These formulas are for a estimate only. If you like to play with numbers these tools can be a major help in starting a new boat building project.

Please keep in mind you can plug any number you like into the fields. It doesn't mean that it will work! You have to have a basic understanding of the limits of your parts.

For example, a Feigao 10xl will spin a 45mm prop just fine in most 25-30" vee bottoms. But that same motor will not turn a 45mm prop in a 50" boat.

Its pretty simple to use the formulas. Anywhere there is the "field" shaded yellow, change the value to a numeric /  number value. Any letters or spaces in the field will cause a error in the formula. 

One of the formula's I have particular interest in is the "Estimate the top speed of a prop". Its amazing to see how much speed our hulls and props are wasting.

Have fun!

Estimate amp draw
 
Enter battery capacity in mAh
mAh
 
Enter run time in seconds
seconds
 
 
Estimated Average current
Amps
 
 
   
Estimate Runtime
 
Enter Cell Capacity in mAh
mAh
 
Enter Discharge current
Amps
 
 
Packs will last approximitly
Seconds
 
Minutes

Cell pack voltage and power output
under load
   
Enter number of cells Enter internal resistance of cell
Milliohms
    example 1.0 milliohms
Enter discharge current Enter Motor Efficiency
Amps %
 
 
Cell voltage under load is Power output @ motor efficiency
Volts Watts
Pack voltage under load is  
Volts Horsepower
Pack power output  
Watts  
 
Estimate your speed between two known points with a stopwatch
 
Distance between check points
Feet
 
Time taken to pass between check points
Seconds
 
 
Speed is
MPH
Km/h
meters/second
feet/second
Estimate the top speed of a prop.
On MOST Octura Props, first two letters or numbers designate the pitch.
Example of an Octura x438 propeller.
The X4 tells us the pitch is 1.4 and the diameter of the prop is 38mm
 
Prop
Diameter
in MM		 mm       Total
Motor RPM
Pitch type Enter Slip %
Total Pitch mm  
Total Pitch inches  
 
 
Pitch Speed No Slip Pitch Speed with Slip
mm/min mm/min
m/sec m/sec
km/h km/h
ft/sec ft/sec
MPH MPH
Speed Estimate by Use of Voltage & Constant "Kv"
 

 Feigao Motor RPM Chart
  Feigao Nemesis
540 S Size
 35.8mm x 51mm 225gr		 Feigao Nemesis
540 L Size
35.8mm x 61mm 282gr		 Feigao Nemesis
 540 XL Size
35.8mm x 75mm 372g
Wind rpm/volt Max
Amps		 rpm/volt Max
Amps		 rpm/volt Max
Amps
6 5915 108 4096 102 2779 110
7 5070 93 3511 88 2382 95
8 4436 81 3072 77 2084 83
9 3943 72 2731 68 1853 74
10 3549 65 2458 61 1668 66
11 3226 59 2234 56 1516 60
12 2958 54 2048 51 1390 55
13 2730 50 1890 47 1283 51
14 2535 46 1755 44 1191 47

 

Neu Motor Chart
Model  RPM/Volt Shaft Size Weight SM/F Length (in) Rm (Ohms) Io@10v
1506/1D 9500 5mm 6.0/6.35 1.8 0.0015 9.0
1506/1.5D 6800 5mm 6.0/6.35 1.8 0.0025 5.0
1506/1Y 5500 5mm 6.0/6.35 1.8 0.003 3.2
1506/2D 5200 5mm 6.0/6.35 1.8 0.004 3.0
1506/3D 3400 5mm 6.0/6.35 1.8 0.008 2.0
1506/3Y 1700 5mm 6.0/6.35 1.8 0.029 1.2
             
1509/1D 6700 5mm 7.48/7.94 2.1 0.002 6.0
1509/1.5D 4500 5mm 7.48/7.94 2.1 0.004 3.8
1509/1Y 3600 5mm 7.48/7.94 2.1 0.004 3.0
1509/2.5D 2700 5mm 7.48/7.94 2.1 0.0082 2.5
1509/1.5Y 2400 5mm 7.48/7.94 2.1 0.007 2.0
1509/2Y 1820 5mm 7.48/7.94 2.1 0.018 1.5
             
1512/1D 4875 5mm 9.88/10.23 2.4 0.002 6.0
1512/1.5D 3200 5mm 9.88/10.23 2.4 0.0035 3.0
1512/2D 2600 5mm 9.88/10.23 2.4 0.006 2.3
1512/2.5D 2000 5mm 9.88/10.23 2.4    
1512/3D 1700 5mm 9.88/10.23 2.4 0.012 1.5
1512/1.5Y 1900 5mm 9.88/10.23 2.4 0.009 1.8
1512/2Y 1400 5mm 9.88/10.23 2.4 0.015 1.1
             
1515/1D 4100 5mm 12.00/12.70 2.7 0.0025 5.5
1515/1.5D 2700 5mm 12.00/12.70 2.7 0.004 4.5
1515/1Y 2200 5mm 12.00/12.70 2.7 0.006 2.5
1515/2.5D 1700 5mm 12.00/12.70 2.7 0.01 2.3
1515/1.5Y 1500 5mm 12.00/12.70 2.7 0.011 1.7
1515/3D 1360 5mm 12.00/12.70 2.7 0.012 1.2
1515/2Y 1100 5mm 12.00/12.70 2.7 0.019 1.1
             
1521/1D 2800 5mm or 8mm 16.93 3.3 0.003 5.5
1521/1.5D 1860 5mm or 8mm 16.93 3.3 0.005 3.5
1521/1Y 1577 5mm or 8mm 16.93 3.3 0.008 2.2
1521/1.5Y 1050 5mm or 8mm 16.93 3.3 0.01 1.2
1521/2Y 780 5mm or 8mm 16.93 3.3 0.025 1.0
             
1527/1D 2160 5mm or 8mm 21.8 4 0.002 5.0
1527/1.5Y 850 5mm or 8mm 21.8 4 0.012 1.5
           
1527/1Y   1250 5mm or 8mm 21.5 4    
1527/2Y 625 5mm or 8mm 21.5 4    
All 15 series motors have 12 slot stators and 4 poles rotors