may have found the ideal board (i think)
recap given i have a
7.4v battery
2x 540 motors
self-right hull design
requirment
500m rc range
GPS for return home when loss of signal or low battery
i have tried to get information on the details of the motor but no luck... so its a guessing game here
using ChatGPT...
Asked the following question....
Question #1
I have to boat, weigh 2.8kg. Powered by 7.4v battery. Driven by 2x 540 brushed motors powering 3cm 3 fan propeller. Traveling at 7km/h.
What is the current draw and rpm of the motors?
Response #1
To calculate the current draw and RPM of the motors, we need to consider a few factors.
- Current Draw:
The current draw (I) of a motor can be estimated using the formula:
[I = P / V]
Where:
- ( P ) is the power (in watts) being consumed by the motors.
- ( V ) is the voltage (in volts) from the battery.
To find P, we need to calculate the power required to move the boat at a given speed.
Assuming the boat is moving at a speed of 7 km/h (which is about 1.94 m/s), we can use the following formula to estimate the power required (ignoring losses for this basic calculation):
[P = F \cdot v]
Where:
- ( F ) is the force required to overcome resistance (we might estimate this based on weight, but it typically needs drag coefficient and other details).
- ( v ) is the velocity in m/s.
Assuming a basic thrust-to-weight ratio and utilizing the weight of the boat (2.8 kg), we can estimate thrust needed might be around the weight of the boat, which translates to approximately:
[F \approx m \cdot g]
Where:
- ( m ) is the mass (2.8 kg).
- ( g ) is the acceleration due to gravity (9.81 m/s²).
So,[F \approx 2.8 \cdot 9.81 \approx 27.5 , \text{N}]Then,[P = 27.5 \cdot 1.94 \approx 53.35 , \text{W}]
Now we can calculate the current draw:[I = P / V = 53.35 , \text{W} / 7.4 , \text{V} \approx 7.21 , \text{A}]
- RPM of the Motors: For brushes motors, the RPM can be estimated with the following formula:
[\text{RPM} = \left(\frac{\text{Voltage} \times k}{\text{Motor Constant}}\right)]
Without the specifics of the motor constant (which depends on the motor design) it's hard to give an exact number. Assuming you have the no-load voltage rating or typical specs for a 540 motor, they often run around:
- No-load: 20,000 - 30,000 RPM at 12V for small RC brushed motors.
If we consider that:
[\text{Electrical Speed Ratio} \approx \frac{7.4}{12}]This would scale down the maximum RPM accordingly.
A rough estimate might yield:If a no-load is around 20,000 RPM at 12V,[\text{RPM at 7.4V} = 20,000 \times \left(\frac{7.4}{12}\right) \approx 12,333 , \text{RPM}]
In summary:
- Estimated Current Draw: Approximately 7.21 A
- Estimated RPM: Approximately 12,333 RPM
These are rough estimates; real-world results may vary based on specific motor characteristics, efficiency losses, and other factors affecting performance.
---------
Question #2:
to keep the current at 10A, what would my propeller size be
Response #2:
To determine the propeller size required to keep the current draw below 10A, we can use the relationship between thrust, power, and current. We will set up the calculation based on the following relationships:
1. Power Calculation
We know that the current ((I)) relates to power ((P)) and voltage ((V)) as follows:
[I = \frac{P}{V}]
Given that we want the current ((I)) to be below 10A and the battery voltage is 7.4V, we can reverse this to find the maximum power:
[P = I \times V][P < 10A \times 7.4V = 74W]
2. Thrust Calculation
The power required to maintain thrust ((F)) can be calculated with the formula:
[P = F \cdot v]
Where:
- (v = 1.94 , m/s) (7 km/h).
3. Calculate Maximum Thrust for 74W
Rearranging, we can find the thrust corresponding to the maximum power:
[F = \frac{P}{v}][F < \frac{74W}{1.94 , m/s} \approx 38.14 N]
4. Determine Propeller Size
Now that we have the maximum thrust requirement, we can relate this back to propeller size. The thrust produced by a propeller roughly scales with its diameter squared (assuming the same motor and operating conditions). We previously calculated that a 3.8 cm propeller could produce approximately 44 N of thrust.
To find the required thrust-to-propeller diameter relationship, we can set up a ratio of thrusts:
Let (d_1) be the original diameter (3.8 cm) which produced approximately 44 N,
Let (d_2) be the new diameter we’re trying to find that produces up to 38.14 N.
Using the thrust area relation:[\frac{F_2}{F_1} \approx \left(\frac{d_2}{d_1}\right)^2][\frac{38.14 N}{44 N} \approx \left(\frac{d_2}{3.8 , cm}\right)^2]
5. Solving for (d_2)
Calculating the ratio:[\frac{38.14}{44} \approx 0.867][0.867 \approx \left(\frac{d_2}{3.8}\right)^2]Taking the square root:[\sqrt{0.867} \approx 0.933 = \frac{d_2}{3.8}]Calculating (d_2):[d_2 \approx 3.8 \times 0.933 \approx 3.55 , cm]
Conclusion
To keep the current draw below 10A while maintaining a speed of 7 km/h, the propeller size should be approximately
3.55 cm or smaller in diameter. However, this calculation is a rough estimate and actual performance will depend on various factors including propeller pitch, motor efficiency, and real-world conditions.
---------
by now my assumption is propeller size can't exceed
3.55cm. (all guesstimates here)
the boards recommended by the seller are as follows
RXG12 -
https://www.aliexpress.com/item/1005005685446335.html
RXG3 -
https://www.aliexpress.com/item/1005006617495908.html
No Model -
https://www.aliexpress.com/item/1005002924309678.html
suggestions?
