a key to succeed in the race.