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  
Input your boats information in the "yellow" fields below.
See below this conversion table for some of the Motor Kv's.
Motor information
Enter Unloaded RPM/Volt "KV". Most times this is the stated Manufacturer Kv
RPM
Torque constant "Kt" is:
in inch-ounces/amp
Enter Motor Efficiency
%    This is an estimated #. 70% for Brushed. 80-90% for brushless, Depending on the Motor quality. Neu, Leopard motors are in the 90% area, while less expensive Feigaos are in the 80% range.
Estimated RPM/Volt Under Load is:
RPM               
Battery Pack information
Pack Voltage   Number of cells in series? Volts/cell?(1.2v for Nimh, 3.7v for Lipo)
 
Estimated loaded RPM is
      RPM @      
Volts
Propeller Information:  Total Prop Pitch is in theory how many millimeters forward the boat will move with each propeller revolution.
Total Prop Pitch is: Enter Prop Pitch Ratio Enter Prop Diameter in Millimeters
Millimeters
Gear Box Info:  Most brushless fast electrics boats are running direct drive, so this area doesn't need changing.
Enter Pinion Gear Teeth (enter 1 if direct drive)
 
Enter Spur Gear Teeth (enter 1 if direct drive)
Final Gear Ratio is:
:1  
Hull Slippage Info:  For each hull there is an amount of prop slip. This make some hull designs faster then others. As a guideline, Estimated prop slippage for Mono Hull/Deep Vee is approx 25% : Full Bodied Hydros appox 20% and Riggers approx 15%
Enter Prop Slippage   
%  
Estimated Top Speed of:  
Km/H
MPH

 

 Feigao Motor RPM Chart
As of June 2009, Feigao has changed KV ratings of their motors. This chart contains the 1st generation of motors.
  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


 

NEW!! Feigao Motor RPM Chart
As of June 2009, Feigao has changed KV ratings of their motors. This chart contains the 2nd generation of motors.
   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 Max Amps rpm/volt
kv		 Max Amps rpm/volt
kv		 Max Amps rpm/kv
kv
4 160 8450        
5 130 7289 123 4893 121 3324
6 108 6192 102 4116 110 2871
7 93 5196 88 3566 95 2370
8 81 4761 77 3201 83 2073
9 72 4332 68 2887 74 1898
10 65 3783 61 2602 66 1734
11 59 3440 56 2326 60 1577
12 54 3180 51 2159 55 1445
13 50 2946 47 1999 51 1339
14 46 2775 44 1828 47 1260
15 43 2590 41 1760 44 1176
16 41 2400 38 1590 41 1092
17 38 2259 36 1450 39 1035
18 36 2064 34 1410 37 950

 

Feigao 580L Chart
Turns Max Amps Resistance RPM/KV IO(a)
6 120 0.0044 2154 6.3
7 102 0.0062 1894 3.5
8 91 0.0078 1649 2.6
9 85 0.0090 1426 2.3
10 74 0.0118 1326 1.9
12 63 0.0163 1051 1.5

 

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/.5Y 4400 5mm 12.00/12.70 2.7    
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 2900 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
1527/1.5D 1500 5mm or 8mm 21.8 4    
1527/.5Y 2500 5mm or 8mm 21.8 4    
1527/1Y   1250 5mm or 8mm 21.5 4    
1527/1.5Y 850 5mm or 8mm 21.8 4    
           
2215 1Y 1440 8mm 24.8 3.3    
2215 1.5Y 1020 8mm 24.8 3.3    
2215 2Y 750 8mm 24.8 3.3    
           
2230 1Y 725 8mm 48 4.7    
2230 1.5Y 485 8mm 48 4.7    
           
           
All 15 series motors have 12 slot stators and 4 poles rotors